Thrust system design tips?
aols...@my-dejanews.com
fully loaded and I need to move it with some authority over a body of water.
Say that I know I'm going to provide power via a 25HP (4-cycle) engine and the
nature of the hull limits me to... say... a 60" diameter propeller.
What is the rule of thumb for pounds of thrust that must be provided by the
thrust prop? 10% of the all-up weight? 20%? Some other starting point
perhaps? How do you determine the number of blades? By personal preference?
Would you possibly be willing to offer the title of a book on the subject?
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SEVTEC
You could use the 60" plastic prop as used on the Vanguard ($225) as these
props are being run on a single 20hp Vanguard with no problems. However, The
preferred prop would be the 72" as you will get more thrust and lower blade tip
speeds, as I have on Surveyor ($430 price is steep, though). Gearing info
will be supplied with propeller purchase. Curiously enough, the hull
dimensions as well as craft weight figure into selection of the propeller
diameter, so the problem is a lot more complicated than you might think if a
near optimum selection (if there is such a thing) is your objective.
I would avoid soft wood props, even if glass covered, as erosion will be a
short term problem, and eventually most wood props rot if stored outdoors. The
hardwood props I used to sell proved that if nature grows anything, nature will
also provide something to eat it. (Under 3 coats of Urethane varnish, no
less). Ask anyone who has run a wood prop on a surface skimmer for long term
and you will find this is no place to try to save money, and the plastic props
will then look to be a bargain. A plastic prop, when matched correctly to the
engine, will perform better than a wood prop due to more precise airfoils,
also.
Ducts are also to be avoided, as while they gain for you at very low speeds,
they destroy performance at even modest speeds, and headwinds, unless the
propeller (fan) is badly overloaded in the first place.
In general, the larger diameter the prop, and the wider the blades, the lower
the tip speed and noise. If you get the blades too wide you will get a "feel
good" acceleration off the line but the installation may be weak when you
really need it, when getting back home into a wind. If you get the propeller
too large in diameter the drive and propeller will use up the added performance
benefits in cost and weight.
You can calculate the static thrust for a well matched propeller with the
formula T=7.27*(P*D)^2/3 where thrust is in lb, P in horsepower, and D in feet.
This does not tell you whether the performance will stay up in that headwind,
however, (or whether or not the engine manufacturer is fibbing about the power
output of his engine.)
Barry Palmer, for <A
HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
SEVTEC
The formula is for propellers designed for static thrust and not encumbered
with guards and rudders, as well as the hull and engine airflow blockage. The
builder should expect to measure from 70 to 90% of the value determined by the
formula if the test is near standard atmosphere and the surface skimmer is
trimmed on a level surface and there is no wind.
Craft that are optimized for cruise conditions may be on the lower side of the
thrust values, as a result of having greater performance in the higher speed
regions.
aols...@my-dejanews.com
Wow, thanks for the wealth of information. I'm going off to play with that
equation but first I have one more question. What's the rule of thumb for the
static thrust value that I'm looking for? It's based on the weight of the
craft right?
- Alex O.
In article <19980925211923...@ng96.aol.com>,
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SEVTEC
The thrust you are after is based on the weight of the craft, its cushion width
and length. Like any design, it is a juggling act to get the right
configuration.
While most people figure out what is required to make "hump" or planeout I use
old NACA propeller performance reports (non-dimensional data) to determine
propeller thrust through the speed range so I can determine thrust over hump
into a headwind, which for a cruiser is really the determining factor for
propeller sizing.
A good rule of thumb is that the static thrust calculated with the formula
should be at least 10% of the craft gross weight. Using this rule of thumb
does not guarantee a planeout if your cushion pressure is too high.
The easiest way to solve the thrust problem is to install lots of thrust power,
using a light weight engine. However, while this will yield high speeds it
results in a poor cruising craft from the standpoints that the high power
installation takes payload away and lots of fuel use causes problems. I know a
guy who cruises a Starship 500XL, who does all right cruising - - - as long as
his two buddies, who have Vanguards, come along to serve as tankers. (The
ultimate will be when he is able to pull up behind for hover to hover
re-fueling.)
Hoverlord
Thats interesting that Barry should be giving advice about thrust technology...
Considering that he's somewhere back in the 70's with his designs...
C. Bailey
group, and I hope he continues to offer his comments. I for one, am quite
interested in what Barry has to say. I am not in a place to comment about
whether or not his comments are "up with the latest technology", but I think
his crafts speak for themselves. Who else can do more with an 18 HP
engine???
Chris
Hoverlord wrote in message <19981010153703...@ng99.aol.com>...
J. Kelley Jernigan
I'm curious why you say Barry is 20 years behind the times? Can you name any other
designer who can get as much performance out of an 18 hp lawnmower engine? Or any
other hovercraft that can carry over 2000 pounds at up to 40 mph on 53 hp? No other
craft has a controllable air cushion. All other skirt designs date back to the 50's
and 60's. His craft are quieter and more fuel effiencent that just about any design
out there.
Kelley
SEVTEC
<<Subject: Re: Thrust system design tips?
<<From: hove...@aol.com (Hoverlord)
<<Date: Sat, Oct 10, 1998 7:37 PM
<<Message-id: <19981010153703...@ng99.aol.com>
<<Thats interesting that Barry should be giving advice about thrust
technology...
<<Considering that he's somewhere back in the 70's with his designs...
Dear Hoverlord: (Mike Chrisman)
Thank you for the criticism. But you are wrong, the propeller technology is
not 1970's but 1938'ish. The problem is that certain "engineeringly
challenged" technologists will not look at what was resolved in the past.
Your first attack was clearly emotional. Now if you wish to attack
strategically, I can be found at <A
HREF="http://www.terraserver.microsoft.com/GetTileByClick.asp?XId=5440&YId
=4416&SrcId=1&ImgDate=08/04/1990&ImgSize=0&DSize=0&W=225&H=150&ClickAt=?11
,48">Microsoft TerraServer Full Resolution Image PagÉ</A> (it's the middle
house in the tight group of three, with the two rock bars in front.)
However, I would much prefer attacks of the intellectual/technical variety,
and maybe we could get something accomplished, rather than have this newsgroup
become one large catfight, as a similar newsgroup about homebuilt aircraft has
become.
CedarDecks
Good one Barry!! Was that a white house, or green? LOLROF
Cedar Decks
Chasmo
much improved. Shunning is a great concept!
I can only echo his sentiment "Hoverlord".
"If you can't say something nice, don't say anything at all".
Chasmo
Change the X to m to reply.
Back to full time lurking mode.
--
Charles Black
Monty Roberts
Just because a technology is old does not mean it is obsolete. A large
diameter propeller is more efficient than a small diameter fan. period.
Whether or not a large diameter propeller is best for a particular
design is dependant on the design goals and overal objective. I would
like to open a discussion on thruster design in this newsgroup because
it is one of the areas people seem to argue about most. I hope the
disscussion will remain constructive.
Question one: Would a ducted Prop/Fan arragement be better for a high
power craft, say one designed with the primary objective, being speed,
acceleration, and minimum noise (while still meeting the first two
primary objectives)
My argument that it is goes as follows:
In a craft with a large power to weight ratio, keeping the disk loading
down requires a prop that is too large to be practical. clearance under
obstacles such as bridges becomes a problem. (at least in my area).
Maintaining a prop type of design that will absorb the horsepower and
maintain dimensional and weight requirements results in excessive tip
speed and noise. This also contributes to tip erosion. High speed
stability should be enhanced by the duct acting as both a horizontal and
vertical stabilizer. Structural efficiency is obtained by using the duct
as a gaurd as well as a mounting point for rudders. Stalling should be
minimal for a properly designed duct because of the pressure gradient in
the inlet of a highly loaded rotor. Tip vortex noise is reduced by the
duct as well as sound print to the sides of the vehicle. I have no hard
numbers at this time, but there must be a point in power to area ratio
where a duct out-performs an open prop. It will never, be as efficient
however.
Monty
SEVTEC
/From: Monty Roberts <montyr...@worldnet.att.net>
/Date: Thu, Oct 15, 1998 2:17 AM
/Message-id: <35AF82...@worldnet.att.net>
/Whether or not a large diameter propeller is best for a particular
/design is dependant on the design goals and overal objective. I would
/like to open a discussion on thruster design in this newsgroup because
/it is one of the areas people seem to argue about most. I hope the
/disscussion will remain constructive.
/I am also curious what was meant by the statement about 1950's tech.
/Just because a technology is old does not mean it is obsolete. A large
/diameter propeller is more efficient than a small diameter fan. period.
The propeller technology I use is 1930's technology and the ducted fan
technology, while somewhat newer, is incompletely done, as it ignores the
extreme sensitivity of ducts to lose performance when seemingly minor flow
disturbances are imposed on them (or structurally reasonable blade tip
clearances are used).
While one may think that efficiency is not important, as you can just get a
bigger, more frantic engine to do the job, the lack of efficiency "ripples"
through the entire skimmer design, making it non-competitive costwise with
other ways of doing a task.
For instance, the added power means more fuel consumption, which means a
larger, heavier hull, which means a larger lift system, and the larger lift
system requires an even larger hull . . . and around and around with everything
else in the system.
While it would be difficult to express the complexity mathematically, here, as
it takes extensive modelling, just think. The Explorer we flew from Anacortes
to Juneau, Alaska with sometimes as much as 1300lbs aboard cruised along at
around 30hp at about the same cruising speed as smaller craft. This is about
what a "two Place" Scat does with 310 lb aboard does. But that does not
complete the comparison.
Change the engine in the Scat from its frantic, unreliable 75 lb engine to 250
lbs (the weight of the Explorer engine) so that you are comparing machines with
equivalent engine reliability at 30hp, and the Scat is now a 120 lb payload
craft. And we still haven't fixed the Scat hull to gain acceptable seakeeping
(to its scale with the Explorer.) and paid for that additional weight.
/Question one: Would a ducted Prop/Fan arragement be better for a high
/power craft, say one designed with the primary objective, being speed,
/acceleration, and minimum noise (while still meeting the first two
/primary objectives)
The low efficiency duct might be preferred only if it is desired to race on
tight closed courses, and the competitors have no desire to flense each other
with the propellers. However, with a little development, a guarded open
propeller machine will easily beat any ducted machine, even on a tight course.
If a propeller that will self destruct benignly is used the guarded propeller
would be safer than a duct as the problem of pushing ducts into thruster rotors
would be much reduced.
/My argument that it is goes as follows:
/In a craft with a large power to weight ratio, keeping the disk loading
/down requires a prop that is too large to be practical. clearance under
/obstacles such as bridges becomes a problem. (at least in my area).
/Maintaining a prop type of design that will absorb the horsepower and
/maintain dimensional and weight requirements results in excessive tip
/speed and noise. This also contributes to tip erosion. High speed
/stability should be enhanced by the duct acting as both a horizontal and
/vertical stabilizer. Structural efficiency is obtained by using the duct
/as a gaurd as well as a mounting point for rudders. Stalling should be
/minimal for a properly designed duct because of the pressure gradient in
/the inlet of a highly loaded rotor. Tip vortex noise is reduced by the
/duct as well as sound print to the sides of the vehicle. I have no hard
/numbers at this time, but there must be a point in power to area ratio
/where a duct out-performs an open prop. It will never, be as efficient
/however.
Not at all. A propeller on a surface skimmer will be quieter, as any
reasonable installation to current power plants will involve a shaft speed
reducer. Typical English Racing type hovercraft, such as Scat, Hovertechnics,
AeroAmerica, et al, have fan propulsors that are around (above and below) the
noise levels of their frantic engines. Ducted thrusters are well noted for
low performance at high speed, which does not get so high on the closed tight
courses these craft use in competition. Added directional stability of a duct
is actually a disadvantage, and that is one of the reasons the English racing
type of craft require lots of "body English" to steer. A duct should never be
considered as a guard, and indeed, for comparison equivalency, to a guarded
open prop it should have some sort of frame around it.
This leaves "looking good" to the uninitiated, and passing under extremely low
bridges the only attributes for the ducted thruster. The second attribute can
be obtained with the open propeller, with a little clever design.
/Monty
aols...@my-dejanews.com
Monty mentioned that a large prop is cumbersome and you mentioned that a large
prop's benefits outweigh its disadvantages compared to a smaller, ducted prop
but would 2 smaller [guarded] open props mounted side by side each offering
around 50% of the thrust (at a similar RPM?) of the large prop be a valid
solution? One of your designs uses this concept doesn't it?
- Alex O.
In article <19981015025243...@ng29.aol.com>,
sev...@aol.com (SEVTEC) wrote:
<snip!>
>
> This leaves "looking good" to the uninitiated, and passing under extremely low
> bridges the only attributes for the ducted thruster. The second attribute can
> be obtained with the open propeller, with a little clever design.
>
> /Monty
>
> Barry Palmer, for <A
> HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
>
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http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
pthe...@mcn.net
obvious that ducts become inefficient at higher speeds. Does anyone remember
Moller VTOL's? His ducted fans have a ring around the tips that rotates with
the fan. This amounts to zero tip clearance. Never seen one on a hovercraft,
but this must work better. Another problem with ducted fans involves
transportation--a strong crosswind might tip over your trailer on the
highway. Also, when hovering, a duct will probably weathervane causing
control problems especially at low speeds in tight areas. The smaller ducts
probably don't experience this effect as much. In article
<7050qa$tgr$1...@nnrp1.dejanews.com>, aols...@my-dejanews.com wrote:
> Barry,
>
> Monty mentioned that a large prop is cumbersome and you mentioned that a large
> prop's benefits outweigh its disadvantages compared to a smaller, ducted prop
> but would 2 smaller [guarded] open props mounted side by side each offering
> around 50% of the thrust (at a similar RPM?) of the large prop be a valid
> solution? One of your designs uses this concept doesn't it?
>
> - Alex O.
>
> In article <19981015025243...@ng29.aol.com>,
> sev...@aol.com (SEVTEC) wrote:
>
> <snip!>
>
> >
> > This leaves "looking good" to the uninitiated, and passing under extremely
low
> > bridges the only attributes for the ducted thruster. The second attribute
can
> > be obtained with the open propeller, with a little clever design.
> >
> > /Monty
> >
> > Barry Palmer, for <A
> > HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
> >
>
bo...@freenet.carleton.ca
I'd like to take issue with a number of things you pointed out in your reply
to Monty.
Before I do, let me add my agreement that an open propeller is a more
efficient propulsive device than a ducted fan when the vehicle is up to a
suitable cruising speed. A ducted fan is far more efficient in a static
condition and up to about 20 mph (depending upon the duct design, tip
clearances and a couple of other items). Recently, I happen to witness the
tip vortices produced by a propellor on a particularly humid day. It was
quite striking to visually see the recirculation at the propellor tips
gradually disappear as the vehicle reached about 20 mph. Statically, I'd
estimate that fully 25% of the rotor was not being used due to the
recirculation of air at the tips of the blades.
I think you have to ask yourself, "What am I trying to achieve when designing
an air cushion vehicle propulsor?" High acceleration or efficient cruising.
You compared a Scat directly to your SEV on what was virtually an open sea
voyage. Of course the SEV will win such a comparison. Do the same comparison
around, say, the US National Rally circuit at Troy and I'm afraid the Scat
will lap the Sevtec before the end of the race. You are, virtually,
comparing apples and oranges and declaring a "hands down" winner.
Your SEV are marvels of technology at being *very* efficient load carriers.
The Scat was never designed to be a load carrier but, rather, a small, fun to
play watercraft toy. Which is what it is. Therefore, it does what it was
designed for and, ergo, it is just as good at doing what it was designed for
as your SEV is.
You spoke of the "disadvantage" of having to use weight shifting to maneuver
the "english type" hovercraft. I directly dispute this as being a
disadvantage but rather, an *advantage*! I'm not sure of your racing
experience but I've raced for quite a few years now in several counries and
it just "feels right" to use your body for fine control when jostling for
position at high speed. I don't think I'd feel too comfortable relying
entirely upon aerodynamic control when the air is blasting at me from all
quarters. The same goes for playing around with a Scat: it just "feels"
right. As you move to a larger machine where you are not surrounded by 20
other wind generators than that is a different story. We're back to the
apple and oranges thing again. You can't compare a small, light, sporting
hovercraft with one designed to cruise.
One final point is the use of "cages" as guarding for propellers. A guard is
good for one thing: keeping things out of the propellor. It will NOT keep a
destroyed propeller within the cage. I have seen many explosions of both
ducted fans and caged propellors (although many more fans than propellors :-)
and the propellor will typically send sharpened spikes right through the mesh
at unheard of velocities. Fortunately, I've never heard of an injury due to
these flying spikes but I have found one embedded in the ground over 100 feet
away from the explosion site. On the other hand, I've yet to see a duct
designed to World Hovercraft Federation specifications fail to keep the
debris from either a fan or a propellor within its peripheral boundaries. I
should also add that I have had the debris from a broken multi-wing
accelerated *rearwards* from the duct to strike me but at a much slower
velocity than it would have had should it have come from the tip of a
spinning fan. I will then argue that a properly constructed duct can
certainly be considered a guard - much more so that some of the frail looking
netting I've seen some builders put on Sevtecs. (Yes, I know of one who
strung a 2" sq. nylon netting around an aluminum frame as his "guard").
Whew, sorry for ranting on.
Your turn, Barry!
Bob Rennick
SEVTEC
;Date: Thu, Oct 15, 1998 5:12 PM
;Message-id: <705ael$8mq$1...@nnrp1.dejanews.com>
;Okay, Barry, my turn to enter the fray !! :-)
;I'd like to take issue with a number of things you pointed out in your reply
;to Monty.
;Before I do, let me add my agreement that an open propeller is a more
;efficient propulsive device than a ducted fan when the vehicle is up to a
;suitable cruising speed. A ducted fan is far more efficient in a static
;condition and up to about 20 mph (depending upon the duct design, tip
;clearances and a couple of other items). Recently, I happen to witness the
;tip vortices produced by a propellor on a particularly humid day. It was
;quite striking to visually see the recirculation at the propellor tips
;gradually disappear as the vehicle reached about 20 mph. Statically, I'd
;estimate that fully 25% of the rotor was not being used due to the
;recirculation of air at the tips of the blades.
So what! The open propeller is still more efficient. You cannot generalize
that the duct is more efficient up to 20mph, as this is a function of both
propeller and duct disc loading and varies depending on the size of the surface
skimmer and top speed potential(Horsepower per square foot of disc area).
;I think you have to ask yourself, "What am I trying to achieve when designing
;an air cushion vehicle propulsor?" High acceleration or efficient cruising.
;You compared a Scat directly to your SEV on what was virtually an open sea
;voyage. Of course the SEV will win such a comparison. Do the same comparison
;around, say, the US National Rally circuit at Troy and I'm afraid the Scat
;will lap the Sevtec before the end of the race. You are, virtually,
;comparing apples and oranges and declaring a "hands down" winner.
You are forgetting, these craft are, first of all, vehicles. If you prostitute
the design for some purpose of ego massage, rather than a real world
usefulness, there will not ever be very many of them. It is not likely that
the Scat was designed for racing, as there is no significant racing market,
but was originally sold to backing as a sports machine. It is obvious to me
that it is merely a copy of an English racing craft, as its "designer" knew no
other technology. If the Scat raced in the real world, rather than on some
golf course and lilly pond environment as usually found in racing, the outcome
of the race with the Explorer would be much different.
;Your SEV are marvels of technology at being *very* efficient load carriers.
;The Scat was never designed to be a load carrier but, rather, a small, fun to
;play watercraft toy. Which is what it is. Therefore, it does what it was
;designed for and, ergo, it is just as good at doing what it was designed for
;as your SEV is.
I do not mind small fun to play watercraft toy, I just do not like it to be
totally obnoxious to the non-participating people who may be around it. After
all you can build such a craft with a $300 engine, rather than a $2500 engine
and although it may not be quite as fast it can be a lot of fun.
;You spoke of the "disadvantage" of having to use weight shifting to maneuver
;the "english type" hovercraft. I directly dispute this as being a
;disadvantage but rather, an *advantage*! I'm not sure of your racing
;experience but I've raced for quite a few years now in several counries and
;it just "feels right" to use your body for fine control when jostling for
;position at high speed. I don't think I'd feel too comfortable relying
;entirely upon aerodynamic control when the air is blasting at me from all
;quarters. The same goes for playing around with a Scat: it just "feels"
;right. As you move to a larger machine where you are not surrounded by
20
;other wind generators than that is a different story. We're back to the
;apple and oranges thing again. You can't compare a small, light,
sporting
;hovercraft with one designed to cruise.
I am just expressing the fact that the standard English racing craft designs
are not "complete" so maybe other designers would quit scaling up the
technology, only to discover that the passengers in the large craft have to
lean to get the fool thing to turn! Lets address surface skimmer problems and
solve them! I would lean in a Sevtec craft in a tight race situation, also, as
control is quicker, but at least one should be able to sit back and relax at
times.
;One final point is the use of "cages" as guarding for propellers. A guard is
;good for one thing: keeping things out of the propellor. It will NOT keep a
;destroyed propeller within the cage. I have seen many explosions of both
;ducted fans and caged propellors (although many more fans than propellors :-)
;and the propellor will typically send sharpened spikes right through the mesh
;at unheard of velocities. Fortunately, I've never heard of an injury due to
;these flying spikes but I have found one embedded in the ground over 100 feet
;away from the explosion site. On the other hand, I've yet to see a duct
;designed to World Hovercraft Federation specifications fail to keep the
;debris from either a fan or a propellor within its peripheral boundaries. I
;should also add that I have had the debris from a broken multi-wing
;accelerated *rearwards* from the duct to strike me but at a much slower
;velocity than it would have had should it have come from the tip of a
;spinning fan. I will then argue that a properly constructed duct can
;certainly be considered a guard - much more so that some of the frail looking
;netting I've seen some builders put on Sevtecs. (Yes, I know of one who
;strung a 2" sq. nylon netting around an aluminum frame as his "guard").
I have a video of an English racing craft upsetting and throwing fan blade
pieces out the front. Isn't that why they wear leathers when they race? As
far as the guarding goes, it should be noted from my plans that I always put
wire in the plane of propeller rotation, and it is ok to omit upper guarding if
the craft is used as a cruiser. The netting is used over the front of the
guard, only.
I have scattered one of my Birch props, an 82 incher, without throwing any
lethal parts. It is all in how you design it.
Why don't we go racing with less expensive, fun machines, rather than catering
to people who are willing to dump their pocketbooks into over-powered, totally
obnoxious craft that are good for absolutely nothing other than racing on
manicured tight little courses? I think the sport might see some growth in the
sport if that were so.
;Whew, sorry for ranting on.
;Your turn, Barry!
;Bob Rennick
Barry Palmer, for Sevtec
SEVTEC
;the affair with more HP.
'Fraid not. To gain efficiency you need disc area, and adding a fan in the
slipstream of another fan is adding a fan that sees a big headwind, that
produced by the other fan.
SEVTEC
;Monty mentioned that a large prop is cumbersome and you mentioned that a large
;prop's benefits outweigh its disadvantages compared to a smaller, ducted prop
;but would 2 smaller [guarded] open props mounted side by side each offering
;around 50% of the thrust (at a similar RPM?) of the large prop be a valid
;solution? One of your designs uses this concept doesn't it?
;- Alex O.
Two props of about 70% of the diameter of a single prop installation will
reduce overall height of the surface skimmer somewhat at the expense of more
complexity. To really get height down you need a mechanism to lay the prop
stand down aft, powered or not. You could shut down the prop, lay it flat,
and open cushion air aft vents to get under the obstacle if your craft has a
high cushion air flow volume and current under the obstacle would allow it. Or
you can merely shut down, lay the prop flat, and get out the boat hook.
SEVTEC
drive
: ;the affair with more HP.
: ; goog...@ripco.com
It is possible, but unless you intend to build a craft to just fit through a
culvert, you would get more static and cruise thrust by eliminating the duct
entirely and putting correctly designed propellers of the same diameter as the
duct bore side by side with the same added power.
Monty Roberts
I noticed a comment in one of the postings asking why airplanes don't
use duct guards. Airplanes fly at much higher speeds than most ground
effect machines will ever see, at least ones that rely on fan generated
static pressure under the craft. The ideal propeller diam. becomes
smaller as speed increases. The drag of the duct also increases rapidly
with speed, ducts therefore become cumbersome. Hovercraft which operate
typically below 80 mph, at the fastest, would have very large propellers
by aircraft standards. Airplane propellers are also not typically
designed for these low speeds. The one exception is ultralight aircraft
propellers. ultralights still cruise quite a bit faster than a typical
hover, but they are comperable. So this is probably the best current
solution to the cruise propulsion issue.
The observation of the tip vortex below 20 mph is an interesting one. It
would be interesting to try to correlate speed, disk loading, and
propusive efficiency in lbs of thrust/horsepower for props and ducted
propulsors. I am hesitant to call them ducted fans, because the design
is different. A fan typically would be designed to produce static
pressure. We need mass flow to produce thrust, and there are some
differences involved. Another issue is the fact that most fans used for
propulsion in hovercraft cannot really be considered ducted. They are
more appropriately called shrouded. The difference is that the duct
length is not enough to prevent recirculation from the back to the front
of the propulsor. This degrades performance at low speed.
I understand Barry's opinion on design. It is true that compromising on
efficiency ripples through the design. I have experienced this myself
while designing craft. You are better off saving weight than adding
power. However, if you are after the ultimate in performance, you must
try some different things. Airliners don't do well in a dogfight against
F-15's and F-15's will never win a distance or payload and comfort
contest with an airliner. It boils down to what do you want to do. I
don't really think that a design is being prostituted if it is designed
specifically for racing. For example: Formula one cars are marvels of
engineering, and LOUD!! Thing is, they don't have to be loud. They could
be made quiet, and just as fast, but it wouldn't be the same kind of
show. People go for the spectical. However, you wouldn't want one of
these guys tearing around your neighborhood while you were having a
barbeque. So those of us who are engaged in cruising should try to keep
our craft as quiet as possible so we don't make a lot of enemies and
thos of us who like to go fast should give people at the races a big
show and a lot of fun.
I think this has been an excellent disscussion so far and hope it will
continue. Some hard numbers would be nice, but who has time to do the
research? Not I. Perhaps after we have beaten this topic to death, we
could attack skirt design.
Monty
SEVTEC
;Date: Sat, Oct 17, 1998 3:24 AM
;Message-id: <35AFF3...@worldnet.att.net>
;Greetings all,
; I noticed a comment in one of the postings asking why airplanes don't
;use duct guards. Airplanes fly at much higher speeds than most ground
;effect machines will ever see, at least ones that rely on fan generated
;static pressure under the craft. The ideal propeller diam. becomes
<cut>
Have you done the numbers to show that the hovercraft should have to have a
duct and an airplane should have a propeller because of the higher speed of the
aircraft? Are you just "arm waving" ( as engineers call discussion that is
unsubstantiated and presented as fact) or have you done the numbers to verify
this.
I have done the numbers and I presented them at the 1984 "CACTS International
Conference on Air Cushion Technology" in Vancouver, and if you wish to pull
this reference you will find that even the AP1-88 should have had propellers,
not ducts.
As far as racing craft go, they are hardly technological marvels, with the
power loadings they have they should be going straight up!
SEVTEC
From: Monty Roberts <montyr...@worldnet.att.net>
Date: Sat, Oct 17, 1998 3:24 AM
Message-id: <35AFF3...@worldnet.att.net>
Greetings all
<cut>
The observation of the tip vortex below 20 mph is an interesting one. It
would be interesting to try to correlate speed, disk loading, and
propusive efficiency in lbs of thrust/horsepower for props and ducted
propulsors. I am hesitant to call them ducted fans, because the design
is different. A fan typically would be designed to produce static
<cut>
All you have to do is go to any serious University library with an engineering
program and you will find this is already done. Do not look at NASA reports,
but do look at NACA reports, as it has been done loooooong ago, and is there
for anyone to view.
SEVTEC
From: Monty Roberts <montyr...@worldnet.att.net>
Date: Sat, Oct 17, 1998 3:24 AM
Message-id: <35AFF3...@worldnet.att.net>
show. People go for the spectical. However, you wouldn't want one of
these guys tearing around your neighborhood while you were having a
barbeque. So those of us who are engaged in cruising should try to keep
our craft as quiet as possible so we don't make a lot of enemies and
thos of us who like to go fast should give people at the races a big
show and a lot of fun.
It seems to me that you could have just as much fun with well designed craft
with smaller, quieter engines that could race out of the lily pond and golf
course environment, and then go cruising. That would be a far more marketable
product than craft that ventilate their pistons and upchuck fan blades along
with an occasional chunck of fan hub.
With an expanding market, parts specially designed for this type of vehicle
could start. Just look at other racing programs, and it will be obvious that
we are microscopic. Look at go-karts and the elegant hardware that is
marketed for them. (But remember, the biggest market is that little old
lawnmower powered kart that you might see at a Fred Meyer or A Sears store.)
David M.
ones done by NASA and by the Army. The reports did not specifically
compare prop vs ducted fans (defined as: duct length >= fan diameter),
but each report was favorable. Some specific implementations were
flawed, but that happens with props, too. This was all for aircraft.
Hovercraft, being a totally different bird, may not yield similar
performance.
I'm not trying to be argumentative. I guess we read different reports.
David M.
SEVTEC wrote:
>
> Subject: Re: Thrust system design tips?
> From: Monty Roberts <montyr...@worldnet.att.net>
> Date: Sat, Oct 17, 1998 3:24 AM
> Message-id: <35AFF3...@worldnet.att.net>
>
> Greetings all
> <cut>
> The observation of the tip vortex below 20 mph is an interesting one. It
> would be interesting to try to correlate speed, disk loading, and
> propusive efficiency in lbs of thrust/horsepower for props and ducted
> propulsors. I am hesitant to call them ducted fans, because the design
> is different. A fan typically would be designed to produce static
> <cut>
> All you have to do is go to any serious University library with an engineering
> program and you will find this is already done. Do not look at NASA reports,
> but do look at NACA reports, as it has been done loooooong ago, and is there
> for anyone to view.
>
SEVTEC
;From: "David M." <dmer...@HiWAAY.net>
;Date: Sat, Oct 17, 1998 10:37 PM
;Message-id: <36291C27...@HiWAAY.net>
;Well, Barry, I have read those old reports, as well as fairly recent
David M.
The reports are there, for you to understand, and then it is your job to do
the comparing. I would suggest NACA TR 640 and NASA TN D-4142, there are
others. Don't believe everything you read. I have an advantage in that I have
tested both ducts and open propellers in real installations, and can compare
with a real world baseline rather than the free unobstructed inlet conditions
in wind tunnels as in the reports.
The upstream interference problem with ducts can really only be studied in a
wind tunnel, an ideal program for some collegiate environment. An arguement
for ducts can be made by assuming the duct sees a lowered inlet velocity due to
body drag of the vehicle hull, whereas a propeller installation might be placed
so that parts of it are exposed to free air, or maybe the propeller is
installed above the vehicle well into the free stream. Also, another
important influence of little interest (except for hover flight) to the
aircraft people is the influence of the ground plane on duct and propeller
performance.
David M.
><<<snip>>>
>
> The reports are there, for you to understand, and then it is your job to do
> the comparing. I would suggest NACA TR 640 and NASA TN D-4142, there are
> others.
Thanks for the specific references. I'll have to dig up the copies I
made and give you the id's.
Don't believe everything you read. I have an advantage in that I have
> tested both ducts and open propellers in real installations, and can compare
> with a real world baseline rather than the free unobstructed inlet conditions
> in wind tunnels as in the reports.
>
Barry, are you saying the document authors lied or were mistaken?
Please, please tell us about your real world experiments. Was this on
hovercraft or airplanes/helicopters? The Osprey? And were you testing
shrouds (duct length << prop diameter) or ducts ( >= fan diameter)? Are
your Canadian reports on-line anywhere? I'd love to study them.
> The upstream interference problem with ducts can really only be studied in a
> wind tunnel, an ideal program for some collegiate environment.
Like the ones going on at Cal Tech, or is it U Cal? Theirs is actually
a shrouded application. I like their thrust vectoring ideas, though.
> Barry Palmer, for <A
> HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
Do you have the book, "Ducted Fan Design", by the de Piolenc & Wright
(the folks at Mass Flow) website:
http://www.sdic.net/piolenc/massflow.htm ?
Good reading, but a couple of the equations are questionable, or my
assumptions are.
David
Nickjokay
>with smaller, quieter engines that could race out of the lily pond and golf
>course environment, and then go cruising. That would be a far more
>marketable
For the past several years the Hoverclub of America,
www.hoverclubofamerica.org, has been giving out "Entry Level Hovercraft"
plans to schools that join the club. The plans detail the construction of a 10
hp machine that the students can choose to race at the National Rally. As a
result the Entry Level Racing class has been the most competative over this
period. There's been a mix of schools, and longstanding club members
participating as well as sons of club members who are not ready for a formula 2
machine. The Entry Level class has now spawned the "Sportsman" Class which is
a 25hp racing/crusing machine. Both the Sportsman and EL hovercraft do well
cruising and participate in the post race cruises and National Cruise. The
Sportsman truly fits this niche of racing/cruising hovercraft.
Back in about 1984 or so Neoteric came out with the "Racer" which was
designed as a racing/cruising machine. However to race you need two people to
show up at the same place with the same class machine. Most of the buyers took
their machines and went cruising, so the racing class never developed. I
bought a Racer in 89 and raced for 3 years and had a lot of fun screaming
around the track while making friends in the club. The Racer was a well built
machine as it was designed for public consumption. I never saw or heard of a
Racer rolling over or bursting a fan (and the ducted fans have a 1/8" tip
clearance!!). I don't race anymore but rather immensely enjoy the Racer's
cruising qualities while hovering along the various rivers of the midwest.
Back to this duct vs prop discussion. There are a few apples and
oranges here when it comes to the hovercraft's application. I do a lot of
cruising on small streams...streams too shallow for me to canoe. Often there
are tree branches hanging down and low bridges every now and again...stretches
with low branches every 10 or 20 yards. The Racer has a two foot dia fan. I'd
be worried about getting a machine with a three foot fan through some of these
spots but the Racer makes it alright. So, my point is, if I wanted to hover
from Seattle to Juneau I'd take a Sevtec, and if I wanted to explore the Ohio
river and all its major and minor tributaries I'd take a Neoteric and trade
efficiency for the ability to fit in tight places.
The Sportsman class is the club's first step in promoting a racing class
that could be used for endurance type racing in an open course environment as
well as cruising. Many things need to happen before this sort of racing will
take place: There needs to be enough machines to race. There needs to be
enough machine owners who want to race. And the most difficult part is,
someone needs to promote, set up, and administer a race.
Nick
SEVTEC
analysis right off. However, since there were others interested in the ducts I
have built a duct for the craft with the excessive hover height in
http://hometown.aol.com/hiitec/kite/trike.html, but rejected its use as it did
not prove out in testing (I also rejected 1 bladed open propellers, which
managed to depart the scene automatically).
I also built a duct and fan for the "Red Machine" (see the Sevtec website) to
see if I could get things working and again, and after getting poor
performance, as the literature on the duct would indicate I re-installed the
open propeller.
I also installed a duct on a small surface skimmer that used only 4 hp (2 for
lift, 2 for thrust. I got what I call root 2 performance in static thrust,
That is the duct rotor was the 1/((2)^1/2 ) of an open propeller for equivalent
thrust but no matter how I nursed the setup, I could not remotely approach
performance of the large machines that are in the NASA literature (no
surprise).
I also tested a Scat 277 and Scat 440 and also got root 2 static performance,
allowing for the 1/3 of the duct blocked by the lift system (and a little
surprising as these craft need about 1/2 inch plus of blade tip clearance to
keep the fan in one piece).
Considering I could get 33mph over water speed for the 277 Scat, and 32mph for
the 12hp Tecumseh Sevtec Scout with open prop and ring guard, and a
(measured from racing video) 23mph over water top speed for the fastest 10hp
Tecumseh powered Universal Hovercraft entry level craft, which uses a short
duct and scoops cushion air out of it, I find the duct just costs too much
performance as well as being a reliability problem.
As far as problems with the literature, there is a whole lot of data to juggle
and sometimes things get mixed up, as they did when I used NASA data on the
program in designing the craft in
http://hometown.aol.com/hiitec/kite/trike.html but things were rapidly cleared
up.
Gordon Hurn
<sev...@aol.com> writes
>;Subject: Re: Thrust system design tips?
>';From: Monty Roberts <montyr...@worldnet.att.net>
>;Date: Sat, Oct 17, 1998 3:24 AM
>;Message-id: <35AFF3...@worldnet.att.net>
>
>
>; I noticed a comment in one of the postings asking why airplanes don't
>;use duct guards. Airplanes fly at much higher speeds than most ground
>;effect machines will ever see, at least ones that rely on fan generated
>;static pressure under the craft. The ideal propeller diam. becomes
><cut>
>
>Have you done the numbers to show that the hovercraft should have to have a
>duct and an airplane should have a propeller because of the higher speed of the
>aircraft? Are you just "arm waving" ( as engineers call discussion that is
>unsubstantiated and presented as fact) or have you done the numbers to verify
>this.
>
>I have done the numbers and I presented them at the 1984 "CACTS International
>Conference on Air Cushion Technology" in Vancouver, and if you wish to pull
>this reference you will find that even the AP1-88 should have had propellers,
>not ducts.
>
>
>
>As far as racing craft go, they are hardly technological marvels, with the
>power loadings they have they should be going straight up!
I wish, I wish !!
Before you continue to knock racing craft technology ;) please remember
we have construction requirements and some very keen scrutineers!! We
are, for example, not allowed props - we have to use ducted fans. Blade
tip speed is also regulated so our development options are much reduced
and inevitably centre on power to weight ratios as well as different fan
setups for lift and thrust a subject not really addressed in this
discussion yet.
Are all USA cruisers non integrated ? By their very nature integrated
craft produce more noise.
Gordon Hurn
>Barry Palmer, for <A
>HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
--
Gordon Hurn
J. Kelley Jernigan
Why are you not allowed props? Would they have an unfair advantage? Universal
Hovercraft holds the world record for hovercraft speed and the UH19P that did it had
a prop. Do you have a racing class without those rules that promotes development of
ACV tech?
Kelley
David M.
talking the same language. Can we agree that:
a "shroud" is a prop covering with its length less than the diameter of
the prop, and
a "duct" is a fan covering with its length greater than or equal to the
diameter of its fan, and
a fan is a prop, but a fan is usually smaller than a prop, and usually
has more blades?
Thanks,
David M.
MURHOVER wrote:
>
> Barry,
>
> Just to clear up a comment on the top speed of the Universal Entry Level craft.
> Our Ducted Entry level craft runs a bit faster than the 23mph as in a previous
> posting. We have seen, with a hand-held GPS, a top speed of 31mph. Would be
> interesting to see if it would run faster or not without the duct though. I
> would think it would not.
>
> Sincerely,
>
> Michael Murphy
> St.Louis Hovercraft
SEVTEC
>Date: Mon, Oct 19, 1998 12:00 AM
>Message-id: <70dve5$c4k$1...@gail.ripco.com>
>SEVTEC (sev...@aol.com) wrote:
: I also tested a Scat 277 and Scat 440 and also got root 2 static performance,
: allowing for the 1/3 of the duct blocked by the lift system (and a little
: surprising as these craft need about 1/2 inch plus of blade tip clearance to
: keep the fan in one piece).
>What (in your opinion) is going on here? Like Nick Jocay...I have had a
>Neoteric Racer and now a Questrek. The stock clearance is around 1/8" but I
>put a "liner" in place to reduce it even more. I run less than 1/16." I've
>hit 7000 RPM; never seen a blade failure.
The Scat had softness built into the engine mount and people do bump their
shrouds against docks, etc, and Scats do occasionally swallow water.
MURHOVER
SEVTEC
<Sincerely,
<Michael Murphy
<St.Louis Hovercraft
I cannot tell much about your speed test of the Universal Hovercraft entry
level craft you made unless I know what engine is being used and whether there
was a tail wind and whether the run was on land, water or ice. Let me know
so I can evaluate it, please. All I know is that the the one I measured on
video had a 10hp (or more?) Tecumseh, was on water with little wind. Since it
was in a race with some 5 or 6 others, and it was videoed over a sufficient
distance to verify that it was likely at top speed, and on water, and it
clearly won the race, I had a more than representative craft to examine. (I
still have the video.)
Also, is your entry level craft one of the newer improved two rotor Universal
designs that uses a belt drive similar to what this designer has been using
(since Fan-Tastic I days, 1975) or is it the single rotor early Universal
design that I measured. If you remove the duct on the single rotor craft you
may run into trouble with the cushion. If it is the newer design, with
separate lift and thrust rotors, go ahead and compare. It would be interesting
to see the outcome, but really a new rotor might be appropriate for the open
propeller version. I would like to know the results.
David Aitcheson
juices of each mixed together, ya' toss in the separator. Oh well.
<GRIN!>
Dave A.
David M. wrote:
>
> Please, guys. Let's agree on some common terms so that we're all
> talking the same language. Can we agree that:
>
> a "shroud" is a prop covering with its length less than the diameter of
> the prop, and
> a "duct" is a fan covering with its length greater than or equal to the
> diameter of its fan, and
> a fan is a prop, but a fan is usually smaller than a prop, and usually
> has more blades?
>
> Thanks,
> David M.
>
> MURHOVER wrote:
> >
bo...@freenet.carleton.ca
Sorry for the delay in response but I was out of town for a while.
Firstly, to answer a question put forward to Gordon Hurn about the use of
propellers in hovercraft racing:
Wooden propellers are expressly banned in UK & European hovercraft racing.
They are permitted in World Hovercraft Championship competitions.
> So what! The open propeller is still more efficient. You cannot generalize
> that the duct is more efficient up to 20mph, as this is a function of both
> propeller and duct disc loading and varies depending on the size of the
> surface skimmer and top speed potential(Horsepower per square foot of disc
> area).
I think I CAN say that the ducted fan IS more efficient given the same
horsepower and the same disc loading. My example was one to show that the
acceleration produced from the ducted fan is superior that the open propellor
from static to about 20mph. It is extremely important (in a racing
hovercraft) as it is always in a state of acceleration. Duh! Of course the
disc loading plays an important part but an unducted propellor has a smaller
"disc" at static and slow speeds. That's what.
> You are forgetting, these craft are, first of all, vehicles. If you
> prostitute the design for some purpose of ego massage, rather than a real
> world usefulness, there will not ever be very many of them. It is not
> likely that the Scat was designed for racing, as there is no significant
> racing market, but was originally sold to backing as a sports machine.
> It is obvious to me that it is merely a copy of an English racing craft, as
> its "designer" knew no other technology. If the Scat raced in the real world,
> rather than on some golf course and lilly pond environment as usually found
> in racing, the outcome
> of the race with the Explorer would be much different.
Now you're talking silly. In my opinion, the Scat was designed exactly for
that: an ego massage. No, it was not designed for racing. Keep in mind that
there were more Scat hovercraft manufactured and sold than any other
hovercraft in history. As to being a copy of "an English sports machine",
the Scat-1 was designed by an Englishman, Kip McCollum, as a single-seat, fun
"ego massager". Therefore, I say again, that it was successful in achieving
it's goals. I also see from your response that you have never competed in a
"real" hovercraft race. I would hardly call either the racing course at the
World Championships in 1993 in France nor the construction site used in 1995
in Portugal or event the circuit used at Mere Brow in England a 'lily pond'.
Frankly, I doubt an Explorer could actually get itself around the race site
in Verneuil. I think you should check out these things before you call them
'golf courses' or 'lilly ponds'.
> I do not mind small fun to play watercraft toy, I just do not like it to be
> totally obnoxious to the non-participating people who may be around it. After
> all you can build such a craft with a $300 engine, rather than a $2500 engine
> and although it may not be quite as fast it can be a lot of fun.
No argument here. I also believe the world community agrees. There is a new
World Federation Class called F25 which is attempting to address this.
Amongst the rules are a 100m/sec tip speed limitation on rotating devices,
stock, four-stroke engines of no more than 25hp total with stock mufflers.
This actually creates a new problem for the racing spectator as the new class
is relatively quiet when compared to the fire-breathing F1 monsters and by
comparison, rather boring to watch. The drivers, on the other hand, really
enjoy the close competition.
> I would lean in a Sevtec craft in a tight race situation, also, as
> control is quicker, but at least one should be able to sit back and relax at
> times.
I agree that when I drive a cruising machine I don't want to "have" to lean
to make it turn. I was recently at a rally in Tennesee and had the chance
to drive several cruisers upon which leaning wasn't a required control
mechanism. On the Scat I drove you had to.
> I have a video of an English racing craft upsetting and throwing fan blade
> pieces out the front. Isn't that why they wear leathers when they race? As
> far as the guarding goes, it should be noted from my plans that I always put
> wire in the plane of propeller rotation, and it is ok to omit upper guarding
> if the craft is used as a cruiser. The netting is used over the front of the
> guard, only.
>
> I have scattered one of my Birch props, an 82 incher, without throwing any
> lethal parts. It is all in how you design it.
The video you saw was probably more than a few years old. I sincerely doubt
you would see a repeat with the newer, tighter rules. By the way, the racers
don't wear "leathers" - just plain coveralls. The Sevtec I saw with the
fish netting used it over the front AND in the plane of rotation. I believe
you were lucky with the Birch prop.
>
> Why don't we go racing with less expensive, fun machines, rather than catering
> to people who are willing to dump their pocketbooks into over-powered, totally
> obnoxious craft that are good for absolutely nothing other than racing on
> manicured tight little courses? I think the sport might see some growth in
> the sport if that were so.
Just as there is expensive automobile racing, there will always be expensive
hovercraft racing. Gary Lutke recently proposed a "stock class" at the
Hoverclub of America AGM this past June. I don't agree that we would see
any growth if we all raced machines whose primary purpose was to cruise. As
to the "manicured tight little courses", see my comment above.
Just for the record. I don't, and never did own a Scat!
SEVTEC
>Bob Rennick
No, I am not talking silly! I am talking about a serious problem for the
industry as a whole. The Scat program may have sold a lot of machines, but
apparently it was not financially viable. It is unfortunate that such a design
has influenced others who copy the technology and cause further problems in the
industry, particularly when things are scaled up. Not only are people turned
off to the machines, they are shown examples of machines that simply do not
work very well in the real world, investors pockets are emptied, and
legislation is caused against the craft.
Admitedly I have seen races only on video, and all of them were on somewhat
benign courses, with nothing nearly so difficult as my backyard rampway. Why
don't these other races ever get videoed. I would have to make a 5200mi round
trip to the US events, all for a few minutes of running.
Yes, it would be fun, but not reasonable (considering I go skimming right out
of my back yard), especially since I would have to build an impractical craft
to compete.
This F25 class from your description sounds like what I am after, using
reasonable hardware. I use a limit of under 395ft per second for fans
(Multiwing), and under 500ft per second for propellers (composite). The
100M/sec limit you suggest is doable probably with a 4 blade propeller, but I
would have to build it as no hardware of this nature is available. Use of nerf
bars to keep craft from overriding each other and proper guarding should
virtually eliminate the propeller as a source of danger.
However, I would suggest that the craft be controlled by an absolute maximum on
noise level, and restrictions in technology should be held to a minimum except
for excess work on engines.
Netting over the periphery of the Vanguard propeller guard is not realistic,
and with 40 acknowledged Vanguards running out there along with perhaps 20
"strays" (people who do not report completions) and a whole bunch more being
built, I cannot control everything that goes on, so such deviations can be
expected.
bo...@freenet.carleton.ca
> Not only are people turned
> off to the machines, they are shown examples of machines that simply do not
> work very well in the real world, investors pockets are emptied, and
> legislation is caused against the craft.
This type of problem has been a thorn in the side of all new acv companies
that have tried to start up in Canada - regardless of the type of product
they are offering. When they approach an investor the common response is:
"Oh no, not another hovercraft company. We know all about you guys".
> Admitedly I have seen races only on video, and all of them were on somewhat
> benign courses, with nothing nearly so difficult as my backyard rampway. Why
> don't these other races ever get videoed. I would have to make a 5200mi round
> trip to the US events, all for a few minutes of running.
This is the biggest problem to racing in North America: geography. I agree
that it wouldn't be worth your while to travel so far to the US races.
Another point: the examples of race tracks I gave you were in Europe. The US
tracks are quite simple by comparison and, in fact, the track at last years
Terre Haute event *WAS* the groomed golf course you mentioned earlier!! Let
me assure you that the European tracks are anything but benign! You should
try to get ahold of Lorenzo Pontelli's "HoverCrash" video tape. It has about
1/2 hour of "how not to do it" on some of these circuits. It definitely
shows how tough some of the tracks are.
> This F25 class from your description sounds like what I am after, using
> reasonable hardware. I use a limit of under 395ft per second for fans
> (Multiwing), and under 500ft per second for propellers (composite). The
> 100M/sec limit you suggest is doable probably with a 4 blade propeller, but I
> would have to build it as no hardware of this nature is available. Use of
nerf
> bars to keep craft from overriding each other and proper guarding should
> virtually eliminate the propeller as a source of danger.
>
> However, I would suggest that the craft be controlled by an absolute maximum
on
> noise level, and restrictions in technology should be held to a minimum except
> for excess work on engines.
This was suggested and there just seemed to be too much opposition. I agree
with you that this would be a better method of ensuring low noise levels but
I still see F25 as a step in the right direction. Perhaps Formula db75 should
be the next step!!
> Netting over the periphery of the Vanguard propeller guard is not realistic,
> and with 40 acknowledged Vanguards running out there along with perhaps 20
> "strays" (people who do not report completions) and a whole bunch more being
> built, I cannot control everything that goes on, so such deviations can be
> expected.
Well said. Perhaps more stress in your documentation upon the importance of
guarding and what it is supposed to achieve?
aols...@my-dejanews.com
bo...@freenet.carleton.ca wrote:
<snip>
> This is the biggest problem to racing in North America: geography. I agree
> that it wouldn't be worth your while to travel so far to the US races.
> Another point: the examples of race tracks I gave you were in Europe. The US
> tracks are quite simple by comparison and, in fact, the track at last years
> Terre Haute event *WAS* the groomed golf course you mentioned earlier!! Let
> me assure you that the European tracks are anything but benign! You should
> try to get ahold of Lorenzo Pontelli's "HoverCrash" video tape. It has about
> 1/2 hour of "how not to do it" on some of these circuits. It definitely
> shows how tough some of the tracks are.
<snip>
*drool* Bob, do you have any information as to where we might get a copy of
this video tape? I'd love to see it (as many others would too I'm sure). :-)
- Alex O.
Gordon Hurn
safety. We are supposed to be a non contact sport but in practice by
accident or design, craft contact happens regularly. Some of our races
are very competitive (in the F2 and F3 categories which normally have
starting grids of 15-20 craft) and racing in close proximity with props
would be a nightmare. My sons web page features photos of the last
juniors race where a novice driver got off line and ended up going right
across the front of a faster craft right on the corner apex when
everybody was committed to a racing line, the photo shows the duct being
heavily clouted. At the end of Edward's races at Tattershall the duct of
his craft had 3 colours of paint on it, ours plus Lukes and Simons craft
following separate incidents involving experienced but competitive
drivers!!
The WHF allowed props at the world championships in F1, the only prop
driven craft at the 1997 Worlds was from Japan (strange all aluminium
machine with rudders in front of the driver - it used a tuned Subaru
Imprezza engine) which was allowed to race after mods to the prop duct
area to improve safety, you could literally walk into the duct.
Incidentally to do the mods they used Kent Gano's shipping pallet
(without asking him) so that after the event Kent had real bothers
getting his craft crated up again for shipping back!! Kent was
definitely unimpressed, relations around the Pacific rim were not the
best!!
I think most UK racers accept that props are more efficient, but
hovercraft racing has an excellent record for safety and we want to keep
it that way. Certainly I would have not allowed my 14 year old loose
with 45 horsepower if that was not the case.
In article <362A7D64...@bellsouth.net>, "J. Kelley Jernigan"
<hover...@bellsouth.net> writes
>> >Barry Palmer, for <A
>> >HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
>>
>> --
>> Gordon Hurn
>
--
Gordon Hurn
J. Kelley Jernigan
Thank you all for a very informative discussion! I am learning allot from it and I
am sure that allot of other folks are as well. I hope we see more discussions like
this.
Barry, I hope mine won't be a stray for much longer. Bryan tells me I may get my
parts in a couple of weeks!!
Kelley
http://www.hovercraftersresource.com
SEVTEC
10.9hp @ 2810rpm). Pluck three of its six fan blades out and it is still more
drivable (but not friendly) than the typical integrated fan craft and now you
can get about 35mph out of it, judging from early runs with Vanguard and
analytics. Toss the terrible Tecumseh and put the shakey 1 cylinder 277 as
found in early Scats into it , 24hp, perhaps, and the same weight as the
Tecumseh. Analytically you get a 47mph Scout, which is probably low as my
modelling, which fits craft quite well up to the SRN4 using the same
empirically developed information from Scout or Vanguard measured data, does
not factor in reduction of drag when the skirt starts peeling itself off the
water. Then why does the Scat top out at about 33mph? What I am trying to say
is there is a lot that can still be accomplished, for both racers and cruisers,
to improve surface skimmers as a whole, and existing technology is far from
mature!
CedarDecks
ED
Cedar Decks
CedarDecks
>
>
>
>
>
Barry,
I asked you a year ago( why don't you design a speed machine) .
Remember most water craft are realy pleasure crafts, and they are bought on
ego, Just like big trucks, motorcycles, ECT. ( like a big SEV that can take you
and your buds to Alaska)
ED
Cedar Decks
John Carter
I believe Barry has raced in earlier Hoverclub of America events in a low
powered 4-cycle sev that won a lot due to the 2-cycle competition breaking
down and dropping out of the races. Since I have personally flown Explorer
in (for REAL) 9 foot seas and other intimidating terrain on the North to
Alaska SEVenture, I'd bet it could run any terrain a WHF champ racer can,
barring overhead clearance issues.
I think the open prop/ducted fan argument will be around for a long time.
It's akin to the old Ford/Chevy, which is better argument. I drive a large
Ford pickup with a full-sized bed. It's a good utilitarian. Some people
drive these short-bed, stepside, chopped trucks that look nice and may be
quick, but aren't quite so utilitarian. Sevs are the same way. Sure,
Scats and Neoterics are pretty, and they're fun to fly when the conditions
are favorable, but give me Explorer for the long haul in bad weather with
everything aboard except the kitchen sink.
I think the main argument that hasn't been addressed as much as it should
be is the noise issue. THAT is the greatest threat to getting us regulated
out of existence! My Dad's Weber Starcruiser is only the 3rd 4-cycle to
grace the Rio Grand in Albuquerque since I had my Sevtec Utility and Norm
Lincoln had his UH-16S (soon to be T). The locals, Scat folks, couldn't
believe how quiet the Weber was, not to mention the payload (which was
affected by altitude, Linda). Enlightened, a few of the locals are now
pursuing the sale of their Scats in favor of a Weber or Sevtec. A high
speed fan will always be much louder than the slower prop given the same
hp, and the duct does a great job as a bull-horn directing that howl at the
pilot/passengers.
Bob, are there any hovernuts in Montreal? I'm flying up there next month
to visit my girlfriend (thank you Balloon Fiesta, and a fellow hovernut!).
Happy Hovering on whatever bubble you're on,
John Carter
Hoverclub of America #2430
http://www.losalamos.com/hovernut
CedarDecks
>I saw that SEV too.
>ED
>Cedar Decks
>
>
>
>
>
>
sorry about that I saw the SEV with netting around the prop.
Cedar Decks
J. Kelley Jernigan
To continue the analogy with cars I'd call a Sevtec a SUV/4X4 and the British Style a
Mustang/Camaro. Vehicles designed for different environments with many people not
realizing that SUV's even exist. There was a UH15P(Mustang) that was following a
Vanguard(SUV) in Washington a few years back. The bottom was ripped off of the UH
craft. You don't take a Mustang off roading.
Kelley
bo...@freenet.carleton.ca
> I believe Barry has raced in earlier Hoverclub of America events in a low
> powered 4-cycle sev that won a lot due to the 2-cycle competition breaking
> down and dropping out of the races. Since I have personally flown Explorer
> in (for REAL) 9 foot seas and other intimidating terrain on the North to
> Alaska SEVenture, I'd bet it could run any terrain a WHF champ racer can,
> barring overhead clearance issues.
John,
9-foot seas are one thing, the tight, twisty, challenging circuit is another.
Forget about the overhanging branches near the island channels, just add 30
other maniacs trying to get ahead of you while you're trying to negotiate the
twisty turns! We're back to apples & oranges. Most of the SEV's are built
for cruising - and they do that well. Most purpose-built racers absolutely
suck in "cruiser mode". I'll go out on a limb here, racing is North America
is significantly different than in Europe where it is a full contact sport.
The courses in N.A. are quite a bit easier to drive (the Island section on
the course at Lazy "L" in Terre Haute was "European" in nature) but the
course at Troy is a whimper by comparison. Additionally, the biggest fields
traditionally seen at Troy are about 10-12 - and that is in Entry Level. A
"small" field in Europe is about 20!!! European competition is much closer
and (flame shield up!) much more difficult and challenging!
Someone asked about the "HoverCrash" video. I have the NTSC master copy but
I'd have to first check with Lorenzo about distributing it. He, of course,
has the PAL master. Unfortunately, Lorenzo is not on the net and I'd have to
telephone him in Italy. It has about 35 minutes of "out-takes" from video
that Lorenzo and his family has shot over about 10-years of European
hovercraft competition. How much of an interest is there? Really.
By the way, John, when will you be in Montreal?
Bob
J. Kelley Jernigan
Monty Roberts
>
> ;Subject: Re: Thrust system design tips?
> ';From: Monty Roberts <montyr...@worldnet.att.net>
> ;Date: Sat, Oct 17, 1998 3:24 AM
> ;Message-id: <35AFF3...@worldnet.att.net>
>
> ;Greetings all,
>
> ; I noticed a comment in one of the postings asking why airplanes don't
> ;use duct guards. Airplanes fly at much higher speeds than most ground
> ;effect machines will ever see, at least ones that rely on fan generated
> ;static pressure under the craft. The ideal propeller diam. becomes
> <cut>
>
> Have you done the numbers to show that the hovercraft should have to have a
> duct and an airplane should have a propeller because of the higher speed of the
> aircraft? Are you just "arm waving" ( as engineers call discussion that is
> unsubstantiated and presented as fact) or have you done the numbers to verify
> this.
>
> I have done the numbers and I presented them at the 1984 "CACTS International
> Conference on Air Cushion Technology" in Vancouver, and if you wish to pull
> this reference you will find that even the AP1-88 should have had propellers,
> not ducts.
>
> As far as racing craft go, they are hardly technological marvels, with the
> power loadings they have they should be going straight up!
> HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
I was not making an argument favoring the use of ducts in ACV's I was
answering a question on why aircraft are not using ducts. (at least not
low speed aircraft, exception: blimps) My argument that props are better
for ACV cruising use is based on momentum theory. You missunderstand my
position. I hold the position that ducts cause excessive weight and drag
penalties in aircraft flying below about 400 mph. At high speeds, the
tip loss on propellers becomes too high. And ducts become more
efficient. I am not "arm waving" (reference: any text on propulsion.)
Propellers are better for cruising hovercraft for the same reasons. I
don't know of any hovercraft cruising at 400 mph. They are limited by
the cushion pressure being overcome by the dynamic pressure of the air.
(yes, no? a seperate discussion)
My question of the prop vs. duct is on small highly loaded rotors for
racing, and I have not done a thorough analysis yet, that is the reason
for my starting this discussion. Not everyone has the benefit of many
years of experience working with hovercraft, or being able to work on it
full time. That is why this disscusion group is so valuable. It lets us
share our knowledge and thereby we all learn more quickly.
Thanks for citing the reference, could you tell me where I can send off
for a copy? I do not currently have access to a good technical library.
Literature on hovercraft is not easy to find. Even in a university
library. Most of the literature is out of date or is not available. I
imagine most other people in this group are in the same boat.
Monty
CedarDecks
>I'd have to first check with Lorenzo about distributing it. He, of course,
>has the PAL master. Unfortunately, Lorenzo is not on the net and I'd have to
>telephone him in Italy. It has about 35 minutes of "out-takes" from video
>that Lorenzo and his family has shot over about 10-years of European
>hovercraft competition. How much of an interest is there? Really.
>
>
I want one!!
ED
Cedar Decks
CedarDecks
You got me thinking.
I have a scat with 26 hp rotax. It go's about 30 mph, and it drinks about 2-3
gallons per hour. If I changed the drive system to a SEV design What would the
specks be, how big of a prop? Keep in mind speed is a needful thing, min 200
pound pay load, need to keep the finger skirts and more thrust power for
climbing hills would be great!
(I wouldn't do all this work to my scat. just thinking)
ED
Cedar Decks
CedarDecks
Cedar Decks
aero...@altern.org
hello alex
hello all hovercrafter.
Each year since 1992 I'am buy the VIDEO lorenzo Pontelli. I'm like the video
"Hovercrash", "hovertop", "EHF 1992" and "EHF 1993" and .... Lorenzo, he 's a
italian hovercrafter pilot F2. He like Hovercraft and Video and 1993 he has 5
video recorder during the week end .all his family used a recorder .
I can echange your USA or CANADIAN video with lorenzo pontelli video after i
make a copy for lorenzo. or i can copy for you a video and send money for
lorenzo.( lorenzo buy his video 20$US each) Have you some video for echange ?
All video are in PAL format (VHS or sony 8mm)
regards
pas...@altern.org
> > bob rennick
> > try to get ahold of Lorenzo Pontelli's "HoverCrash" video tape. It has
about
> > 1/2 hour of "how not to do it" on some of these circuits.
> *drool* Bob, do you have any information as to where we might get a copy of
> this video tape? I'd love to see it (as many others would too I'm sure). :-)
>
> - Alex O.
>
CedarDecks
ED
Cedar Decks
Hoverlord
Properly designed and opperating ducted fan
systems are greatly more efficient than any open prop. It's was proven in the
70's as clearly explained in the Canadian tech
symposium handbooks and proven endlessly ever since by Hoverworks, Neoteric,
Hovertechnics, Can-Air, US Hovercraft Inc, and many more. Anyone still
promoting open props is living back in the 60's... :(
Michael Crisman
Designing since the late 50's.
CEO of The World Hovercraft Exchange
Hove...@aol.com
J. Kelley Jernigan
are more efficient I'm sure airlines would use them as they would save a fortune
in fuel costs alone! Ducted fan/prop aircraft are virtually nonexistent when
compared the the numbers of open prop aircraft.
Hoverlord wrote:
J. Kelley Jernigan
The Hovercrafters Resource
http://www.hovercraftersresource.com
-------------------------------------------
Hoverclub of America
Member # H-4806
-------------------------------------------
"There is never enough time and money to do
it right but there is enough time and money
to do it over."
SEVTEC
<Date: Mon, Nov 2, 1998 8:38 PM
<Message-id: <19981102153842...@ng145.aol.com>
<For the record folks...
<Properly designed and opperating ducted fan
<systems are greatly more efficient than any open prop. It's was proven in the
<70's as clearly explained in the Canadian tech
<symposium handbooks and proven endlessly ever since by Hoverworks, Neoteric,
<Hovertechnics, Can-Air, US Hovercraft Inc, and many more. Anyone still
<promoting open props is living back in the 60's... :(
<Michael Crisman
<Designing since the late 50's.
<CEO of The World Hovercraft Exchange
<Hove...@aol.com
Would you please identify some specific references as you have reviewed in the
above to make statements about open versus ducted thrusters. I would like to
review them.
david1
oop's SPELL
SEVTEC wrote in message <19981102165717...@ng93.aol.com>...
Steve Lyons
very distinctly noticed that the craft equipped with PROPERLY designed
professionally manufactured props produced far more thrust than the
ducted fan equipped craft. My RX 2000 with Rotax 52hp 503 and 9 blade
28 inch ducted fan did not produce thrust that was anything near what my
ultralight trike produces equipped with the same engine and 66 inch 3
blade prop. And riddle me this hovernauts, why does Bob Wendt run a
prop on his fastest craft if that ducted fan is supposed to be so much
better? Why don't the airboats in the Florida everglades run ducted
fans if the ducted fan is so much better? If I tried to put a 28 inch 9
blade ducted fan on my trike, it would never get off the ground. That's
a fact. I'll take a professionally designed and manufactured prop any
day of the week. Get a test stand, mount an engine, get a ducted fan
and a prop, and measure the thrust produced by each. The figures will
tell you that the prop puts out a lot more thrust and is a lot quieter.
Period. So forget those ducted fans, just get an open prop if you want
the most thrust.
bo...@freenet.carleton.ca
The answer is: they do. They're commonly referred to as jet engines!!
The typical jet flying today is actually using a turbine powered fan operating
in a well designed duct (the nacelle around the engine). The shape of the
nacelle is idealised to match the cruising speed of the aircraft. The same
is true of the ducted fan powered hovercraft/airboat/whatever. For the
maximum in *static* thrust you would design a duct with a wide, flaring intake
bellmouth. For maximum thrust at a specific forward speed, the flare on the
bellmouth would taper and the outside profile of the duct should be of a
high-lift aerofoil design. Properly designed and constructed, the ducted fan
*IS* much more efficient than an open propellor of the same diameter and the
duct, while admittedly adding drag the correctly designed and constructed
duct can *add* forward thrust. The trick is to construct the whole thing with
minimal tip clearances and no distortion.
Bob Rennick
J. Kelley Jernigan
That was me. I didn't think I needed to bring up jet engines as that is an apples
and oranges comparison.
(The following is my opinion without the benefit of formal training)
Jet engines are designed for high speeds and that wasn't part of the discussion
that I was aware of. I also don't think the discussion was for same size
diameters. Hoverlord was stating that ducted fans are more efficient that open
props and that anyone who designs open prop craft is living in the stone age. A
small ducted fan is not more efficient than a larger diameter prop.
If jet engines are the most efficient then why do craft with propellers exist? Why
don't we have jet engines on hovercraft? Can you imagine the fuel it would guzzle
and people think hovercraft are noisy now!
bo...@freenet.carleton.ca wrote:
J. Kelley Jernigan
Bill
Steve Lyons wrote:
> 28 inch ducted fan did not produce thrust that was anything near what my
> ultralight trike produces equipped with the same engine and 66 inch 3
> blade prop. And riddle me this hovernauts, why does Bob Wendt run a
>
You should be comparing a 66" 3 blade ducted prop to your 66" 3 blade open
prop.If you compare your 28" ducted fan to a 12" open prop The ducted prop
would have much more thrust and this is a closer comparison than what you
used.
> day of the week. Get a test stand, mount an engine, get a ducted fan
> and a prop, and measure the thrust produced by each. The figures will
> tell you that the prop puts out a lot more thrust and is a lot quieter.
> Period. So forget those ducted fans, just get an open prop if you want
> the most thrust.
Just curious. Have you done this Steve?
SEVTEC
Date: Tue, Nov 3, 1998 4:21 PM
Message-id: <71nai0$fm5$1...@nnrp1.dejanews.com>
<Someone asked why airlines don't use ducted fans if they are more efficient.
<The answer is: they do. They're commonly referred to as jet engines!!
<The typical jet flying today is actually using a turbine powered fan operating
<in a well designed duct (the nacelle around the engine). The shape of the
<nacelle is idealised to match the cruising speed of the aircraft. The same
<is true of the ducted fan powered hovercraft/airboat/whatever. For the
<maximum in *static* thrust you would design a duct with a wide, flaring intake
<bellmouth. For maximum thrust at a specific forward speed, the flare on the
<bellmouth would taper and the outside profile of the duct should be of a
<high-lift aerofoil design. Properly designed and constructed, the ducted fan
<*IS* much more efficient than an open propellor of the same diameter and the
<duct, while admittedly adding drag the correctly designed and constructed
<duct can *add* forward thrust. The trick is to construct the whole thing with
<minimal tip clearances and no distortion.
<Bob Rennick
The "ducts" on jet aircraft are inlet flow diffusers, and are for the purpose
of getting the fan inlet blade tip Mach number down. This type of device has
nothing in common with hovercraft design, unless the hovercraft is very fast
and has a peculiarly high Mach number, say, .83.
A subsonic jet aircraft (commercial) is a surprising device, from the technical
standpoint. The propulsive efficiency of even the large fanned jet engines,
(or high bypass ratio engines) is terrible, around 45%, even at cruise speed.
The engines are responsive to improvement in performance by way of the inlet
flow diffuser only due to their low propulsive efficiency. The aircraft more
than makes up for this, though, in that typical airframes are low drag, and
they do their thing at high altitude, in low density air.
GLutke
Ducted systems are more effecent at low speeds, under 80 mph, they become
usless after 180 mph (most current designs) where they typically induce more
drag than the thrust they produce.
A duct allows you to use a smaller fan/prop to produce the same amount of
thrust.
Most airplanes don't allow for an easy mounting of a duct and most fly at a
speed where the duct would be of little bennefit for the trouble. Most
hovercraft run a lower speeds where the duct can be of use. All factory made
hovercraft (that I know of) use a duct for two reasons;
- SAFTEY - a ducted fan or prop is much easier to guard
- Disk area - smaller thrust systems are more acceptalbe to the general public
as many people use craft in more confinded area's
Gary
Hoverlord
But the simple fact is that ducted technology
produces more thrust than a basic two bladed
open prop per horsepower. That's why 80% of the hovercrafts in the world use
it...
:)
Hoverlord
:)
Hoverlord
For the casual reader here.. My father and Barry were like the Hatfields and
McCoys
of Hovercraft design in the Seattle area back in the 60's.
Barry - The comments I make about ducted efficiencys come from studying
advanced hovercraft designs world-wide, my own experiences, and the The
National Research Councel of Canada. They concluded.. and I quote here, . . .
Page 7.
"Finnally, the cheapest and and simplest device is the plain unducted
propeller. However, as one might expect it is a lot less efficient than a
ducted propeller, and if we try to get the same thrust out of it as we can from
a multibladed fan we have to drive it at very high tip speeds, at which the
noise increases violently, the efficiency falls off drastically, and the danger
of flying apart becomes very real."
This came from the "Thrust systems for light
air cushion vehicles" by HS Fowler. 1974
Hoverlord
fan way to small and the blades way to low a pitch.
Yes.. it's one of the worst functioning small crafts around. It's an unfair
comparison.
:)
bo...@freenet.carleton.ca
are more efficient is interesting, certainly the one factor which should
influence your design decision the most is *SAFTEY*! As Gary pointed out, it
is far easier to effectively guard a ducted fan/propellor than an open one.
I don't care how well you design and manufacture your propellor - the chance
of a catastrophic failure is ever present and so you must prepare for it.
Open meshing on the rotional plane of a spinning propellor is insufficient to
contain an exploding propellor. Period. Pieces will always find their way
through. If you feel you can live with the thought that you could have done
something to prevent an injury to a bystander (let alone the "sue-me-sue-you"
attitude so prevelent in the USA today) then go ahead and take a chance on an
improperly guarded, open propulsor (I'm sure a lawyer would describe it more
effectively!) So if it is necessary, if only for safety reasons, to surround
your propulsor with a solid, why not make it a well designed, efficient duct?
By the way, Barry, I only mentioned the jet engines as used on aircraft like
the 767 to point out that they are, in fact, turbine powered ducted fans. I
didn't, for a minute, suggest they be used on hovercraft.
Bob
mik...@my-dejanews.com
by Bob Windt that it has more thrust than a 60 inch ducted (shrouded)
two-blade. I also read in his construction manual that a multi-bladed prop is
much slower to rev up or wind down. Now before you fly off your seat, I
realize that his props are made from wood and have a large chord (this helps
keep it together when an occaisional bolt or other small part goes through
it). A lighter weight multi-blade will not have as much problem winding
up/down but may pay the price when a bolt goes through it. So this is not
just about thrust but also reliability. I won't argue that if you had a 74
inch duct you would have more initial thrust when travelling in a straight
ahead direction but this is a bit impractical and unwieldly. You can bet that
when your craft is sliding sideways the duct will lose a lot of efficiency
and cause more drag than an open prop. Face it, a large propeller arc is
biting a lot more undisturbed airspace. Furthermore, many ducts are not
perfectly built/maintained. Why is it that the pictures always show the pilot
wearing jet-engine ear protection. Is this due to a slow tip speed?--Mike In
article <19981104034422...@ng18.aol.com>, hove...@aol.com
(Hoverlord) wrote:
-----------== Posted via Deja News, The Discussion Network ==----------
SEVTEC
<Date: Wed, Nov 4, 1998 8:44 AM
<Message-id: <19981104034422...@ng18.aol.com>
<Hello Barry:
<For the casual reader here.. My father and Barry were like the Hatfields and
<McCoys
At risk of being accused of being out of subject on this thread, I should set
the record straight. " My contact with the Crismans ("Hoverlord") is a single
trip by myself to see a surface skimmer of theirs, one powered by a V8 engine,
3 place. They may have visited me perhaps 4 times over a period 20 years.
This is hardly a feud!
The Fowler reference as stated by "Hoverlord" is clearly incorrect, all one has
to do is observe a Vanguard, or better yet, an Explorer, to prove it. As far
as I know to this date the Canadian Government sponsored efforts have yet to
produce a viable surface skimmer, and they have "bought" English technology and
imported it.
SEVTEC
<Date: Wed, Nov 4, 1998 2:15 PM
<Message-id: <71pnii$p84$1...@nnrp1.dejanews.com>
<Bob
It should be noted that a ducted fan must have tip clearances of the order of
1/8- 3/8 inch to work really effectively, in the case of the open propeller,
guard to propeller clearances are the order of 3 inches. It is a no-brainer to
figure out which is more suceptible to damage by external impact. (In larger
machines, the open propeller can be safely used without much in the way of
guarding.)
With proper design, the odds of an open propeller unloading are about nil,
except that wood propellers can rot over time. Combined with low tip speeds,
and low blade mass, Sevtec propellers really did not represent a major danger
unless someone deliberately reached in or slipped off a guard while pushing a
craft (Do not push sevs, as in the Sevtec manual.) These dangers also exist
for a fan, and we are exposed to similar dangers in many other endeavors, as in
driving, train platforms, etc.
However Sevtec went away from wood propellers and now uses composite ultralight
propellers, operating them at power levels only a fraction of the power levels
for which they are run in ultralights.
Maybe we should recognize that there is probably a little "propaphobia" by some
observers, who also adhere to the "lawnmower effect", if you can't see it, it
isn't dangerous.
bo...@freenet.carleton.ca
> With proper design, the odds of an open propeller unloading are about nil,
> except that wood propellers can rot over time. Combined with low tip speeds,
> and low blade mass, Sevtec propellers really did not represent a major danger
> unless someone deliberately reached in or slipped off a guard while pushing a
> craft
>
> Maybe we should recognize that there is probably a little "propaphobia" by
some
> observers, who also adhere to the "lawnmower effect", if you can't see it, it
> isn't dangerous.
It seems to me that you've stuck your head in the sand yourself, Barry.
*ANY* rotating device is a potential hazard and should be appropriately
guarded. Granted, you may be using a well designed and constructed propellor
operating well within those design limits but what happens when you literally
"throw a wrench into the works". It *does* happen. All kinds of things go
through propellors. If the propellor itself doesn't self destruct you can
pretty much bet on bits of something coming out the insufficiently guarded
rotating device. By merely stating "don't push my SEV" doesn't mean it isn't
going to happen and I think you're being pretty naive to think that it won't!
I've seen too many problem hovercraft over the years to believe that a
chicken wire frame around a rotating propeller will protect me in the event
of a catastrophy.
Bob
bo...@freenet.carleton.ca
sev...@aol.com (SEVTEC) wrote:
> The Fowler reference as stated by "Hoverlord" is clearly incorrect, all one
has
> to do is observe a Vanguard, or better yet, an Explorer, to prove it. As far
> as I know to this date the Canadian Government sponsored efforts have yet to
> produce a viable surface skimmer, and they have "bought" English technology
and
> imported it.
Barry,
I've also had my disagreements with Howard Fowler over the years (I also work
at Canada's National Research Council from where Howard is now retired).
Outside of Howard's research hovercraft (that were designed and constructed
in Canada), I'm not aware of what hovercraft you are referring to? The only
"English Technology" machines that the Can Gov't has bought are the N5, N6's
and AP-1/88's used by the Coast Guard and the two new AP-1/88's made here in
Canada (albeit totally English designed). What are the gov't sponsored
efforts to which you refer?
J. Kelley Jernigan
Neoteric):
"Diameter of the fan or the propeller, more than any other criterion, effects
the thrust output. Large diameter helicopter rotors can achieve 12 lb/HP of
static thrust. Such large diameters cannot be incorporated into small
hovercraft. The next best alternative is the propeller. This gives the best
static thrust and, consequently, acceleration."
Also according to the Universal Hovercraft pamphlet you get more thrust from a
prop as well.
So I guess, Barry, Bob, Chris and Robert are all wrong eh?
Hoverlord wrote:
J. Kelley Jernigan
SEVTEC
<Message-id: <71qfpo$sh3$1...@nnrp1.dejanews.com>
Barry,
<I've also had my disagreements with Howard Fowler over the years (I also work
<at Canada's National Research Council from where Howard is now retired).
<Outside of Howard's research hovercraft (that were designed and constructed
<in Canada), I'm not aware of what hovercraft you are referring to? The only
<"English Technology" machines that the Can Gov't has bought are the N5, N6's
<and AP-1/88's used by the Coast Guard and the two new AP-1/88's made here in
<Canada (albeit totally English designed). What are the gov't sponsored
<efforts to which you refer?
<Bob
(Rennick)
That is the problem!!!!
For years the Canadian Government had maintained a bureaucracy involving
hovercraft. All that was produced was what they eventually called a "Skirt
Test Rig" (C'mon, give me a break!) look for it in Jane's, and guidelines
and specifications for legal commercial hovercraft that basically described
English technology and certainly would discourage anyone who wished to get into
the business, due to its bulk.
It would be nice if the government had spent at least some of that money on
knowledegeable technologists, that could have produced real results, rather
than squander it on do nothing bureaucrats.
Steve Lyons
The fan pitch on my craft was properly set. My Rotax 503 powered craft
performed a lot differently than what the early models did. My craft
was only about half as loud as most early RX2000's.
Why don't Cessna and Beechcraft airplanes use ducted fans for thrust if
they are so much better than an open prop? I would be willing to bet
that the major airplane manufacturers know quite a lot about prop
thrust.
Seems like the only way to settle this disagreement between props and
ducted fans is to conduct a properly designed experiment and measure
lbs. of thrust produced by each design. Surely this has already been
done by someone.
willwork4play
they could install on their aircraft. However, it costs so expletive
much money to get them recertified by the FAA (in the millions), that
they just can't afford it.
David M.
bo...@freenet.carleton.ca
sev...@aol.com (SEVTEC) wrote:
> guidlines and specifications for legal commercial hovercraft that
> basically described English technology and certainly would discourage
> anyone who wished to get into the business, due to its bulk.
>
Well Barry, now's your chance to change things up here! I'm currently
involved with Transport Canada in setting up new legislation that will add
air cushion vehicles to the Canada Shipping Act (removing them from the
Aeronautics Act). The working group is re-examining *EVERYTHING* about acv's:
construction, operation, crew licensing as well as establishing new
guidelines for private acv operation as this will soon be legislated
(currently it is not). (Note: the new legislation is not singling out acv's
but, rather, *all* privately owned powered "boats" will soon require licensed
operators in Canada.) We've had but one meeting to set the initial guidelines
but all should be in place by late spring as the new legislation goes before
parliament in Sept 99. Check the last page of our newsletter for the
announcement of the last meeting and my request for club members to put in
their 2-cents worth. If you have things you want to change in our legislation
you'll never have a better opportunity than now. Let me know if you want to
attend the next Transport Canada meeting and I'll let you know when and
where.
Bob Rennick
bo...@freenet.carleton.ca
and open propeller debate is safety. My own personal opinion is that a
ducted fan/propeller is more efficient at the slow speeds used by hovercraft
and for open cruising at higher speeds I believe that an open propeller is
more efficient. *BUT*, I think safety has to take a higher priority over
performance.
I'll give you a real-life example of how even a well designed, safely set up
open propeller can be dangerous:
While in Portugal for the World Hovercraft Championships, Speed Run and World
Cup racing, the day after Bob Windt set the World Speed Record in his big
monster UH-26, we were invited to cruise down the Douro River to a town some
distance away who put on a big spread for us. The UH-26 has a large, open
propellor set well behind the occupants - separated by a V-6 automobile
engine which is in turn set inside the body cowling. There is no way that
the occupants can get hurt by this propeller. I was in the UH-26 with Bob,
Billy Zang, Jeremy Wilson and a Portuguese political big-wig. (Incidentally,
during our run down the river we were chased by several cars containing
hovercraft race crew who for some reason or another chose to drive to the
luncheon. I was told afterward that they couldn't keep up to us on the
straight road in spite of their automobile speedometer showing well over 100
mph!!!! Bob's air speed indicator topped out over 90 mph (I think) but we
were really flying and arrived about 20-minutes before anyone else I think
that the *real* hovercraft world speed record was set that day). Anyway, upon
arriving at the village, I jumped out to clear the crowds away from the
slipway. The slipway way cut into a rock wall about 8-feet high. I almost
ate my heart when several children standing atop the wall leaned over to get
a better look as Bob approached and the open propeller whizzed about 2" from
their heads!! This in spite of the fact that I had cleared what I thought
was a large open swath in the crown for Bob to manoeuver into.
Moral of the story:
If there is even the remotest chance of someone hurting themselves on your
hovercraft, they will. Plan for it to happen.
Given this, I'd much rather protect my open propeller with a duct than leave
it open.
Hoverlord
which one is better...? has caused some
argument. .. I probably started it with my comment that open props are ancient
technology. I appologize for that, in that I ment no offense. Just wanted to
let the begginers know ducted designs have been very efficient in some cases.
It's been intriging to see all the statements on both sides by everyone and
frankly.. I think there all correct. What we're seeing here is there are
many many successful ways to build these amazing crafts.
Fair enough..?
Michael Crisman
Hoverlord
sorry for any
offense that I may have caused by calling open props ancient technology. It's
been very interesting to see all the comments about the subject. I was merely
trying to let the begginers who brouse here know that there are many ways to
build these great crafts.
Fair Enough..?
Michael Crisman
Steve Lyons
aircraft, but due to liability reasons and certification reasons, those
innovations never make it to the market place. But in the experimental
aircraft category, just about anything is possible. I have seen very
few ducted fans on experimental aircraft.
J. Kelley Jernigan
Steve Lyons wrote:
--
J. Kelley Jernigan
I for one think this is a great discussion. This newsgroup hasn't seen so much
activity in a long time. I for one am also learning a few things. BTW, I would
like to see photos of your craft.
Hoverlord wrote:
J. Kelley Jernigan
J. Kelley Jernigan
or not. Thank you for the story.
bo...@freenet.carleton.ca wrote:
--
Bryan Phillips
from an exploding rotor and is safer? In many cases I have heard about
or seen on video, pieces of the fan from an integrated craft come
forward into the passenger compartment. One reason for this potential
danger of shrapnel is that the duct is such a poor guard for the rotor.
Since clearances are so tight even the slightest impact to the duct can
cause the rotor to fly apart. Or just as common the wire or net in front
does not prevent objects from entering the duct. This happened to my
neighbor on a factory built Neoteric Questrek. A rag went in and out
came the fan! It seems to me that if ducted thrusters want to be "safer"
than open props they need guards! Yes, a metal frame work outside and
beyond the duct.
When it comes to safety in my boat my first concern is is keeping the
people out of the water. Then it is keeping the prop in one piece. My
reasoning here is that if it comes apart someone could get hurt by
pieces of the prop. With the way my craft is setup the persons hurt
would likely be a bystanders, not a passenger. The next reason I want
the prop in one piece is that being stranded in a remote place could
also pose danger to the passengers.
All my passengers are in the craft before it ever starts so they are
safe from the rotating equipment. My main concern is bystanders. I
still believe that safety is mostly a function of the driver, not your
type of guard.
The best way to protect my open prop in my opinion is with a metal
frame, wire/net guard. Would I use a duct because it is safer from the
standpoint of a rotor failure? No because I don't see that it is. But
that is not an issue (duct vs. prop) for me because I also know the best
all-round performance for my craft is with its 7 foot prop. How else
can I get this kind of performance:
Hull size, 20'x10'
base area, 171 sq. ft
prop, 84" 4 blade, power at prop 29 hp
fans two, 24" 23 hp (11.5 each)
engine 1.4L 4 cylinder subaru EA-65 55 total hp@4250 rpm max
fuel burn average over 1400 miles 2.2 gallons per hour
typical cruise speed 24 mph averaged over great distances
In tests 24 mph is at about the 50% throttle setting which corresponds
nicely with:
50% of 55 hp=27.5hp times .55 lbs of gas per hour per hp or
.55*27.5=15.1 lbs per hour/6 lbs per gallon or 2.4 gallons per hour fuel
burn (theoretical).
top speed, 40 mph
craft empty weight, 1250 lbs.
max. planeout in deep water test under ideal conditions, 2100 lbs,
includes fuel/gear/people.
No that is not a typo, 3350lbs gross up on step with 52hp at the rotors.
Look ma no duct!
Running my ride business I keep the load to 1500 lbs though.
Pictures are at:
http://www.fidalgo.net/~bphillip/amt.html
and
http://www.losalamos.com/hovernut/alaska.htm
Look for a craft called "Explorer".
All for now.
Bryan
Steve Lyons
hovercraft. Safety issues regarding open props on an aircraft are much
different. For one thing, an airplane obviously doesn't spend nearly as
much time on the ground as does a hovercraft. Personally, an open prop
doesn't scare me nearly as much as those wooden reduction drives that
some of these guys are running. IMHO.
Bryan Phillips
dock? Nothing I would say, perhaps a bird might die. At idle speeds
which I maintain in a marina the prop is not going to unload from bird
strike. It might make enough noise and feathers to cause me to look.
Anyway I am sure no birds have been killed this way lately. Even at full
throttle settings this prop is far below its normal capacity on its
designed ultralight application. It is far less likely to unload than
some of these high speed ducted thrusters you see. Spontaneous failure
of the prop is just not a likely scenario, running someone over is far
more likely in my mind. In that case my prop guard is 6 feet off the
ground, far safer than that helo tail rotors or lawn mowers. I get far
closer to my band saw blade than my prop and it is designed to cut. It
is all a matter of perspective, the most obvious dangers are over
emphasized while other more ominous threats are ignored.
Well we could talk about this one till the cows came home.
If we want to assess risks how about we all list the hover accidents we
know of on a new thread. Then we can better see where the problem is. I
can think of quite a few, many of which did not involve the rotors.
Bryan
bo...@freenet.carleton.ca
Bryan Phillips <bphi...@fidalgo.net> wrote:
> Hello. Why is it that people often say the a duct will protect them
> from an exploding rotor and is safer? In many cases I have heard about
> or seen on video, pieces of the fan from an integrated craft come
> forward into the passenger compartment. One reason for this potential
> danger of shrapnel is that the duct is such a poor guard for the rotor.
No, I think you're wrong here. Rather the metal mesh guard on the front of
the duct is the poor guard in this particular case. Obviously, the duct
contained the bits in your example but the guard on the front of the duct was
insufficient. I have also seen this type of failure on older designs - never
on more recent designs that adhere to hoverclub safety standards. I have,
however, seen the plastic shrapnel propel out the rear of the duct.
> Since clearances are so tight even the slightest impact to the duct can
> cause the rotor to fly apart. Or just as common the wire or net in front
> does not prevent objects from entering the duct.
With a properly constructed duct you won't have deflections into the fan.
concerning objects entering the fan due to insufficient front guarding, you're
absolutely right!
> When it comes to safety in my boat my first concern is is keeping the
> people out of the water. Then it is keeping the prop in one piece. My
> reasoning here is that if it comes apart someone could get hurt by
> pieces of the prop. With the way my craft is setup the persons hurt
> would likely be a bystanders, not a passenger. The next reason I want
> the prop in one piece is that being stranded in a remote place could
> also pose danger to the passengers.
> All my passengers are in the craft before it ever starts so they are
> safe from the rotating equipment. My main concern is bystanders. I
> still believe that safety is mostly a function of the driver, not your
> type of guard.
Makes good sense to me. With most cruising hovercraft I've seen with open
propellers it is usually not the occupants who are in danger, but, as you
pointed out, the bystanders. This is my primary concern. Nothing could be
more damaging to our little industry than to have an injury to someone who
has nothing to do with acv's.
> The best way to protect my open prop in my opinion is with a metal
> frame, wire/net guard. Would I use a duct because it is safer from the
> standpoint of a rotor failure? No because I don't see that it is. But
> that is not an issue (duct vs. prop) for me because I also know the best
> all-round performance for my craft is with its 7 foot prop. How else
> can I get this kind of performance:
I don't question the excellent performance you are getting, Bryan, but as you
pointed out, there is still an inherent danger to bystanders with an
uncontained propeller. Since you recognise this I believe you are most
likely taking extra care to see that a mishap does not occur. But what
happens when something as wierd and strange as a bird enters your big 7' prop
just as you are departing one of the picturesque docks as you used during the
Alaskan adventure. And just maybe some kids are fishing off the end of the
dock as this occurs. Okay, it seems like a long shot, but I've learned long
ago that if something can go wrong - it probably will!
J. Kelley Jernigan
If you were to put a full guard on your prop how much do you think that
would affect performance?
J. Kelley Jernigan
The Hovercrafters Resource
http://www.hovercraftersresource.com
-------------------------------------------
Hoverclub of America
Member # H-4806
-------------------------------------------
"There is never enough time and money to do
it right but there is enough time and money
to do it over."
Bryan Phillips wrote:
> Hello. Why is it that people often say the a duct will protect them
> from an exploding rotor and is safer? In many cases I have heard about
> or seen on video, pieces of the fan from an integrated craft come
> forward into the passenger compartment. One reason for this potential
> danger of shrapnel is that the duct is such a poor guard for the rotor.
> Since clearances are so tight even the slightest impact to the duct can
> cause the rotor to fly apart. Or just as common the wire or net in front
> does not prevent objects from entering the duct. This happened to my
> neighbor on a factory built Neoteric Questrek. A rag went in and out
> came the fan! It seems to me that if ducted thrusters want to be "safer"
> than open props they need guards! Yes, a metal frame work outside and
> beyond the duct.
>
> When it comes to safety in my boat my first concern is is keeping the
> people out of the water. Then it is keeping the prop in one piece. My
> reasoning here is that if it comes apart someone could get hurt by
> pieces of the prop. With the way my craft is setup the persons hurt
> would likely be a bystanders, not a passenger. The next reason I want
> the prop in one piece is that being stranded in a remote place could
> also pose danger to the passengers.
>
> All my passengers are in the craft before it ever starts so they are
> safe from the rotating equipment. My main concern is bystanders. I
> still believe that safety is mostly a function of the driver, not your
> type of guard.
>
> The best way to protect my open prop in my opinion is with a metal
> frame, wire/net guard. Would I use a duct because it is safer from the
> standpoint of a rotor failure? No because I don't see that it is. But
> that is not an issue (duct vs. prop) for me because I also know the best
> all-round performance for my craft is with its 7 foot prop. How else
> can I get this kind of performance:
>
bo...@freenet.carleton.ca
Bryan Phillips <bphi...@fidalgo.net> wrote:
> What happens when a bird hits my prop as I pass by this hypothetical
> dock? Nothing I would say, perhaps a bird might die.
hazard still exists.
> Well we could talk about this one till the cows came home.
> If we want to assess risks how about we all list the hover accidents we
> know of on a new thread. Then we can better see where the problem is. I
> can think of quite a few, many of which did not involve the rotors.
I agree. My name was "mud" at the US Hoverally two years ago when, as
scrutineer, I refused to okay two of the top competitors hovercraft when they
showed up with exposed drive shafts and pulleys situated directly behind
their heads. Their quick solution was to install chicken wire located about
1/8" away from the rotating device - which, of course, I also disallowed.
Their argument was similar to what I've been hearing from many open prop
owners: I'm safe in the cockpit. But, again, my concern was more from them
driving at close quarters to passers-by in the paddock area. Someone
stumbling against their hovercraft would put their hands down to prevent the
fall and guess which part was on the uppermost structure of the body directly
in line with the theoretical falling person?
Hoverlord
Philippe Marcil
Its better to be safe than sorry!!!!
Serving the world:
www.angelfire.com/biz/aerotour
AeroTour Canada
1279 boul. Hurtubise
Gatineau, Quebec, J8P 7C2
Tel.: (613) 762-3167
Voice and fax: (819) 669-8454
J. Kelley Jernigan
was a figment of my imagination? WOW! What was I smoking that day?
Hoverlord wrote:
> Mr Rutan has never created a successful craft.
J. Kelley Jernigan
J. Kelley Jernigan
it, and I won't be using any chicken wire anywhere! The idea of a 1/2
guard while it has proven successful in the past scares me.
Philippe Marcil wrote:
J. Kelley Jernigan
rwth...@my-dejanews.com
UH12R with a thrust duct, 4ft-2blade wood prop powered by a Kawasaki 440. I
have noticed that in static conditions at high throttle settings I can
actually see the duct sucking air in from 4ft to the side and behind the
duct. (The reason I can see it is exhaust smoke) My question is this the
reason the duct works? It literally stops the pressure created behind the
duct from escaping around the blade tips to be sucked in again? If this is
the fact, it makes sense that a duct will produce more thrust than an open
prop in a static situation. It seems however that the prop would gain
efficiency as speed increases and the pressure can no longer escape around
the prop tips due to forward motion. If this is true then I can see that the
duct would be a drag liability at high speeds. Any thoughts? R. Thomas
<19981104121016...@ng-ft1.aol.com>,
sev...@aol.com (SEVTEC) wrote:
> <Subject: Re: Thrust system design tips?
> <From: hove...@aol.com (Hoverlord)
> <Date: Wed, Nov 4, 1998 8:44 AM
> <Message-id: <19981104034422...@ng18.aol.com>
>
> <Hello Barry:
> <For the casual reader here.. My father and Barry were like the Hatfields and
> <McCoys
>
> At risk of being accused of being out of subject on this thread, I should set
> the record straight. " My contact with the Crismans ("Hoverlord") is a single
> trip by myself to see a surface skimmer of theirs, one powered by a V8 engine,
> 3 place. They may have visited me perhaps 4 times over a period 20 years.
> This is hardly a feud!
>
> The Fowler reference as stated by "Hoverlord" is clearly incorrect, all one has
> to do is observe a Vanguard, or better yet, an Explorer, to prove it. As far
> as I know to this date the Canadian Government sponsored efforts have yet to
> produce a viable surface skimmer, and they have "bought" English technology and
> imported it.
>
> Barry Palmer, for <A
> HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
>
-----------== Posted via Deja News, The Discussion Network ==----------
Lou Hinshaw
>I'm planning on going with the full guard on my Vanguard when I finish
>it, and I won't be using any chicken wire anywhere! The idea of a 1/2
>guard while it has proven successful in the past scares me.
>
Yah. Chicken wire. Chicken ploy.
I have never seen it used on airboats or GEVs, but he said it as of
some authority.
That was tossed in so the tosser could win a point without bothering
to be right. I nearly droppes the NG from my list till I cooled off some
and realized that he was a very small part of the NG.
"The learned Fool writes his nonsense in better language,
but 'tis still nonsense."
B.Franklin 'Poor Richard Improved' (1754)
Lou H.
Prolific inventor,
Lousy Salesman,
Incompetent businessman.
(Typical of the Breed)
Looking for both around
Tulsa OK, USA