Prop/Fan Questions
Steven L. Yarak
A number of friends of mine and I have taken it upon ourselves to build a
hovercraft as our summer project. None of us have any practical experience
with hovercrafts, only an AP level knowledge of physics. The chief concern
for all of us is cost. Time we have lots of, and lots of people to divide
it over. So anyway, here are our "newbie" questions thus far:
1) Would it be possible to use the fan assembly from a whole house fan for
lift? The unit we looked at was a 3ft, 4 blade design that made 6900 CFM
when coupled with the included 120VAC motor. I believe this was at 3000
RPM, but I am not certain. Is 6900 CFM enough to lift a hovercraft of say
6.5x12-13ft? The plan would simply replace the electric motor with a
vertical shaft small engine. This would be rather simple given that this
was a belt driven model. The main reason for this would be do avoid having
to balance the prop. This is something that none of us have any idea how to
do. Which brings us to question
2) What exactly is involved in balancing a prop, and how difficult is it? I
understand perfectly the need to perform such an operation, but am unsure
how one would go about it to such precision that one would feel safe in the
proximity of such a device spinning at thousands of revolutions per second.
3) How fast is the maximum speed that the thrust prop should spin at? Small
engines rev only to about 3,600 RPM, but 2-stroke engines easily attain
10,000 RPM. Or, if one were to use a car transmission to couple the engine
and the prop, who knows how fast the prop would spin in 5th gear? I know
that with airplanes, the performance of certain designs of props taper off
when spun above a given speed. What are the real world advantages of
spinning a hovercraft prop at 10,000 RPM, vs 3,600 RPM? Will there be 277%
more air moved as result?
4) Is it more efficient to have two, or even three, smaller props for
thrust, rather than one huge one. It would seem that this would focus the
airflow more at the level of the hull, and thus make more thrust. It would
also seem that with more numerous smaller props it would be possible to
cover the area missed by one large one, again, giving more thrust. Is this
the case? Please explain.
5) How difficult is it to implement water cooled automobile engines in
hovercraft? Is the water system and radiator location too much effort to be
worth it? What are the pros/cons?
Answers to any of these questions, or even expansions of them by others,
would be greatly appreciated. I have learned a great deal by lurking in
this newsgroup. I hope through this post that you can teach me even more.
Thank you.
-sly
Steven L. Yarak
TERENCE DUNN
I am also fairly new, and can only answer a couple of your questions.
The scout may not look as sexy, but it has better payload (can carry 2
people)Vanguard says it is 2-3 person. I have talked to people who cary
4 people with no problem. Your budget should be between $1000 - $2000.
Hope things work out.
Go to these sites and order plans for:
Universal Hovercraft www.bews.com/hover/catalog.htm
uh 10f entry level 19.95
uh 13p sportsman 29.95
prop fan construction 15.00
Sevtec http://members.aol.com/sevtec/sev/skmr.html
sevtec scout $29
sevtec vanguard $44
"Steven L. Yarak" wrote:
>
> Hi,
> 2) What exactly is involved in balancing a prop, and how difficult is it? I
> understand perfectly the need to perform such an operation, but am unsure
> how one would go about it to such precision that one would feel safe in the
> proximity of such a device spinning at thousands of revolutions per second.
Ok ballancing is not that hard I have done it. as for safty if the line
the thrust duct with 1/8" plywood. For the full blurb, spent the $15.00
and buy the plans for prop construction. It also covers balancing.
http://www.bews.com/hover/catalog.htm
> 3) How fast is the maximum speed that the thrust prop should spin at? Small
> engines rev only to about 3,600 RPM, but 2-stroke engines easily attain
> 10,000 RPM. Or, if one were to use a car transmission to couple the engine
> and the prop, who knows how fast the prop would spin in 5th gear? I know
> that with airplanes, the performance of certain designs of props taper off
> when spun above a given speed. What are the real world advantages of
> spinning a hovercraft prop at 10,000 RPM, vs 3,600 RPM? Will there be 277%
> more air moved as result?
>
you use a belt and pully reduction system to bring your prop tip speed
down to around 400 feet per second.
> 4) Is it more efficient to have two, or even three, smaller props for
> thrust, rather than one huge one. It would seem that this would focus the
> airflow more at the level of the hull, and thus make more thrust. It would
> also seem that with more numerous smaller props it would be possible to
> cover the area missed by one large one, again, giving more thrust. Is this
> the case? Please explain.
>
You want to go with the largest prop you can fit. one large prop with a
high pitch is quite efficient for a given horsepower.
> 5) How difficult is it to implement water cooled automobile engines in
> hovercraft? Is the water system and radiator location too much effort to be
> worth it? What are the pros/cons?
>
sorry havent tried this.
Earl
I havent heard of anyone using a house fan for a lift fan in their hover. I
dont know if this would be a safe thing to do or not. My inclination is that
since I have never heard of anyone doing this, there may very well be a good
reason why. You said the fan is 36 inch diameter and turning at 3000. That
would give you a tip speed of about 470 ft/sec. Recent checking I did revealed
lift fan blade tip speeds with a low of 280 ft/sec to a high of 500 ft/sec for
the quieter craft. Average seems to be around 400 ft/sec.
Some quick calculations show that a hovercraft measuring 6'6'' X 13' with a
gross weight of 700 lbs would need about 5200 CFM to sustain a 1/2 inch
hovergap. That would require a minimum of about 2 1/2 HP. You will have to
check the balance on the prop regardless. I think it is a very bad idea to run
a prop without doing so. Also, your blade area and pitch will need to be
matched to the rpm/power curve of the engine you choose to use. All ventilation
fans I have seem for home use utilize metal blades. I don't know if repitching,
modifying blade area and balancing one of them is possible.
On prop speeds, generally you would want as much thrust as you can get at the
low end of the scale. It will make it much easier to get over "hump". In this
case, you would want the biggest prop you can reasonably fit to your craft. In
a generalized "nutshell", you can think of it as prop diameter is efficiency and
pitch and blade area are power absorption. As for tip speeds, anything much
over about 500 ft/sec is going to start to be a bit noisy. If I remember
correctly, the structural limit for a wood prop (assuming it is made from
straight grained birch, maple or other suitable wood) is about 900 ft/sec, but
you would'nt be able to stand the noise level. To find your blade tip speed,
multiply rpm X diameter in inches X constant of .00436 The answer will be
in feet per sec.
On water cooled automotive engines: I am installing one in the hovercraft I am
building. The good points are: 1. they are dependable 2. They are easy to
work on and parts are readily available. 3.they are fuel efficient 4. they can
be cheap if you dont mind doing your own rebuild
The bad points are: 1. they are heavy 2. you have extra plumbing and
constructing to do to for the radiator.
As for difficulty, my auto engine installation is pretty straightforward. I
would'nt consider it difficult at all.
Hope this helps
Earl
Ken Roberts
props. i make no claim to expertise, only to a limited amount of experience and success.
> 1) Would it be possible to use the fan assembly from a whole house fan for
> lift? The unit we looked at was a 3ft, 4 blade design that made 6900 CFM
i would say not. you're trying to lift a human, presumably. a prop is not hard to make. mine were
made from wood i got at the local lumberyard. i think my two props averaged $20 each in materials
cost. while the first was a bit off, the second worked well and is still working fine. you'll want
to fiberglass it if you want it to last.
you can go to universal hovercraft or to sevtec to get plans for props. although, on second
thought, i think sevtec uses a prefab prop. universal is almost certainly not the only one out
there with prop plans, though. barry palmer from sevtec has offered his wisdom on the topic to this
list repeatedly.
you can get the prop templates from universal for about $20. i'd get the construction booklet too.
it's about $2. while you're there, you may want to get the plans for one of the UH-10F machines.
there are a few to choose from and they're very cheap to build. i rode one a few months ago, and
while it wasn't remarkably fast it was a whole lot of fun.
construction comes down to gluing and clampign some boards together, marking them according to the
instructions with the template, and carving it out with saws/grinders. i skipped the saws and went
straight to the grinders, since i have no saw and do have a really kick-butt pair of grinders.
> 2) What exactly is involved in balancing a prop, and how difficult is it? I
> understand perfectly the need to perform such an operation, but am unsure
> how one would go about it to such precision that one would feel safe in the
> proximity of such a device spinning at thousands of revolutions per second.
when i glass my prop, i statically balance it by putting a 1" diameter rod through the hole (the
size of the hole) and then adding more fiberglass to the blade that always drifts up. it probably
won't take much.
> 3) How fast is the maximum speed that the thrust prop should spin at? Small
the general rule of thumb is to have a big prop that spins slowly. this cuts down on noise,
increases safety, and decreases prop wear. most crafts i've seen use a belt reduction on the thrust
prop. the prop should be built in such a way that the engine can't redline, but can come close.
this assures maximum use of power.
> 4) Is it more efficient to have two, or even three, smaller props for
> thrust, rather than one huge one. It would seem that this would focus the
> airflow more at the level of the hull, and thus make more thrust. It would
> also seem that with more numerous smaller props it would be possible to
> cover the area missed by one large one, again, giving more thrust. Is this
> the case? Please explain.
the simpler the design, the less cost, work, chance of failure.... you heard of the KISS concept?
look at the web pages for lots of hovercrafts. since you're talking about building, look especially
at the homebuilt pages. buy a set of plans or two. plagiarism is the sincerest form of flattery.
> 5) How difficult is it to implement water cooled automobile engines in
> hovercraft? Is the water system and radiator location too much effort to be
> worth it? What are the pros/cons?
if you use an auto engine you'll want to make a 19' or larger craft. this will probably hold 5-7
people, and will be very fast. these could possibly be built for approximately the same cost as my
uh-12r since you can get a used auto engine for as little as $100 and still have something
quasi-reliable. my 45hp 2si engine costs $2k.
the biggest thing is weight. you need much surface area to support the weight of an auto engine.
10 lbs/in^2 is the pressure you're shooting for. also, hovercrafts don't really need that much
power in order to be very, very fast.
john
plans and build to the plans. These other points can be discussed while you
are building your craft.
Steven L. Yarak wrote in message <92821432...@news.remarQ.com>...
Nickjokay
>hovercraft as our summer project. None of us have any practical experience
>for all of us is cost. Time we have lots of, and lots of people to divide
>it over. So anyway, here are our "newbie" questions thus far:
>
for a summer project. What's your goal? Do you just want to hover, or do you
want to relish in the design and construction process of the hovercraft?
In the first case I recommend pooling your money and buying a kit from Sevtec
or plans from Universal. If Weber has kits then consider them too. By the end
of the summer you'll be hovering, and I mean really hovering at at least 30mph
over grass and water.
The best advice I can give to those getting involved in the design and
innovation process is to first find out everythng possible about the existing
products. And this applies to the design of anything. There's two things I
recommend you do. First, take a road trip to the National Hovercraft Rally at
Troy Ohio (near Dayton) this June 19 and 20. You can see the past 40 years of
American hovercraft design effort rolled up into many successfully functioning
hovercraft. And there you can see first hand how many whole house fans are
used for hovercraft lift as well as answer the rest of your questions.
The second thing I'd do to learn about design is buy a kit from Sevtec, Weber,
or plans from Universal. You'll have a good time building it. You'll learn
first hand about prop balancing and many other skills used in properly building
light weight structures. The project will add to your skills and knowlege for
designing your own hovercraft. And in the end your hovercraft will almost
certainly work.
Good luck,
Nick
Ken Roberts
> plans and build to the plans. These other points can be discussed while you
> are building your craft.
> Steven L. Yarak wrote in message <92821432...@news.remarQ.com>...
> >Hi,
> >
> >hovercraft as our summer project. None of us have any practical experience
> >with hovercrafts,
i agree with this. i bought three sets of plans before i ever owned a hovercraft. these things are
a wealth of knowledge on keeping it simple and cheap. even if you don't intend to build exactly to
the plans, you can still get a huge amount of info and ideas.
of the two plan-suppliers i've seen a craft of, either would be a good recommendation:
-universal hovercraft
-sevtec.
each has strengths and weaknesses, i'm sure. i've spouted my two cents worth on this list so often
people probably wonder if i'm a dealer, so i won't go into that here. there are lots of places to
get plans, those two are just the only ones i've seen a real craft of. i can assure you that either
will be a very workable craft, assuming you build to the plans.
jdem...@wisc.edu
> Hi,
>
> A number of friends of mine and I have taken it upon ourselves to build a
> hovercraft as our summer project. None of us have any practical experience
> for all of us is cost. Time we have lots of, and lots of people to divide
> it over. So anyway, here are our "newbie" questions thus far:
>
> lift? The unit we looked at was a 3ft, 4 blade design that made 6900 CFM
> RPM, but I am not certain. Is 6900 CFM enough to lift a hovercraft of say
> 6.5x12-13ft? The plan would simply replace the electric motor with a
> vertical shaft small engine. This would be rather simple given that this
> was a belt driven model. The main reason for this would be do avoid having
> to balance the prop. This is something that none of us have any idea how to
> do. Which brings us to question
>
> understand perfectly the need to perform such an operation, but am unsure
> how one would go about it to such precision that one would feel safe in the
> proximity of such a device spinning at thousands of revolutions per second.
>
> 10,000 RPM. Or, if one were to use a car transmission to couple the engine
> and the prop, who knows how fast the prop would spin in 5th gear? I know
> that with airplanes, the performance of certain designs of props taper off
> when spun above a given speed. What are the real world advantages of
> spinning a hovercraft prop at 10,000 RPM, vs 3,600 RPM? Will there be 277%
> more air moved as result?
>
> thrust, rather than one huge one. It would seem that this would focus the
> airflow more at the level of the hull, and thus make more thrust. It would
> also seem that with more numerous smaller props it would be possible to
> cover the area missed by one large one, again, giving more thrust. Is this
> the case? Please explain.
>
> hovercraft? Is the water system and radiator location too much effort to be
> worth it? What are the pros/cons?
>
>
> would be greatly appreciated. I have learned a great deal by lurking in
> this newsgroup. I hope through this post that you can teach me even more.
> Thank you.
>
> -sly
> Steven L. Yarak
I am also looking into building my first hovercraft and have very little knowledge. Regarding fans, I was looking around and the cheapes pre made fans I was able to find ran about $200 from slipstream hover. They are metal which from what I've seen on this forum sounds really dangerous. so what is the deal with these Hascom fans? Are they safe for my diy craft. otherwise it sounds like it would be much more cost effective to build my own out of wood. the craft I plan on building would be roughly 4'x8' and use a 6.75 hp engine for lift and a 5 hp engine for thrust. Any advice is greatly apprciated.