Centrifugal Fan
Greg Tomlin
centrifugal fan would greatly simplify the design.
However, I haven't notice very many uses of such a fan (actually only
found 1 use) and I was just wondering what I might be overlooking.
I would appreciate any information on the subject.
Rick
Greg Tomlin wrote:
There is a section on centrifugal fans in "Thurst Systems for Light
Air-Cushion Vehicles" by Howard S. Fowler, published by The Canadian Air
Cushion Technology Society. Fowlers summary includes the following
advantages: 1. quiet; 2. possible to integrate powerful steering and
reverse thrust; 3. blower characteristics permits control without danger of
fan stall. He lists the following disadvantages: 1. Bulky; 2. poor
thrust/horsepower efficiency; 3. volute design is difficult and involved
much cut-and-try testing.
In the end, Fowler suggests in general that they not be used, because
better alternatives exist.
Centrifugal thrust systems were used on some medium-size experimental
hovercrafts several years ago. Designing one would clearly be within the
realm of creative tinkering. If I were to contemplate a centrifugal thrust
system, then I would first build a test bench setup before committing to
any hovercraft design, and be prepared to spend lots of money designing,
discarding, redesigning, etc. Centrifugal fan impellors tend to be quite
heavy and, therefore, are difficult to build and balance. It is doubtful
that you will find a pre-built fan to suit your needs for use as a
thruster. However, because they are less likely to stall than axial fans,
centrifucal fans can be great for lift, and something off the shelf is more
likely to be found for that purpose, than for thrust.
Centrifugal fans were used succesfully for lift on large craft (i.e. 11
foot diameter impellors), and I've flown two-seater prototypes that had
roughly 12 inch twin centrifugal fan units spinning on a horizontal axis on
each side of the craft for lift. The "stealth hovercraft" built by Leo
Cruikshank uses a centrifugal fan that spins on a vertical axis for lift.
He's a sheet metal worker, and he built the fan himself. It's heavy, and
once going, takes a long time to slow for the craft to come off cushion. I
believe that he added a brake to it to allow the craft to come off cushion
more quickly.
I myself gave serious thought to using twin centrifugal fans for lift on my
hovercraft, but the more I though about having to build the fan from
scratch (which I would likely have to do), the more leary I was of the
idea. But if a appropriate pre-built fan fell into my hands today, I would
certainly spend the time thinking hard about using it for lift. Of course,
I would do this with the realization that all would be experimental.
Andrew Barber
all together. He can also keep the engine at one throttle position and
regulate the lift with the dampers. However weight is beginning to become an
issue and he has been contemplating some changes to the craft.
BTW the web page is up again but the address is a bit different:
http://www.pegasusus.com/djdewey/hovercraft/leo/
Andrew
"Rick" <rgh...@ns.sympatico.ca> wrote in message
news:3B856EFD...@ns.sympatico.ca...
Greg Tomlin
skirt, and rear airflow being either diverted into the bag skirt, or (via a
flapper type valve) exhausted out the back for thrust.
That way, I would have 100% airflow for the initial lift and then up to 50%
being diverted for thrust.
I have already built 2 fans - the first WAS very heavy (6 blade, 20" diameter).
The second is a 3 bladed fan and weighs at least 2/3s less. Unfortunately, my
test engine is a 4 HP B&S with a pull start and I was unable to crank the thing
(direct shaft connection). I'm in the process of getting an electric start 8 HP
and hope that will work out better.
I am not looking for speed here. 5 or 10 mph would be fine for now. I don't
trust my daughter that much.
We have a river by the house that is really a knee deep creek (except during
floods), and I am concerned about noise and running over people canoeing. So it
sounds like if I can get the proper fan design, the centrifugal fan fits the
bill nicely.
Thanks for the information.
Nickjokay
>and hope that will work out better.
>I am not looking for speed here. 5 or 10 mph would be fine for now.
mph would be fine for now. I don't
>trust my daughter that much.
>We have a river by the house that is really a knee deep creek
To answer your original question about why there are so few centrifugal fans.
Centrifugal fans simply don't fit the application for recreational hovercraft.
Axial fans move air at high volume-low pressure. Centrifugal fans move air at
intermediate volume-intermediate pressure. The typical recreational hovercraft
operates with a cushion pressure of about 10 pounds per square foot so the
axial fans are sufficient. In contrast the military hovercraft operate at
about 100 psf and therefore need centrifugal fans to reach that pressure.
Having a top speed limited on water to 5-10 mph will be difficult to achieve
because the maximum power is used to get over hump, which will be between
5-10mph. Once over hump then the hump drag forces drop off...leaving more power
left to go into top speed.
The only company I know of that deal in small hovercraft is Universal
Hovercraft. Universal makes a parking lot hovercraft (UH-6) that goes ok on a
5 hp engine. They also make a UH-10F that uses a 10 hp engine. It might work
with an 8hp on water with a small person. Universal should be contacted on
this.
If you decide to go with axial fans, you might contact Universal Hovercraft as
they make a range of fans and propellors specifically for hovercraft and at, I
believe, the lowest prices of any fan dealers.
Nick
Ralph DuBose
Historically speaking, centrifical blowers have always been a
intermediate step on the way to axial blowers, except when really high
pressures are needed. Real performance is with straight- ahead
laminar flow systems.
SEVTEC
>From: rdu...@pdq.net (Ralph DuBose)
>Date: 8/23/2001 10:42 PM Pacific Daylight Time
>Message-id: <cb5b2d4e.01082...@posting.google.com>
Use of centrifugal fans I feel is a mistake and one of the reasons these craft
are not more prevalent. Serious performance degradation in large craft is a
result of these fans, along with mixed flow fans, and the highest cushion
pressures can be handled by a single stage axial flow fan, and if quiet is
desired, a two stage axial fan still beats the centrifugal or mixed flow fan
all hollow from the standpoint of weight and bulk, as well as a desireable head
capacity characteristic.
I gave a technical paper on this in 1984 (CACTS CASI, Vancouver) that spelled
out the gruesome details of this design problem along with others in large
craft, but those who control the large programs (and the money for them) seemed
to have torpedoed large machine development through a lack of understanding of
engineering basics.
Barry Palmer, for <A
HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
NWCedarDecks
>pressures can be handled by a single stage axial flow fan, and if quiet is
>desired, a two stage axial fan still beats the centrifugal or mixed flow fan
>all hollow from the standpoint of weight and bulk, as well as a desireable
>head
>capacity characteristic.
>
>I gave a technical paper on this in 1984 (CACTS CASI, Vancouver) that spelled
>out the gruesome details of this design problem along with others in large
>craft, but those who control the large programs (and the money for them)
>seemed
>to have torpedoed large machine development through a lack of understanding
>of
>engineering basics.
>
>
>
>
Could you explane a 2 stage to us rest of us.This is some thing New we haven't
beat to death and back again. :-)
Really
Could we make the Scat thunder Hover even louder? Come on Barry give it up!!
LOL Just a joke People
This does sound like a low speed pusher for the hole shot !!!!
My .002 LOL
ED
Greg Tomlin
SEVTEC
>From: nwceda...@aol.com (NWCedarDecks)
>Date: 8/24/2001 3:43 AM Pacific Daylight Time
>Message-id: <20010824064333...@mb-cj.aol.com>
>
>>cushion
>>pressures can be handled by a single stage axial flow fan, and if quiet is
>>desired, a two stage axial fan still beats the centrifugal or mixed flow fan
>>all hollow from the standpoint of weight and bulk, as well as a desireable
>>head
>>capacity characteristic.
>>
>>I gave a technical paper on this in 1984 (CACTS CASI, Vancouver) that
>spelled
>>out the gruesome details of this design problem along with others in large
>>craft, but those who control the large programs (and the money for them)
>>seemed
>>to have torpedoed large machine development through a lack of understanding
>>of
>>engineering basics.
>>
>>
>>Barry Palmer, for Sevtec
>>
A two stage axial fan is a fan that pumps air into another fan, the flow swirl
being taken out by intermediate stators, or stationary blades. Two stages will
pump air at roughly 70% of the blade tip speed of a single fan. You could use
a single stage but it would be noisy, and there would be two much flow swirl if
there were no stators. Even four stages will be far more compact than the
centrifugal or mixed flow design, and be exceptionally quiet, running at about
50% of a single stage. Listen to your Scat at 1/2 rpm, or 1/8 power. Its fan
is quiet, but it won't move, either.
Machines that require pressures approaching 100psf for cushion waste power as
air must pass through the bag before entering the cushion, and this is
necessary for a proper head capacity characteristic, as the centrifugal fan
alone will not deliver that characteristic, unless hopelessly undersized.
The added needed power feeds upon itself, when you system analyse the whole
craft, and this, along with ducted thrusters or aircraft propellers, and to a
lesser extent the bag-finger skirt, makes for an inefficient craft that is
being displaced by catamarans due to its high operating costs.
The surface skimmer can be a viable commercial craft, as well as a recreational
craft for cruising, but until the engineers start using engineering basics,
and start working a real surface skimmer problem, like drag in a seaway (which
has been done by this writer, but not developed enough as it really is for
large craft,) the industry will languish.
Ken Roberts
the uh-6 goes on water too, and if you're in water more than a foot or so deep
it can't maintain plane, which is by design. there was one at muscoda this
last time. he could get above plane in an inch or two of water, but if he went
deep he lost plane. this is quite possibly the craft you want.
Ken Roberts
>A two stage axial fan is a fan that pumps air into another fan, the flow swirl
>being taken out by intermediate stators, or stationary blades. Two stages will
>pump air at roughly 70% of the blade tip speed of a single fan. You could use
>a single stage but it would be noisy, and there would be two much flow swirl if
>there were no stators. Even four stages will be far more compact than the
>centrifugal or mixed flow design, and be exceptionally quiet, running at about
>50% of a single stage. Listen to your Scat at 1/2 rpm, or 1/8 power. Its fan
>is quiet, but it won't move, either.
>
>HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
the idea of a multistage lift fan is interesting. the crafts i play with would
have a problem accommodating that long of a thruster without slapping water to
get off cushion, but for something a bit larger i suspect you might get a
significant noise reduction for a front-mounted lift fan. that engine is the
primary cockpit noise source if you have a twin engine craft and a front lift
system. it's still audible with a single engine twin fan of the same
configuration.
going to a twin fan on each might make the engine be the primary noise source
on something like a uh-13p. speaking from the cockpit perspective.
Ralph DuBose
When I was priviledged to talk to LCAC engineers I got the
impression that they felt a lot of pressure to stay well within what
was taken to be accepted practice for large hovercraft. In other
words, their company was guaranteed to make money if their product met
the expectations of their only customer[which had been formed by
existing BHC designs] whereas building a revolutionary, much improved
type of craft was not something they had been asked to do. Hence the
decision to take BHC design concepts and just pump them full of
steroids.
When I ruminated out loud about centrifical fans often being,
historically, a kind of intermediate phase I meant that it has usually
been much quicker and easier to build or adapt a centrifical blower
than to design from scratch a suitable multistage axial blower to make
higher pressures. I believe that the first gas turbines/ jet engines
had centrifical compressors. The first engines with good performance
had multistage axial compressors. I was not there, but I get the
impression that it was much harder to build that way.
A working, full time hovercraft designer once described to me the
"business" ,literally, of designing a large low cost craft for the
gov. of India. The lift was to be from cent. blowers for the simple
reason that reliable blowers with known performance were available and
that could be designed around and cheaply purchased off the shelf. The
appeal of that route from a business standpoint is hard to argue with.
To me, one of the charms of the hovercraft world is that an
individual can still push the envelope by taking chances on projects
that a commercial enterprize would certainly pass on. I bet that the
first suitable high pressure multi stage axial flow hovercraft lift
fan will be built in a barn by a retired postal worker.
Ralph DuBose
Forgot to say something. When I spoke about "really high
pressures", I was not thinking about hovercraft lift at all but
several times atmospheric. Like in the final stage of a gas turbine
compressor or an aircraft turbocharger. Should have been more clear.