Build your own prop.
Buy_Sell
how to build your own propellers.
http://www.aerodyndesign.com/PROP_10/PROP_10.htm
I like the idea of making your own propellers because I am always
searching for a new design that is more efficient and quieter than
anything currently available. If anyone has any ideas on how to
achieve this, I'm listening. I always start by studying computer fans
as they seem to be getting quieter.
Another thought that crossed my mind today, was if the propeller had
not been invented yet, can you think of another method of moving large
quantities of air to propel a vehicle such as a hovercraft or
aircraft. I thought of maybe the pulse jet but these things are
really noisy, so I gave up on that idea. I am looking for a method of
propelling a hovercraft in stealth mode. This might not be possible
but without a dream, reality becomes just a concept.
Ken Roberts
They sure did it the hard way, with all that whittling. They came up
with a nice looking prop though.
I enjoy making props. The next one I make, I'll design it myself.
However, it'll probably look a lot like a UH prop since that's the
simplest way, it'll just be my numbers.
Go get a book on propeller design. Hopefully your math background is
strong enough to understand it. Mine isn't, but I still got a lot out
of the text anyway.
I would suggest a test harness with an electric motor, and a wind
tunnel of sorts. The basic requirements are:
- The power source (motor) which turns at the desired RPM
- A speed control to adjust the RPM/power input.
- A prop to test
- A way to measure results.
As usual, the measurement is what is probably going to be hardest.
You'll want a way to measure:
- Static thrust
- Thrust at various inlet air speeds
- Thrust at various power settings (for a given inlet speed)
- Noise
- Exit velocity along thrust axis
- Exit velocity crosswise to the thrust axis
- Probably a lot more.
I would say working full size is going to be difficult, but working in
the 24" diameter range is probably best. The props will be big enough
to represent a real prop, and the size is small enough that you can
build it cheaply and quickly.
Buy_Sell
now that you mention it, I just happen to have a 525 lb. electric
motor sitting in my garage. It was once used to power an elevator of
a building. It runs on 240 VDC but I have successfully tested it on a
12 volt car battery and even a 48 volt UPS battery pack. For the life
of me, I cannot understand why it was discarded. I got it for $100
from the scrap metal yard and it works fine. I had planned on using
it for a wind powered generator because when I spin it, I get power
from it. I've been working on the VAWT (vertical air wind turbine)
concept generators lately. Racecom makes these wind generators and
they produce no noise (0 dB).
I suppose a bathroom scale would make a great static thrust analog
instrument.
PS: As for the wind tunnel idea, I found a Multi-Wing fan unit in the
metal recycling yard that would be ideal for this. It is way too
heavy to put on a homebuilt hovercraft. This fan unit is at least
four feet in diameter and the hub is heavy. I paid 5 bucks for it and
it looks cool hanging on the wall in my garage.
Ken Roberts
> Well Ken, I never thought of testing props with an electric motor but
> now that you mention it, I just happen to have a 525 lb. electric
> motor sitting in my garage. It was once used to power an elevator of
> a building. It runs on 240 VDC but I have successfully tested it on a
> 12 volt car battery and even a 48 volt UPS battery pack. For the life
> of me, I cannot understand why it was discarded. I got it for $100
> from the scrap metal yard and it works fine. I had planned on using
> it for a wind powered generator because when I spin it, I get power
> from it. I've been working on the VAWT (vertical air wind turbine)
> concept generators lately. Racecom makes these wind generators and
> they produce no noise (0 dB).
>
> I suppose a bathroom scale would make a great static thrust analog
> instrument.
>
> PS: As for the wind tunnel idea, I found a Multi-Wing fan unit in the
> metal recycling yard that would be ideal for this. It is way too
> heavy to put on a homebuilt hovercraft. This fan unit is at least
> four feet in diameter and the hub is heavy. I paid 5 bucks for it and
> it looks cool hanging on the wall in my garage.
Somehow I think testing props using an elevator motor is a bit
overkill, Even with full-sized props. That said, it wouldn't really
stop me either.
I don't know much about it, but maybe elevators have a lot of
regulations to worry about like the FAA does with airplanes. Maybe it
just had too many hours on it? One thing I remember from scrap yards
in South Dakota growing up, a lot of companies get rid of stuff that's
on their shelves too long. We picked up a hydraulic cylinder for a
payloader, still had all the factory markings on it and a
manufacturer's tag on it. It had never been used, but it was on the
shelf and they didn't want to inventory it. Got it for 10 cents a
pound and converted it into a log splitter of gargantuan proportions.
I'd love to have a wind turbine, but my house is not in a good spot
for that. Solar might be pretty good though.
Anyway you'd want a motor speed controller suitable for your type of
motor. From the sounds of it you have a brush-type motor with
permanent magnets for a field. I think a plain-old variac would work,
if you could find one big enough, but probably you'll need to look at
the funny boxes at that scrap yard for a while to get a real speed
controller.
Buy_Sell
Transistors) I have an entire 5 gallon bucket of these from the
recycling yards. These devices have the high speed switching feature
of a MOSFET along with the current handling ability of a bi-polar
transistor. A simple pulse width modulation circuit with IGBTs could
be used to control the speed.
http://en.wikipedia.org/wiki/IGBT
As for solar systems, one of the most effective for the money is the
evacuated tube water heaters. I purchased mine from http://www.wearesolar.com/
in the USA. They had the best price anywhere with the exception of my
contact Angela in China where they are being made. Angela wants to
sell me a sample shipment of 20 units of 30 tubes each for $500 each.
This is insured and delivered to the Port of Vancouver in Canada. I
opted out of this arrangement because I didn't need that many units.
Bill Fitch sold me my unit for $700 and it was delivered to my
doorstep for a very reasonable price. It survived the trip with no
breakage at all.
We get plenty of wind here, that is why wind power interests me so
much. When I discovered how quiet the VAWT squirrel cage type of wind
generators were, they had me sold on the concept. Jay Leno has one of
these on his garage. The design has to be dead quiet or municipal
noise bylaws will come into effect. 0dB is perfect.
As for testing prop designs, I am wondering if I can use the program
'Solidworks' to test this. Software has some pretty advanced analysis
features. It might be worth looking into. I have an engineer friend
from England who might be able to help me out with this.
Buy_Sell
explained a few concepts to me that I wasn't really aware of. Gordon
was asking me if I had noticed the propellers on airboats recently and
I responded that I had noticed them in Florida a few weeks ago when I
was on holiday. The airboats are using a much wider blade on their
props in order to put more power thru them. Gordon said that if you
took the same type of propeller and put it on a hovercraft that it
wouldn't need to turn as fast to move the same amount of air. This
makes sense because each blade would be taking a bigger scoop of air
compared to the conventional hovercraft props. I thought about this
idea quite a bit and wondered if it had been tested as a method of
making a stealth air vehicle. Have a look at the propeller on this
stealth night observing aircraft used in the vietnam war. It seems to
confirm what Gordon has stated. The next step would be to try to make
one of these type of propellers light enough to fit on an ultralight
hovercraft.
Buy_Sell
technologies. Apparently, the owl is the quietest bird on earth
because of its tattered trailing edge feathers. I got thinking about
this idea and another idea came to my mind. Don't they put dimples on
golf balls to make them go farther? I wonder if this has anything to
do with noise reduction also. It seems that air likes to stick to a
clean surface and when it lets go of this clean surface, it creates a
sudden shock wave. By breaking up this adhesion to the propeller by
putting dimples on it, it might achieve the same thing and reduce the
noise of the propeller. I also thought that maybe a brush type
material on the trailing edge of a propeller might achieve the same
feature as the owl's tattered wings. Lots of ideas but the real facts
with be in the testing.
Ken Roberts
What you're talking about in this post is called solidity. Solidity
is the percentage of the swept area which is covered by a prop. In
other words, say you have a 48" diameter prop. So you draw a circle
48" diameter, set the prop in it and compare the area of the total
circle (pi*r^2) with the area your prop covers from the front view.
Solidity relates directly to the amount of coupling between the engine
and the air. Too little solidity makes you unable to move enough air,
or even load your engine safely. Too high a solidity figure and your
engine is forced to work outside its design torque curve. Hovercrafts
have a problem with this when they use a two stroke engine and there
is too much solidity. The two stroke tends to not have enough
midrange torque, which means they can't spin the prop up into high
RPMS to get full engine power.
You can turn the prop slower when you have a higher solidity, as long
as you increase pitch accordingly. There are a lot of considerations
here, and while I know how some of it relates I can't really design a
system that will work without trial and error. The way I understand
it, turning the prop slower also makes the air seem more fluid, in a
way that works as an inverse to solidity. So yes, you can make a huge
prop with a high solidity and turn it slower and you get load
characteristics that look very similar, and you get lower noise and
less tip wear.
One thing that airboats and hovercraft have in common is the use of a
prop in dirty conditions. Water spray and sand erode props badly,
especially at higher power settings. Turning them slower makes a huge
difference on that. I went from a 48-36 to a 48-50 and the difference
was astounding, both in noise and in tip erosion. The 48-36 had a tip
speed of around 675 fps. Any dirt or water would cut through it like
butter, I had to repair my tips every other time I went out. The
48-50 allowed me to run without ear protection and I could go maybe 2
months without repairing the tips.
Ken Roberts
There are some people experimenting with quiet props, I'm just trying
to get my hover done right now. :)
David Bosworth
years back. not sure what ever happened with that, maybe he is keeping a
secret! heh
I believe most of the noise comes from the tip not the trailing edge so that
is where you might need the feathering. I thin leading edge and sharp
trailing edge make a big difference in both performance and noise.
--
David Bosworth
From the rocky shores of the Skykomish river
http://www.premier1.net/~daveb/
"Buy_Sell" <werk...@hotmail.com> wrote in message
news:aff5f8e8-50e5-40ff...@s33g2000pri.googlegroups.com...
Ken Roberts
> I think it was Matt Starbuck that was experimenting with dimple tape a few
> years back. not sure what ever happened with that, maybe he is keeping a
> secret! heh
>
> I believe most of the noise comes from the tip not the trailing edge so that
> is where you might need the feathering. I thin leading edge and sharp
> trailing edge make a big difference in both performance and noise.
>
> --
> David Bosworth
David,
I don't know if it was Matt or someone else. There was a discussion
on the HCA forum if I remember right. George (kach22i) posted a bunch
of links to new prop designs for low noise. Some of them are great
ideas, I think, but others probably don't make any sense in our
application. I think Matt participated in that discussion, if I
remember right.
Most of the noise does come from the tip because that's what moves
fastest, but which part of the tip?
Evidence from some of the sites George posted suggest that most comes
from the trailing edge. (of the tip).
Personal experience says a fat leading edge makes a whole lot of
noise. Personal experience says a blunted trailing edge makes a whole
lot of noise. This is not an instrumented test, this is me with a
prop I made, listening to the results after I changed props.
From the bit I've read on it, it seems that breaking up the sound
waves (pressure waves) coming from the prop will reduce noise. That's
sort of what the owl is doing, but more importantly one way that
manufacturers make jet engines and turbofans quieter is by placing
noise reducing structures at various distances from the source of
noise, so that by the time the noise hits an observer there is enough
destructive interference in the truly offensive frequencies that it
reduces the far-field noise.
David Bosworth
--
David Bosworth
From the rocky shores of the Skykomish river
http://www.premier1.net/~daveb/
> I don't know if it was Matt or someone else. There was a discussion
> on the HCA forum if I remember right. George (kach22i) posted a bunch
> of links to new prop designs for low noise. Some of them are great
> ideas, I think, but others probably don't make any sense in our
> application. I think Matt participated in that discussion, if I
> remember right.
>
> Most of the noise does come from the tip because that's what moves
> fastest, but which part of the tip?
>
> Evidence from some of the sites George posted suggest that most comes
> from the trailing edge. (of the tip).
>
> Personal experience says a fat leading edge makes a whole lot of
> noise. Personal experience says a blunted trailing edge makes a whole
> lot of noise. This is not an instrumented test, this is me with a
> prop I made, listening to the results after I changed props.
>
It was here or hoverlovers, I am not a member over on HCA so I know I didn't
read it there.
Yes, no evidence other than my own trial and error building props for me
either, but my thin leading edge sharp trailing edge props are quieter and
outperform their counterparts.
I though it was from the end of the prop so to speak, I could be wrong but I
seem to remember that this is part of what a duct does to quite things down,
I don't have a duct so maybe it is just redirecting the sound more out the
back.
Buy_Sell
wide bladed propellers are variable pitch and constant velocity. (28
- 50 degrees.) This would definitely help with the proper loading of
the motor. It takes a lot more energy to get something moving than to
keep it moving. I've read somewhere that a typical car only requires
20 hp to keep it moving along down the highway. I thought that this
was hard to believe but after looking at my diesel Jetta of only 48
hp, I can assume that this figure might be correct. Since a
hovercraft propulsion system doesn't have gears like a car, a variable
pitch propeller system makes sense. This would be another avenue to
pursue in the desire to have a stealthy running hovercraft. I live on
the edge of a wilderness environment, with grizzly bears, cougars,
elk, etc. A stealthy hovercraft would be much more appreciated than a
noisy machine thundering thru the wilderness. There is truly a lot of
beauty left in this world and exploring it can be quite rewarding.
Ken Roberts
> I've noticed on the YO-3A stealth aircraft websites, that the three
> wide bladed propellers are variable pitch and constant velocity. (28
> - 50 degrees.) This would definitely help with the proper loading of
> the motor. It takes a lot more energy to get something moving than to
> keep it moving. I've read somewhere that a typical car only requires
> 20 hp to keep it moving along down the highway. I thought that this
> was hard to believe but after looking at my diesel Jetta of only 48
> hp, I can assume that this figure might be correct. Since a
> hovercraft propulsion system doesn't have gears like a car, a variable
> pitch propeller system makes sense. This would be another avenue to
> pursue in the desire to have a stealthy running hovercraft. I live on
> the edge of a wilderness environment, with grizzly bears, cougars,
> elk, etc. A stealthy hovercraft would be much more appreciated than a
> noisy machine thundering thru the wilderness. There is truly a lot of
> beauty left in this world and exploring it can be quite rewarding.
You're right about the goal of a quiet craft, and about a constant
velocity (CV) prop, and for that matter about a lot of really
beautiful wilderness area.
However, I think you might be missing my point.
If your solidity is too high, the propulsion system is too sensitive
to inlet air speed. Pitch and solidity and gearing can be selected so
that the engine couples nicely with the air at any speed which is
reasonable for a hovercraft to go, and still not turn too fast. I've
driven and also ridden in several machines which are quiet enough the
wildlife doesn't hear you coming. My UH-18sp is, when everything is
adjusted right, very close to this quiet. I'm still working on that.
Variable pitch props, including CV props, are for adjusting to wildly
different speeds like an aircraft sees. You might improve your noise
performance somewhat with a hovercraft, but I'm betting it won't be by
much. The reason IMO is that there is only one setting where that
prop will have the same pitch all along the blade. Any other pitch,
your blade segments are working against each other and creating
turbulence.
IMO, a really well designed static pitch prop (not a fan) will work
well at any speed the craft might go, and have really good noise
performance in most circumstances.
At the very least, I think the constant pitch solution should be
explored first before going through the effort and expense of VP.
On my craft, the loudest part is when (if) the blade is banging
against the duct. :D Seriously though, if everything is adjusted
right my loudest point is still the engine. I have only the catalytic
converter on it right now, and people still have told me they can't
hear my craft coming up the river from the beach. I have an after-
market catalytic converter and muffler I'm going to use instead, and
I'm making an exhaust header as well. By the time I'm done, the
loudest thing will be the prop and lift fan.
It won't stop there, I'll keep tweaking until it's all very quiet
without sacrificing speed. I'll replace the prop and lift fan as
necessary, and I'm also considering centerline slipstream adjustments
like spinners and fairings. We'll see how far it goes.