Scott Speirer wrote:
I've got two different propellers I'm looking at and I'm trying to
figure out which
one to buy. I'm soliciting your educated opinions as to which would be
better ..
which would provide more thrust in the airspeed range 30 mph to 90 mph.
1.) 68" Warp Drive 5 or 6-blade ground adjustable propeller.
Approximately 2" chord
for each blade. (Originally designed for the ultralight/microlight
market.) About
$800.
2.) 68" Sensenich 2-blade composite ground adjustable propeller.
Approximately 8"
chord for each blade. (Designed for the airboat market.) Manufacturer
reports it
is most efficient between 2500 rpm and 3000 rpm. About $1500.
Remember my aircraft is different than most of yours ... it's a two-seat
tandem
gyroplane, not an airplane. Think "slow flight". This thing is only
going to fly
between 30 mph and 90 mph. I have a '91 Mazda 13B turbo, and a 2.85:1
gear reduction
drive. Gyroplanes need more thrust to fly than do airplanes of
comparable weight. I
need at least 600 lbs of static thrust. 800 lbs would be better. I
cannot go bigger
than a 68" diameter, otherwise I would be sawing into my vertical fins
on either side
of the prop.
I've posted this question to "the rotorcraft conference", at
www.rotorcraft.com, and
received mixed reports. Some say they had better performance from a
Warp Drive
multi-blade, others a large chord 2-blade. None of them had all the
details/parameters I was looking for ... engine make and model, engine
and prop rpm
for max static thrust, engine and prop rpm at cruise, propeller
diameter, propeller
chord, number of propellers, propeller make.
The responses I received were as follows:
1. Machine with Subaru Legacy EJ-22 having a 4-blade 68" Warp Drive puts
out more
than 600 lbs static thrust easily. Ross 17:1, 12 degrees pitch, 5200
rpm on climb
out, 5000 static.
2."A friend was running a five blade warp drive on a 150 hp Lycoming.
It had the
most thrust of any prop he tried, but the noise was too much so he went
back to a
Sensenich."
3. Used both a 72" 2-blade Sensenich and a 72" 4-blade Warp Drive on a
180 HP
Lycoming, in a McCullough J-2. The Warp Drive produced more static
thrust, more
thrust in-flight and had less vibration.
4. Used both a 68" 2-bladed Prince prop and a 68" 3-bladed Warp Drive on
an RAF 2000
gyroplane, having an EJ22 Subaru. User estimated the engine put out
90-110 hp.
Prince prop chord about 5". The Prince prop produced more static thrust
and less
vibration. Much preferred. The RAF 2000 has a belt reduction drive.
(The reduction
ratio was not reported.) Max engine rpm with the Prince was 5000 rpm,
and 5150 rpm
with the Warp Drive. (The Prince is also ground adjustable, I think.)
I bought a copy of "Airplane Aerodynamics and Performance", by Roskam
and Chuan-Tau.
They have a chapter on propeller performance estimation. Downloaded a
copy of NACA
TN 1338, "Propeller Efficiency Charts for Light Airplanes", July, 1947.
This
technical note provides some insight, but is not tailored to slow flying
aircraft.
It does give data for engines of 50, 100, 150 and 225 HP, at airspeeds
of 50, 100 and
150 mph, and with different number of prop blades with 6 and 8-foot
diameter. That's
the low end charts. It goes on to 300 and 400 HP engines, and 10 and
12-foot props,
and 300 and 400 mph. It is interesting how it shows the trends for
increasing the
number of blades ... the resultant blade system is efficient over a
wider rpm range
at a particular airspeed, and all other parameters fixed.
Most of the charts assume a solidity per blade of 0.0345. A rough
estimate for the
solidity per blade of the Warp Drive would be 0.019, and the Sensenich,
0.075. The
charts just don't address either of these solidities. I don't think one
can
interpolate the data here for these.
So ... what are your educated opinions?
Thanks,
Scott
Scott,
All I know about warp drive blades is that they are flat, and flat is no
good. They also have a constant chord, which means that the tips will be
loaded much more highly than the rest of the prop. When they are "warped"
there is no way that the shape attained will remotely approach a correct
shape, and if it does, the aerodynamic loading will "warp" it out of shape
again. The miracle of warp drive props lies more in the fact that they can
be economically mass produced and a blade unit can be used with many
different engine combinations, simply by adding more blades. And they have
the great advertising appeal of being "adjustable". So it can be made to be
serviceable with almost any conceivable flying contraption. The prop will
work, however, it will be nowhere near optimum efficiency.
The airboat propeller will more than likely be built for very slow
operation, I doubt seriously that an airboat does 90 mph, more like 0-60
mph. The most important design point in an airboat is hump speed, the speed
at which you overtake your bow wave. Until you are "on plane" your drag is
very high, then it goes down. The prop is therefore probably designed to
make a lot of thrust at low speed. However, there are some crazy Cajuns in
the world, and they just might have a big block supercharged swamp buggy
capable of such speeds; who knows :^) My guess is it is designed to be quiet
and have a LOT of static thrust. Just make sure that you are not sacrificing
cruise performance for static thrust. You don't want to get airborne just to
find out you have run out of prop and have to overspeed the engine just to
stay airborne. This has happened to some people who chased after the
mythical static thrust number. It is just like gearing in a car. That granny
low gear sure does pull hard, but it runs out awful fast. Run the numbers on
it. If it checks out, given the choice of these two, I would go with the
Sensenich, but...................
I think you would be far better off with a properly designed wood prop for
your application.
A simplified procedure for designing a propeller for your application:
1.) For slow speed, larger diameter is better, so go with the 68" prop
diameter.
2.) Using diameter, calculate the rpm based on the speed of sound. You want
the tips to be subsonic. (You really can't do this since your rpm is fixed
due to the redrive. Prop should be about 2000-2500 rpm with your redrive.
You'd have to turn it up to about 3500 for the tips to go super sonic, but
forward speed will reduce this number and you will have to factor this into
the equation)
3.) Design a blade that has the proper twist and the proper chord width for
the rpm and airspeed you will be flying.
4.) Perform strength calculations to insure a sound design.
5.) Now that you have your blade design, calculate how much HP it will
absorb.
6.) The number of blades is now determined by how much power your engine
makes. If you get an oddball fractional number of blades, start varying the
chord width until it comes out an integer number.
The problem with prop design is there are a lot of independent variables and
there is usually more than one way to reach a design goal. There used to be
a company in Meeker OK that specialized in airboats and props called
Arrowprop, their ph was 405-279-3833. This is old info, but they seemed to
be a good source for slow speed wooden props. One of my friends ordered a
prop from them for his powered parachute, and was very satisfied with what
he got for his money.
Don't know if this helps much, just my $.02
Monty
Excellent dissertation on the subject Monty! Thanks.
Paul Lamar
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