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
I have Warp Drive props. They aren't actually flat; they do have a
little twist to them.
Also, Warp Drive says they have the same or better efficiency in the
speeds he wants. They lose a small amount of efficiency above about 160
mph. At 200 mph, we will lose about 7 mph compared to an optimised
fixed pitch prop, and lose nothing compared to the average fixed pitch
prop.
AI Nut
Seven MPH is a large loss of net HP out of the prop and a huge loss in efficiency.
Speed is proportional to HP cubed. I suspect they lose efficiency throughout
the speed range as Monty says. Their only advantage is low cost and the ability
to add blades to absorb more HP.
Paul Lamar
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