ELIPPSE propeller, Contact! magaine article

Corky Scott wrote in message . ..

He also wondered why folks were using flat bottomed turbulent airfoils
for high speeds. The flat bottomed airfoil coupled with a sharp
leading edge makes for a prop that has a narrow range of peak
efficiency, and also creates a lot of drag and noise. After all, the
tip approaches the speed of sound, what airfoil do you know for near
supersonic flight has a flat bottom?

Look at the bottom of an A-4 - only slightly supersonic, but still.
Dead-ass flat. this is a trick answer, having to do with the delta
planform, which is notoriously airfoil insensitive.

And generally, prop tips DO NOT get supersonic. Mach .8 at most.

Finally, prop airfoils do not seem to make much difference. Look down
mid-page of this link for a discussion of real-world results with prop
airfoils: http://users.lmi.net/~ryoung/Sonerai/Carve_Prop.html
"Sensei" is a practical man of wide experience familiar to readers of
this newsgroup. Warnke props had/have an excellent rep, but this IS
about a 3rd hand rumor.

Paul decided to use laminar flow symetrical airfoils.

There are people who do it. Having carved a prop meself, I will say
it's a WHOLE lot easier to manage a flat back side than a hollowed-out
one. Plus, you end up with a pretty thin trailing edge, which is
pretty hard to manage in wood.

Dr. Martin Hepperle, a German Aerodynamicist, has created a whole
family of laminar prop airfoils.
http://www.mh-aerotools.de/airfoils/index.htm I looked at using his
airfoils for my prop (which hasn't flown...), but gave up - for the
time being.

I don't know if they have ever been built into a full scale prop, let
alone tested.

Lift force on a wing is proportional to the square of the velocity.
That means if you double the speed, the lift force is quadrupled.
That being the case, you don't need a wide cord at the tip of the
propeller because it's moving at near subsonic speed. Going that
fast, it does not have to have a wide cord to produce lift.

Also, because the tip is moving at such high speeds, you need to
minimize it's size to reduce drag and noise.

All this is true, but real live props need tips both wider and thicker
than ideal for reasons of structure, and flutter. Carbon fiber and
CAM has made possible shapes that could only dream about in the past.

Another interesting point he makes is that the prop definately IS
capable of creating lift even right next to the hub as long as it has
been given the proper angle of attack.

True, but not much lift, again, because the airspeed is so low in
close to the spinner. And what is the efficiency, I.E. Lift to Drag
ratio of that portion of the prop? It may be absorbing more
horsepower than it is worth in thrust, even at high speeds.

Is this the same Paul Lipps that designed Light Speed Engineering's
ignition system?
http://www.lsecorp.com/Company/TheTeam/LSE_Team.htm

I'd love to see Paul's airplane run the CAFE Triaviathon, which
measures both climb and cruise performance, and compare it against a
"stock" Lancair 235, which I suspect the CAFE Foundation has data on.
Paul is near enough to Santa Rosa to make it possible.

I don't mean to spit on the man's props at all. I'm just engaging in
the dialogue, and showing my skepticism. It looks to me as if Paul's
props MAY show some improvment on his airplane, but I'm skeptical of
how much.

I've been meaning for some time to subscribe to CONTACT! Articles
like this one leave me no excuse.....hurray to Pat Panzera for finding
and publishing such interesting information.