Bill Eslick has run a bunch of interesting prop tests and
published the data on his web site. http://www.weslick.com/
I reproduced some of it here and added some comments.
The airplane is running into a brick wall at 165 MPH.
Increasing the power does not help and just burns more
fuel. It does not result in an increase of airspeed.

My theory is the prop tips are stalling and running
into the high drag transonic region at the same time.

Quoting Bill:
"I used a Props, Inc. 67 x 68 wood prop, a Felix Bi-Cambered
68 x 72 wood prop, and a Warp Drive three-blade ground adjustable
carbon-fiber prop."

Quoting Jordonoff:
"If the propeller were turning in solid material, as is the case with
a screw in  wood, it could act without slipping, and so the distance
traveled in one revolution would be equal to the theoretical travel
of the propeller. Inasmuch as the  propeller is turning in the air,
however, there is a certain amount of slip; there fore its actual
travel distance falls short of its theoretical travel distance."

The ratio of how far it would screw into wood to how far it would
screw into air is called the Advance Ratio. IMHO these props are
at the limit of the advance ratio.

The Warp Drive have no twist so the tip drag is higher.

The tip thickness is as follows.
 Felix: 0.235" Props, Inc: 0.420"  Warp: 0.272"

I am guessing here. The solution might be more blade area with
a shorter prop. The Warp drive blades can be modified by
cutting off the aft part of the blade at an angle and sanding
them in such a way as to reduce the tip angle of attack and hence
the drag. However one would probably have to add another blade.
I'll do a 3D shortly

What is your theory on what is happening?

Paul Lamar

The Warp Drive prop has very little twist - about 13 deg and was designed
and
optimized for ultralights and 100 mph operation.  At higher speeds, the
inboard
sections of the prop are actually going into reverse thrust.  Look at the
prop
and see if you have any bugs on the forward face of the prop.  There should
be none, only on the back of the prop (this is near the hub).
I had the same problem with my 125 hp Soob EA81 powered Europa and
after doing the research went to a Catto custom fixed pitch.  WOT speed
instantly jumped from 150 to 175  mph with no other changes.  Climb stayed
the same.
The Catto has 40-45 deg twist at the root.  This is so at higher speeds, the
hub area is still at a positive angle of attack to the relative wind.
I had no idea what was going on and I emailed Stuart Gort at Powerfin
Props and he was
very helpful with the following:

 The Powerfin B-model has only about 2 more degrees of twist than the
Warp. The twist is important on your application because as you approach the
fast part of the flight envelope, the inboard sections of the prop begin to
go into beta mode. This means the air is being accelerated over the lower
camber faster than over the upper camber, essentially making reverse thrust
(or at least making enough drag to be the main factor in achieving
equilibrium - no more acceleration). Our prop, because of the 2 more degrees
might have some positive effect but I wouldn't bet the farm on it. Chances
are that any improvement would be negligible. For airplanes in the 135 mph
range, a proper design might have about 40 - 50 degrees of twist from the
tip to the most inboard definable sections. A Warp, at 64" has about 13
degrees. So, the lack of twist on the Warp and the resulting drag on the
inboard stations is certainly slowing you down but there is no off the shelf
design out there yet that can well accommodate your plane.

                                                      Stuart Gort
                                                      Powerfin Props
GLENN CROWDER

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