We should be able to demonstrate an improvement in BSFC if and when
we get an effective variable pitch prop. In my C182 the best
economy is obtained by increasing the pitch and throttle opening and

decreasing the RPM. Most people will tell you not to exceeded
"square" in
an aircraft piston engine.

   What square means is 2200 RPM and 22" of manifold pressure.
Supposedly if the pressure rises above the RPM you are lugging the
engine and it is bad for it. You are risking bearing or rod failure.

Since the rotary is more robust than any piston engine we may be
able
to open the throttle wide (at low altitudes), increase the prop
pitch
and decrease the RPM. The lower RPM will result in less engine
friction
and the wide open throttle less pumping looses resulting in better
BSFC or lower fuel burn for the same speed.

This principle is widely used to improve the MPG of cars. My Ford
Cougar cruises down the freeway at about 70 MPH and 2000 RPM.
Nothing new. Over drives have been used for decades.

Paul Lamar

Virgil N Salisbury wrote:

        Lindberg increased range and lowered fuel flow by
        flying the P-38 oversquared in the pacific.  Virg

Virgil is right, Lindberg showed the Army Air Corps how to
dramatically increase the range of the P-38, this is how they
killed Yamamoto.  Oversquare operation being bad is an old pilot's
tale.  There is absolutely nothing magic about the MAP in
inches Hg being the same as the hundreds of RPMs, this is
just a units coincidence.  What if we measured MAP in
psi and engine speed in rev/sec?

My Dad tells me that the max range for the F8F Bearcat was
obtained with the prop turning so slowly that you could nearly
count the blades (a geared motor) and the MAP pushed up
pretty high (multispeed supercharged).  Many of the internal
losses are square functions of RPM, so lowering the speed
moderately can reduce the parasitic losses substantially.
Also, I agree that the rotary seems plenty robust enough to
eliminate this as an issue, even tho I don't think it is real
even in A/C piston engines - they have SO much more bearing
area than a car rod.

Bill

 
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