>>
>> >
>> >      > The pitch of my old wood propeller which I am retiring is given
>> > in
>> >      > inches, while my new Warp Drive uses degrees to set pitch. No
>> >      > reference
>> >      > to inches.
>> >      > What is the relationship between the two methods of
>> > measurement?
>> >      > Can you direct me to reference material (for the layman)?
>> >      > Yvon

   There is only one true measure of the pitch of a propeller */as
installed on a particular airplane!/* That is the effective pitch, which
is the amount the airplane goes forward, in inches per revolution, in
horizontal flight. It's an easy formula: TAS, mph, times 1056 divided by
rpm.

At most airspeeds and rpm, */in level flight, /*the effective pitch
of my strange prop is 76". It has to be taken in level flight since the
prop load is different climbing or descending. If your prop's effective
pitch changes much at different speeds and rpm, its twist and/or chord
distribution is not optimum. As you can see from the previously shown
helix-angle tables, most fixed-pitch props have insufficient twist as
compared to the tables, and actually generate reverse thrust in the root
area which, besides considerably reducing the prop's efficiency, blocks
the cooling airflow, requiring larger cooling inlets, thus more drag.

I've heard many stories about how, even though someone has given his
plane more throttle and rpm, the speed didn't change. If the rpm
increased and the speed didn't increase proportionally, then the prop is
on the downward-slope of its efficiency curve. So a 1% rpm increase
*/should result from/* 1% more speed, not the other way around*. Your
speed increases from increased hp, and your prop now turns faster! Now
if you have a 100 mph TAS plane and you advance the throttle to where
your rpm goes from 3000 to 3030, your speed should increase to 101 mph;
that's pretty hard to see! However, if your rpm increases, say, 5%, from
3000 to 3150, then you should see 105 mph TAS. If not, your prop has
insufficient twist and you are generating reverse thrust inboard which
works against increased thrust outboard.

*I modified the tips of my friend's prop on his Lancair to reduce tip
drag. The speed increased from 215 mph at 2600 rpm to 219 mph at 2650
rpm. To increase speed that much required (219/215)^3, 5.7% more power.
But 2650 rpm/2600 rpm is only 1.9% more rpm, so only 1.9% more power.
And the speed increase was 1.9%. So, 1.9% more speed, 1.9% more rpm! The
speed increase was due to the prop's increased efficiency due to much
less tip drag!

Paul [Lipps]

Thanks Paul! Very very informative.

Paul Lamar ...No rotor no motor.

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