prop pitch vs/ altitude

[centmont]

I'm aware of the relationship between density-altitude and declining performance in both the prop and the engine. My questions are: is this decline linear for both engine and prop.... or does one decline faster than the other? The important question is: do pilots who regularly fly in high density altitudes (between 4000-10000 ft. DA here in Montana) attempt to regain some loss in the prop efficiency by using slightly higher pitches? I can increase the prop pitch...can't do much about the horse-power loss in the engine.

[flynlow]

I can't answer your question but I read a study of a Supercub with constant speed vs borer climbing to 10,000'. The performance was almost the same.

I expected the constant speed to significantly outperform the borer at high altitudes but it didn't happen. The difference in climb rate was 26 ft./min. (6%) in favor of the constant speed (probably the difference was the rpm difference which relates to power developed) I doubt a coarser fixed pitch would have kept up with the finer McCauley, for one thing you would be reducing your rpm/power.

Here is a link to the test.

http://sparky.supercub.org/photopost...sBorerO320.pdf

[KenyaCub]

The higher you go, the more pitch you will want up to a certain point.
With higher altitude or higher temps, more pitch is needed. Not concerned in cruise here, just climb.

[centmont]

Let me ask my question directly. If you moved from an airstrip at 500' MSL to one at 5000' MSL and had to buy a new prop, would you expect to add add pitch to the new prop because most of your flying was in greater altitudes?

[Ruidoso Ron]

Let me ask my question directly. If you moved from an airstrip at 500' MSL to one at 5000' MSL and had to buy a new prop, would you expect to add add pitch to the new prop because most of your flying was in greater altitudes?

Yes, that has been my experience. Currently repitching my Borer from 41" to 43" here at 7,000'.

[flynlow]

Ron, are you underpitched at the lower altitudes, are you exceeding redline ? Does your engine gain rpm as you climb into the thinner air?

I understand the arguement that the air is thinner at higher DA's but I know that the engine is also producing less power. I would think it would balance, maybe not. I am certain that you don't want to be 'lugging' the engine by being overpitched.

It just seems to me that the most efficient climb prop at sea level would also be the most efficient at higher altitudes.

The comparison that I posted above suggests that the constant speed set at 2700 rpm would out pull the same prop set at a lower rpm (in a best rate climb).

Don't know for sure, just makes sense to me...

[Ruidoso Ron]

I think your reasoning is valid. It will be a while before I get to test mine, but at 41" I seem to be making a lot of noise and not developing a lot of thrust. Cable tach is so bad that I am replacing it with electronic. I don't think that I am exceeding redline though. I should add that there were several other factors that could effect my performance: cracked cylinder, plane out of rig, etc. I am correcting all of them at one time so I really hope for an improvement.

[Lance]

How much longer Ron?

Lance

[Ruidoso Ron]

Lance,
I can only hope to live long enough, at this rate.

[jr.hammack]

howdy,
not to argue with ron,but having a pa-12-160 h.p ,and a pa-18-160 h.p.
both based at 6500' to 7000', i alway's was over pitched at 82'' x 44'',
and was wanting to pitch down to 42''.brought the pa-18 to alaska and still
need to pitch down to 43'' or 42''. either place i havn't been able to excede 2700 r.p.m.
seen a lot of work cub's up here with 41'' & 42'',still depend's on what your trying to accomplish.
ron want's to go as fast as his old c-180 and land and take off like a helo.,
but then that's just my personal observation,he may still have a set of
6.00 x6.00's .

jr.

[Cajun Joe]

he may still have a set of 6.00 x6.00's .

OMG Jr. You must have crushed him with that parting shot.

Joe

[Ruidoso Ron]

That's O.K. I'll just move over to the 172 site. They like 6:00 X 6's. By-the-way, my goal wasn't to go as fast as a 180. It was to go as fast as the J-3's and PA11's!

[jr.hammack]

howdy,
i've alway's been told for ever 1,000 ' of altitude gain a normally aspirated engine lose's around 5% h.p.,i agree that you gain a little a/c performance because of less drag,but still belive we are being hurt more
by loss of engine performance,less dense air would require more pitch if you had sufficient h.p./torque to pull it.
i guess my point is i'am having a hard time computing how less h.p.
mean's you need more pitch.
7,000' msl x .05= 35% loss
.35 x (example) 160 h.p = 56.0 loss leaving you only 104 h.p to pull a
44'' prop.


jr.

[mmr]

115 hp @ 7000'
Horsepower vs. Altitude Calculator
http://www.slowgt.com/Calc2.htm#BasAltCal
Mark