prop size vs altitude?

[allesg]

I'm not completely sure how much to compensate for altitude in
prop selection other than bigger when going from sea level
to say 5200 feet or 6800. Let me suggest that I have three
planes that optimally turn a 10x6, an 11x6 and a 12x6 prop
respectively at sea level.

I now take all three planes to 5200' and 6800'.

Keeping all else constant, how much larger should I go assuming
I can only buy x6 pitch props?

What are the right formulae?

Thank you.

Gary

[Sail 'n Soar]

Suggest you use Motocalc, etc., and not try for rules of thumb.

[Ollie]

At higher altitude planes must fly faster to generate the necessary lift. Keeping the pitch constant is the worst possible assumption. Pitch should be increased in preference to diameter when going to a higher altitude.

Internal combustion engines produce less power at higher altitudes. This may show up as lower RPM which, in turn, makes increasing the pitch even more of a priority. Depending on the circumstances it may even be necessary to decrease diameter to keep the engine operating near peak power.

[allesg]

Ok,

Then it makes more sense to go from a 10x4.7 to a 10x6 or 10x8

than from a 10x4.7 to an 11x4.7 when going to a significantly
higher altitude keeping all else constant. The motor would be
wanting to turn more turns due to lower air density.

Seems I need to find a thrust table for different
perhaps idealized props with different pitches and
diameters at a given RPM.

Thank You

Does motocalc include a parameter for altitude?

Gary

[Sail 'n Soar]

OK, enough esoteric discussion. Let's look at some MotoCalc numbers based on a design I'm flying. The following table compares data for seal level and 5,000 feet. The ultimate answer is to go to EITHER one size larger diameter OR a slightly higher pitch DEPENDING on what you are trying to achieve!

Baseline
Alt: sea level
Prop: 11x7 APC
P,in,static: 196 watts
T,static/W: .90
T,excess/W @ L/D,max: .555
V,prop/V,stall: 2.49
N,prop: 6836 rpm

Case 1
Alt: 5,000 ft.
Prop: 11x8 APC
P,in,static: 194 watts
T,static/W: .82
T,excess/W @ L/D,max: .525
V,prop/V,stall: 2.65
N,prop: 6860 rpm

Case 2
Alt: 5,000 ft.
Prop: 12x7 APC
P,in,static: 212 watts
T,static/W: .99
T,excess/W @ L/D,max: .547
V,prop/V,stall: 2.20
N,prop: 6510 rpm

Case 1 has about the same static current draw & watts, lower thrust to weight ratios and a higher prop speed to stall speed ratio. This combo would have a higher top speed relative to stall, but weaker acceleration and climb.

Case 2 has a higher static current draw, better static thrust to weight and a higher rate of climb, but at a slightly lower angle, compared to the baseline case. However, top speed would be less and, for this notional model, outside the 2.5 - 3.0 V,p/V,s range recommended for sport aircraft.

Which way you want to go will depend on your specific model and what you are wanting your model to do. Get yourself MotoCalc, etc., and make a more rationale decision.

[allesg]

ok, I'm convinced,

then again, the last answer I got told me more than what I
probably would have found all by myself.

Maybe I need to read some books too!

I understand much better now.

Motocalc it is! Thank You!

I already visited the motocalc site after the first response
and I'm convinced it isn't too hard for me.

Again, Thank you very much for your help.

[AndyOne]

Gary,

I attended a talk by Maynard Hill at the Chesapeake Bay RC club a while ago and he explained how he flew a fairly standard RC plane to about 30,000ft to atttain a world record.
I guess the moral of the story is that unless you are very marginal on power you are not going to notice the difference.

Andy.