A thought I have been pondering is , at 5500ft. altitude where I fly, how the thinner air affects prop pitch. Should I be using a prop with more pitch when flying at this altitude and expect to use less pitch at a lower altitude, such as sea level?

Take off props are low pitch because they allow higher RPM and thus more HP. Cruising pitch is higher because you do not cruise at full power. Cruise is usually at higher altitudes and the adjustments are made using manifold pressure for max performance unless you have a fuel injected engine. All the above is true for full scale. I too fly at 5,000' ASL and generally use about one or two inches higher pitch for fun flying. For quicker takeoff and more power, lower pitch will do the job. If your engine will not deliver the power you want you can up engine size or go to lower pitch. I usually choose the next size up on the engine. The recommended engine size on the box for most models must be calculated for 2995 inches of mercury at
65 degrees F at sea level, (standard atmosphere). Most models flown at higher altitudes are usually underpowered when using the engines specified by the manufacturer.

I've noticed when I compare my tach readings at 4000' to those of folks flying at sea level, I'm down up to 1000 rpm with the same prop and nitro. Adding any more pitch would lower these rpms even more. The engine has less air available for combustion. What I do is just go with more nitro.

I suggest staying at the same pitch, but go up 1" in diameter. More diameter will grab more air, give you more pulling power. Pitch gives you speed, but diameter gives you power. You probaly can even go 1 pitch and 1 diameter at that altitude. This is an area to experiment some. Try a few sizes of props. i reccomend APC props as they are the most efficient design out there.

Mark Shuman

The opposite. For R/C use a lower pitch at higher altitude for propellor efficiency. R/C spends most time climbing, turning, manoeuvreing & descending. Our cruise content of the flight envelope is minute unless we are racing pylon.

Increased altitude = decreased air density.

A propellor is simply a spinning aerofoil. For practical purposes you can ignore the effect of density upon either the prop or the wing in terms of density within the lift or drag equations. In both cases V² has by far the higher impact upon resulant performance, and that is restricted more by reduced hp output from the engine itself than prop or wing efficiences due to reduced density which both achieve a balancing tradeoff due to resultant diminished drag.

In simple terms, the engine will be significantly down on hp due to the reduction in air density. To accomodate this diminshed hp, prop efficiency needs to be increased. You need to reduce the pitch for the same diameter or reduce the diameter for the same pitch to accomodate this hp loss. Which you choose will be role requirement dependent. Low pitch offers superior acceleration, braking and climb performance. High pitch top higher top end speed and (pitch chosen dependent) lower rpm at full throttle.

I generaly fly what guys at sea level do. The prop does have less air to bite but the engine also has less air to breath thus producing less power and esentially it's a wash.

Darren

I appreciate all the input guys. Gives me something to think about and do some experimenting with.
I'm a R/C retread from 20 years ago and relearning just about everything from scratch, given all the new technology.
I have been flying C/L for a few years, got back into that while recouping from 6 bypasses on the heart, but I found myself getting a little bored, started with a Park Flyer-Slow Stick, and got bored with that, now I'm building a Sig Midstar 40.
When I first moved to Colorado 30 years ago and then got into R/C, the local guys in the club said go up one engine size and 5% more nitro for the altitude. With the new engines these days I don't think going up in size is that important, but I learned from C/L flying, about 15% nitro helps a lot.
Props have always been a guess and experimenting is the best way to get the proper engine run.
Again, thanks for all the input,
Jerry Bohn Longmont, Colorado

Standard atmosphere is 29.92" Hg and 59* F (15* C).

Well I am at the same height too. I generally use a high pitch prop for speed but i am aware of it and compensate by making my takeoff roll longer. Last year I made some comparisons with a couple guys at sea level over in California with identical planes, weight, engines etc. There is about a 3% loss inperformance for every 1000ft altitude or around 17% loss for our area.

I have a F-20 with a Jett 50 and a 9/7 apc turning 17400rpm at around 120mph average which is great here but over on the beach they're pushing 150 mph with the same setup. Have a Shoestring with a G90 getting around 90mph. Don't sound like much but it is here.

Be careful when you ask others for numbers as to where they live at.

Another trick is to go with the higher of the recommended engines sizes. Buying a plane that calls for a .60-.90? Go with the .90