Altitude Effect

In HVAC, electric fans area considered as constant volume devices. They move the same amount of air, the only thing is at elevation, the density of air is less. The data from the RCM trial runs at the two different elevations more or less bear this. Since the aircraft weight is the same but density is less requires a faster velocity to keep airborne. Hence this is why needing more RPM.

The difference is more notable on air breathing glow engines. With lower density air requires a greater combustion chamber to produce the same amount of power.

I've more or less equated efficiency by simply taking the ratio of the air density at elevation divided by sea level. Where we are at, 4,300 ft. elevation (1311 m), air density is around 12.5 psia. At sea level it is 14.7 psia. 12.5/14.7 = 0.85 or 85% of sea level performance. In other words, there may be a 15% reduction in available HP at my elevation.

When I was in Gallup, NM at 6,500 ft. (1981 m), air density is around 11.5 psia or 78% of sea level, so loss was around say 25%.

This is why we go a size up on engines, to help compensate. A .19 at sea level is compensated with a .25 - .28 at elevation. A .35 is replaced with a .45, and so forth.

Anyway, that is my simplistic way of viewing it, right, wrong or indifferent.  

With electric motors, you don't have combustion efficiency loss, just need to up the rpm (seems more electric powered planes use more power than glow to start with) or go with a prop slightly larger in diameter or increased in pitch, IMO.

Oh, and I wanted to add. In Gallup, I was able to still fly my Golden Bee powered Q-Tee, but it would fly okay on a 6x3 prop. The Q-Tee has very generous wing area, which allowed it to do this. At sea level, a 5.25x4 prop flew it the same as a 6x3, no perceptable change in velocity during level flight. But at elevation, the difference was remarkable. It barely lumbered along with the smaller diameter prop, but moved out fine with the 6x3.

Using more nitro content fuel at high altitude is a problem solver? Instead of using larger diameter propeller?

With half-A engines, at least I was already using high nitro fuel (25%). On a Golden Bee, a 6x3 Masters prop matches best HP thrust output as in the air, it is at max HP RPM. Q-Tee has generous wing area and with smaller fuselage cross section, is also reasonably clean a