ORIGINAL: yard-dart
What I was getting at earlier was that an engine is going to produce more RPM in an atmosphere that is less ''humid''. Yes, humidity makes the air feel more hot during the summer, but humidity doesn't make the air more thin, it makes it more thick. What is humidity? It's water vapor. Water being in the air makes the air more dense, regardless if the outside temp. is hot or cold. Look at it this way. An engine in the 100 degree Arizona desert may turn 15K. That same engine on the same prop/fuel setup brought to Louisiana (100 degrees) and ran will probably only turn 14K. Even though the temps in the two locations are the same, the air in Arizona is less humid, causing the prop to spin more freely, with less drag. The air in La. is less dense heat wise, but more dense by humidity.
You may, or may not, agree. It really doesn't matter either way. But, I do feel that my explaination is correct.
John
Actually, Sport Pilot was right. Humidity makes air LESS dense. Here's why. You might remember that equation from high school Physics:
PV=nRT
Forget about units for now, if you rearrange the equation:
n/V=P/RT
This means that, if pressure (P) and temperature (T) are held constant, a gas will contain the exact same number of molecules (n) per unit volume (V). So every molecule of water takes up the same amount of space as every molecule of nitrogen (N2) or oxygen (O2). Now.. every molecule of water (H2O) weighs 18 atomic mass units (amu), while every oxygen molecule weighs 32 amu and every molecule of nitrogen weighs 28. So you're replacing heavier molecules with lighter ones. We're so used to thinking of water as being "heavy" compared to air because we're used to liquid water and gaseous air.
Air which is less dense makes for worse performance - ask any full size pilot. Engine power goes down (water displacing oxygen, as noted), but lift created by the wing and thrust created by the prop also diminishes. Takeoff rolls are longer, takeoff speeds increase. It doesn't matter whether the thinner air is caused by humidity, high temperatures or high altitudes. Yes, it is true you can fly faster in thinner air (if your engine has supercharging), and you can get better fuel economy (which is why airliners like cruising at 30,000 feet).
Actually, I came here, like everyone else, to find out if these engines are any good, since there are quite a few used ones around. If I had to buy new, I'd get one of the blue head ASP .15s. Sounds quite promising, I'm not afraid of throwing the muffler away or hogging it out, same with the carb (replace it with a big venturi and remote needle). Too bad, though, that it isn't legal for Indonesian Rules Slow Combat (plain bearing engines only).
Iskandar