Flingwing207
"Actually a rotor and a constant-speed prop act the same (as far as MP is concerned)."
It does only cause it is and engine too. They behave differently once you put the govenor on because a constant speed prop keeps the speed constant by altering the pitch of the blades (not something i'd want in my helicopter
). Where as a helicopter controls the amount of power the engine produces to keep the engine speed constant.
22clipper
Fourth point. When you climb MP drops. Pull carby heat MP rises. So why don't we just pull carby heat on climb to maintain MP??
Because you are not realy trying to maintain MP. You are trying to maintain Rotor/ Engine RPM. If you pull the carb heat to maintain MP you are making the engine less efficient so greater fuel consumption.
Flingwing207 You are correct - the motor does run out of "grunt" at 12,000' - because the MP isn't available.
Think of it this way. Take an engine to 6,000' MSL, read the MP gauge before you start it. Say it reads 24". Now we start it, and run it at wide-open throttle. It shows 24" (we will ignore losses through air filters, ect). We measure the horsepower - say we get 140 HP at 2700 RPM.
Yep or to put it another way.
At sea level you have 30" of map avaliable.
6000' above MSL you have 24" (Say)
12000' above MSL you have 20" (Say)
So this is the maximum the maximum MP you can get with the butterfly fully open. So there comes a point when there is just enough MP avaliable to maintain level flight.
Just had another thought the other thing that will affect things is the rotor. As you climb the air beromes thinner (less pressure) so there is less drag on the rotor. Therefore the power required to keep it at the same rpm is less. Therefore MP will drop as you climb (assuming you keep the colective in the same place). The reverse is also true. As you decend the air gets thicker (more resistance) and so more power required so more MP required.
As you may have noticed these things are not simple and are affected by many things.
