Originally Posted by Keef256
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.
But unfortunately, as the drag decreases due to the lower air density, so does the lift !
Originally Posted by Flingwing207
Originally Posted by R22 Clipper
Third point. When I descend the air density in the manifold increases causing a rise in MP.
The small change in air density would not make a readable difference against the vacum of the running engine. What you are seeing is the governer opening the throttle to compensate for power loss due to the higher back pressure. Remember, for a given MP, you get more power at a higher altitude and less power at a lower altitude.
Originally Posted by R22 Clipper
Fourth point. When you climb MP drops.
Because the engine makes more power for a given MP, the rotor RPM would rise for a fixed collective setting, so the governer closes the throttle to maintain RPM.
Not sure that's right about atmospheric pressure not affecting MP readings 207. 1000ft is worth around 30mb at low levels and that's just under an inch of mercury - ie significant. In my fixed wing with a constant speed prop, I have to open the throttle every 1000ft on the way up and close it on the way down to maintain MP. As I go up, the prop will be coarsening in order to keep the RPM constant, but the MP is dropping due to atmospheric pressure.