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Originally Posted by fourierman When on the ground vs in the air, why is the following true?
Ground - throttle controls RPM, prop control directly affects MP (manifold pressure)
Air - throttle is used to change MP, prop control is used to change RPM
During Runup - Pull back prop control, MP pressure increases (why not decrease?) |
The throttle controlls MAP allways and forever, even in fixed prop engines (for that matter your car, boat, & lawn mower too).
When the throtle plate is closed you are chokeing the engine's air supply. Since the piston is sucking in air, but it can't get enough the pressure drops. This is why you see low MAP at idle.
Since the engine can't breathe very well, it dosen't turn very fast = low RPM.
When taxining on the ground, the prop governer dosen't come into play beecause RPM is to low.
If you were to really play around with full power runups and move the prop controlls you would see the governor act just like it does in the air. DON'T DO THIS to your poor engine however, it really is bad for the prop, cylinders, and just about everything else/.
Why does MAP rise when you pull back on the prop durring runup?
You haven't changed the amount of restriction in the intake, but you have reduced the number of breaths the the engine is taking allowing the pressure to rise slightly.
Deakins articles are the best expaination out there covering these concepts. They should be required reading for CFIs.