MP gauge

[22clipper]

I'm sorry I'll try that again;

When I pull carby heat the intake air is thinner so the engine slows (or the govenor opens the throttle) causing a rise in MP.

When I climb the intake air is also thinner which should tend to raise the MP. But other factors (less atmospheric pressure on the exhaust, blades easier to turn) overcome this trend leading to a net fall in MP.

Its interesting isn't it when there are competing trends how one wins out. Like when you climb the air gets thinner, its gets colder too which should make it more dense, but the loss of molecules is greater than the decline in their speeds which means ultimately less density.

[rotorfloat]

I've always wondered why we can wind our little O-320's (in a robbie, say) all up and down the MP range at constant rpm, while the planks have to stick to a square or near square settings on their engines fitted with constant speed props?

Why can't they just set an rpm and then pitch the prop/gov to whatever the desired MP is, say, at cruise, full MP?

[puntosaurus]

We and the planks have different control objectives, so we end up with different control mechanisms. We have highly variable flight regimes and want to maintain rrpm (because it keeps us alive) without having to play with the throttle all the time (because we've got too much else to think about). The planks have much more fixed flight regimes, so they focus on prop efficiency for airspeed (fuel economy). I could turn your question round and say why would you want to target a particular MP setting ?

R22 Clipper, I think you're there on the pulling carb heat, but I think the climb stuff would be better stated along the lines of

"As you climb, atmospheric pressure reduces which reduces (among other things) rotor drag, thrust, manifold pressure, and back pressure on the exhaust. If the governor is on it will sense any changes to rrpm resulting from the balance of changing engine power and rotor drag and adjust the throttle to restore it. The pilot may then choose to compensate for the reduced performance (rotor thrust) by raising the collective, and if she does, the governor will respond by opening the throttle to meet the demand, thus increasing the MP. Exactly how much the pilot needs to raise the collective to maintain performance will depend on a balance of factors, but if the pilot happens to reproduce the MP that existed at lower altitudes then the engine will be producing more power than it was as a result of lower exhaust back pressure"

Great question BTW, really makes you think. It just shows why lawyers end up with such convoluted prose when they try and answer a simple question !

[Keef256]

Quote:
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.

Originally Posted by Flingwing207

Er - I'm pretty sure that's what I said...

Not the way i read it you talked about the exaust pressure being lower which will create more power which means engine rpm will increase. Therefore smaller butterfly opening and lower MP.
I was sugesting that the drag on the rotor would decrease as the air gets thinner causing a rise in engine rpm. Therefore smaller butterfly opening and lower MP.
Same result different cause.

[Keef256]

Originally Posted by 22clipper

I'm sorry I'll try that again;

When I pull carby heat the intake air is thinner so the engine slows (or the govenor opens the throttle) causing a rise in MP.

If you are talking about during the carb heat checks then only the engine slowing. As below 80% RRPM the govenor on a robo is not working (if on primed but not engauged).

Originally Posted by 22clipper

When I climb the intake air is also thinner which should tend to raise the MP. But other factors (less atmospheric pressure on the exhaust, blades easier to turn) overcome this trend leading to a net fall in MP.

Yes all these things affect the MP but also the govenor and the pilot. To name but a few.

[Flingwing207]

Originally Posted by Keef256

Not the way i read it you talked about the exaust pressure being lower which will create more power which means engine rpm will increase. Therefore smaller butterfly opening and lower MP.
I was sugesting that the drag on the rotor would decrease as the air gets thinner causing a rise in engine rpm. Therefore smaller butterfly opening and lower MP.
Same result different cause.

Yer right! I was thinking it though, I'm pretty sure...

[22clipper]

Now that you guys have brought me up to speed on the MP gauge, I've written the definitive training aid.....
http://www.ma1.net.au/images/mp.jpg