Constant speed props

[skydraulic]

What's the main reason for decreasing MP before RPM's; and increasing RPM's before increasing MP? Sorry to sound so stupid but I can't recall what I was told.

 

[nosehair]

Think of MP as "Power to Drive the Prop". If you increase the MP while the Prop is still set at a fixed RPM, it is like pushing down the accelerator of your car with the brake on. First, release the brakes; ie. increase the RPM's, then increase the power(MP). When you want to stop or slow down, first release the accelerator(MP), then add the brakes(reduce the RPM's).
Now, the "Why". Think of holding your car brakes and increasing the accelerator, or keeping the RPM setting low and adding lots of more power. The increase of fuel/air(combustion) is increasing but the engine is not allowed to turn faster. The pressure builds in the cylinders to the cracking/explosion point.

 

[SkyGuyEd]

It raises the pressue in the cylinders because it raises the volumetric efficiency (the ratio of the amount of air going into the cylinder in the intake stroke over the amount of air able to enter it) because the engine runs slower allowing more time for the cylinder to suck the air / fuel in.

Higher pressures mean more efficient operation, as long as the engine doesn't exceed the operating limits. That's why on descent its better for the engine to have lower RPMs and higher MP (as opposed to low MP and high RPM) - it keeps the temps up and increases fuel efficiency.

Along with the increase in pressure, the temperatures will increase. If the temperature gets too high, you'll get detonation.

In practice though you just gotta know your engine and what it will tolerate; for example when you pull the power back during climb in a NA engine you can pull the prop back to 2500 and keep throttle full, and in many cases this is preferable to using 25/25 that a lot of people like so much.

 

[skydraulic]

Thanks for the responses. I understand what both of you are saying. I just can't recall the reasoning an examiner gave me on my CFI-A checkride. For some reason it seemed a little different but made sense. I understand that there are situations where MP will go over square ie. takeoff and initial climb out on both fixed and constant speed props, some turbocharged aircraft even in cruise etc...

 

[Alchemy]

You can cause detonation if you have very high manifold pressures and low RPM's in direct drive engines. Peak pressure in the cylinder chamber occurs earlier than it should in the cylinders' cycle. The cylinder does not move "fast enough", causing stress at peak pressure during compression stroke, damaging the engine.

From http://www.avweb.com/news/columns/182084-1.html

http://www.avweb.com/newspics/pp18g.jpg
In any case, at maximum MP and reduced RPM, the peak pressure will be much higher, because the combustion chamber will be much smaller when PPP occurs. Since the pressure will be much higher, the temperature will be much higher, and this increases the risk of detonation. More on this later, but here's the same picture, this time illustrating classic detonation:
http://www.avweb.com/newspics/pp18h.jpg

However, most people are more cautious than necessary about always having RPM's "higher" than MP on small, direct drive engines. It takes some pretty extreme settings to really cause problems on those types of engines. Radial engines are more sensitive to this issue, and that's probably a big reason why we're told to operate the engine the way we are....it's a holdover from the days of the radials.

I reccomend reading John Deakin's "Pelican's Perch" articles athttp://www.avweb.com/news/columns/182146-1.html, specifically numbers 8, 15, 16, 18, and 19. I keep all these articles in my CFI binder for reference. He gives an easy to understand break-down of what the engine controls on a piston-powered airplane really do.