Constant speed prop

[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?)

[Baronman]

Come on all you CFI's! Give the guy an answer.....

[BZNflyer248]

Check out this link - its pretty technical and a little on the dry side - but I suggest my commercial students read it... It deffinitly helps shed some light on it....

Why Manifold pressure sucks! -
http://www.avweb.com/news/columns/182081-1.html

Here's a good one on the prop too -
Lots of info on large recip props etc, but a lot of the basic principles are the same

http://www.avweb.com/news/columns/182082-1.html

[ananoman]

"Ground - throttle controls RPM, prop control directly affects MP (manifold pressure)"

When you are idling around on the ramp, and taxiing at lower power settings the prop is on it's low pitch stop and the governor has no effect on RPM. So, any power changes will cause a change in RPM.

If you are at low power settings (1000 RPM) on the ground, you can move the prop control throughout its range and it will have no effect on RPM or MP. Next time you are out, try it. Just don't pull it into the feather detent.

"During Runup - Pull back prop control, MP pressure increases (why not decrease?)"
Because when you reduce the RPM, you are sucking less air through the same sized opening. This results in less vacuum in the intake manifold, which equals a higher MP. The same thing will happen in flight, so you might want to revise this one:

"Air - throttle is used to change MP, prop control is used to change RPM"

[USMCmech]

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.

[MidlifeFlyer]

The most simple version of the answer I know is that below a certain MP, there is not enough power to maintain the high rpm set by the prop. If the prop control doesn't have an effect, you are essentially sitting there with a fixed pitch prop, so it reacts to changes in MP just like a fixed pitch prop. It will do it in the air just like on the ground.

I'll stick with ananoman's answer to the rise in MP during the prop cycling. Can't get any clearer than that.

[CAVOK]

...but when I when I pull the MIXTURE all the way out, how come both manifold pressure AND prop rpm go to zero?? Are you saying that the MIXTURE can control both MP and RPM!?!?! Sooooo confusing!

Without getting into flyweights and superchargers and blah blah blah... Just think about what could change an engines manifold pressure, and how that relates to turning a constant speed prop. Your questions should answer themselves...

Or just read above. There are some very good explainations up there.

[CAVOK]

Glad I caught myself being a retard before somebody else beat me to it.

Manifold pressure wouldn't go to zero after shutting the engine off, so just insert local barometric pressure where I said zero...and...well, whew... dodged a self induced bullet

Later.

[MidlifeFlyer]

Originally Posted by CAVOK

Glad I caught myself being a retard before somebody else beat me to it.

Manifold pressure wouldn't go to zero after shutting the engine off, so just insert local barometric pressure where I said zero...and...well, whew... dodged a self induced bullet

Um, so your quetion was "why does the prop stop rotationg when you shut off the engine?" and you call your self a retard because of the manifold pressure mistake?

[CAVOK]

Sarcasm dude...

[pilot602]

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?)

Who cares ...




Fly a turbine.