Some questions about RPM and prop pitch management.

[69th_Mobile_BBQ]

It seems to me that different planes manage RPM and Prop Pitch by using differing mechanisms.  What are those mechanisms, and how do they work? 

It's obvious that a Pe-2 manages RPM and Pitch quite differently than a P-40, and planes like the Me109 seem to have their own way of doing it.  What were the various methods used and how do they effect how the respective planes should be flown? 

I gave 3 example planes, but if other planes operate differently than those 3, I'd like to hear about them as well. 

 

Thanks!

Edited January 29, 2019 by =AVG77=Mobile_BBQ

[montag]

The pilot manages RPM via a governor that varies the prop pitch to maintain a constant speed in both the Pe-2 and P-40. This is why they are called constant speed propeller systems. You also have your throttle, which manages manifold pressure.

 

In the Pe-2 you set the throttle and it maintains a constant manifold pressure if possible even when changing altitude. If you go to higher altitudes it may not be possible to maintain though. Then you have RPM control where you set RPM and the governor does its best to maintain that speed by varying the pitch of the props. It is the same for the P-40. Where the two differ though is in control of the manifold pressure.

 

The P-40 throttle position will not maintain a constant manifold pressure for a given throttle setting if you change altitude. So if you have 42 inches at 5000 feet and then descend your manifold pressure will increase if you do not change the throttle position.

 

The Bf-109 manages manifold pressure and prop speed based solely on the throttle position. So for a given position it will maintain a certain manifold pressure and prop speed. You can switch to manual prop pitch control which will allow you to play with changing the prop position. If you go more fine (clockwise on the clock like instrument) your prop speed will increase assuming you leave everything else the same. If you go more course the opposite happens because it is getting a bigger "bite" out of the air. Switching the 109 over to this manual mode is a great way to ingrain in your head how prop pitch affects prop speed, torque, acceleration, etc.

 

Here is a good video by a player of this game, current ATP (airline transport pilot), and a former flight instructor. I would also check out the other 4 videos in this playlist. It might be a good idea to watch them in order.

 

I know I only really covered the operation of these systems, but I don't know much about the actual mechanisms besides some basic principles that I learned from engineering courses. There might be some people around though that are pretty knowledgeable about that. Current constant speed props might not be much different. A lot of mechanical systems used today are surprisingly similar to systems developed in the 30's and 40's.

 

 

 

Edited January 29, 2019 by TheKillerSloth

[WheelwrightPL]

22 minutes ago, TheKillerSloth said:

The P-40 throttle position will not maintain a constant manifold pressure for a given throttle setting if you change altitude. So if you have 42 inches at 5000 feet and then descend your manifold pressure will increase if you do not change the throttle position.

 

 

Even FW-190 does this which caught me by surprise because that plane is supposed to be worry-free and fully automated. It isn't, so if you set your manifold pressure to 1.3 ATA combat-power and then descend it can quickly go into 3-minute emergency range (1.4 ATA) which will result in an engine blow-out if you're not monitoring the gauges.

[montag]

2 hours ago, WheelwrightPL said:

 

Even FW-190 does this which caught me by surprise because that plane is supposed to be worry-free and fully automated. It isn't, so if you set your manifold pressure to 1.3 ATA combat-power and then descend it can quickly go into 3-minute emergency range (1.4 ATA) which will result in an engine blow-out if you're not monitoring the gauges.

Was it a fairly fast descent?