Constant speed prop while taxing

C

Christopher

Filing Flight Plan
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Jul 1, 2015
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Christopher
Need a good explanation of why the constant speed prop uses throttle to adjust rpm on ground but when up it uses manifold pressure
 
Because your prop has low pitch limits. Even at the flat limit you can set power low enough to not move the prop off the low pitch stops.
 
I believe all ground operations are prop full, therefore only the throttle adjusts your power/speed.
 
Because when the governor is in the "underspeed" condition it reduces properler blade pitch to make the prop turn easier, but blade pitch can be reduced only as far as the prop's low pitch stop allows (set by the manufactuer).

Until the governor reaches an overspeed condition it will not increase prop pitch from the low pitch stop. Until that happens it acts as a fixed pitch propellor.

http://www.mccauley.textron.com/von_klip_tip_cs_propeller.pdf
 
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Stewartb is correct. Oil pressure has absolutely nothing to do with it; not sure where that idea came from. The prop governor is working just fine on the ground, but the prop is already at the flattest pitch, so it's got nothing to do.

Think about it this way: say you have the prop control set to 2,500RPM, but the prop is only turning, say, 1000 RPM. The governor is going to pitch the blades flatter to try to get the speed up. However, what if there's simply not enough torque to spin the blades that quickly? Well, then the governor will go as fine as it can and then effectively give up. As you open the throttle, more torque (twisting force) is applied to the prop. The blades speed up. Once you hit 2,500RPM, then the governor starts increasing pitch to hold 2,500. When you're reducing power on final, the same thing happens in reverse: you pull the throttle, the governor flattens the blades, but eventually there's just not enough torque to spin the blades at the commanded speed. The blades hit the fine pitch stops.

In theory, the same thing could happen the other way: you could push so much torque into the props that the engine will overspeed despite the blades' being in feather. In a piston engine this is almost impossible due to design limits, and in a turboprop you'll tear up the gearbox. But, in theory, it's possible. Once the governor taps out (i.e. puts the prop in feather), your throttle would, in theory, then control RPM again. So basically, prop governor controls the RPM _within the limits of the governor_. Outside of that, the throttle controls the speed.
 
it is past the set parameter were the prop governor is in play. low rpm,low mp
 
Yep.. I deleted my comment on oil pressure right after I thought about it for about a second... :)
 
starglider said:
Stewartb is correct. Oil pressure has absolutely nothing to do with it; not sure where that idea came from. The prop governor is working just fine on the ground, but the prop is already at the flattest pitch, so it's got nothing to do.

Think about it this way: say you have the prop control set to 2,500RPM, but the prop is only turning, say, 1000 RPM. The governor is going to pitch the blades flatter to try to get the speed up. However, what if there's simply not enough torque to spin the blades that quickly? Well, then the governor will go as fine as it can and then effectively give up. As you open the throttle, more torque (twisting force) is applied to the prop. The blades speed up. Once you hit 2,500RPM, then the governor starts increasing pitch to hold 2,500. When you're reducing power on final, the same thing happens in reverse: you pull the throttle, the governor flattens the blades, but eventually there's just not enough torque to spin the blades at the commanded speed. The blades hit the fine pitch stops.

In theory, the same thing could happen the other way: you could push so much torque into the props that the engine will overspeed despite the blades' being in feather. In a piston engine this is almost impossible due to design limits, and in a turboprop you'll tear up the gearbox. But, in theory, it's possible. Once the governor taps out (i.e. puts the prop in feather), your throttle would, in theory, then control RPM again. So basically, prop governor controls the RPM _within the limits of the governor_. Outside of that, the throttle controls the speed.
Click to expand...
Nice way to explain it
 
I was waiting for somebody to pick up on this. Careful, operating a constant speed prop while taxing might be viewed as cockpit distraction and poor CRM. The FAA might come down hard on individuals practicing such bad habits.
 
The prop has a "governing range" and it is below it when taxiing. So the prop is at the stops. The prop governor has to have more 20% power or so to start governing much. It might govern a little bit if you pull it out all the way at fast idle.
 
coloradobluesky said:
The prop has a "governing range" and it is below it when taxiing. So the prop is at the stops. The prop governor has to have more 20% power or so to start governing much. It might govern a little bit if you pull it out all the way at fast idle.
Click to expand...

And you know what comes with "governing"? Taxes! :)
 
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