Flat pitch or feather???

[apcooper]

In a piston twin if you lose all oil pressure will the prop seize in the feather position even if you leave the prop control full forward? I've heard this is the case but am puzzled by this since every time you shut down the engines on the ramp therefore depriving them of oil P the prop is in the flat pitch. Someone enlighten me!!

 

[Workin'Stiff]

Most "featherable" props have anti-feathering pins which fall into place for that very reason. You shut the engine down, the oil pressure drops, but these pins engage and prevent the prop from feathering on the ground. I know on the Duchess you can override this by placing the prop handle in feather. I just don't know why you'd want to do that. Not only that, it's hard on the starter to get it going again with the prop in feather.

 

[MTpilot]

The pins are hooked to squat switches on the gear.

 

[Flechas]

The pins are hooked to squat switches on the gear.

It depends on the plane, in most light twins the pins are spring loaded and driven by centrifugal force, that's why you have to make sure to feather the prop above 800 rpm.

 

[paulsalem]

In our Barron 55 the pins are held disengaged until about 300 RPM by centrfigal force (actually inertia, but thats another rant). So as long as the blades are feathered either by the prop control or the sudden loss of oil pressure in the govs you'll be fine.

There is no relation to the squat switch in the baron


That is how I understand it atleast.

 

[minitour]

I know on the Duchess you can override this by placing the prop handle in feather.

Wow...Didn't know that...I guess they didn't teach it to us because it is tough on the starter...

-mini

 

[Flechas]

Is not that you can override that. That's what the prop handle is for, to feather the prop.

 

[minitour]

Is not that you can override that. That's what the prop handle is for, to feather the prop.

wow...when you say it like that, I guess I did know that...

-mini

 

[ackattacker]

This thread needs clarification... the feather position does not "override" anything...

In your typical piston twin the oil pressure provided by the prop governor acts to bring the propeller to a flatter pitch (higher rpm). Moving the propeller control to feather directs the prop governor to drop oil pressure to the prob hub, the same as if would happen if you ran out of oil or the oil pump failed (or the engine seized).

Aerodynamic forces normally would act to bring a spinning propeller to flat pitch, however in a piston twin this is counteracted by some combination of springs/air pressure/counterweights, so that the loss of oil pressure brings the propeller to feather.

However, the "feather locks" will lock the propeller in a non-feather position if the prop is stopped or at a very low RPM at the time that the oil pressure loss occurs (be it from actual loss of oil pressure or moving the prop control to feather). These feather locks are actual physical hooks on the blade hubs which drop into place when the prop RPM is very low.

So, when you stop the engine on the ground, the engine stops turning before the oil pressure has a chance to drop significantly, so the feather locks drop into place and prevent the prop from going into feather. This makes the engine easier to start. If you accidentally stop the engine while it is feather ed (or land with the engine feathered), you will have to start the engine to bring it out of feather unless you have an unfeathering accumulator.

If the engine seizes suddenly in the flight the propeller will not feather, nor will it be possible to feather it due to the feather locks. It cannot be "overriden". This doesn't matter, since a stopped prop creates very little drag whether feathered or not.

If oil pressure drops suddenly in flight the propeller will feather, and there's nothing you can do to stop it.

If the engine fails in flight, without losing oil pressure, and you get slow enough that the engine is just "barely" windmilling, then it may not be possible to feather it because of the feather locks. You may have to increase airspeed to bring propeller RPM above the point where the feather locks engage, then feather the propeller.

got it?

 

[Lead Sled]

Changing the subject just a little...

The next time you guys are out on the ramp, notice the different ways the Garrett vs. the Pratt & Whitney turboprops are started up and shut down. The PT-6s are either put into feather before they are shut down or they go into feather as they are spooling down and the oil pressure bleeds off if the pilot forgot. They are started in the feather position.

During shutdown, Garrett-powered airplanes keep the propeller in flat-pitch using "start locks" or pins in the propellor hub. This is done by moving the power levers into beta (reverse) as the engine is spooling down allowing the pins to move into position. If the pilot fails to do this the blades will move into feather as the engine spools down and the oil pressure bleeds off. If the pilot screws up, he must use the electric unfeathering pump to put the blades in flat pitch and pop the start locks into position proir to start. After the engines are started the pins must be moved out of the way, centrifically, by momentarily going into beta after the engines are started.

'Sled