Page 24 - Volume 14 Issue 3

On the other hand, if the propeller blade angle goes a bit past 90° now the relative wind leads to a CCW, backward rotation.
Although it is pleasing to have the propeller come to a dead stop when shutdown and feathered in flight – like it was on the piston twin trainers in which we learned – it is not at all uncommon to experience slight rotation in either the normal or the backward direction. Don’t worry about this being harmful to the engine ... it’s not. Why? Because the oil supply and scavenge pumps are driven by the compressor’s rotation: N1 or Ng. Unless something jams the compressor, the ram air through the cowling and engine causes sufficient compressor rotation to supply a continuous supply of oil to the rotating propeller, its RGB and the power turbine’s shaft: N2 or Nf.
When Raisbeck Engineering developed the first four-blade propeller for the King Air 200 model – the excellent “Quiet Turbofan” propeller – it had a very pronounced twist designed into the propeller blades. As I have mentioned before, this prop doesn’t even come close to stopping rotation when feathered on a shutdown engine. Expect to see 10 to 20 RPM in the normal, CW direction ... which is enough to create sufficient propeller oil pressure – from the pump inside the propeller governor – to allow the prop to unfeather itself if the propeller lever is not kept in the feathered
position, keeping the path open for propeller oil to exit back into the engine. Even if your King Air has autofeather, be certain to complete the full shutdown procedure and move the prop lever manually into the fully aft, feather position!
The second situation of windmilling in reverse that I wish to present is more obtuse, by far. We all expect that a fixed pitch propeller will turn faster when subjected to either more power or more airspeed. Of course, that is correct and it is what a constant speed propeller governor is meant to overcome. Namely, whenever the propeller momentarily goes faster (i.e., it overspeeds), the governor sends the blades to a larger angle, a bigger bite. The bigger bite creates more rotational resistance, slows down the propeller, and restores the on-speed condition. Vice versa, when the propeller underspeeds, the governor decreases the blade angle to restore the on-speed condition.
As a mental imaging exercise only (Don’t try this at home, folks!), consider what happens to the propeller when we cut off the fuel to a PT6 in cruise flight. Momentarily, due to the total reduction of power, the propeller underspeeds and the governor flattens the blade angle. Because the propeller of a PT6 has so little resistance to rotation – since it is only connected to the RGB and the power turbine, as well as to three accessory drive pads – at any airspeed above about 140 KIAS, it can return to constant speed operation at any speed selected by the prop lever ... even takeoff RPM! Only when the airspeed drops below 140 do the flattening propeller blades finally reach the Low Pitch Stop (LPS). Now the blade angle becomes fixed, cannot flatten any more, and indeed the propeller speed starts to slow down.
As I hope you realize, when the pilot moves the power lever into the Beta and Reverse ranges, he or she is moving the LPS to a progressively flatter angle. Ignore the tremendous drag that is being produced as we continue our mental imaging exercise and theorize what would happen if the blade angle went totally “flat.” Does it make sense that our windmilling tendency would be lost? To make it simpler, imagine blades with no twist ... just flat boards replacing the actual propeller blades. It those boards were at a zero-degree blade angle, the relative wind would be producing lots of drag but no rotational tendency. It’d be like holding your hand out of the car window on the freeway with your palm facing forward. There is lots of force pushing your hand back but no tendency to make your arm move up or down. Now rotate your hand such that your thumb moves forward. Hand wants to rise, right? Vice versa, rotate your hand to move the thumb back a little and now the arm wants to move down. (Like me, you did do this in your parents’ car as a kid, didn’t you?)
I hope that the conclusion being reached is that a totally flat propeller blade has no windmilling tendency, same as a feathered blade. Given enough time, all prop rotation would stop.
22 • KING AIR MAGAZINE
MARCH 2020