Aerodynamic/Centrifugal Twisting force

unclenobby

Well-Known Member
Maybe I'm just not reading it correct but the ASA book - A pilots guide to AC and their systems states re; forces acting on a prop....

"Aerodynamic twisting force tries to twist a blade to a higher pitch angle and centrifugal twisting force tries to twist them to a lower pitch angle"

while the following in the the Jepp ME book re; counterweights and constant speed props...

"...counterweights are attached to the blade shank so that when oil pressure is reduced in the hub, centrifugal force causes the blade angle to increase......the counterweights provide enough force to overcome the aerodynamic twisting force of the blades"

Seems to contradict?
 
R

Roger, Roger

Guest
The Jepp book is wrong about Aerodynamic twisting moment. ATM will cause blade angle to increase.

The "centrifugal force" on the blades themselves will cause BA to decrease, however, when you add counterweights to the prop the CF on them counters the CF on the rest of the blade.
 

tgrayson

New Member
Seems to contradict?
When you apply a force to a wing (or a propeller), the wing will tend to twist unless you apply the force at a precise position on the chordline. The location of this position will be determined by the wing structure and its resulting stiffness. When you look at the whole wing, there is a line from the root to the tip defining where these precise points are for every specific chord line. This line is called the elastic axis, or the torsional axis.

If you apply a force ahead of this axis, the wing will tend to rotate to a higher AOA. If you apply the force behind this axis, the wing will rotate to a lower AOA. One of the phenomena you get with high speed swept wing flight is "aileron reversal"; when you deflect an aileron down to increase lift to bank the airplane, the force lies behind the elastic axis and thus rotates the tip to a lower AOA, decrease lift, rather than increasing it. The aircraft rolls the wrong way.

So the propeller is the same, with the addition of the centrifugal force. The question is, where do these forces act with respect to the elastic axis of the propeller blade? The basic physics says that you could construct a blade so that either ASA or Jepp would be right, but that doesn't mean that you necessarily run into both sorts of constructions in the real world. The only book I have that addresses this question in a scientific manner is a book on helicopters, which I bought for this specific question. The book is "Helicopter Theory" by Wayne Johnson. On page 415, he describes all the moments on a helicopter blade. Leaving out what only applies to helicopters (i.e. flapping), he says:

1) a nose-down centrifugal moment around the elastic axis, and
2) a nose-up aerodynamic moment around the elastic axis

Now, one reason that #2 may not be 100% applicable to propeller blades is that helicopter blades are often symmetrical airfoils, whereas propeller blades are not. Symmetrical airfoils do not have an inherent nose down pitching moment around the 1/4 chord point (aerodynamic center), whereas cambered airfoils do. Those down down moment would tend to counteract at least some of the nose up pitching moment around the elastic axis of the propeller.
 

unclenobby

Well-Known Member
Thanks for the answers above answers. I think I'll have to go with the "canned" answer for now but will definitely dig deeper on the topic - will make for good bedtime reading!!
 
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