# Why does the centrifugal twisting force move variable pitch propellers to fine?

In my research on the forces acting on and influencing a single engine, non feathering variable pitch propeller's ability to change blade angle, I've come across a bit of a roadblock.

Aerodynamic forces try to move the blades into a coarse pitch, as the blade's centre of pressure is ahead of the blade's pivot point. That much is obvious and makes perfect sense.

What stumps me is the centrifugal twisting force. How does this send the propeller into a fine pitch?

Several sites talk about this concept, but only to the extent that they acknowledge it exists, and in what context it occurs. Here are some examples of the only relevant sections per site:

Centrifugal twisting force, being greater than the aerodynamic twisting force, tends to force the blades toward a low blade angle.

Source

The centrifugal force acting on the blades tends to twist the blades such that the blade angle reduces. This force is somewhat opposed by the aerodynamic twisting force.

Source (Also comes with this image that has no explanation)

Centrifugal And Aerodynamic Twisting. Any asymmetrical spinning object generates a centrifugal twisting force, the propeller is no different with the force of its spinning action twisting the blades to a fine pitch.

Source

I could go on.

Given that the force refers to 'centrifugal', I suspect centre of mass has something to do with it. If so, where is the centre of mass located? Forward, or aft of the pivot point? Seems like it would be slightly forward, given that a blade's airfoil shape would require more mass at the leading edge.

• Reference Commented Nov 25, 2021 at 14:13
• "centrifugal twisting force" cannot be understood if you don't provide the principle used for pitch adjustment. The centrifugal force acts on some mass, which in turn can twist the blade. To understand how this happens, you need to look at and disclose here the mechanism used.
– mins
Commented Nov 25, 2021 at 20:20

The centrifugal twisting force in variable pitch propellers does not come from the blades themselves. It is induced by ballasts connected to the root of the blades via a mechanism such that when the spinning motion of the propeller induces a force on the ballast, it will (in most cases) force the blades into fine pitch.

• Do all propellers have this? I've seen some with fairly obvious counterweights, but nothing on others. Unless they are hidden behind the spinner? The confusion I have is that if the centre of mass is behind the point where the blades pivot for pitch, it would make sense, but without those counterweights, CoM would have to be ahead of the pivot and would want to move coarse instead, as the blades would be thicker closer to their leading edges. Commented Nov 28, 2021 at 5:22
• No, on small aircraft adjustable propellers are not very common, as they are quite expensive, bot to buy, and to maintain. For "hobby" use they offer little advantage for the price, but for passenger planes they add considerable amount of economy and safety in form of much improved efficiency and performance. Commented Nov 28, 2021 at 8:30
• If you mean do all adjustable propellers have the counterweights, I mus say I'm not certain. The weights can be quite small however, and may be hidden inside the spinner. Commented Nov 28, 2021 at 8:41

There (indeed) are single acting and double acting blade fineness adjustment systems. In the single acting design, the hub counterweight is set to use centrifugal force to move blade to coarse. Oil pressure, in the form of a hydraulic piston, is used to move the blade to fine.

In a double acting system, oil pistons move the blade both ways.

Confusion may arise in the statement "moves the blade to a lower angle", as for feathering: the "low blade angle" is to the wind, not to the axis of rotation (very coarse).

For the propeller blade itself, it is entirely possible that more mass may be ahead of the pivot point which would cause a centrifugal twist force towards fine.

"Aerodynamic forces try to move the blade to a coarse pitch"

Yes, and this is counterbalanced by the blade center of gravity trying to move to fine from centrifugal force. This helps balance forces on the blade shaft.

But we must consider all components of the rotating plane, including the hub and attachments. In order to be able to "feather" the prop in the event of engine failure, a counterweight is added to the hub to fully coarsen the prop pitch.