Need information to use in designing a variable-pitch propeller

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piolenc

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I've been exercising my neurons about variable-pitch propellers for some time, because it turns out that a VP prop is essential to several projects that I plan to undertake. After a fair amount of skull sweat I have come up with a mechanism that I think will work, but now I need solid numbers to use in sizing its components.

The centrifugal force will be taken by the propeller mount bearings, and all bearings will be low-friction rolling element bearings. The remaining friction forces should be calculable from manufacturer's data. There is however another force which I know exists, but don't know how to calculate. This is the force that tends to force an unrestrained propeller to take the full flat pitch position. It is caused by the fact that, in that position, the propeller has the lowest potential energy with respect to centrifugal force (i.e. all mass elements are at their furthest outward point). Unfortunately, I haven't been able to figure out how to calculate the force driving the prop to that position, and I need to know it to know the actuation force for each blade and for the operating shaft. If it is too great, it is possible to partially compensate for it using counterweights, and those too need to be calculated.

If anybody knows of a book or a technical report that can help, I would be grateful to hear about it.
 

TiPi

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That force (moment) would be the difference between the centre of lift of the blade and the axis of pitch rotation and should be able to be calculated from the blade lift data? Moving the axis of blade pitch rotation would be the first approach to reduce the moment, counter weights to possibly compensate for any remaining moment.
With hydraulic pitch actuation, counterweights are used to move the blade pitch to full fine in case of loss of hydraulic pressure (hydraulic pressure is used to move the pitch to coarse).
 

rv7charlie

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Governor oil pressure
300 psi times piston area times the length of the actuating 'arm' for the blade (inside the hub) should get you pretty close to a valid number.

I think that 'standard' c/s props increase pitch with increased oil pressure, and will go flat pitch (high rpm) as oil pressure is reduced. Air resistance on blade pitch forces pitch flat when oil pressure is reduced/lost.

Aerobatic and multi-engine props have counterweights to force the prop into max course pitch (cruise for Aerobatic; 'feathered' for multi-engine props) when oil pressure is reduced or lost, and oil pressure is used to take them to flat pitch.
Acro/multi props

Having said all that, designing a c/s prop makes designing an engine gear reduction look like a kindergarten exercise. Tread carefully. A failed engine or gearbox usually results in a forced landing. A thrown blade from a prop often results in the *literal* loss of the engine, and the 'forced landing' happens in a vertical spin.
 
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Dan Thomas

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Forces on a prop:

Thrust (obviously)
Centrifugal forces (much bigger than you think)
Torque bending force, flighting drag plus engine pulsations (prop is also flywheel)
Aerodynamic twisting force (from CP being ahead of rotational axis of the blade in the hub); tries to increase pitch
Centrifugal twisting force, as weight near leading and trailing edge tries to come into central plane of rotation (bigger than aerodynamic twisting force)

1642352012279.png

The thrust bending force is mitigated by the centrifugal force; the blades are designed to want to swing back into line with the center of the blade roots. Place a fixed-pitch prop backside-down on a table and see how the majority of the mass is ahead of the blade roots just a bit.
 

TFF

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I wouldn’t mind carving a copy of a standard prop. Improving one, not likely. Think of the thinking that went into the Aeromatic. It would be nice to have one for my little biplane.
 

piolenc

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Mindanao, Philippines
That force (moment) would be the difference between the centre of lift of the blade and the axis of pitch rotation and should be able to be calculated from the blade lift data? Moving the axis of blade pitch rotation would be the first approach to reduce the moment, counter weights to possibly compensate for any remaining moment.
With hydraulic pitch actuation, counterweights are used to move the blade pitch to full fine in case of loss of hydraulic pressure (hydraulic pressure is used to move the pitch to coarse).
I've got a pretty good handle on aerodynamic forces. It's the inertial forces that I'm having trouble with.
 

piolenc

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Joined
Feb 17, 2013
Messages
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Location
Mindanao, Philippines
Forces on a prop:

Thrust (obviously)
Centrifugal forces (much bigger than you think)
Torque bending force, flighting drag plus engine pulsations (prop is also flywheel)
Aerodynamic twisting force (from CP being ahead of rotational axis of the blade in the hub); tries to increase pitch
Centrifugal twisting force, as weight near leading and trailing edge tries to come into central plane of rotation (bigger than aerodynamic twisting force)

View attachment 120614

The thrust bending force is mitigated by the centrifugal force; the blades are designed to want to swing back into line with the center of the blade roots. Place a fixed-pitch prop backside-down on a table and see how the majority of the mass is ahead of the blade roots just a bit.
Thrust, centrifugal force and torque oscillations will be handled by the bearings used to mount the blades to their hub. Not worried about them. I know how to calculate them.
It's the last one, the centrifugal twist that occurs when the mass of the blade tries to reach its lowest potential, that is giving trouble. It has been fifty years since I had Mechanics in high school, so I know the energy approach, but don't know how to translate that into force. Obviously this is a problem that has been handled many times, but I can't find a trace of it in the technical literature that I have seen. Hence the post...
 

piolenc

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Feb 17, 2013
Messages
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Location
Mindanao, Philippines
I wouldn’t mind carving a copy of a standard prop. Improving one, not likely. Think of the thinking that went into the Aeromatic. It would be nice to have one for my little biplane.
The Aeromatic is a very clever design, and a good solution for someone who needs a propeller that shifts from "climb" to "cruise" and back again automatically, but it is not relevant to what I am trying to do.
 

piolenc

Member
Joined
Feb 17, 2013
Messages
12
Location
Mindanao, Philippines
Governor oil pressure
300 psi times piston area times the length of the actuating 'arm' for the blade (inside the hub) should get you pretty close to a valid number.

I think that 'standard' c/s props increase pitch with increased oil pressure, and will go flat pitch (high rpm) as oil pressure is reduced. Air resistance on blade pitch forces pitch flat when oil pressure is reduced/lost.

Aerobatic and multi-engine props have counterweights to force the prop into max course pitch (cruise for Aerobatic; 'feathered' for multi-engine props) with oil pressure is reduced or lost, and oil pressure is used to take them to flat pitch.
Acro/multi props

Having said all that, designing a c/s prop makes designing an engine gear reduction look like a kindergarten exercise. Tread carefully. A failed engine or gearbox usually results in a forced landing. A thrown blade from a prop often results in the *literal* loss of the engine, and the 'forced landing' happens in a vertical spin.
I am treading very carefully. I intend to calculate every aspect of the project, then overspeed test the result in a safe place.
 

terke

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Dec 4, 2009
Messages
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Calgary, AB, Canada
I've been exercising my neurons about variable-pitch propellers for some time, because it turns out that a VP prop is essential to several projects that I plan to undertake. After a fair amount of skull sweat I have come up with a mechanism that I think will work, but now I need solid numbers to use in sizing its components.

The centrifugal force will be taken by the propeller mount bearings, and all bearings will be low-friction rolling element bearings. The remaining friction forces should be calculable from manufacturer's data. There is however another force which I know exists, but don't know how to calculate. This is the force that tends to force an unrestrained propeller to take the full flat pitch position. It is caused by the fact that, in that position, the propeller has the lowest potential energy with respect to centrifugal force (i.e. all mass elements are at their furthest outward point). Unfortunately, I haven't been able to figure out how to calculate the force driving the prop to that position, and I need to know it to know the actuation force for each blade and for the operating shaft. If it is too great, it is possible to partially compensate for it using counterweights, and those too need to be calculated.

If anybody knows of a book or a technical report that can help, I would be grateful to hear about it.
 

autoreply

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Rotterdam, Netherlands
Having said all that, designing a c/s prop makes designing an engine gear reduction look like a kindergarten exercise. Tread carefully. A failed engine or gearbox usually results in a forced landing. A thrown blade from a prop often results in the *literal* loss of the engine, and the 'forced landing' happens in a vertical spin.
About equal risk/complexity I think, having designed and flown both.

Biggest challenge is covering all the resonance frequencies for all modes. Precession loads are simple in comparison.
 

Jay Kempf

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Warren, VT USA
I am treading very carefully. I intend to calculate every aspect of the project, then overspeed test the result in a safe place.
Jay Carter built an entire earth circle, like a mote without the castle, to contain the testing of his rotor system. A safe place is easy to say fast :)

It's a bunch of math to size all the components properly for a CS design. If you are going to drive it hydraulically that is well known. But the tough part is how the whole assembly will work together that is the hard part. All the big prop guys have experience going back decades and over a lot of installs that were tested to their maintenance limits and then optimized for assembly harmony. That is the rub with all this stuff. Over build a part to solve a problem and then it becomes the place where it won't break or wear. Then the break or wear moves to another part in testing where you find it. Lather, rinse, repeat over a lot of failures and you eventually get it. Anyone can do the mechanical layout of the parts. Any competent engineer can do the vibe and stress but some have specialized in that stuff and are few and far between so normally expensive. It can be done. And if you go in with eyes wide open and go through the process you should prevail.

I went through part of the process of figuring out an electric variable pitch prop that could fold. Went through enough of the design and sizing process to know it was feasible and had no real show stoppers. But to make one and make it robust enough to work and light enough for the application would be a lot of work and some sort of test bench to do first static testing of all the moving bits and then all the dynamic testing. All doable but sheesh. A lot of work.
 

Dan Thomas

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Sep 17, 2008
Messages
6,991
All the big prop guys have experience going back decades and over a lot of installs that were tested to their maintenance limits and then optimized for assembly harmony. That is the rub with all this stuff. Over build a part to solve a problem and then it becomes the place where it won't break or wear. Then the break or wear moves to another part in testing where you find it. Lather, rinse, repeat over a lot of failures and you eventually get it.
Yup. The AD history of CS props reveals that even the experts don't always get it right before they release it for certification and production. Pilots are rather adept at finding new ways to break something.
 
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