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Are there variable pitch propellers, where pitch is set so that the blades constantly are at their maximum lift to drag ratio AoA, whatever the incoming relative wind speed & direction?

Such a variable pitch propeller would be powered by some electric engine, able to equally produce torque for clockwise and anti-clockwise operation.

Such a variable pitch propeller would be free on its pitch axis, able to rotate 360°, each blade being mounted on two ball bearings, inside propeller's hub. This way maximum aerodynamic efficiency could be reached whatever the needs:

take off at low relative pitch, cruise at some higher pitch, ability to produce efficient reverse thrust after landing. Each of these at maximum lift to drag ratio AoA.

Each blade is balanced about this pitch axis, so that it is statically stable, and aerodynamically always trying to pitch up by its own aerodynamic configuration, towards its best lift/drag ratio angle of attack.

Each blade becomes an individual glider/flying wing, trying to pitch up.

For the sake of understanding, imagine a propeller with trailing edge tabs set at an angle allowing only thrust generation, regardless of the direction of rotation of the propeller.

te tab

This tab is not a viable way to build such a blade, it may instead look like some conventional blade, having some negative incidence blade tip combined with swept back tip, or some standard geometry, with a reflex airfoil, or both, or something else.

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The idea here could be to have an automated variable pitch prop with the least amount of moving parts, where pitch is set by aerodynamic forces at its optimal angle. Instead of going for constant rpm, this one goes towards constant maximum lift.

For instance, cutting power off would induce some reverse windmilling, adding power in windmilling direction will generate reverse thrust.

During forward flight, pitch would adjust on resultant airplane's velocity + blade's rotational velocity.

Question is : Are there propellers of this kind? If not what are the drawbacks I don't see?

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  • $\begingroup$ I imagine it is hard to design a blade such that every portion of the blade fit your condition at each aircraft and/or rotational speed. GIven a rotational speed, the air speed vary by a great amount along the blade, thus I guess the blade twist must also vary with aircraft's speed. $\endgroup$
    – Manu H
    Jan 8, 2020 at 15:36
  • $\begingroup$ This idea would seem to have something in common with the "freewing" concept, where the wing is free change angle-of-incidence in flight in order to maintain some given constant angle-of-attack in response to gusts, etc. $\endgroup$ Jan 13, 2020 at 23:07
  • $\begingroup$ In the propeller context, does the fact that the whole prop blade can never be designed to have a constant angle-of-attack from root to tip, at least over the whole envelope of possible airspeeds and prop rpm, possibly negate some of the possible theoretical advantages of this idea? $\endgroup$ Jan 13, 2020 at 23:08
  • $\begingroup$ What problem are you trying to solve, and how would you actually monitor and set the optimal L/D ratio? I.e. how would you measure and display the information? Because optimizing prop efficiency is the goal of constant speed props, it just happens that measuring RPM is way easier than mounting a miniature AOA sensor on each blade… (Unless you have a better idea!) $\endgroup$ Oct 19, 2021 at 2:04
  • $\begingroup$ @MichaelHall I'm not trying to solve anything that already works, this was about knowing if something had been attempted along these lines. RPM doesn't either tell anything about optimal AOA, it just tells about RPM being constant relative to maximum available power output $\endgroup$
    – qq jkztd
    Oct 19, 2021 at 15:31

1 Answer 1

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Yes, considering a prop more analogous to a stabilator than a horizontal stabilizer and elevator (and trim tab).

Optimal AoA can be found in cruise by comparing prop pitch to forward airspeed at a given RPM or, most accurately, fuel consumption. For props, lift is thrust. Maximum cruise pitch efficiency has already been solved.

A major factor in efficiency is engine friction. This is why lower RPM and coarser pitch may actually burn less fuel per mile.

Also, prop pitch may need to be other than optimal AoA, such as feathering.

With take off and go around, you don't care about efficiency, max thrust is needed. Higher engine RPM and finer pitch can squeeze a bit more power out of the engine/prop.

So, some good thinking here, in search of an application niche unfilled. A main issue to reconsider is that there are many throttle and forward airspeed variables for climbing, level flight, and descending flight. Prop AoA must be controlled, rather than to freely pitch because any rpm other than optimal rpm will produce a non-optimal AoA.

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  • $\begingroup$ Can you provide an example of one please? $\endgroup$ Mar 30 at 15:25
  • $\begingroup$ Robert, I think you missed the central premise of the question. It's asking if any variable pitch props exist that establish and maintain pitch solely from aerodynamic forces. i.e. free rotating at the hub, not held in place of moved by springs, oil pressure, or counterweights. You answered "yes" to that question, but offered no actual example. Your answer is really just generic discussion about a few topics associated with standard constant speed props. You haven't actually answered the question that was asked... $\endgroup$ Mar 31 at 0:55
  • $\begingroup$ @MichaelHall the question is: do constant maximum lift to drag ratio variable pitch propeller(s) exist? My answer is yes, just not like that one. Trimming for aerodynamic force requires constant speed. What's being missed here is that the engine/prop rpm and forward speed are so variable that having that thing flopping around in the breeze won't work. That is why there are no known examples of that application, going back 120 years. (Unless someone wants to change the trim in a way that is not like a stabilator). $\endgroup$ Mar 31 at 2:56
  • $\begingroup$ Just as an afterthought, some WW2 trainers had a "2 speed" prop pitch adjust that might be great on recreational aircraft (takeoff and cruise). As with trucks, more gears enable a smaller, lighter, more fuel efficient engine (although 10 speeds with a stick just might be a bit much). $\endgroup$ Mar 31 at 3:06

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