Propeller dihedral

In articles on airplane wings the amount of upsweep or downsweep is often mentioned as having a significant effect on that wings performance and drag characteristics. Propellers are described as rapidly spinning wings and often have many of the same adaptations applied to them that wings have such as swept tips to reduce drag. What I want to know is have there been any experiments with giving the blades of a propeller any form of dihedral or anhedral? Such a configuration would naturally case a great deal of stress on the prop so I can see why it may never have been tried before modern high-strength materials became available but has the idea ever come up?

Oh, sorry. Misread the title, thought it said “Propeller Diarrhea”. Thought maybe you had a few beers and invented some weird new party game.

I’m not sure what purpose prop dihedral would serve.

(But then, I am not an aircraft designer/engineer)

Dihedral acts as a stabilizing feature on a wing, but since the prop is (one would hope) securing fastened to the airplane this shouldn’t be a problem requiring a solution. Dihedral in a prop would also deflect some of the thrust (prop-generated “lift” that propells you forward) from the ideal thrust-line, reducing efficiency. It just doesn’t sound like it would nbe of any benefit.

Maybe someone else with more technical knowledge will be along and give you a more detailed answer.

Yes, that article had part of what I was thinking of, a pusher prop with rear-swept blades that would sqeeze the air into a cone shaped area creating a narrow high-speed stream of air. I suppose if the dihedral were adjustable the way pitch is, one could use little or no dihedral for takeoff and maximm for high speed cruise.

(Im just talking out of my ass here so I may be totally wrong but it sounds pretty good.)

Any other aerodynamically inclined dopers want to chime in?

No ideas yet, but i wanted to thank Desmostylus for that link. I’ve been trying to remember Flettner’s name because I wanted to make a windmill based on his Flettner Rotor.

Ah, the good ol’ days of aeronautics and agriculture!

I also fail to see the advantage of dihedral on a propeller.

Dihedral in aircraft wings gives the aircraft more stability around its roll axis. The reason is because if the airplane rolls in one direction, the force vector of the wing in that direction more directly opposes gravity, while the other one moves more to the horizontal. Like this (greatly exaggerated:

Airplane in normal flight: /

Airplane rolling in one direction: __|

Now, what would dihedral gain you in a propeller? I can think of a bad thing it would do - if the airplane yawed, it would cause one side of the disc to create more force than the other, which would put a pretty good bending moment on the prop and the propeller shaft. In addition, in normal flight a certain amount of the propeller’s thrust would be 90 degrees to the direction of flight, reducing propeller efficiency.

Your ‘cone of air’ thinking presumes that focusing the air with dihedral would cause a faster stream and more thrust. I don’t think that’s the case. I think you’d just get a reduction in thrust from the angled force vector, and a lot of really turbulent air.

Sam Stone that’s not how dihedral works… Looking at the aircraft from head-on doesnn’t allow one to resolve any torques. Dihedral functions from a change in AOA on the two wings. That plus the change of airflow to the horizontal stab.

Perhaps a small amount of dihedral could help; as the propeller spins it swirls a mass of air; a part of the energy of the propeller is used to swirl an amount of air outside of the propeller downstream; so a bit of dihedral (the propeller tips pointing back) could prevent that energy loss… maybe; at times like this is when I wish I had a wind tunnel. :stuck_out_tongue:

Yes, it is.

No, it doesn’t. Dihedral doesnt change the AOA in any way, nor does it change airflow to the hort stab. The AOA is controlled by the elevator, not the wings. (It’s also affected by incidence and wash-out, but neither of those are dihedral.)

In the classic Stick and Rudder, Wolfgang Langewiesche gives an explanation of the effect of wing dihedral. He says it is a design feature that makes an aircraft tend to self-correct a slip. In a slip, one wing will have a higher effective AOA, so will tend to roll the plane in a way that reduces the slip.

None of this, of course, has any value for a propeller.

Any transient yaw or pitch, or any steady state in which the propeller axis is not parallel to the local airstream, will cause the prop’s center of thrust to be eccentric from its center of rotation. The blades will have a higher effective AOA on one side than the other, producing more lift on that side. It’s called “p-factor”, and it does put a bending moment into the shaft and radial loads onto its bearings. On a single-engine plane, p-factor forces some rudder correction to keep the plane going straight ahead during takeoff and climbout.

Centrifugal loads on a dihedraled prop would place very substantial bending moments on the blade roots, with the increased moments reducing away from the hub. The added loads would require thicker and less-efficient airfoils throughout the blade span to keep maximum stresses acceptable. The more dihedral, the worse the problem. It does seem unlikely that any “cone concentration” effect, which furthermore would be available only to a pusher configuration, could overcome that disadvantage.

The word dihedral doesn’t seem to fit well for a prop. A blade set that is angled one way or the other would sweep out a cone. In the helicopter business, we call it “pre-cone”.

There seems to be little reason for doing it on a propeller, but it is something that some helicopter hub designs use. Since the blades will be expected to cone upward under load, the hub can be designed to aleviate some stresses, or make hinges line up a little better, if the hub has “pre cone”. I don’t think this buys you anything with propellers, however.

There’s an interesting discription of dihedral for fixed wings on this page. .

Now, there are some aerodynamic tricks to be played with tip shapes. (probably not what you’d call dihedral). Hartzel (I think) makes a Q-tip prop that has tips bent about 90 degrees.

Hm, my link didn’t quite get to the target. Trythis one

Interesting links and all. I won’t get into the dihedral of wings and all that but…

Some fixed pitch metal props of the kind on 100 - 200 HP type engines and on something like an 1958 C-172 or the later C-175 models with the longer props did / do have a small amount of dihedral in a FWD direction.

The reason is that as the rpm goes up, the blades will be pulling all along the length, producing lift if you will, and the rotation will attempt to pull the center of the blade FWD into alignment, in other words, straighten it out, but the prop is constantly trying to fold FWD on itself so there seems to be a smell amount of added thrust.

On more powerful engines, standing at the side watching the prop in cross section during an run up, you can see the blade bending FWD. They do not need to be pre bent because the power is such that they will bend FWD on their own just from the pull on the air. They don’t start out negative if you will, because they would end up straight and it seems that the other is slightly better.

I don’t know if they still do this or not.

The arguments will never be over just as the arguments will never be over on the idea that you can put an aircraft on the ‘step’ and get more out of it. Tests with machines say no. Timed flight with different pilots says you can, as the auto-pilots can’t fly SOME aircraft as fast as SOME pilots can in the same aircraft at the same power settings.


In a FWD swept prop wouldn’t the centifugal force at least partly balance out the air pressure bending it foreward?

Sock Munkey, yes absolutely. This is the principle I mentioned that’s used in some helicopter rotors. Even with hinges near the center (so there’s no bending stresses), the blades will seek a balance between the centrifugal forces and the thrust forces.

Propeller blades are typically light and stiff, so I don’t imagine that the “coning” angle is very much, but I don’t doubt GusNSpot’s observations.

I’d just like to point out that “propeller dihedral” would make a great band name.

Interesting but not quite helpful.