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7 Answers

Propeller Geometric Twist

Asked by: 10449 views , ,
Aerodynamics, Aircraft Systems, Commercial Pilot, Flight Instructor

Question Given: "The reasons for the geometric twisting along the propeller blades is that..."

Answer explination:

"The reasons for variations in geometric pitch (twisting) along a propeller is that it permits a relatively constant angle of attack along its length when in cruising flight. Sections at the tip are moving faster than sections near the hub, causing the relative wind along the blade to vary."

 

Above is a statment as a correct answer to a question from the CFI question bank, brought to us by the King DVDs. 

 

I cant seem to wrap my head around this...and it might be because im making a poor assumption. 

In my understanding, the geometric twist creats an equal amount of thrust along the profile, not an equal angle of attack.

If it had an equal angle of attack along the length, and the outter parts moving faster, it would creat more lift at the tips.

The assumption that I am making, that may be why the answer given by King (and presumably the FAA) is that I am assuming the relative wind is contant along the length of the propeller, which is not a perfect assumption (more interruptions at the roots- spinner, air intake, etc)

 

Any help would be great, I dont have an exact question, just help explaining to me about the answer given for this qustion.

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7 Answers



  1. Radu Poenaru on Apr 19, 2013

    I think it’s not a problem of distributing thrust as it is a problem of preventing blade stall toward s the tip. If you increase the rotation speed (which is what happens along the blade length) the angle of attack will continously increase. At some point the relative flow along the length of the blade might reach a critical angle of attack, so to prevent this the blade gets twisted.

    In fact, as I remember it, the blade tip will create more thrust, which is why a blade tip warps forward quite a lot, if you have ever seen a high-speed photo of a moving prop.

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  2. Nathan Parker on Apr 19, 2013

    “If it had an equal angle of attack along the length, and the outter parts moving faster, it would creat more lift at the tips.”

    So? It actually does and there is some bending when the propeller generates thrust.

    Any airfoil that has a constant lift coefficient (AoA) along its span generates less induced drag than if there were variation in the lift coefficient. That’s because induced drag varies with the lift coefficient, squared.

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  3. Chris Carlson on Apr 19, 2013

    I think I got it now, Radu. The relative wind around a propeller is the combination of two vectors added together, the vector of the wind coming straight at it from the forward motion of the airplane, and the vector perpendicular to that which is created by the spinning of the prop(with the plane of the prop) The smaller angle at the tips is because it has a greater amount of wind coming from the “plane or the prop” which creates a total relative wind vector that is more parallel tothe plane of the prop, as compared to the relative wind at the root which is being affected by a relative wind vector at an angle closer to straight on.

    Is the above correct?

    A follow up question…does the warping of the propeller tips (as both responses above mentioned) reduce the propeller efficiency? Or have an effect on the props lifespan?

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  4. Nathan Parker on Apr 20, 2013

    Your understanding is correct.

    I doubt the bending has much effect on efficiency; only twisting would have an effect on the local AoA, not bending forward. There probably is a slight reduction in the effective aspect ratio of the blade, but I can’t imagine it would be significant.

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  5. Brian on Apr 21, 2013

    “In fact, as I remember it, the blade tip will create more thrust”

    Not exactly. The root and the tip generate close to no thrust. The location along the prop to generate the most thrust is approximately two thirds out from the root. Graphically, the thrust curve across a propeller blade looks like a bell curve where the peak of the curve is positioned over the two thirds point just mentioned.

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  6. Radu Poenaru on Apr 21, 2013

    The thrust curve will be skewed towards the outward (tip) of the blade, it will not be a symmetrical bell, won’t it?

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  7. Brian on Apr 21, 2013

    Yes, it will be skewed so that the peak is located at approximately two thirds outward from the hub. It starts at nearly zero at the hub and falls back to nearly zero at the tip.

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