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A helicopter rotor blade will generate more lift at the fast-moving tip than at the slow-moving root, unless it is twisted or tapered. Wikipedia calls this "asymmetry of lift", however I cannot find any sources calling it this. In fact, "asymmetry of lift" is used in several sources to mean the same thing as "dissymmetry of lift", which affects helicopters in forward flight.

The sources I've found refer to the use of twist and taper to mitigate the otherwise unequal distribution of lift across a rotor blade, but they don't give it a name. Does it have one?

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  • $\begingroup$ Wikipedia no longer calls it "asymmetry of lift". "Asymmetry of lift" means the same thing as "dissymmetry of lift", as multiple sources attest. Wikipedia now calls the phenomenon where the tips generate more lift than the hub "Unequal rotor lift distribution". $\endgroup$ Mar 23 at 10:15

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To clarify, the question is apparently asking about the distribution of lift along the length of the blade, rather than across its profile. The question seems to boil down to the following, how to appropriately reference a term for non-uniform thrust loading along the blade, on the one hand, and, on the other hand, that for a design feature to mitigate the consequence of non-uniform thrust loading along the blade. To address the first question, the general term for this unequal lift distribution across a helicopter blade seems to be, non-uniform lift distribution. The alternative, proposed incidentally by the o.p., seems quite acceptable, namely, unequal lift distribution. References reviewed in assessing this issue do not seem at odds with the use of such terms.

The second question is embedded within this context -

The sources I've found refer to the use of twist and taper to mitigate the otherwise unequal distribution of lift across a rotor blade, but they don"t give it a name. Does it have one?

This part of the question seems to indicate a specific aspect of blade design intended to mitigate the "otherwise unequal distribution of lift across a rotor blade." This progressive spanwise reduction in angle of attack is literally called washout or twist. Specifically, the twist is a means of delaying or eliminating tip-stall. However, twist (as noted in the question) is also used to manage lift.

Saunders (referenced below) mentions this problem -

...if all sections have the same pitch and approximately the same $v_i$ [induced velocity], the tip sections have both high velocity and high angle of attack. Consequently, the tips produce a disproportionate amount of lift compared to the inboard sections. This is not only aerodynamically inefficient, but creates large moments about the hub with accompanying large coning angles... Designers therefore keep the blade pitch toward the tip about 8 or 10 degrees smaller than that toward the hub, thus altering the angle of attack distribution and obtaining a more uniform lift distribution.

Nevertheless, Wortmann mentions this within the whole context of managing the supersonic field developed along the span of the rotor. Specifically, design considerations not only regard managing lift along the span of the rotor, but also the onset of, and management of, shockless supersonic flow over the crest of, and along the span of, the rotor blade.

References are many regarding aircraft propeller and helicopter blade and section design considerations. Here are two that are readily available addressing helicopter blade issues -

Saunders, George H., Dynamics of Helicopter Flight. John Wiley and Sons, Inc. New York. 1975.

Sloof, J. W., Wortmann, F. X., and Duhon, J. M. The development of transonic airfoils for helicopters. 31st Annual National Forum, American Helicopter Society, 1975.

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    $\begingroup$ I think this misses the question. As I read it, the question is about a name for the "asymmetry of lift" generated by a helicopter blade, which is combated by twist, not the name given to the twist in the blade itself or "how to address non-uniform thrust loading along the blade". $\endgroup$
    – FreeMan
    Jan 18 at 19:28
  • $\begingroup$ @FreeMan, correct. I'm asking for a reference which gives a name to non-uniform load along the blade. Of course, "non-uniform load along the blade" is such a name, albeit a long one without a reference. $\endgroup$ Jan 18 at 19:43
  • $\begingroup$ Failing that, can someone invent a good name? $\endgroup$ Jan 18 at 19:49
  • $\begingroup$ "Asymmetry of lift" is a bad name because 1. It is hard to distinguish from "dissymmetry of lift", 2. It has already been used to mean the same thing as "dissymmetry of lift", and 3. The phenomenon has nothing to do with symmetry. $\endgroup$ Jan 18 at 19:51
  • $\begingroup$ Ok... Ok... I attempted some further clarification... Sometimes, language is never adequate. $\endgroup$ Jan 18 at 22:06

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