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The problem with retreating blade stall is that it limits the VNE and the overall top speed of helicopters. Retreating blade stall is basically the tip of the retreating blade having too much AoA at a lower speed, causing it to stall.

In general, coaxial counter-rotating helos are faster and mitigate retreating blade stall effects (they experience increased drag). Supercritical airfoils are designed to operate at far higher speeds than conventional airfoils (thus can produce more lift), have a high maximum lift and docile stall behavior, and generally stall at higher AoA.

Would this combination allow to reduce or eliminate retreating blade stall?

There have also been a couple of studies on the use of supercritical airfoils on helos. Here's one:

https://www.techscience.com/iasc/v33n1/46153/html

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Counter-rotating rotating rotors is enough to effectively eliminate the retreating blade stall problem. Note that the CH-47 is considered the fastest helicopter in the US military.

Retreating blade stall is strongly related to a requirement for roll equilibrium. The left half of the rotor needs to create as much lift as the right half -- otherwise, the aircraft will roll.

The retreating blade is less able to produce this lift (as it has lower effective velocities) and so (on a single-rotor helicopter) you must increase the angle of attack on the retreating blade to make this possible. This blade therefore stalls before the advancing blade. This dramatically increases the power required to turn the rotor and limits the speed the aircraft can achieve.

Any helicopter with left-right symmetrical lifting systems (i.e. counter rotating rotors) can balance the lift from one rotor's advancing blade with the other rotor's advancing blade. This allows you to set the angle of attack of the retreating blade to the minimum drag setting. The retreating blades will not contribute to lift, but they also will not cause an early dramatic rise in drag.

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  • $\begingroup$ Wasn't the Westland Lynx the one holding the world record for straight flying? $\endgroup$ Sep 19, 2023 at 15:21
  • $\begingroup$ So a supercritical airfoil could be detrimental or helpful? $\endgroup$ Sep 19, 2023 at 15:22
  • $\begingroup$ Also, what are possible solutions to Dynamic and Static rollover, Ground resonance, Dynamic stall and Vortex ring state? (from a design standpoint) $\endgroup$ Sep 19, 2023 at 15:26
  • $\begingroup$ What about adding winglets or using blade shapes like airbus' blue edge blades? Or something similar like the scimitar shaped fan blades in the GE90? $\endgroup$ Sep 19, 2023 at 15:40
  • $\begingroup$ @BanzaiFighterbomber: you'd better post these comments as questions $\endgroup$
    – sophit
    Sep 19, 2023 at 16:12
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Supercritical airfoils are unsuitable for helicopter blades due to their high pitch moment coefficients.

The idea of a supercritical airfoil is to separate the effects of thickness and lift creation on its pressure distribution. There is more to them, so this is probably an oversimplification, but it should help to understand why supercriticality is connected to high pitching moments.

Those pitching moments will twist the high aspect ratio helicopter blades in uncontrollable ways. Given the variation in lift over the rotor disk, the consequent variation in the local center of pressure will make it impossible to control the local angle of attack of the blade. Only with pitch-neutral airfoils like the venerable NACA 8H12 or the Hughes HH02 will a helicopter rotor become controllable.

Yes, there are papers out there which claim an advantage for supercritical airfoils. But when I read sentences in them like this one ("The supercritical airfoils are specially used in the supersonic aircraft to cross the sonic barrier"), the authors are evidently clueless and I wonder how this nonsense survived a peer review.

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  • $\begingroup$ Can this be solved by using washout? $\endgroup$ Sep 19, 2023 at 1:49
  • $\begingroup$ @BanzaiFighterbomber: washout is abundantly used on helicopter blades $\endgroup$
    – sophit
    Sep 19, 2023 at 7:17
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    $\begingroup$ "and I wonder how this nonsense survived a peer review" they even forgot to properly attribute the figure they used (Helicopters aerodynamic by Leishman) so I don't even know if this went through a peer review actually... $\endgroup$
    – sophit
    Sep 19, 2023 at 8:33
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Supercritical airfoils are designed to operate at far higher speeds than conventional airfoils (thus can produce more lift), have a high maximum lift and docile stall behavior, and generally stall at higher AoA.

These premises about supercritical airfoils in respect to conventional airfoils are not 100% correct:

  • They are not designed to operate at far higher speed than conventional airfoil rather to retard the generation of shock waves and therefore the relevant drag increase; they generate less drag at higher Mach number but the flying speed is only determined by the available power; most of the jet fighters out there (F16, F18, F22,...) use quite standard NACA airfoils but they can happily fly supersonic.
  • They do not have a higher maximum $C_l$; being thin and with a small camber, the opposite is actually true, they have a lower maximum $C_l$.
  • And for the same reason they also stall at lower AoA.
  • Stall is indeed docile but at the expense of a higher pitching moment which might limit the manoeuvrability and gives for sure high control loads.

Supercritical airfoils (with a bit of tweak of the camber) can and are actually used for the tip of the blades (last 10 to 15% of the bladespan) but not to mitigate the retracting blade stall phenomenon rather to reduce drag at the high Mach numbers typical of the advancing blade. The rest of the blade is designed around a more thick and cambered airfoil.

Please note that the paper you linked tested the airfoils at Mach number "0.3, 0.4 and 0.5" i.e.quite far from the actual conditions seen by the tip of the blade, where the Mach number can be as low as 0.1 on the retracting blade and as high as 0.8 on the advancing blade... They also forgot to calculate drag and pitching moment which are obviously also of paramount importance in blades design: they claim for example that "the rotor thrust is increased about 5% to 10%"; this is a very good result... but what if drag increases of 15%?

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  • $\begingroup$ Depends on the profile though, NASA SC(2)-0714 doesn't have a thin camber at all. $\endgroup$ Sep 18, 2023 at 23:36
  • $\begingroup$ @BanzaiFighterbomber The NASA SC(2)-0714 is not a helicopter airfoil. $\endgroup$ Sep 19, 2023 at 1:54
  • $\begingroup$ Doesn't mean it can't be used on one. It's the most extensively tested supercritical airfoil. Either on a plane or a helo, still answers the same aerodynamics $\endgroup$ Sep 19, 2023 at 15:19
  • $\begingroup$ @BanzaiFighterbomber: nope, the aerodynamic environment seen by a rotating wing is completely different than a fixed wing. Just the fact that a supercritical airfoil stalls before a conventional airfoil and generates a higher pitching moment is a no-go for its use on a blade (except maybe on its tip but after having tuned its camber) $\endgroup$
    – sophit
    Sep 19, 2023 at 16:07
  • $\begingroup$ @sophit Doesn't make any sense. None of the meaningfully tested supercritical airfoils is thin. None of the Sikorsky airfoils has a small camber, like the SC2110 and SC1095. $\endgroup$ Sep 19, 2023 at 20:46

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