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I'm designing a small RC plane and I'm currently at the first iteration of airfoil selection. I have converged to Clark-Y, NACA-2412, NACA-2415, and S-8036.

In comparing the four, I noticed that all of them have a negative Cm up to the critical angle of attack. Clark-Y's Cm values, however, are consistently more negative for all angles of attack, compared to the other three. I was wondering if this can be considered as an advantage for Clark-Y as less downward pitch would be required from the horizontal stabilizer. Could this be a disadvantage at level flight since it can cause the plane to nosedive? I noticed that at AoA = 0, Cl = 0.4 which could stop the nose from pitching down?

I have included a picture of Clark-Y aerodynamic graphs for your reference.

Thank you in advance for your helpenter image description here

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  • $\begingroup$ Bear in mind that you probably don’t have the capability to reproduce that airfoil section to the level of precision required to do anything more than approximate those graphs. So the small differences in performance characteristics will likely be lost. $\endgroup$
    – Jim
    Jan 26, 2022 at 20:28
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    $\begingroup$ I’m voting to close this question because it belongs on Drones.SE. $\endgroup$
    – Ralph J ♦
    Jan 27, 2022 at 5:28
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    $\begingroup$ @RalphJ -- we have a "model aircraft" tag here. $\endgroup$
    – quiet flyer
    Jan 29, 2022 at 8:10
  • $\begingroup$ @quietflyer Anybody can create a tag, and Drones.SE hasn't always been around. It is now, and it's a far better fit for this question. $\endgroup$
    – Ralph J ♦
    Jan 29, 2022 at 13:08
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    $\begingroup$ @RalphJ -- but is it really? Have questions about airfoil design, pitching moment, and stability gotten a good treatment on Drone SE? A search through ASE Meta suggests the issue of model airplane or drone postings is unresolved -- aviation.meta.stackexchange.com/questions/4072/… -- I support this answer -- aviation.meta.stackexchange.com/a/4085/34686 -- it might be time for a new Meta discussion -- $\endgroup$
    – quiet flyer
    Jan 29, 2022 at 13:14

1 Answer 1

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Camber in the main wing airfoil means that the wing's center of pressure will move forward with increasing angle of attack (AoA). Since the reference point for the pitching moment coefficient c$_m$ is the airfoil's quarter point, this drives the c$_m$ to negative values. By choosing the quarter point as the reference point, the value of c$_m$ will ideally stay constant over the range of AoAs with fully attached flow.

This movement of the center of pressure destabilizes the airplane slightly and in consequence requires a slightly larger tail surface in order to trim the airplane over its useful AoA range. But it will only make the airplane nosedive when the tail is missing - the large lever arm of the horizontal tail gives it a strong stabilizing effect.

Now it might seem that a large, negative c$_m$ causes more drag from the larger tail surface when you compare the same design with an uncambered wing airfoil, but camber will also allow the wing to reach a higher maximum lift coefficient. For the same minimum speed, the cambered wing can be made smaller and lighter, so in total camber reduces overall drag.

The Clark Y is popular because it has a flat lower side which makes building the wing simple if you use traditional construction methods. It is also, together with a few other airfoils like the Göttingen 398 and 449, the basis of the thickness distribution of the NACA four digit range. However, airfoils designed with a desired pressure distribution in mind (as opposed to the pure geometry) will yield better results. Maybe you should look into Eppler or Quabeck airfoils instead which have been designed with the use in model airplanes in mind.

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