wing design

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scott mckenzie

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Feb 23, 2003
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a while back i was reading an article on wing shapes but cant remember where. the piece showed the stall areas i think on a taper wing and a elipticle wing verses a rectangle type wing and my question is what is the characteristics of the stall on these wings? does the taper and elipticle have more violent wing drop or what?
 

AVI

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Feb 25, 2003
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Scott,

I'd really recommend Dan Raymer's new book, "Simplified Aircraft Design For Homebuilders" which is available at discount prices from Amazon. If this book had been available a few years ago it would have saved me countless months of research! It is written on the basic level for the non-engineer amateur and in my opinion is far more helpful than Martin Hollomann's "Modern Aircraft Design" or the classic, "Light Airplane Design" by Pazmany. I had even purchased the expensive series published by Roskam, but those volumes remain on a shelf in one of my bookcases.

Alex Itenson
 
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orion

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Dan Raymer has written some of the better references out there. I haven't seen the one mentioned in the last post but we have used his "Aircraft Design: A Conceptual Approach" a number of times.

The stall behavior of a wing depends on a number of items, some of which include the airfoil section(s), the planform, the loading, the effects of the fuselage or engine nacelles, and of course any aerodynamic or physical twist.

Although not complete, a good reference and discussion of the stall is in Harry Riblett's publication "GA Airfoils".
 

Holden

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Scott,

Harry Riblett points out that the taper ratio (above 2/3) was not the main issue for a sharp stall found on production airplanes. The use of thinner foils on the outboard wing, along with symmetrical foils caused the tips to stall first.


On figure I-4 of "Spin Resistant airfoils," by Riblett he states: "Airfoil thickness in %C and airfoil series (position of maximum airfoil thickness in %C) are the only two airfoil design parameters that have a significant effect on the post-stall lateral stability of the airplane (slope of the post-stall lift curve)."

Many airplane designers used %12-15 foils inboard and 9-12% foils outboard. That would require around 3 degrees of tip twist just to make them stall at the same time. Even with flaps down (effectively twisting the wing) it is not enough to tame the wing.

Holden
 

mogren

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Jul 18, 2003
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TRy about 2degrees foreward sweep and tell us what the stall acts like !! MM
 

wsimpso1

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One of the interesting things about wing theory is how people have come to believe that taper and elliptical shapes cause tip stall and the associated terrible stall behavior.

When NACA did all of the work on these topics, they did some things that we now know are somewhat funky:

The rectangular wingss that gave nice progressive root stalls also used the same foil from root to tip. These gave nice stall progession patterns;

The tapered and elliptical wings tested had thinner wing sections (% thickness) at the tips than at the roots. The method for developing the leading edge of their foils also tended to make a foil have poorer stall behavior if it was a thinner section. These gave poor stall progression patterns;

Now this is correlated variables (constant sections and rectangular wings, thinner sections and tapered wings), and is known to confound. We can not know from their data if it was the thinner sections, the leading edge development method, or the the tapered planforms that gave the poor stall behaviour.

Still, many persist in the theory that to stall nicely, you need a ractangular wing and washout at the tips. Other folks have broken the convention. Piper Commanche has the same section from root to tip, same airfoil form and % thickness, and no taper. I understand that it stalls nicely, and recovers easily. Burt Rutan has much the same philosophy, with the section being the same, with the whole foil scaled to the plan form...

It seems that there is much more to this than meets the eye. I will tell you that my airplane uses a 15% Riblett foil, one foil from root to tip, tapered from 48" at root to 32" at the tip. If I were starting now, I would have used 17% or 18%. My wings will have slight washout in them for a simple reason - my spar is at the max depth of the wing, which is far enough aft that 6 g's will twist my tips by about 1.5 degrees, which is enough to cause tip stall... So I am washing out my tips by 1.5 degrees to give me some margin from tip stall. The thicker wing would have reduced this effect and made the fuel system simpler too. And no one accuses Burt Rutan of using draggy wings while the Boomerang has 17% thick foils...

Yeah stalls are more complicated than just selecting a plan form and foil...
 
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