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Riblett on Twist

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mcrae0104

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In Riblett's GA Airfoils section "Design Notes for Tapered Wings," he asserts that NACA's conclusion that tapered wings stall at the tip first is invalid. The reason he gives is that not only the planform of the test specimens was tapered; the section thickness was tapered from root to tip. He says the thin (percentage) sections near the tip were responsible for the tip-stalling tendency, not the planform taper. His conclusion is that a tapered wing without variation in the percent thickness of the sections does not require washout (or additional camber at the outer section) in order to have good stalling characteristics.

I am tinkering with a tapered planform, and it sure would be nice not to have to deal with washout or blending to a more cambered foil at the tip. But I'm not sure I buy his conclusion. I think he's probably right that the thickness variation muddies the waters, but it's not clear to me that an untwisted, tapered, constant percent-thickness wing will automatically have pleasant manners. Riblett points to the example of the PA-24 Comanche, which has planform taper only (no percent thickness taper) and no washout, with its docile hadling, as evidence of his position.

What do you think?
 

Voidhawk9

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Reynolds number may be very important here. If your tip has a particularly low Reynolds number, it's stalling characteristics probably will be considerably worse than the root. Depends on the airfoil characteristics at low Re too.
I'd expect this to be a bigger issue with smaller aircraft (and thus smaller tips). On larger types where the tapered tip Reynolds number doesn't get too low, it shouldn't be much of an issue.
 

Speedboat100

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I recall someone claiming the tip ought to have percentage wise thicker foils.....just the opposite what is being done.
 

wsimpso1

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Other folks have built wings per Harry Riblett have claimed good behavior, soft stall, and attempting spin entries find they transition to spirals instead of spins. Our own beloved Orion (Bill Husa) liked and used Harry's airfoils, and opined that wings done correctly probably do not need twist. Orion was a fan of gentle aerodynamic washout... Advanced Search on Orion and words like twist, washout, and Riblett - there are a lot of posts with his thoughts on Harry's foils and wing design.

I drank the Koolaid. My wings use one Riblett foil from root to tip. I analytically found that at Va and +6 g my wing tip can twist to wash in about 0.7 degrees, which might make for interesting stall departures. So, I built with 3/4 degree of washout to prevent that misbehavior in positive g stalls. All bets are off for negative g stalls, but I am planning on very little negative g flight and no exploration of negative g stalls.

Billski
 

fly2kads

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I've been tinkering with this, myself. Fortunately, software and spreadsheets make this a little easier to evaluate, even for an amateur like me. Snorri Gudmundsson, in his book General Aviation Aircraft Design, says this:

If inviscid design methodology is used to tailor the stall progression, the goal should be to ensure the section lift coefficient (Cl) at the 70% span station is no higher than the maximum lift coefficient (Clmax) of the airfoil at that station. Furthermore, from 70% to 100%, Cl should gradually fall to zero.

On my own learning project, I can meet this criteria with no washout. (No sweep, AR = 6.45, taper = .67, constant 15% airfoil.) My effort isn't far enough along to predict any aeroelastic behavior, like Billski has done.
 

Norman

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In Riblett's GA Airfoils section "Design Notes for Tapered Wings," he asserts that NACA's conclusion that tapered wings stall at the tip first is invalid. The reason he gives is that not only the planform of the test specimens was tapered; the section thickness was tapered from root to tip. He says the thin (percentage) sections near the tip were responsible for the tip-stalling tendency, not the planform taper. His conclusion is that a tapered wing without variation in the percent thickness of the sections does not require washout (or additional camber at the outer section) in order to have good stalling characteristics.

I am tinkering with a tapered planform, and it sure would be nice not to have to deal with washout or blending to a more cambered foil at the tip. But I'm not sure I buy his conclusion. I think he's probably right that the thickness variation muddies the waters, but it's not clear to me that an untwisted, tapered, constant percent-thickness wing will automatically have pleasant manners. Riblett points to the example of the PA-24 Comanche, which has planform taper only (no percent thickness taper) and no washout, with its docile hadling, as evidence of his position.
Planform effects and airfoil cross section effects both contribute to stall behavior and Mr Riblett was simply wrong to claim that taper isn't responsible for tip stall. In vortex lattice methods the actual thickness of the wing isn't even considered in the simulation (think of a paper airplane with camber but no thickness) and this stall progression due to taper shows up anyway.
 

BBerson

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Larger nose radius strongly delays stall. And the dropped nose with the larger radius is the best. NASA found these added drooped cuffs only on the outboard wing is best for spin resistance. No change in airfoil thickness.
Tapered wings naturally have a smaller radius at the tip.
 

David L. Downey

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Not remotely similar but symmetrical control-line precision aerobatic wings respond favorably to both taper and increasing thickness (of course blunter leading edge radius ) at the tips...less wing waggle at the entry/exit of square maneuvers.
 

BJC

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Not remotely similar but symmetrical control-line precision aerobatic wings respond favorably to both taper and increasing thickness (of course blunter leading edge radius ) at the tips...less wing waggle at the entry/exit of square maneuvers.
I’m not familiar with control-line aerobatics; how do you control the yaw axis?


BJC
 

fly2kads

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I’m not familiar with control-line aerobatics; how do you control the yaw axis?
It's normally not controllable in-flight. Either the rudder is deflected, or the whole vertical fin is offset, to produce a yaw to the outside of the circle. This keeps tension on the lines. On some aircraft this is ground-adjustable, but on many it's just built-in into a fixed position.
 

Swampyankee

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In this case (and in the case of the 230xx airfoils) he's wrong; the increase in lift coefficient towards the tips of a finite wing is predicted by vortex lattice, lifting surface, and lifting line analyses and supported by wind tunnel test data; it happens even if the wing is not tapered.
 

TFF

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CL models are pretty good in the world of induced drag and stall. The taper and airfoil has to be spot on to cut the corners required to be judged 10s. It is a different world, but in that one axis, they are close to perfection at their size.
 
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