Increasing Angle of Incidence

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Dan Thomas

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1. In regular configuration, a 3-point landing is still at a higher speed than level stall speed. In other words, a normal 3-point does not approach the stall angle of attack because the tail wheel hits first. That is supported by noting that you can take off in a 3-point attitude, so obviously the wing is not stalled at that angle.
I've many times pointed that out on forums. I measured the chordline angle on our Citabrias in the three-point ground attitude, and found them at 12 degrees. Those wings stall at around 17 degrees, so the airplane, flaring to three-point attitude a few inches above the runway and travelling roughly parallel to the surface, has the wing at around 12 degrees, far short of stall. So we cannot call these "full-stall" landings. Trikes are similarly built; you bang the tail on the runway before you get to stall angle. The manufacturers designed their airplanes this way on purpose; you don't want the airplane stalling a couple of feet up, with attendant wing and nose drop. Could get ugly.

Besides that, at altitude there is upflow ahead of the leading edge that tends to increase actual AoA a little. In ground effect that upflow is damped, reducing the effective AoA a small amount.

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Dan Thomas

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The infamous NACA study that produced the above illustration was a classic case of correlated variables producing misleading results:

Rectangular wing used same chord and same % thickness at root and tip with good progression;
Tapered wings used same camber curve but thicker foil (by %) at root than at tips with poor progression;
Tapered swept wings also used same camber curve but thicker foil (by %) at root than at tips with even poorer progression;
Elliptical also used same chord and % thick throughout with a decent progression (the tiny Re of the tiny tips is known to bias stall outward);
Missing was any attempt to build with tips at same and greater % thickness at tips.

The result is called "correlated variables" and easily results in drawing a conclusion about one variable when a different variable could easily account for the behaviors observed. This is classic poor experimental design and is railed against in training folks on the topic. Generations of airplane designers have "learned" this erroneous conclusion.

Starting with the PA24 Comanche, we have had strong evidence that a tapered wing using the same % thickness throughout can have excellent stall behavior without fixes and washout. Since then, a number of airplane designs have been flown with tapered wings and good behavior. Our own Bill Husa (Orion here on HBA.com, now passed away) has advocated both same foil and % thickness from root to tip and built airplanes this way with excellent behavior.

Let's try to quit spreading the false conclusions coming from a flawed study.

Billski
I stand corrected.
 

wsimpso1

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Just visited The Incomplete Guide to Airfoil Usage, and Dave Lednicer lists the foils for the Questair Venture as 23017 root and 23010 tip. The wing as designed has an extended and drooped leading edge towards the tip, which is effectively a front loaded more cambered foil, so that would qualify as delaying stall on the outboard portions. That would keep the aileron alive into the stall, and make spins kind of tough to get into.

The scary part about these extended drooped leading edges is that while it delays stall on the outboard portion in positive g, it makes the outer parts of the wing stall early when in negative g. A negative g stall will probably turn into an inverted spin, and it would really want to enter a negative g snap roll. Maybe we just don't go there ...

Billski
 

wsimpso1

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I stand corrected.
Sorry, did not mean to single anyone out. The teaching that tapered wings makes for bad behavior is WIDELY established. Looking in more detail shows they had stuff to learn about designing experiments at that time. I would sure hope we get smarter over time... And the data is really clear on what a bad idea the combination of thin tips and tapered planform is.

Billski
 

Lendo

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Hallelujah Billski, you addressed that perfectly - Same thickness is required!!
Many designers reduce the tip thickness for aesthetic reasons (perhaps), from my point of view necessitating wash-out, reducing lift and increasing Drag.
George
 

rv7charlie

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I did too, and while I have no excuse for mixing up the two planes, I think I found my distorted memory.
Bede BD-4 NACA 64-415 NACA 64-415
Bede BD-5 NACA 64-212 NACA 64-218
Bede BD-6 NACA 64-415 NACA 64-415

Shame that Jim wasn't as good at that Business Ethics course in college as he was in designing a/c.
Charlie
 
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