Tailwheel aircraft taxiing aerodynamics

jrh

jrh

Well-Known Member
I'm doing some tailwheel training with a very experienced instructor (he's got about 12,000 hours TT, 3000+ hours in tailwheel) and he brought up an interesting point today.

He told me to turn the ailerons *away* from the wind while taxiing with a quartering headwind. What I mean is, if I'm taxiing with a quartering headwind blowing from left to right, then I should turn the ailerons to the right.

This would effectively change the saying, "Climb into the wind, dive away from the wind" to "Climb away and dive away from the wind."

When I pressed him for why this is better, he admitted he couldn't explain why it worked, he said it just did. He agreed that it completely goes against any formal "FAA" way of teaching, but he's taxied numerous taildraggers in 30, 40, even 50 knot winds and everything becomes more stable when he positions the ailerons in the way he does.

I hypothesized (is that a word?) that it might work under the same principle as what happens when turning the ailerons against the direction of rotation in a spin. In such a case with a spin, the inside aileron goes down, increasing the angle of attack, putting the wing deeper into a stall, and vice versa on the outside wing. This would be a bad thing in a spin, but could be good on the ground.

If a taildragger naturally sits with the wings at a stalled AoA, would turning the ailerons away from the wind make the upwind wing more stalled and the downwind wing less stalled, thus helping controllability on the ground?

I asked my instructor about my theory and he basically said he had no idea. What works, works, and he's always left it at that.

What are your thoughts? If I start teaching in a taildragger someday, I want to have a true explanation for why they work this way.
 
jrh said:
I'm doing some tailwheel training with a very experienced instructor (he's got about 12,000 hours TT, 3000+ hours in tailwheel) and he brought up an interesting point today.

He told me to turn the ailerons *away* from the wind while taxiing with a quartering headwind. What I mean is, if I'm taxiing with a quartering headwind blowing from left to right, then I should turn the ailerons to the right.

This would effectively change the saying, "Climb into the wind, dive away from the wind" to "Climb away and dive away from the wind."

When I pressed him for why this is better, he admitted he couldn't explain why it worked, he said it just did. He agreed that it completely goes against any formal "FAA" way of teaching, but he's taxied numerous taildraggers in 30, 40, even 50 knot winds and everything becomes more stable when he positions the ailerons in the way he does.

I hypothesized (is that a word?) that it might work under the same principle as what happens when turning the ailerons against the direction of rotation in a spin. In such a case with a spin, the inside aileron goes down, increasing the angle of attack, putting the wing deeper into a stall, and vice versa on the outside wing. This would be a bad thing in a spin, but could be good on the ground.

If a taildragger naturally sits with the wings at a stalled AoA, would turning the ailerons away from the wind make the upwind wing more stalled and the downwind wing less stalled, thus helping controllability on the ground?

I asked my instructor about my theory and he basically said he had no idea. What works, works, and he's always left it at that.

What are your thoughts? If I start teaching in a taildragger someday, I want to have a true explanation for why they work this way.
Click to expand...

Its a very interesting theory. It does go against the text though. Aerodynamically, the AoA is the angle formed between the relative wind and the chordline. Tail wheels aircraft do infact sit closer to the critical AoA but are not actually in a stalled state. The text I've read with regards to taxiing a tail wheel airplane suggests taxiing it the same way as a tricycle gear airplane.

For a left quartering headwind, the ailerons shouls be turned into the wind with the yoke back as a the only primary difference.

If you had a left quartering headwind, I'd still turn the aileron into the wind. In this instance the up wind will experience a greater AoA from the wind whereas the right wing will experience minimal effect from the wind.
By turning the aileron into the wind, lift is destroyed on the upwind wing while increasing lift on the right wing.

I think the fact that the wing is already close to its critical AoA has very little significance since the wing is not infact stalled. Also consider the outboard section of the wing has washout built into it and carries a lower AoA when compared to the inboard section of the wing.
 
jrh said:
If a taildragger naturally sits with the wings at a stalled AoA, would turning the ailerons away from the wind make the upwind wing more stalled and the downwind wing less stalled, thus helping controllability on the ground?
Click to expand...

I don't think a taildragger is actually at the stall AOA while sitting on the ground. I think you'd have a hard time taking off in a three-point attitude if it were.

You're right to be cautious about accepting his recommendation uncritically. My own tailwheel instructor has similar times and told me that you couldn't do wheel landings power off. I knew this to be a lie. Sometimes these guys pick up a habit early in their careers and simply never explore other techniques.

That said, reasons that what he said *might* be true could be that the profile drag of the upturned aileron is generating more drag at low airspeeds than the induced drag of the down aileron. Or perhaps the downturned aileron is generating more downwash that the crosswind blows toward the vertical stabilizer.

Still, I'm skeptical. Mavericks are more often wrong than right. Perhaps it's simply the placebo effect that he sees; the perception of a stabilizing effect is very subjective without a means of actually measuring it.

Perhaps he's been successful in spite of his technique, rather than because of it.;)
 
meritflyer said:
I think the fact that the wing is already close to its critical AoA has very little significance since the wing is not infact stalled. Also consider the outboard section of the wing has washout built into it and carries a lower AoA when compared to the inboard section of the wing.
Click to expand...

Good analysis.
 
http://www.fly-aerobatics.ca/taildragger_course.pdf

The diagram on page 3 pertty much sums up what I was always taught. I have about 300 hours or so in Tail Draggers, and the one thing I have learned is the elevator position is really the one to pay attention to. The ailerons I can't really tell you in each situation where to put em. But you will get a feel for where they need to go especially if it's gusty and the wind is all over the place, that's when I seem to feel the effect of aileron position.



PS Not a PHD or a CFI, and I am probably a little aerodynamiclly(sp) retarded.:)
 
flynik said:
it's not hard to make a three-point take-off because the relative wind changes as you speed up (Thus lowering the effective AOA)
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Ah, that doesn't even make sense. Sitting still on the ground, there is no relative wind. As the aircraft moves forward, the RW is parallel to the ground and that won't change until the aircraft lifts off, no matter how fast the airplane rolls.
 
You are totally correct tgrayson, my mistake, disregard what I said. What I meant to say was that the speed of the relative wind changes as you speed down the runway, thus allowing the wing to produce lift.
 
flynik said:
What I meant to say was that the speed of the relative wind changes as you speed down the runway, thus allowing the wing to produce lift.
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You're right, though, the RW will speed up and even stalled, the wing will generate some lift, but the drag would be enormous. You'd never get off the ground.

Every tailwheel pilot has landed at least once with the tailwheel touching down first; that alone shows that the aircraft can achieve a higher AOA than its ground attitude without being stalled.
 
tgrayson said:
Perhaps it's simply the placebo effect that he sees;
Click to expand...

This is what I think.

Ask him, when he lands with a real rippin' quartering headwind/crosswind with the downwind wheel on the ground with almost full aileron into the wind, just when does he feel comfortable in 'switchin' the ailerons over to the other side.
 
I don't think he is correct. I wouldn't suggest practicing his method, and definitely not teaching it to your students. There isn't a good aerodynamic reason and experience has told me otherwise.

From my 1k hrs tailwheel I would say the big three lessons are 1. never stop flying the plane (after landing, during taxi, etc), 2. don't let the nose get even a hair to the windward side of centerline 3. as soon as the tailwheel touches the ground, keep it down with full aft stick.
 
nosehair said:
Ask him, when he lands with a real rippin' quartering headwind/crosswind with the downwind wheel on the ground with almost full aileron into the wind, just when does he feel comfortable in 'switchin' the ailerons over to the other side.
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I asked him exactly that. He said he switches techniques once he's off the runway. He'll hold ailerons into the wind during the landing roll, taxi clear of the runway, then switch over to his other method of control placement while he taxies to parking.

When I pressed him for why he keeps them in the traditional location during rollout, he said it was because he might still have flying speed--while taxiing, he's going slow enough to not have flying speed.

I thought his explanation was pretty "out there," but he's taught me a lot in other areas, so I just discounted it as a quirk. He must've survived his thousands of accident-free hours somehow, so he can't be too crazy of a guy. As for me in my personal flying and teaching, I'll stick to the traditional methods.
 
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