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Old Feb 06, 2013, 07:12 PM
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Portland oregon
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washout, root stall and other gadgets

I favor a bit of wing tip wash out on my models to avoid those nasty
tip stalls when turning slow for landings. Or just gentle stalls.

I just assembled a ARF AT-6 and tried to warp the wings a litte with heat and overnite jigs but the thing just keeps coming back ... so....
I recall that several full scale airplanes I have seen over the years like the Globe Swift, used a small turbulance causing device riveted to the leading edge of the wing near the root. This would cause the root to stall ahead of the rest of the wing giving the pilot a nice warning before a wing might drop.

My question is , how might I arrive at a size and location to install such a device?
I am sure it will be trial and error but I need a starting point. All comments welcome.
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Old Feb 06, 2013, 08:41 PM
"...certainty is absurd."
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I think you are looking for root stall strips. They are basically just a small 90 degree triagle added to the leading edge of the root airfoil.

From this paper, it looks like having the point of the triangle right on the chord line is probably close to optimum, or perhaps just slightly above (upper most position shown). The triangle should be about 1.2% of the wing chord on each side of the triangle.

"CFD Study of a NACA 63-415 Aerofoil
Fitted with Stall Strips"

http://130.226.56.153/rispubl/VEA/veapdf/ris-r-1370.pdf

(right click and save as...)

Note that the stall strips work by reducing the max lift coefficient of the airfoil. This means you don't want to make them too long, or the stall speed of the airplane will increase substantially.

Kevin
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Old Feb 07, 2013, 12:22 AM
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Some big planes have them also.
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Old Feb 07, 2013, 04:40 AM
An itch?. Scratch build.
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How about raising the ailerons slightly for a little reflex, that could give a similar affect to washout.

But, the real answer is don't slow it down too much. A model prone to dropping a wing tip, (tip stall), often in that dreaded 'down wind turn', just need to be flown faster and further out on the down wind turn. Once lined up and flying straight with the runway, then start to slow down, (flaps etc ? ).

The higher the wind speed, the easier it is to start relating ground speed to the model, and slow down too much on the down wind leg.
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Old Feb 07, 2013, 04:42 AM
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That taxiing dolly on the Dragon Lady had me confused for a bit.
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Old Feb 07, 2013, 11:09 AM
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That's the ground handling dolly.. they're towed backwards...
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Old Feb 07, 2013, 01:31 PM
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Yes. For a bit I thought that it was a main landing gear, but I know that the U2 doesn't have that gear configuration. so I thought it could have been this one instead:

but the wing is mounted differently, the intakes are bigger, and it has two fins and a high tail, so it didn't fit anyway. I know the U2 has outrigger pods it drops once airborne, but I don't think it can use those to actually taxi to the head of the runway, so I assumed they fitted some sort of taxing dolly and that was what I was seeing in the picture. I presume it is towed to the starting position instead, which makes sense since taxiing would be a waste of fuel. Actually, I wonder if they ever thought about aerotowing it to altitude to extend the mission range.
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Old Feb 07, 2013, 01:35 PM
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Some U-2 pilots eschew ground handling to position themselves on the runway for takeoff, and will taxi out... and after the pogos are reinstalled after landing, taxi back.
NASA has been flying an ER-2 out of here every day this week. Noisy thing!
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Old Feb 07, 2013, 09:50 PM
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Portland oregon
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Thanks to All of you for the replies.

I am now convinced that the best approach is ..... find a way to warp that wing and get the
washout I really want.

Wish me luck and listen for cracking sounds.
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Old Feb 07, 2013, 10:07 PM
Really?!?
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How about some simple turbulator strips further out on the wing? A piece of tape cut in a zig-zag and adhered just in front of the high point of the wing for the last 1/3 of the wing span may work wonders.

Just a thought

Chris
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Old Feb 07, 2013, 10:25 PM
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1960 I was flying a PAA Load 1/2A free flight at the Dallas Nats... I'd test flown it several times back home in VA.
On its first circle over the runway at Dallas after takeoff, the warp I'd doped out of the wing came back... Short flight...
I have warmed some MonoKote wings on both sides and held them just past the unwarped position until the wing cooled.
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Old Feb 08, 2013, 03:01 PM
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Quote:
Originally Posted by ChrisS View Post
How about some simple turbulator strips further out on the wing? A piece of tape cut in a zig-zag and adhered just in front of the high point of the wing for the last 1/3 of the wing span may work wonders.

Just a thought

Chris
Expand on that a litle, How does that work?
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Old Feb 08, 2013, 04:03 PM
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Any rough edge that generates some skin turbulence downstream from the trip point is called a turbulator or invigorator or any of the other buzz words you've read about.

The tape method is a pretty common method of making a turbulator for a model. In fact there's really no need to make it a zig zag shape either. BUT.... it does need to be tall enough to produce the desired effect. The times I've done a tape turbulator I've used a double or triple layer of 1/8 wide automotive pinstriping tape. It needed the two layers in one case and three on the other to be thick enough to produce the turbulation and perk up the low speed flying. I can't say if it aided in delaying the stall or not as I was more concerned about draggy low speed performance and a glide slope that looked like a duck with a belly of birdshot. In both cases adding the turbulator strips perked things up and made these two sailplanes into great low speed performers.

There's another way to make the center stall first. A stall strip or shaping the leading edge so the inner area has a sharp small radius shape and the tips set up with a relatively generous radius will work to some extent similarly to washout in the tips.

Keep in mind too that you're also ASSUMING that the model will encounter trouble. You haven't even flown it yet. It may be just fine.
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Old Feb 08, 2013, 07:11 PM
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Correct, I have not flown it yet. It is a world models AT-6 44 inspan ep.
I have read several reports and they all say its its a fun to fly model but has a habit of tip stalling in slow turns.
I had a larger model of the same plane World models puts out with a 91 four stroke on it.
I suspect it was the same but only larger design.
I lost that one in a low turning tip stall turning finel. That is why I am so suspect of this bird.
Most of my flying fun come from shooting landings.
So..... like they say "an ounce of prevention....

I like the tape idea. Thanks all
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Old Feb 08, 2013, 07:35 PM
Really?!?
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Lost ...on the 3rd rock out
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Nearly any tapered wing will do that. The lesson is to keep the speed up a bit...especially on downwind turns. Learning to use rudder for downwind turns will help too.

Chris
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Last edited by ChrisS; Feb 08, 2013 at 07:47 PM.
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