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Old May 12, 2012, 07:06 AM
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dihedral as cure for tip stall?

just came from the field after testing dihedral on a new sailplane. before it had no dihedral, and easily got into tip stall. then i added dihedral (same as the full size) and now the plane is way more stable. no tip stall.
could be that some planes that do tip stall can be cleared from it by just adding dihedral? that is what i am trying to learn.

if some1 has any experience or knowledge about it, polite, positive and if possible precise comments are welcome.
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Old May 12, 2012, 02:29 PM
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Not much of a help, weighing my words, all I know about it is that dihedral is a overall help as you discover, meaning a progressive curve upwards starting the app. last fourth of the wing. You know that curvy wings at the "tips"..

In my world lift is made to conquer gravity, and since the dihedral not is inline with it, it fades the effect gradually as it in dihedral is bent upwards. The more dihedral, the more lift will be tuned down, and the effects that causes trouble. Forfull this bend upwards to 90, zero lift, and no trouble at all in terms of surprices.
If wings was 90 deg upwards, they would have no effect to conquer gravity.
This is a extreme way to put it, but also a scale to understand my thoughts in it..

This way of making wingtips makes an elliptical fade-out in lift at high AoA also as I see it, anyway enough major/minor (choose the way you see it) to make the difference.
You might say it`s a genius way to fade the lift away, quite another way than use zero lift airfoils with a constant V thruout the span. Or washout for that matter. But scale is scale, it`s in this game we have a task to do.

I have seen a RC model with a full cirkle O as the wing somewhere. Meaning the O is actually the wing.
All I have written now can be explained in this O model, talking about the lower half of the O. But of course in sailplane terminology this O is hopeless..
But it might give an idea how lift is made and how it effects. Full lift is in a straight wing, no dihedral.
The more you curve the tips, the less trouble is yours.
Stalling it makes it even less a problem, that`s why it functions as it does.

Put it otherwice, anhedral.. Now you get some seeerious truble, guess why?

In my head..
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Old May 12, 2012, 03:01 PM
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It also makes a perfect V this way, as the most of the wings are "flat/even" with gravity, the rest on the tips is bent upwards and fades trouble away, the more the tips are off direct faced gravity.

The point is to find the correct "alternative" V, this might be affected to the airfoil used..
All I know is that there is something in it, and my letters here are just my thoughts about it.
And would be interested to hear others thoughts in it.

Thinking model sailplanes every day for 37 years, hopefully I`m not lost in the forest?
Don`t know for sure .
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Old May 13, 2012, 10:36 AM
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But don`t think the ordinary V with a straith wing can affect tip stall.
Anyone?
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Old May 14, 2012, 01:09 AM
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Tip stall is exactly that, the wingtip portion of the wing stalling before the rest of the wing. Most wings have a little washout in the tips to make sure that the root stalls before the rest of the wing.
If dihedral has cured your issues, I don't think it was tip stalling to begin with. A stall is not always just dropping the nose when the lift runs out, it can also be dropping a wing, depending on the design. A tip stall is usually more violent and spiral shaped.
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Old May 14, 2012, 04:10 AM
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Quote:
Originally Posted by Robbie d View Post
Tip stall is exactly that, the wingtip portion of the wing stalling before the rest of the wing. Most wings have a little washout in the tips to make sure that the root stalls before the rest of the wing.
If dihedral has cured your issues, I don't think it was tip stalling to begin with. A stall is not always just dropping the nose when the lift runs out, it can also be dropping a wing, depending on the design. A tip stall is usually more violent and spiral shaped.
thank you for your comment. agree. the plane am talking about is the 2 mt asw 28 from hobby king. it went into snap rolls unexpectedly, at the slightest signal or even without me sending any command. now it does not. i only added 4.5 degrees dihedral. i was under the impression it was snap roll (or tip stall) but maybe not. the thing is that dihedral helped, whatever it was doing. am just trying to find the explanation of this.
regards
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Old May 14, 2012, 05:21 AM
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First I did not assume you had a absolute flat wing, but I should have considered other options for your question..
Adding V makes the plane "sit" in the air a whole different way.
The weight of it comes deeper and will stabilize the overall performance as we all know.
Lets say you added 30 deg V.. Now it will never flip in a stall no matter what, as the overall weight have come so deep compared to the lift that makes it fly. So deep that the weight of it always will win no matter what a stall is trying to do.
My point in my previous post is that adding V not makes a aerodynamical change of the "picture", it does very little to the airflow itself. Adding V is purely about moving one of the two components so the weight of the model becomes a help, not the opposite.
I misunderstood the question from the start, as I only had aerodynamics vs V in mind..
I don`t say my thoughts here are the textbook, but purely as I see it.
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Old May 14, 2012, 05:34 AM
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Originally Posted by Gudmund View Post
Adding V is purely about moving one of the two components so the weight of the model becomes a help, not the opposite.
It was ment as "moving the TWO components (weight and lift) further apart".
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Old May 14, 2012, 06:32 AM
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right. dihedral, among other things, lowers the cg and hence increases the pendulum effect, so the plane becomes more stable, and less prone to get into snap rolls.

also, i got some advice on adjusting the stab incidence. i have been shimming the rear to reduce its incidence and therefore increasing the decalage. still the plane needs some up, so i have to add a little bit more, but am heading into the right direction, and very close to have it trimmed for thermals.
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Old May 14, 2012, 01:31 PM
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Originally Posted by Gudmund View Post
First I did not assume you had a absolute flat wing, but I should have considered other options for your question..
.
I actually didn`t read your question good enough, Phil.
Just took the headline as the question without reading the rest.
Sorry about that..
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Old May 14, 2012, 10:32 PM
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More likely it raises the CG, especially on an unpowered airplane. The plane becomes more stable mostly due to the dihedral.

A little up trim won't hurt a thing.

Quote:
Originally Posted by phil alvirez View Post
right. dihedral, among other things, lowers the cg and hence increases the pendulum effect, so the plane becomes more stable, and less prone to get into snap rolls.

also, i got some advice on adjusting the stab incidence. i have been shimming the rear to reduce its incidence and therefore increasing the decalage. still the plane needs some up, so i have to add a little bit more, but am heading into the right direction, and very close to have it trimmed for thermals.
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Old May 17, 2012, 05:31 PM
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Originally Posted by Robbie d View Post
Tip stall is exactly that, the wingtip portion of the wing stalling before the rest of the wing. Most wings have a little washout in the tips to make sure that the root stalls before the rest of the wing.
If dihedral has cured your issues, I don't think it was tip stalling to begin with. A stall is not always just dropping the nose when the lift runs out, it can also be dropping a wing, depending on the design. A tip stall is usually more violent and spiral shaped.
This post is spot on. Based on the information provided your problem is not tip stall, but spiral instability. With no dihedral the ship will want to roll one way or another, awaiting some small perturbation to initiate the roll. Once in a roll, the large % vertical stab area associated with high AR wings will cause the spiral, once initiated, to steepen. This is a relatively slow process when compared to a tip stall induced roll and would require a different recovery technique. By adding a little dihedral you have cured the spiral instability.
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