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Tip stall avoidance question.

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David Hall 921/09/2016 21:09:22
262 forum posts
15 photos

I'd like to indulge in a little own design build and thought that a flying wing in Depron might be fun. I would want to try to design/build a more efficient wing than my combat kit, so a higher aspect ratio with tapered wing is my target.

I would try for a flat bottomed wing with a root to tip width ratio of 1 : 1.5 or thereabouts. Given that for ease of building with Depron, incorporating any (deliberate) wing twist might be difficult, I thought that if I use a proportionally thicker section at the tip compared to the root, this might lessen any tip-stall tendency. Can anyone suggest a good a good starting point for this, say 10% thick at the root and 13% at the tip.....or should I be thinking of more of a difference?

Denis Watkins21/09/2016 21:22:59
4449 forum posts
113 photos

A dog tooth jetstyle leading edge at the tips will delay tip stalling considerably David

Google it

David Hall 921/09/2016 21:38:07
262 forum posts
15 photos

Is that effectively a jump up in the wing chord (width and thickness) towards the tips, or is it a change in the LE shape only?

Josip Vrandecic -Mes21/09/2016 21:42:32
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2993 forum posts
260 photos

Hi David,listen to Denis and I want to help you find it :

**LINK**

All the best

Edited By Josip Vrandecic -Mes on 21/09/2016 21:43:07

Simon Chaddock21/09/2016 22:56:36
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5716 forum posts
3034 photos

David

I am not sure that in real life your supposition is correct that a thicker wing section is going to stall later than a thin one, particularly at model sizes, but it would be interesting to try.

The plan shape of a wing has a big impact on the actual airflow particularly if it is swept back. In addition Depron is quite flexible so this too can have an effect on the way a wing actually behaves under load.

It quite possible to achieve effective washout by simply changing the wing section between root and tip.

For example a Clark Y section at the root changing to a simple flat plate section at the tip has several degrees of effective washout even if the underside remains flat..

In some respects the 'spectre' of tip stalling is worse than the actual occurrence as most models recover very rapidly provided the correct control inputs are made promptly before the wing drop develops! wink 2

Edited By Simon Chaddock on 21/09/2016 22:57:26

PatMc21/09/2016 23:38:15
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4407 forum posts
530 photos

Just make the LE blunter towards the tip. This has the effect of automatic washout as the AoA is increased.

David Hall 922/09/2016 09:08:34
262 forum posts
15 photos

Thanks All for your relies and suggestions. I'll work out how to incorporate some simple changes to the wing towards the tip.

Colin Leighfield22/09/2016 21:19:48
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5996 forum posts
2503 photos

In trying to understand why a sharply tapered wing made from flat balsa sheet with profiling only on l/e and t/e refuses to tip stall, the only reason I could think of was the progressive increase in thickness/chord ratio resulting from the constant actual thickness from root to tip (e.g. Nigel Hawes's Tucano and my Seafang). I can't think of any other explanation.

cymaz23/09/2016 00:52:38
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9254 forum posts
1196 photos

However you do it, washout will reduce tip stall.

onetenor23/09/2016 01:06:23
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1901 forum posts

Tip fins / endplates ?

Charles Smitheman23/09/2016 07:35:10
226 forum posts
18 photos

Just make the LE blunter towards the tip, as PatMc says.

The shape of the leading edge is much more important than the rest of the wing.

I dont know about deltas, but it really works on straight wings, and thinner lifting sections.

Colin Leighfield23/09/2016 09:51:20
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5996 forum posts
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In the case that I described of constant actual thickness from root to tip with sharp taper, with the factor of increasing thickness/chord ratio you will also get the effect of a blunter leading edge in relation to section profile as you move towards to the tip. Perhaps there's a co-incidence here with Charles experience of thinner lifting sections.

With thicker (but still relatively thin in some cases) wings I believe that wash-out fundamentally solves tip-stalling, my Mixmaster has 3 degrees because of concern about the swept leading edge and stalls absolutely straight, no tip stall. I think it's possible that progressive wash-out from root to tip might have an improving effect on the wing anyway because of some effect on span-wise air flow. Just a personal inclination to think that.

dsc_0113.jpg

David Hall 928/09/2016 12:37:44
262 forum posts
15 photos

Thanks all for your input, all very good info and appreciated.

For my wing (and my first foray into Depron models), simplicity is important. The wing section will be folded sheet over a spar. Creating a blunter LE will be difficult unless I use a balsa LE rather than a fold in the sheet (under consideration ). The wing section will be very simple indeed, possibly a little rough too. Changing the section from root to tip might involve more production skills/innovation that I had envisaged.

Your suggestions have given me much to think about, I have a couple of ideas that I could work on. To maintain some simplicity, I will build it as a simple sheet, folded over a spar with the LE created as a (probably sharper than ideal) fold in the 6mm Depron. The constant thickness spar and the wing taper will provided a thicker tip, so this will be my starting point. If tip stalling is a problem (assuming that I can build it straight and true ), then I will incorporate a blunter LE and a dog-tooth step by adding a new LE to the outer 1/3 or 1/4 of the wing.

Colin Leighfield28/09/2016 12:54:01
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5996 forum posts
2503 photos

Good luck David. I've just seen some work that describes how sharply swept forward trailing edges are a remedy for tip-stalling. This was the theory used by the Handley-Page designer Lachmann on the Hampden, which had this characteristic and apparently didn't tip-stall. It did have retractable leading edge-slots though!

That's not much use to you though, I'm sure you will get a result from what you are doing.

eflightray28/09/2016 13:24:09
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622 forum posts
132 photos
Posted by David Hall 9 on 28/09/2016 12:37:44:

Thanks all for your input, all very good info and appreciated.

For my wing (and my first foray into Depron models), simplicity is important. The wing section will be folded sheet over a spar. Creating a blunter LE will be difficult unless I use a balsa LE rather than a fold in the sheet (under consideration ). The wing section will be very simple indeed, possibly a little rough too. Changing the section from root to tip might involve more production skills/innovation that I had envisaged.

Your suggestions have given me much to think about, I have a couple of ideas that I could work on. To maintain some simplicity, I will build it as a simple sheet, folded over a spar with the LE created as a (probably sharper than ideal) fold in the 6mm Depron. The constant thickness spar and the wing taper will provided a thicker tip, so this will be my starting point. If tip stalling is a problem (assuming that I can build it straight and true ), then I will incorporate a blunter LE and a dog-tooth step by adding a new LE to the outer 1/3 or 1/4 of the wing.

I've been building models using Depron for close on 10 years, if you do manage to fold 6mm Depron to form a leading edge, (I assume you bend back on itself), I would love to know how.

It can be done with the softer foams, polystyrene etc, but with Depron ...... , even 3mm is difficult.

Ray.

Simon Chaddock28/09/2016 14:43:30
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5716 forum posts
3034 photos

"fold 6mm Depron to form a leading edge"?

I cannot imagine why you would even try unless you cut in lots of deep slots or you are building to a huge scale!

Surely simpler to use top and bottom skins and use a separate Depron(?) leading edge.

6mm LE

I appreciate in this case the skin is much thinner (2 mm) but the 6 mm Depron leading edge allows a 'proper' nose radius to be created.

I only once used a true 'folded over' leading edge and even then with 2 mm Depron.

Tortion box

Hard to get a good glue join.

Light and looks good but any leading edge repairs are a nightmare!

 

David Hall 928/09/2016 14:48:33
262 forum posts
15 photos
Posted by eflightray on 28/09/2016 13:24:09:
Posted by David Hall 9 on 28/09/2016 12:37:44:

 

I've been building models using Depron for close on 10 years, if you do manage to fold 6mm Depron to form a leading edge, (I assume you bend back on itself), I would love to know how.

It can be done with the softer foams, polystyrene etc, but with Depron ...... , even 3mm is difficult.

 

Ray.

 

Oh no! I assumed that it was routine! Here is a link to Andrew Newton's video on his wings built in 6mm Depron. He coats the outer with tape and effectively crushes the inner along the fold by pressing a dowel into the Depron. This "looks" to work well, but leaves a fairly sharp LE.

 

 

 

Edited By David Hall 9 on 28/09/2016 14:53:53

Peter Miller28/09/2016 15:06:20
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11094 forum posts
1309 photos
10 articles

My elliptical wing designs (Rhapsody, Symphony, Harmony, Melody and Jezebel all have fairly thin sections. An elliptical wing is a type that begs to tip stall.

I use 2 degrees of washout on the wing.

ot one of those designs will tip stall and one can take amazing liberties with them and yet they will do fantastic flick rolls whichare basically caused by forcing the wing into a stall.

I have yet another on my drawing board at the moment.

David Hall 928/09/2016 16:02:25
262 forum posts
15 photos
Posted by Peter Miller on 28/09/2016 15:06:20:

My elliptical wing designs (Rhapsody, Symphony, Harmony, Melody and Jezebel all have fairly thin sections. An elliptical wing is a type that begs to tip stall.

I use 2 degrees of washout on the wing.

ot one of those designs will tip stall and one can take amazing liberties with them and yet they will do fantastic flick rolls whichare basically caused by forcing the wing into a stall.

I have yet another on my drawing board at the moment.

Many thanks for taking the time to add to the suggestions/experiences.

Many years ago, I built a "Starlet" from kit. It had so much washout that it looked as if the wing was built wrongly, but..it flew amazingly well and was well mannered. I have no doubts that washout would be the solution.

The problem for me is that (as far as I can see), incorporating 3 degrees of wash out in a Depron/hotglue folded wing of this type will be very difficult. Far better, I guess, is to build a traditional rib and sheet (although in Depron) where I could jig the LE and TE on the board to build in some washout. This might be the way to, but then it departs from the idea of a super simple wing build......

Perhaps I should jump in and hot-glue it, then discover how it flies...

Peter Miller28/09/2016 18:08:16
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11094 forum posts
1309 photos
10 articles

Perhaps you could lay it on the board with the lower surface propped up by the required degrees at the tip and then hot glue the trailing edge together while held down

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