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Dihedral and aerofoil profile

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Chris Walby07/01/2019 12:10:56
943 forum posts
226 photos

I have a small electric balsa model that has a flat wing with no washout and a tendency to be rather unforgiving as it approaches stall (okay it bites like a saber-toothed tiger).

Hence the question is two fold, firstly does dihedral effect the stall characteristic and do different profiles exhibit more/less abrupt stall tendencies. No doubt a very complex area that will be well over my head, but if we could keep it simple it would be appreciated.

PS The existing wing has a low profile so perhaps I just need to fly into the ground faster!

Erfolg07/01/2019 12:25:45
11239 forum posts
1118 photos

I have no idea with respect to dihedral. Other than arguments put forward with respect to how dihedral helps in rudder turning models.

The issue will be related to the wing loading. Yet I guess no weight can realistically be shed.

It is well known that swept back wings are liable to stalling, at low speeds, due to air moving along the span (energetics).

Washout at the tips helps greatly with a controlled stall.

Yes some are able to build in washout by use of aerodynamics, that is using sections that will tend to stall at lower speed at the tip, at the same AoA. A bit late on an already built wing.

It sounds that you have a 1970s club 20 pylon racer type handling model, that is flown onto the ground or better still long arrestor type grass.

Nigel R07/01/2019 13:27:07
2933 forum posts
470 photos

Assuming it is accurately made, Piers covers it I think.

Yes, different profiles can have different stall characteristics, the chief bit is the LE sharpness, the sharper the LE the more vicious the stall.

On the weight front. My understanding, is that weight simply determines the apparent speed at which stall occurs, it does not change whether you have a nasty tip stall (or not). I believe. Happy to be corrected.

Worth noting that with tapered wings, the smaller tip is always operating at a lower Reynolds number than the root, and this has a knock on effect of making sure it stalls at a lower angle of attack than the root - airfoils get progressively less useful at slow speeds.

A relatively more blunt LE, at the tip only, can fix that though. Hence 'stall strips' - nice sharp triangular bits - applied to wing roots. Another trick I have seen used is to have, e.g. 15% thick section at root, and 18% thick at tip - same effect, tip LE is blunter than root, by virtue of the whole wing section being thicker.

Yet another trick is to use a symmetrical root and a section with a couple of % camber at the tip. A cambered section stalls (typically) after a symmetrical section.

However, you don't say if it is tapered or not. A rectangular plank shape, then the stall, by default, starts at the root and should be benign. It is one of its defining features. And one very good reason conventional type trainers tend to have plank wings, or wings that are very close to a plank.

The only way you can make a rectangular wing tip stall is bad building, or by choosing a wing section that stalls before the root, which would have to be some kind of deliberate and perverse choice.

Edited By Nigel R on 07/01/2019 13:29:55

Chris Walby07/01/2019 13:44:14
943 forum posts
226 photos

Thanks guys, I think I'll have to re-read your advice and see how it applies to my straight LE tapered wing with rounded tip!

I have identified a build issue where I have induced a problem near the wing root that won't be helping things, so while it is back on the building board I'll sort that out.

Soft ground and long grass sounds the best option as it has folding prop due to continued motor bulkhead damage.

Odd as it seems to fly quite slowly until it gets close to the ground where it appears to want to lawn dart its way in!

Nigel R07/01/2019 13:53:06
2933 forum posts
470 photos

Is the TE straight?

If so, you have a moderate sweep back. This increases chance of tip stall by a hair, but not a significant amount. Far outweighed by taper - and if the taper is something moderate, like the tip = 2/3 root, then that sort of wing is still quite benign, as I think with taper of around 66%, the whole wing tends to stall simultaneously. Add the moderate sweep, and you get a fairly crisp stall. It is quite an ideal shape for an aerobatic model where you want to control the stall. See numerous F3A designs. They tend, my experience, to be generally very pleasant to fly with few bad manners.

If the taper is increased (see, many warbirds, particular wood wing types like the Russians) then the wing starts to get a bit tip happy. Then washout, different sections, stall strips etc, start to be useful. Some warbirds had taper so extreme (and worse yet, they had relatively thinned out tip sections for low drag) they could not sustain high speed tight turns without stall problems.

Extreme sweepback - such as EE Lightning - becomes very tip happy, too.

If you have found a building issue, it is most likely the answer.

My aeronautics is a bit amateur, so corrections are welcomed from more knowledgeable members!

Edited By Nigel R on 07/01/2019 13:55:14

Denis Watkins07/01/2019 15:58:24
3742 forum posts
179 photos

Dihedral stabilises the model Chris

Just by gravity alone, as well as airflow, the fuselage tries to stay below the wing if a high winger

From models I have built

For small !models, 12" - 20", 100grm to 200grm, a flat plate wing does work best with a parallel chord

And as you found, a taper wing is harder to land as the model just looses lift so abruptly

Edited By Denis Watkins on 07/01/2019 15:59:15

Erfolg07/01/2019 17:32:06
11239 forum posts
1118 photos

As has been high lighted small chords, small models (wing area) tend to fly faster than you like. it is the V^2 that is the problem in W/S = 0.5 RoeV^2Cl(coefficient of lift). There are a few rearrangement about of this relationship. Generally a bit of speed is required just to balance the weight and wing area, just to keep flying, with the V^2 being there, all the lift can disappear in an instance when landing, with no amount of aerodynamic devices, preventing the sudden stall.>>

That is why most successful small models are light as can be. >>

I have tried the blunting of the wing LE, or that should really be well rounded. For me it did not noticeably work. As the triangular wing root idea suggests, sharp leading edges suffer from a sudden transition from a streamline type flow, to a very disturbed turbulent flow. Although I am sure it does work dependant to some degree on Re numbers etc., I am really suggesting broadish chords and reasonable aspect ratios.>>

I have tried wing fences, but they seem to need being large, to do anything on small models, at our speeds.>>

I have raised the ailerons, although this seems to work best with tip types.>>

I think why plank/solid small wings work OK, is that the thickness ratio is often greater than you at first think, approaching 10%. If the wing is tapered, this increases. Probably more importantly the camber increases. The one issue I have played with solid wings is where the LE radius is, how high and how well rounded. Both affecting camber and flow around the LE. I have noted that some chuck gliders have a sharp LE, although the thrower, has often used the stall characteristics in conjunction with the other trim settings to get a one speed glide. Sometimes these models do not do what was intended or seen in previous flights.>>

It would be interesting to know how small we are talking>>

J D 807/01/2019 17:46:35
1197 forum posts
74 photos

You could try adjusting both ailerons so they are up a bit in the center position,sort of a poor man's washout.

I have an aircraft [ OS 15 power ] with dihedral on a plank wing [ no washout ] but has similar symptoms to your's Just have to fly straight in and not muck about on landing. Wing section [ small leading edge] and a high loading I recon are the reasons.

It isonly rudder elevator but with that wing slap the rudder hard over you can do flick manouvers like nobody's business.

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