Tip Stall

[Mike Morcher]

Hi

I am building a GWS P40, the kit looks good but I hear that when flying it can tip stall.

Question, will feeding in some flaparon help on the stall?

I will be fitting a 2217 motor so it should have some kick to it.

> >

Many thanks

Mike.

[James40]

Do you have flaps on this model ?

 

I should imagine a flaperon will induce a tip stall rather than prevent it?

[Biggles' Elder Brother - Moderator]

James is right, one of the worse things you can do in a tip stall is lower the aileron - it just increases the effective angle of attack of that wing and deepens the stall!

 

Change of wing section and/or washout are the main building tricks to try to avoid tip stalling.

 

BEB

[Tim Mackey]

I agree with James - those outboard ailerons will likely make things worse if you set up for flaps, effectively introducing wash in.

[Eck]

Try setting the ailerons so that, at neutral, the trailing edge is raised by approx 3mm. This adds washout to the tips and helps prevent tip stalling.

[Mike Morcher]

Thanks Chaps

Yes I wondered if it would be wash out, can I bend and set the wing to give it some wash out, or try Eck trick on the ailerons?

 

[Biggles' Elder Brother - Moderator]

You could bend it - but you'd have to get the same bend in each wing - that could be tricky! Washout is usually incorporated at build time.

 

The slightly upward set aileron idea from Eck sounds more practical.

 

BEB

[Chris Bott - Moderator]

Mike I've been through this with another model and tried all those tricks. What you end up doing is messing with a design that's already been thought through.

I think the biggest issue with tip stalling is being so scared of it that it affects your flying.

 

Just don't slow right down as if it's a 3D model and you should be OK.

Keep the speed up a little, not drastically, in the turns.

Large inputs of elevator can also provoke a tip stall, so with a model like that I tend to set it up so there is only enough elevator movement for scale manoeuvres.

 

Someone like BEB will explain why too much elevator can make your horizontal wing suddenly roll to vertical. Something to do with G forces effectively increasing the wing loading I think.

[Alan Cantwell]

3mm up on the ailerons, go have fun i never use down for flaps, daft idea,

[Mike Morcher]

Well I think I will keep the speed up and try the up ailerons,

Its always fun with a new plane

Many thanks all

Mike.

 

[Biggles' Elder Brother - Moderator]

I think Chris has the right idea - live with it. The P40 doesn't have a really bad reputation for tip stalling - compared to some!

 

Picking up on Chris's last point. The problem is the elevator shouldn't really be called the elevator - cos its not for elevating! Some folks think that the purpose of the elevator is to make the model climb or decend - it isn't, that's what the throttle is for! The real function of the elevator is to control the model's angle of attack. Back stick - bigger angle of attack.

 

Above someone advised against trying to fly the P40 like a 3D model and advocated keeping the speed up. Good advice - but its not the full picture. The time when a tip stall catches most pilots out is not when the model is flying slow (they know the dangers then and are careful). Ironically its when the model is flying very fast!

 

You're bowling along really fast, you pull a high-g turn. In this turn the wings have not only to support the weight of the model, as they do in level flight, they now have to support the G-force induced weight as well. This G-loading could easily be 2-3 times the weight of the model. Where does the extra lift come from? Easy, you pull harder on the stick "to hold the model in the turn" - but what you are really doing is increasing the wing's angle of attack to get more wing force to support the increased "weight". In short the model goes round the turn with its nose pointed inside the line of flight - to keep the angle of attack up. Its under these circumstances that the tip stall is most dangerous.

 

Suddenly one wing "lets go" - simply because the angle of attack requested is too high. Result - the plane flicks out of the turn and plunges earthward. "Radio failure" some will shout...."Mechanical failure" from others. The fact is it was a high speed stall pure and simple.

 

The moral is beware high angles of attack were you don't expect to see them and remember what causes a stall is not low speed - its excessive angle of attack, so a stall can happen at any speed if you are pulling a lot of elevator - hence the wisdom of Chris's remarks about limiting elevator travel on this kind of model.

 

BEB

Edited By Biggles' Elder Brother - Moderator on 11/03/2011 13:43:26

[Mike Morcher]

Thanks BEB

That seems to make sense; we will fly it and see

All the best

Mike.

[Steve Houghton 1]

That was interesting BEB, I shall try to remember that.

[James40]

I'm trying to locate the video clip from the RedBull air races where he has a high speed stall going through a chicane type obstacle.

[James40]

[James40]

Notice he has full right aileron input during the stall but it doesn't respond until just before impact where luckily the wing starts working again and he gets it wings level JUST in time!

[Biggles' Elder Brother - Moderator]

Nice example - I bet that was the longest 3 seconds of his life!

 

BEB

[John Privett]

Posted by Biggles' Elder Brother - Moderator on 11/03/2011 13:42:13:

The moral is beware high angles of attack were you don't expect to see them and remember what causes a stall is not low speed - its excessive angle of attack, so a stall can happen at any speed if you are pulling a lot of elevator - hence the wisdom of Chris's remarks about limiting elevator travel on this kind of model.

 

 

If people remember nothing else from this thread - please remember what BEB wrote above.

[Tim Mackey]

Isnt it fair to say that its actually

"an excessive angle of attack relevant to the air speed.".... that causes a stall?

EG: if flown slowly enough, a wing can stall even though the only angle of attack is that which is built in through incidence, or airfoil shape, and to all intents and purposes the wing is "flat".

Not being clever...just wondering BTW

[John Privett]

Tim, I'm not sure quite what you mean by "relevant to the air speed"

 

The angle of attack is the angle between the wing and the airflow, the airspeed is pretty-much irrelevant - though I suspect there may be minor differences in the angle at which the stall occurs at very high or very low air speeds. The important point is that when the angle of attack reaches approximately 15 degrees (exact figure depends on wing section) then the wing will stall - and that happens whether the plane is doing 20 mph or 120 mph.

 

[Mike Morcher]

Ok Chaps

You are getting away from a tip stall and looking at the whole

wing stalling not just the tip. A full wing stall can be very benign.

It’s the design of the wing which will give it a tip stall ?.

 

 

[Tim Mackey]

Posted by John Privett on 11/03/2011 23:58:18:

Tim, I'm not sure quite what you mean by "relevant to the air speed"

 

The angle of attack is the angle between the wing and the airflow, the airspeed is pretty-much irrelevant - though I suspect there may be minor differences in the angle at which the stall occurs at very high or very low air speeds. The important point is that when the angle of attack reaches approximately 15 degrees (exact figure depends on wing section) then the wing will stall - and that happens whether the plane is doing 20 mph or 120 mph.

 

 

Fair enough, but the actual result will be different wont it? - in so far as at 120 mph, the 'plane will likely continue to "fly" (as in move forward through the air) due to inertia etc. Whereas at very low speed, it is more likely to simply drop and crash. I think its this scenario that alarms us, IE we see a model flying low and slow, and the AOA is increased to the point where it no longer lifts - and it drops like a stone.

Surely at a higher speed for the same given AOA the lift produced is higher and therefore the model will continue to fly?

....thats where people get confused between airspeed and AOA as being the cause for the stall?

Edited By Tim Mackey - Administrator on 12/03/2011 08:24:56