Tip-stalling

[Bandit]

I have a Comet DH88 which is an awesome looking model, large too I think the span is about 7foot.

Trouble is flying it is always a risk which bothers me as I'd like it to see a life of flight instead of hangar rash.

I was wondering if anyone had come up with a simple way to held stop the dreaded tip stall, it's only really a big issue at landing.

I was thinking of a wing fence of clear plastic just to stop the air flow disappearing off the end of the wing, I have fitted counter rotating props and wondered which way would be best that they turn.

The fish tailing thread inspired to bring up this subject and the input there was great!

[Chris Bott - Moderator]

On the counter rotating props Bandit, run them towards the fus at the top. This means that if you lose one side, the torque reaction from the remaining one is in opposition to the rolling effect of the asymmetric thrust. This is a bit better than having both effects rolling the plane in the same direction.

The tip stalling is a function if the hugely tapered wings. With mine, I've come to the conclusion that any changes, such as washout, could cause as many problems as it might solve. So I've decided to live with it and fly it accordingly. As you say, landings have to be quick, but then you get the danger of a bounce followed by an uncomfortable moment of slow flight. Perfecting that perfect bleeding off of speed just above the ground is a great challenge though.

Have fun, it certainly is a great looking aircraft.

[Bandit]

Yes I had a bounce on landing.

Scared the daylights out of me

I'm glad you mentioned washout as one of the club members mentioned it. There's nothing I can do about it anyhow I thought as the wing has to have washout built into it.

I was also wondering about differential aileron control might help.

Edited By Pete B - Moderator on 19/11/2013 11:59:51

[Chris Bott - Moderator]

Adjusting both ailerons so they are slightly up may give a washout effect. But washout also has an effect similar to reducing wing area, so the wing loading effectively increases, therefore the likelihood of stalling increases. A catch 22 really.

Differential aileron is to reduce the effect of drag from the down going aileron which causes adverse yaw.
With tip stalling It tends to be the down going wing in a turn that stalls first. This is the wing with the up going aileron, so I'm not sure that differential would help with tip stalling?

[Dave Miller]

Would it be possible to mix in a little up aileron with the lowering of the landing gear?

[Ady Hayward]

If the plane is not going to be doing aerobatics I would try 100% differential so the ailerons only move upwards. This at least leaves you with the best wing profile at the tip.

Ady

[Bandit]

I see Chris's point about the differential.

The DH88 seems to only really suffer when landing so both wings should be going the same speed but then I see your point Ady as you would be using aileron on occasion to keep the wings level and like you sqay best to keep the best wing profile at the tip.

I think I might try differential to some degree next flight.

[Bandit]

I am sure it is possible to mix some up when the gear goes down Dave, would that be like adding washout?

[Biggles' Elder Brother - Moderator]

Differential ailerons might help you a little with tip stalling - although they may be more beneficial in other regards. The argument being that a down-going aileron causes a local increase in angle of attack - but Chris is right it tends to be the inner wing in the turn that "let's go" first - and that will have an up-going aileron - so not much assistance there!

I'm afraid there really isn't very much you can do. As you say an attempt to create true washout (post building) is going to be very difficult and you are just as likely to end up with uneven twisted wings and that's never going to help.

A little reflex (both ailerons slightly up) could possibly help - as Chris says it does reduce effective wing area - but I would have thought that effect would be fairly marginal TBH. The main impact would be a slightly increased flying speed - but then as fast landings are part of your problem I guess that isn't too helpful either!

Wing fences would help - on the upper surface aimed at straightening the flow across the top of the wing and eliminating the cross flow component. But to really work I think you would need two or three on each wing and isn't that going to be something of an eyesore - even made of transparent plastic?

Does the model have flaps? If not it may be worth considering them. Or if it has them ensuring they can deploy to a very high angle - 70 degrees plus. Flaps could help indirectly in that, while they will not reduce the tip-stalling tendency, they will help you avoid straying into the territory where its going to happen! To guarantee no tip-stall you must keep the nose down all the way through the approach - its got absolutely nothing to do with speed - its all about angle of attack. No aeroplane ever stalled without the elevator stick being pulled back! Slightly nose down in attitude you cannot stall - its impossible! But of course you gain unacceptable amounts of speed that way. Deeply deployed flaps will help you to commence the approach from a higher altitude cross wind, make the final turn a descending turn and to hold a nose-down attitude right upto the flair, without gaining unacceptable levels of speed.

Other than that I'm afraid its just nose-down and keep the AoA low or negative and cope with the resulting airspeed! It in the beast's DNA - so it can't really be "cured" short of redesigning it!

BEB

[Chris Bott - Moderator]

Good point about the flaps BEB. Yes it does have them and mine go down a good way.

They certainly help, allowing a nice steep approach without speed building too much. The problem I had then was speed bleeding off quicker than I expected once we're flying level in the flare, just above the ground.

I guess it's just a practice and judgement thing. Trouble is, it has to survive the learning phase!

[Dave Miller]

Posted by Bandit on 19/11/2013 13:21:22:

I am sure it is possible to mix some up when the gear goes down Dave, would that be like adding washout?

It would, but the key here is in the word "little" for the only real solution is exactly as per the full size and that is as BEB has stated and learn to handle the faster landings that this craft is famous for.

[Bandit]

Confession here, I didn't think about flaps and didn't use them.

I shall be checking angles tonight

[Peter Christy]

A couple of points here: The DH88 along with some other DH designs (Mosquito, for instance) have the entire fin and rudder ABOVE the fuselage. This means that when held in a nose-up attitude for a 3-point landing, the fin and rudder are very much in the wake of the fuselage, which will reduce their effectiveness considerably!

This certainly won't help with "tip-stalling"!

Whilst tip stalling will be an issue with such a sharply tapered wing, its just possible that what you are experiencing is the fin and rudder becoming ineffective in the wake of the fuselage!

I would suggest that a "wheelie" landing rather than a 3-pointer would help enormously with both issues!

Back in the 60s, the Americans used to run what they called "Class 2 Aerobatics", a schedule for aeroplanes with only rudder, elevator and throttle controls. There was a design called the "Aristo-Cat", which featured a large sub-fin under the fuselage, and a rudder that extended all the way from the top of the fin to the bottom of the sub-fin. Despite having very little dihedral, this model would roll beautifully on just rudder. I had a locally produced version of it, and after a couple of seasons I added ailerons - only to discover that it STILL rolled better on the rudder than it did on ailerons!

Ever since then, I've been a big fan of models with substantial amounts of rudder down to the bottom of the fuselage at least! Something the DH designers in the 30s and 40s should have considered, perhaps?

--

Pete

 

 

Edited By Peter Christy on 19/11/2013 14:41:11

[Bandit]

Trust me if you try a three pointer with the dh88 I have you have already tip stalled.

It would be great to 3 point it but there is no chance of that. I guess as it comes time that I look to flying it more often I am going through all the possibilities in my mind with help from here to make sure I have thought everything through properly.

I find because the gear is fairly far forward of the CG it tends to lift off again (bounce) just after it touches down so the speed has to be just right, not too fast nor too slow

Flaps seem to be the key, fast in downward nose, slowing quickly once on the ground hopefully.

[Seraph]

Can anyone explain why it should be the inside wing which stalls first in a turn? Since the outer wing has a down-going aileron and, therefore an increased angle of attack I should have thought the plane would tend to fall outwards.

[Bandit]

Inside wing is going slower.

[Chris Bott - Moderator]

Seraph, my take on the inside wing dropping may be completely wrong but it's this:-

For a start, once a turn is established the ailerons will be centered, otherwise the bank angle will just keep increasing.
In a steady turn the inner wing will be travelling slower than the outer one. Because of this, there will be less lift on the inner wing, so the model continues to roll. To stop this roll we apply opposite aileron, and this is the point where the inner wings angle of attack becomes too high.
The chain of events is then that drag is increased because of the down going aileron, the wing doesn't follow our command input because of both the aileron drag and the loss of lift due to reducing wing speed, so we apply even more opposite aileron.

Before we know it, this has all added up to one wing pointing at the ground, and a complete loss of aileron control.
We can even add to the woe by pulling in some up elevator at any point in the event.

[Bandit]

Ah now I am beginning to see why differential wouldn't help much at all as it would slow the inner wing even more.

[Shaunie]

I can see the advantage of differential would be this:-

In the situation where one wing is low for whatever reason and the pilot attempts to lift the wing by opposite aileron, the inner wing is slow as well as low, down-going aileron will will increase the effective camber and AoA of the low wing which will increase the likelihood of stalling. With lots of differential then the up-going aileron on the opposite side will first depress the high wing and the aircraft will roll about its CofG thus raising the low wing, secondly the increased drag will cause a yaw towards the high wing increasing the speed of the low wing and thus reducing the risk of stalling.

So it probably won't help when entering the turn but when leveling up or countering gusts it may make all the difference.

Shaunie.

[Rick Tee]

You could try turbulators, 3mm - 4mm wide strips of PVC tape 2 - 3 layers, 100mm - 200mm long should do. May take some experimenting to find the best position, start off close to the wing tip 15% of the chord from the leading edge. I've had some success reducing tip stall with them.

[Bandit]

Would it be possible to see a picture of your turbulators Rick?

[Rick Tee]

Sorry I don't have any atm, the model they were on erm bit the dust a while ago. A quick google of turbulators should reveal a few images. Just stick 3 layers of pvc tape together to the length you want and cut a 3mm - 4mm (roughly) wide strip with pinking sheers.

[Dane Crosby]

Hi. Another remedy, although not to scale, would be to add leading edge slots. When correctly designed and installed, they increase their effect as alpha increases. By re-energising the boundary layer more lift is supplied. These may not look as obvious as the turbulators might. Fit the slots to the leading edge and make them the similar length to the ailerons. Leading edge plaps would do the same trick but the engineering is a bit complicated. Fixed slots would be better I reckon.

[Simon Chaddock]

Bandit

One relatively simple trick to achieve a modest washout effect is to rig both ailerons with their trailing edge raised by a small amount as it reduces the effective incidence (and lift) of that portion of the wing so the tip is less likely to stall before the root.

[Bandit]

I like the idea of the trailing edges up a bit and I also like the idea of turbulators.

Would be good to hear input from others on the subject because they are handsome planes and would be good to keep them looking just as they are, cant see turbulators or lifting the trailing edges a bit would change the look.