Tip-stalling

[Piers Bowlan]

Hi Bandit, Reflexed ailerons may help, as will flaps to some degree, although they do have the effect of moving the centre of lift inboard and therefore making the aircraft laterally less stable. Not so good.

Wing washout is the real solution but short of building a new wing this is not much help to you.

Have you considered fitting a gyro to the rudder channel?

At the point of stall if the aircraft is not in balance (skid or slip) it will tend to drop a wing in the direction of the yaw and auto-rotate into a spin if not checked. Many factors can cause the aircraft to yaw, from adverse yaw caused by the ailerons, slipstream and lateral balance to name but three. However if the aircraft is in perfect balance it should stall straight ahead, even in a turn. However in reality very rarely is the aircraft in perfect balance. Other factors that can affect this behaviour is whether the aircraft is climbing/descending, turning, C of G position, configuration and power. The DH88 does not appear to have a very big rudder so perhaps loss of rudder authority at slower speeds may exasperate the loss of directional stability and so a tendency to yaw in the critical landing phase. Fitting a bigger rudder to a graceful scale model is not an option but a gyro to help tame this wing dropping tendency might be of help. When all else fails keep your speed up! Just my 2p worth.

[Bandit]

Hi Piers yes a gyro might be an option.

Up till now I haven't been a fan of gyros but might be worth considering and adding that the reflex moves the lift inboard is interesting, never heard about that before.

I hope to assemble the Comet again this weekend and sit and have a good think about it, check everything and see maybe I had something wrong.

The reason I started the topic was as a result of it stalling in the air and I thought it was going fairly quickly.

The landing was ok with just one bounce.

[Biggles' Elder Brother - Moderator]

Posted by Bandit on 22/11/2013 09:46:56:

The reason I started the topic was as a result of it stalling in the air and I thought it was going fairly quickly.

It is really important to the survival of your model that you stop equating speed with stall-safety! As I have said many times before speed is no protection whatsoever against stalling - the aeroplane will stall perfectly well - just the same - regardless of how high your airspeed is.

Stalling occurs because the angle of attack is too high - not because the airspeed is too low! The "stall inducer" on your Tx is the elevator control - not the throttle!

As an old lecturer of mine was fond of observing "no aeroplane ever stalled with the stick forward". Or as another author (name I can't remember!) said "if every aircraft's control column was fitted with chain from the top to the instrument panel that prevented the pilot from using up-elevator - then no aircraft would ever stall"!

BEB

[Dane Crosby]

Quite right BEB. An aphorism i was told during flying training was "pull back gently, go up. Pull back more, go down!"

[Simon Chaddock]

"The reason I started the topic was as a result of it stalling in the air and I thought it was going fairly quickly"

.At the risk of being a bit contentious I think the important bit is "I thought it was going fairly quickly"

The human brain is pretty good at doing complex relative speed calculations (i.e. catching a ball) but poor at judging true speed.

Thus when stationary objects are in view (landing approach?) the relative speed tends to swamp the more difficult actual speed calculation leading to the situation that up elevator is applied when the wing is unable to accommodate it with the inevitable result.

As BEB points out monitoring the angle of attack of the wing (and it takes practise to do it well) is the only sure way to keep out of its stall region.

[Chris Bott - Moderator]

Another good trick that I used to tame a hand launched spitfire I had, was purely and simply to reduce the elevator throw. This had the effect of making it much harder to quickly increase the angle of attack, whatever the speed.

It also had a secondary effect of making the model much smoother to fly, and didn't really have any downside.

[Piers Bowlan]

Yes an aircraft can stall at any speed and in any attitude. Is this helpful and stop Bandit writing off his DH88 - Er, no!

BEB has the answer- don't move the elevator. Job done!

[Biggles' Elder Brother - Moderator]

Posted by Piers Bowlan on 22/11/2013 22:32:44:

BEB has the answer- don't move the elevator. Job done!

I tell you - on this model that might be very good advice!

BEB

[Piers Bowlan]

On a more serious note Bandit, if you have noticed some squirelly behaviour in flight then I assume that you have a reasonable amount of differential dialled in to your ailerons either mechanically or on your Tx? Aileron differential will counter adverse yaw if it is present. Also, have you tried some coupled aileron and rudder? Another thought is to try moving the C of G forward a tad (technical term), this can have a secondary effect of improving the yaw-roll couple (although it will improve pitch stability at the expense of elevator authority). I think someone mentioned counter rotating propellers as well, - this is worth a try if not very scale. In fact anything that reduces the aeroplane's tendency to yaw will be helpful in reducing the naughty wing dropping tendencies that the aeroplane may exhibit.

Nice smooth flying is helpful. Don't over bank on the base turn and, like I said, keep your airspeed up. Like my (full sized) flying instructor said to me many years ago whilst discussing false landings, 'it is better to over run the landing strip at taxying speed than to stall the aircraft in on the approach. The former you will probably survive, the latter you won't!'

As for people who can 'see' the pitch attitude of a model aeroplane when it is on the approach, - they must have better eyesight than me!

[Julian Thacker]

Another great thread (alongside the fishtailing one). FWIW, I have had two models with extreme tip stalling tendencies - a Kyosho Cap (many years back) and an EFlite 70mm EDF Hawk (known for its unfriendly tendencies unless flown with great caution in the landing phase). Both were transformed by mixing in slight aileron reflex with up elevator. On both this could be switched in and out, partly because I was concerned what the mix effect might be. Eventually I left the mix permanently activated as I could never detect any negative effects and the positive effect on reduced tip stall tendency was dramatic. I used about 2mm reflex on the cap and about 1.5mm on the Hawk.

I should emphasize both models could still bite, but they were changed from frankly nerve wrackingly unpleasant machines to land to pleasingly challenging ones.

[J D 8]

Posted by Chris Bott - Moderator on 22/11/2013 17:40:09:

Another good trick that I used to tame a hand launched spitfire I had, was purely and simply to reduce the elevator throw. This had the effect of making it much harder to quickly increase the angle of attack, whatever the speed.

It also had a secondary effect of making the model much smoother to fly, and didn't really have any downside.

Did the same with a Seagull SNJ [NAVY AT6] previous owner found it a bit of a pig and when I first flew it,found that one had to be very gentle on the elevator or a tip stall/flick would happen at any speed !

[john stones 1 - Moderator]

Posted by Dane Crosby on 22/11/2013 12:30:27:

Quite right BEB. An aphorism i was told during flying training was "pull back gently, go up. Pull back more, go down!"

spot on

on a tippy model its elevator that kills you

[Rick Tee]

I know this thread is old but Bandit did ask for a picture of turbulators and as I now have some fitted to my Seagull Extra 300s I thought I'd pop a pic in here:

[Colin Leighfield]

As this one has come back, I will speak up for wash-out. I appreciate that once a plane is built it isn't a very practical option to introduce it and although simply putting in some upwards reflex on the ailerons might help a bit, I don't think that it is an ideal solution. If I was building a DH88 from scratch I would not think twice about it, I would put in 3 degrees of wash-out, progressive from root to tip. I don't think that tip-stalling would be any issue. Neither do I go along with the idea that wash-out will increase the stalling sped. At any given speed in level flight the lift needs to balance the weight. If you have a wing with an angle of attack of say 1.5 degrees, then build an identical one with 3 degrees at the root and 3 degrees of wash-out, it will also have a mean angle of attack of 1.5 degrees. The stalling speed will almost certainly be the same, but the tip-stalling risk will have been greatly reduced, if not eliminated. At any equivalent speed in level flight, each wing will need to provide the same lift.

I have tested the effectiveness of wash-out in the design of my XB42 Mixmaster. It has 3 degrees from root to tip. With a tip chord of 50% of root chord and all of the taper in the swept leading edge, there is not the slightest evidence of tip-stalling. Also, it has a very low-stalling speed, allowing gentle landings onto the sub-fin of the cruciform tail. While others might not agree, I have thought for a long time that a progressively washed-out wing must have a beneficial effect on the span-wise air-flow that is normal and controlled on modern airliners by winglets. Correctly designed, I think that a wing with progressive wash-out may be superior to one without it. I accept that I might be an idiot, but I'm happy all the same.

While on the subject and referring to some of the discussion on this subject related to flat-plate wings in the Tucano build-threads, I have found that the Seafang, with a Tucano style flat-plate wing, double taper and no wash-out, has absolutely no tip-stalling tendencies. Slowed right down, it eventually stalls straight ahead and recovers easily. Although Nigel's original design had reduced wing taper compared to the full-size, when altered later to reproduce the scale taper, I understand that there was no sign of tip-stalling. This appears to be a characteristic of flat-plate wings that hasn't been explained, though I can guess at the reason. Certainly at sizes up to say 50" span, they seem to have a lot going for them, whatever the purist in us (me included) might otherwise think about the apparent contradiction of our conventional thinking about the importance of wing section design.

[Piers Bowlan]

Totally agree Colin, nothing like a dollop (about 3 degrees) of wash-out to sort out a recalcitrant tapered wing from tip stalling. Modern airliners - don't their wings all have wash out? As for winglets, aren't they just to delay the formation of wing tip vortices and therefore reduce induced drag (so they burn less fuel - ££££££).

Flat plate wings - they work, but is it because of the low Wing loading of these models (no landing gear or heavyscale finish etc.) or because of the low Reynolds number ( fleas fly but elefants don't) ?

Looking forward to your Seafang being published Colin, looks very nice.

 

Edited By Piers Bowlan on 11/10/2015 03:03:37

Edited By Piers Bowlan on 11/10/2015 03:09:33

[onetenor]

Fast flying planes need to fly FAST and that includes taper wings and swept wings et al. When I was learning to fly I was told that some aircraft have to be flown onto the ground (or Carrier decks ---- them in particular   Watch planes landing on a carrier.They are flown on fast for just the reason to avoid tip stalling .They are literally dumped on the deck. It is not absolutely the same for land planes but it is pretty close. The undercart on the DH88 is fairly well forward of hte Cof G and I think it was designed that way for that very reason. To stop nosing in during wheeler landings. and with some power still on. Practice touch and go's to get the feel is the best advice I can give. Good luck.

John

[Colin Leighfield]

Hi Piers and onetenor. All good stuff! As I understand it the reason for wing tip vortices is that because airflow moves span wise as well as flowing backwards it leaves the wing tip at an angle and that is what causes the vortex. Carefully designed winglets direct it upwards and outwards in a controlled manner so that it leaves the wing in a much tidier fashion and reduces drag. My thinking is that progressive wash-out must have some effect on the outwards movement of the air-flow and perhaps reduces the tip vortices a little as well.

Overall, that leaves me to think that there is far more benefit from having wash-out than disadvantage and the point about a modern airliner wing is spot-on, I'm sure. The shape of the wing progressively washing out towards the winglet gives a very high degree of control of the direction of airflow over the wing and that must be what it is all about. It isn't just about what happens at the stall, it affects the efficiency of the wing at all flying speeds. The only way to find out on a model is by doing it.

The flat-plate wing is an enigma because at these sizes it seems to work amazingly well. I can't see that the Hawes Tucano or this Seafang would fly any better if the wing was built up. I have wondered that because the constant thickness of the wing from root to tip combined with reducing chord means that the thickness/chord ratio is progressively increasing and perhaps having some improving effect on the lift (coefficient)? as you move outwards? The effect must be small, but maybe it is all that you need to stop a tip-stall. BEB might shoot me down with a far better explanation, but I can't think of anything else.

[Piers Bowlan]

 

Yes Colin, it is the span-wise flow that causes the vortexes, not only at the tips but also all along the trailing edge of the wing, although it is at the tip where most of the drag is produced. The wing produces lift and the by-product is induced drag. Induced drag can not only be reduced with washout (improving the lift drag ratio) but also with a higher aspect ratio. Wash-out tends to reduce lift at the wing tips to produce the optimised lift distribution (elliptical). This also ensures that the root tends to stall before the tips to produce a benign stall characteristic. The Spitfires classic elliptical wings were very low drag because of their plan form but did not have any washout (contrary to what Wikipedia says!), hence it's vicious stall and departure into a spin for the unwary pilot. Washout however may increase the stalling speed to some degree for any given wing loading, depending on the wing section used. On balance the main downside to washout is just the extra complication and possibly weight of building a twisted wing, whether it be model or full sized.

Richard Whitcombe developed winglets in the seventies to reduce the size of the tip vortex and hence induced drag. The winglets on a B747 reduce fuel consumption by 1% per winglet, so on a flight LHR-HKG you are talking about nearly two and a half tonnes of fuel saved. You could probably get the same effect by increasing the wingspan but it would make the aircraft more tricky to park! For future Blended Wing Body designs they are talking about folding wings. I would not have thought that winglets would have any effect on tip stalling but I could be wrong.

I think you are spot on Colin with the flat plate wings and their lack of tip stall. The wing section is thicker at the tips relative to the chord so the wing root stalls before the tips.

 

 

 

 

Edited By Piers Bowlan on 11/10/2015 14:10:44

[Piers Bowlan]

Just found a very authoritative piece about the Spitfire's '2.5 degree washout'. Ops! What do I know? (Not a lot!)

Edited By Piers Bowlan on 11/10/2015 14:58:54

[Dave Bran]

Posted by onetenor on 11/10/2015 04:56:15:

Watch planes landing on a carrier.They are flown on fast for just the reason to avoid tip stalling .They are literally dumped on the deck.

Planes are flown fast(Ish) onto carrier decks as they have to be going quickly enough that if they do not catch the wire or the wire breaks they can lift out and go round. They actually land with the throttle opening or their jets engines would not spool up soon enough.

It has little or nothing to do with Tip Stalling.

[Ben Kenobi]

Confirmed, nothing whatsoever to do with stall, speed/power is maintained to ensure sufficient power / energy to climb out if they miss the ropes - otherwise they'd need to land below the stall speed of the aircraft to prevent flying off the end should they miss.

I'd actually like to see the term 'tip stalling' abused a little less - it is much abused in my opinion, limited to only certain profiles of wing (sharp leading edge and low thickness to chord ratio i.e. swept back wings) - other than that the wing is either stalled or it isn't - many people repeat the terminology without trying to understand it. There are some wings such as the Cirrus SR22 that have a different profile on the outer wing than the inner to help keep the ailerons alive at low speeds - makes it more forgiving but it is still possible to stall.

How many slow down too much on downwind legs and stall on turn to base leg as a result - this isn't a tip stall it is a stall full stop, perceived 'ground' speed versus actual airspeed - more pronounced with strong tailwinds which will compound the issue when turning onto the base leg - they're already close too or stalling before they make the turn, the turn stalls the inside wing because the airspeed is too low, by which time many are too low to recover or try to recover using aileron or elevator - ailerons on a stalled wing don't work and elevator makes matters worse by scrubbing even more speed - power is the only solution. The other stall is the one caused by drag from 'excessive' aileron slowing a wing that was already close to stalling anyhow - but again this is a stall full stop.

Higher wing loads need higher speeds to prevent stall is how I'd put it, the winglets thing if I understand it correctly keeps the wing tips flying instead of allowing the air to spill off the tip creating both drag and reducing lift in that area, nothing to do with 'tip stall' - as I understand it. I'm sure that I read somewhere that the A380 wings would need to be 30% longer without the winglets and it would burn more fuel.

I could of course be totally wrong, I think it would be nice to have a kind of Wiki forum / section on such topics.

[Don Fry]

Many years ago I used to have a drink occasionally with a bloke who flew Mosquitoes in the Second World War, lot of experience, a couple of years as a reconnaissance pilot and instructor. I was drinking with him one day when we heard that a Mosquito had crashed (at an air show?). His fairly instant verdict, and he was right, was with that wing shape, you did not change direction close to the ground, never side slipped, mess about, or try to do anything else but put it down at speed, straight and level, on its main wheels. If you can't do that, go around again, or die. Not a quote, but fairly close to word for word. Stuck in my mind because he was fond of them and liked them.

[Colin Leighfield]

Piers, yes, the Spitfire had +2 degrees incidence at the root and -1/2 degree at the tip, giving wash-out of 2 1/2 degrees. The talk about the Spitfire having a vicious stall is a total myth, almost certainly originated by models of Spitfires built without wash-out. The best reference on this is Jeffrey Quill himself. He describes how you could keep pulling the stick on the Spitfire back until it just stalled, the nose would drop and it recovered straight ahead. Interestingly, the Hurricane did drop a wing at the stall. The combination of wash-out, elliptical shape and thin section make the Spitfire wing exceptional and almost certainly were factors in relatively low tip vortex losses. It turned out to have a better transonic performance than the laminar flow wings on the Spiteful or P51 and the head designer of North American later said that the high level speed of the Mustang wasn't because of the laminar flow wing, but the very efficient ventral cooling system designed on Meredith principles gave positive thrust worth 35 mph.

Dave, you're right of course. Eric Brown also said that against his advice the Fleet Air Arm after the war decided to change from the previous system of flying the plane onto the carrier deck to the American practice of pulling back hard on the stick and stalling it on to make sure that it stayed down. It was the straw that broke the camel's back for the Seafire and was responsible for the fuselage wrinkling problems with the Seafire 47 that shortened its' service life, in spite of a change to stainless steel for the fuselage longerons.

Ben, I think you are right. Tip stall is only genuine if a wing drops when the plane is flying straight ahead, if it stalls in a turn the lower wing is almost certain to drop regardless. Winglets are all about reducing drag and increasing fuel efficiency, although they will certainly contribute to a reduction in any tip stalling tendency.

Donald, it sounds as if you are bang on about the Mosquito, a wonderful aeroplane that could bite at low speed and was particularly risky in engine out situations, when I believe the advice was to bale out rather than try to land. Anthony Wedgewood (Tony) Benn's elder brother Michael was a top Mosquito pilot. In 1944 he was killed at Thorney Island. He had an a.s.i failure and because of the concerns about the risks associated with landing the Mosquito at too low a speed, came in deliberately fast to avoid the risk of slowing down too much without any speed indication. He got down all right but ran off the end of the runway. I'm not sure what stopped him, but it caused the seat armour to break away and severely injure him, he died shortly afterwards.

[ron evans]

Some years ago I read about a modeler who helped cure a tip stalling model by giving a portion of the wing section at the root a sharp entry. It would be simple to try a temporary fix, although if it did work the stall speed would probably be increased, but at least it would be straight ahead.

Ron

[Colin Leighfield]

Ron, perhaps that has a similar effect to the horizontal strips on the leading edge of the Chipmunk on the inner section of the wing, designed to disrupt the airflow at sharp angles of attack to help ensure that the root stalls before the tip and also give some stall warning.