Hi

I read many explanations about this stuff.
The more common one talkes about the side-slip
which causes the A/P to roll back

But when we fly 2 channel glider with wing dihedral
in order to turn we create yaw (side-slip)
by the rudder --->> dihedral translate it to bank (roll)
into the turn !!!

So how roll-back moment is created by the dihedral when having
gusts?

If I understand properly what "roll-back" means ( English is not my first langage), here is how dihedral works: In straight and level flight, both wings have the same projected surface, which is smaller than the actual surface. Under gust, the lower wing being closer from "flat" has a greater projected surface, hence generate more lift, so the lower wing goes up....That's simply how it works, and why dihedral give roll stability.

In a turn on a 2 channel, you increase the external wing speed, thus increasing the lift. As soon as you let go on the rudder, the plane retrieve straight and level attitude by itself for the same reason.

Bernard.

That's pretty much right Bernie.

But just to clear up the language a little the wing will only return to level when there is a slide slip component. In a turn when we return the rudder to straight the model is left side slipping a little so it returns to level. When a gust tips the model it side slips again and so the dihedral returns the model to level.

The idea of the projected area has been used for a long time but if you think about the wing always producing lift striaght up from it's upper surface then anytime the model tips it HAS to be side slipping. The only time this doesn't happen is when the rudder is displaced to compensate for the bank.

All these explanations are for steady state. Obviously when you first move the rudder or a wind gust picks up a tip there is a few seconds of transition from one state to another.

All this is hard to see on a poly glider because the dihedral has such powerful and quick response but if you ever fly a 4 channel trainer model and try to do your turns with the rudder and no aileron you'll see the small amount of dihedral exposes all these transition conditions much more clearly.

Not sure i agree here. Sideslip, as you say, is a result, not a cause. If you put ailerons on a wing with quite a lot of dihedral, in a perfectly (and theoretical) calm air mass, and bank slightly, then release, the wing will go back to level, without any other sideslip than the one induced by the bank itself (Wing with more lift induce more drag than the other, hence "sideslip", normally compensated by the fixed rudder.)

Bernard

I think you're getting caught up in that transition time here. The wing lifts upward in a vertical line along the center line of the wing. If we roll the plane then the wing no longer lifts directly upwards. Instead the wing is now lifting to one side at the angle of bank. The resulting side component to the lift is going to accelerate the model sideways and thus we get our side slip and our dihedral induced righting force. Granted there will be a brief moment where the force has not yet accelerated the model but it's not long enough for anything to happen. Certainly the model doesn't snap back to level during THAT moment but rather after a 1/2 second or so when the side slip is well developed.

Other proof that there is a side slip happening when you bank like this is the fact that the nose of the model falls off into a spiral dive. This is the fin doing it's job as a result of the side slip. If there was no side slip happening then the nose wouldn't fall off into a spiral. Rather the model would just continue happily onwards with the wings tilted.

Take out fictitious model. You bank the wings with the ailerons then neutralize them. The model tilts then it starts to fall off into a diving turn (the first part of a spiral dive induced by the side slip), the model then levels the wings thanks to the side slip acting on the dihedral after some part of a diving turn has occured and the extra speed from the dive brings the nose up past level into a partial stall after which the model returns to stable flight with a oscillation or two. If there was no slide slip the model would not enter the spiral. Rather it would continue straight ahead.

Fair enough?

Sorry,but This is a totally wrong explanation :"In straight and level flight, both wings have the same projected surface, which is smaller than the actual surface. Under gust, the lower wing being closer from "flat" has a greater projected surface, hence generate more lift, so the lower wing goes up....That's simply how it works, and why dihedral give roll stability"

if you just rotate the wind axis, you will discover your mistake
(you can also rotate your viewing position...)


I have talked with some aerodynamic engineer at my company,
and he finally succeeded to give me a good explanation.

dihedral do not roll back the A/P !!!
it's just create a Croll_beta derivative (Rolling moment coefficient due to side-slip).
That means, It couples between yaw and roll.
Having a sudden gust, A/P sideslip and then dihedral causes
a roll moment into the turn !!!
what happens is that beta angle (sideslip) become alfa.
Dihedral actually elimnates sideslip by turning it into alfa angle,
and this is a real directional stability.
finally the A/P rolls back due to it's low C.G position,
this has nothing to do with dihedral...

This is similar to the longtitudinal axis, static stability is the ability
of the A/P to eliminate sudden increase in alfa due to gust
(negative pitch moment is created in order to drop
the nose down)

I will be happy to get some feedback, to see if my
theory-about dihedral affect is correct

B.T.W
I am an aeronautical engineer myselfe...

If it is, by what sort of miracle this happens to the whole wing in a turn ( projected surface smaller than when flat, hence less less lift), so much so that you have to compensate loss lift by a little elevator (increase lift) to not loose altitude ? if it's true for the whole wing, why would not it be for half of it ?

Bernard

Bernard,
In a coordinated turn (with no sideslip)
what makes the plane stay in it's bank angle
is the deflection of the ailerons.
the inner wing has lower airfoil camber (lower CL) due to aileron up, and the outer wing has an increase in airfoil camber
due to aileron down (higher CL)

if you live the sticks while in a turn,
low c.g A/P will roll back while others like EXTRA 300 (middle c.g)
will stay in it's current bank angle.

please explain in a more scientific manner what is wrong
with my previous explanation.

Obviously, the deflection of the ailerons induce the bank (not the turn), but, as consequence, if you don't want the plane to sink, you have to apply elevator to counteract for loss of lift.

I don't care a bit about the scientific langage, as if you can't explain and understand in simple terms, it means you don't have a good grasp on the concepts. In French "Ce qui ce concoit bien s'enonce clairement".

You should re-read my previous message, and answer to it, this answer do not address the point.

Bernard

ddekel, you say my explanation is totally wrong but then go on to pretty much repeat what I just said using a lot of fancy terms and force names.....


And as for your last statement I've flown a couple of low wing models that have some dihedral and the still show a tendency to roll back to level when disturbed or when the controls are neutralized at the end of a turn without actively leveling the model. A low CoG will certainly help to level the wings thanks to the pendulum action but I've got first hand proof that neutral or even high CoG's will self level also.

In these discussions, I usually find out that more than one phenomenon is at work, so what I say here is probably not the whole answer. Having said that, no sideslip or yaw angle is required for dihedral to encourage recovery in the roll axis, through the mechanism that BernieG mentioned ( projected area of a wing half ). To convince yourself, do the following thought experiment: imagine that you have a plane with extreme dihedral, say 90 deg., and it finds itself rolled to the left, 90 deg., with no yaw angle, sideslip, or aileron deflection. What will happen? In this extreme case, it is obvious that the right wing is generating no lift ( lift is the force in the opposite direction from gravity ), and the roll will tend to correct until the two wing halves generate equal lift. Before you start chiming in, I realize that the side force generated by the right wing-half will probably generate some sideslip and yaw, but those effects are not necessary for recovery. For those of you with engineering backgrounds, draw a little free-body-diagram with all the forces on it. My impression is that this phenomenon is the primary mechanism by which dihedral gives roll recovery, even though other effects are certainly present. Having said that, I'm overdue to go do some reading on this topic and get a more thorough answer.

Cheers,

banktoturn

Excellent ! I should have thought of that myself as example. Now, I am the first to admit that I forgot to mention, that, indeed, there is secondary forces, just like drag is a secondary force on lift, so that it seems certainly logical that a little more drag is generated by the wing gaining more lift, inducing a yaw effect, and the rotation of the plane around it's roll axis, when it recover, must have some secondary effect like sideslip or something like that. But, by definition, secondary effects are....secondary....

Bernard

This really isn't a valid example. It's impossible for such a model to fly straight ahead with all it's wing on one side.

Actually it would still respond with a side slip as with the lift centered on the horizontal panel and the weight very close to the fuselage the fuselage would immediatley begin to drop bringing the rest of the wing with it which would give you a side slip condition in any event........

FWIW, I too subscribed to the "classic" dihedral explanation for many, many years. But if you look at it in real life there is just no way for the unequal projected length factor (which I admit is still valid IF it could operate in isolation) to be the dominating condition for more than a few milliseconds just at the completion of the roll in action until the model accelerates into a side slip condition.

This is fun...... Can we debate the temperature of inter gallactic space next?????

OK, so now take a fully flat, symetric wing, with ailerons, apply ailerons, release. What happens ? You have a bank, no turn. what the plane do ? Following the degres of the bank, the plane loose altitude, more or less quickly. Why ?

I think you will agree with me that it is because the projected surface of the whole wing is smaller than normally, hence you need to apply elevator, or ailerons in reverse to fly straight and level ( lower) again.

Now, why is that, that something true for the whole wing would not be equally true for half of it ?

Or is there also another explanation for the need to apply elevator when you bank ?

Bernard

You are absolutly right ! But why the fuselage would fall down ( what you name sideslip) in the first place ? BECAUSE THERE IS NOT ENOUGH LIFT TO FLY STRAIGHT ! hence..... see above.....

Bernard

Bernie-
You're right on this. A model like you are describing here would indeed maintain the same bank but it would fall off into a spiral dive and change it's heading as it spirals down. The steepness would be determined by, I believe, the degree of bank and the resultant lift vector being aimed off to one side. It wouldn't just sideslip into the ground while pointing in the original direction. If it was shaped such that there was no roll resulting from other factors then it would keep the same bank angle all the way to the ground. I've had models built like you describe and they fly pretty much just as you're saying. But the spiral they fall into thanks to that bank is a product of side slipping.

Actually you can get spirals like this even with lots of dihedral. I've seen more than one free flight model where the fin area was too large and the center of gravity was too far back and it made the model spirally unstable. The model would fall off into a spiral dive and it would either keep the same size or, more usually, it would tighten the spiral into a vertical death dive and matchsticks would be the final outcome. And I'm talking about free flight models with LOTS of dihedral. But this is an extreme case. I thought you might appreciate the fact that you can have gobs of dihedral and still have a spirally neutral or unstable model. And trust me, it's a sad, sad sight to see a big 60 powered old timer with all that wood and silk bite Mother Earth like this.........

On that other "pigeon" model with the 45 degree dihedral the reason the fuselage would drop is because our lift is out in the center of the one wing rather than at the CG. If you stuck enough wingtip weight on it then it WOULD fly straight ahead if we didn't look at the drag of the fuselage... or if the lump of Plasticene was big enough to match the fuselage's drag........

Ya know, at this point I'm not sure if we aren't agreeing but in different words. Or is there still something that we can use to keep this thread going.......... :spinnyeye

Not sure why you talk about spiral. If you fly a pattern plane, bank it, slightly, say 25 degres, and do nothing else, it will continue to fly, straight like an arrow, on a slight dive. If you bank it, 60 degres, and do nothing else, it will continue to fly, straight like an arrow, but the dive will be much more obvious, and a bit shorter...

if the model do not do what I describe, it's simply because it's not built, or setup, properly, If the nose fall down in a "spiral", it would be because the center of gravity is too much forward.

You are talking about secondary effects all the time, maybe because you have never had the opportunity to experiment a plane "pure" enough to have no, or very small, secondary effects.

All pattern planes that I have seen and flown behave that way, and do not change heading a bit ! (Would make knife edge problematic, if they did.)

Bernard

I must admit that I have zero experience with pure pattern planes. I've flown a couple of sport models that were very close to dead neutral and yes they did fly as you're saying. One even used to try to tuck under slightly in a long dive. So yes I'll admit that a neutrally set model will do as you say. Most of them have a touch more stability built in and wil fall off into the spiral I described thanks to the trim that provides that stability.

We seem to be coming closer to the same line with each post.... :surprised

hehe So, now that we agree about what happen on a full wing, how come an half wing would be that different ? For the sake of the discussion, imagine the fuselage is a tiny wire, no aerodynamic influence from it. Now imagine a 90 degres dihedral (each half wing has 45 degres angle), and you bank 45 degres, that is, one wing is horizontal, the other is vertical. What will happen ? (Beside the dihedral, the plane setup is as neutral as the pattern plane we were talking about before)

Bernard

OK, OK, you finally got me.... Given THIS set of circumstances your model would respond to the classic idea of dihedral and just roll back to upright. I still say there's a little side slippin' goin' on as the fuselage settles into that big V groove the wings are cutting through the air but with the upward pointing wing producing no initial lift the horizontal wing is doing the driving and it's going to lever that model back to level pretty quick.


ARE WE ALL HAPPY NOW....... LOL :spinnyeye

PS: Actually thinking it over we have a very unique situation here. With wings at 45 degrees the effective projected span of the model in level flight is going to be the same as the model banked 45 degrees with the one wing horizontal. So the area in both cases will be the same. This means the fuselage WILL NOT settle as the model rights itself back to level AS THE WING AREA WILL REMAIN CONSTANT THROUGHOUT THE ROLL TO LEVEL..........

I'VE BEEN SET UP :devious: CURSE YOU RED BARON :boxing:



You may take your bow now......

You just forget the position of the center of gravity. The fuselage WILL settle itself back to level once the lift is equal both side of the CG.

Bernard

Well, let me start with a lame observation, so that my humble pie tastes a little better: my assertion was that no sideslip or yaw is necessary for dihedral to give roll recovery. This, I think, is true. However, when I consulted my favorite aerodynamics reference, Model Aircraft Aerodynamics, by Martin Simons, I discovered that sideslip actually makes a bigger contribution than the "projected area" mechanism, at least for larger roll angles. According to Simons, when the roll input disappears ( aileron deflection, wind gust, etc. ), the dihedral results in a sideslip toward the lower wing, which raises the effective AoA of the lower wing, and lowers the AoA of the higher wing, which gives higher lift for the lower wing, which acts to correct the roll. Let this be a lesson to me!

banktoturn

I would tend to disagree, as the sideslip is not a cause, but a result. Ok, I am in a bank, and release the control. The flight is assymetric. Because I have less lift (projected surface), the plane tend to dive and accelerate (this IS a sideslip) The lower wing "see" the relative airstream at a higher AOA ( of faster, it's the same), lift increase, it correct the roll, ok.

Now, make the same bank, but imagine we are in a perfectly symmetric flight. That is, once banked, I put aileron in neutral, and give just the correct rudder input to keep the ball centered (admit it's almost impossible on a model, talking full scale here). The plane will, by itself, go back to straight and level flight, and if there is no other input, i will be able to slowly release the pressure on the rudder, to keep symmetric all the time until I am out of the bank/turn. (Done that, more than once, on the wonderfully flying machine named ASK-13. The most delightfull harmony of controls I have ever flown. )

The sideslip theory is valid in only one case, assymetric flight, while the projected surface theory is always valid, so I still believe that the sideslip is a secondary effect.

Bernard

BernieG,

I have not done any experiments or analysis that tell me the sideslip is the main recovery mechanism, I am just believing Simons. His assertion is that the side slip is indeed caused by the non-aligned lift vectors resulting from a banked wing with dihedral, as you say, and that it also ends up being a stronger roll recovery mechanism than the "projected area" mechanism we have discussed. I agree entirely that wings with dihedral would correct from rolls without the sideslip mechanism, but I am inclined to accept Simons' assertion that, in practice, the sideslip mechanism is the primary mechanism. My preference is to be able to sit down and "prove" these things to myself using a piece of paper and a thought experiment, but sometimes an accurate analysis requires some data that I don't have, and then I have to trust the most credible source I can find. This is one of those times, for me.

Thanks,

banktoturn

Ah, now we are back to the basics......

Bernie, the side slip is a result but it's an unavoidable result. If you bank any airplane and it's not in a coordinated turn then it WILL slip. The nose may drop but there is still a slip to the low wing side. It doesn't matter if there is dihedral or not. If you use rudder to correct the flight path as you are suggesting then the model yaws to bring the flight direction to what we want but the air is still coming at an angle from the side with the low wing thanks to the yaw induced by the rudder. This doesn't matter if it's a poly glider or our dead neutral pattern ship. If you tip the wings then there is a lift vector off to the side of low wing and the model will slip in that direction. The nose will drop too but that's another issue. I have no doubt that the greater projected area on the low side of a dihedral wing would produce some righting force but the increase in the angle of attack thanks to the dihedral geometry would be much more powerful. Look at the lift curves for wing sections. A one degree change in AoA makes a BIG change in the lift coeficient. When you figure if the low wing gains one degree and the high wing looses one degree from the same slip angle that's a big difference.

I think this was the original subject that we've finally gotten back to.