Weathercock effect in turns
 

Hey can someone explain this so called weathercock effect while in a turn? It seems new to me and I've heard a few people throwing the term around a bit but never really explaining haha (maybe they don't even know what it really is!)

My understanding is this effect is apparently the reason airplanes will go around in a turn and without it, the aircraft wouldn't turn.

To me it sounds interesting... I'd have to say it's got nothing to do with weathercocking. I'd say the elevator and rudder have a bit part to do with it. Even in straight and level flight the elevator might be deflected up to keep level, depending on the airspeed and aircraft. As you bank the aircraft you progressively need more up elevator to keep the nose up, but also that helps you to point the aircraft into the center of the turn coincidentally. Now for theory purposes, supposed you could stay level at 90 degrees of bank, all that turning action would be from the elevator IMO. Now in straight and level, the rudder is in the same position as the elevator in the 90 degree bank, so it would be the one to point the aircraft into the center of the turn. So progressively that elevator is getting more and more control of the rate of turn of the aircraft as the bank is increased. If you let go of the elevator, the airplane will kinda leave the turn... if this was weathercocking, why doesn't it keep in the turn??

Plus, based on what I know of things that weathercock, they're one piece and don't have "control surfaces". So they wouldn't be able to alter the air, they just point parallel with the wind. So I'd say the actual majority of the turning is caused by elevator.. the more bank, the more elevator required and the tighter the turn. Obviously you have the horizontal lift vector which pulls it into the center too. But the elevator is the one "pointing" the nose in the turn.

That's my take on it... but if anyone has some good explanation of this weathercocking theory I'd like to know a bit more about it.

Cheers!
J

How do you expect to stay level in a 90 degree bank? What exactly do you think is counteracting the force of gravity in this situation?

As a non-pilot, who's only experience of 'flying' is in air-combat computer games, I find the lack of basic understanding of physics displayed by some on this forum as frankly worrying. I'll probably get banned from the forum for this, but I'll say this anyway: aeroplanes exploit the laws of physics, they don't circumvent them. The only thing that keeps them in the air is the interaction with the atmosphere, and there aren't any magical attributes of aircraft that make them better at staying airborne than a dustbin-lid or a discarded lottery ticket. To be able to stay in the air, an aircraft needs to generate sufficient lift to balance its weight. To be able to manoeuvre, it needs excess 'lift'/thrust to change direction, and sufficient control response to apply this lift in the desired direction. End of story.

None of this would matter a damn to me were it not for qualified airline pilots apparently not understanding that one can recover from a stall by pushing forward on the control column/sidestick/whatever planes are controlled with now....

Someguy... Did you read my quote you used? Or the thread I posted? I was talking about a hypothetical situation.I'm a commercial pilot and well aware of what you're saying. My point was to emphasize that the elevator would be pointing the aircraft in the turn in that situation. In real life the aircraft would need to have an incline with the horizontal sufficient enough to produce enough lift via the body, rudder and vertical component of thrust to keep the aircraft level.

Wiz... See last sentence above

Isn't there some quote about a little knowledge being a dangerous thing?

Imbracable Crunk has answered the original question clearly, putting basic Newtonian dynamics in qualitative terms. Other forces might come into play, but the question is which is the most influential, and this answer shows it.

Now for the intriguing question. How do flying wings such as the B2 , which have no find or rudder, coordinate turns? If you think the B2 does it with sophisticated electronics, then answer the same question for a flexwing microlight.

Concerning the contribution from SomeGuyOnTheDeck, I agree with him that a basic understanding of physics is helpful, but a basic understanding of aerodynamics is even more helpful!

Some airplanes can maintain knife-edge flight. All that is necessary is that the lift produced by the body flying sideways, coupled with the vertical component of engine thrust, equals or exceeds the weight of the aircraft, and that the rudder has enough aerodynamic authority to hold the nose up. What no airplane without thrust-vectoring can do is maintain knife-edge flight without yaw.

One should also be a little careful about such statements as It is not as simple as that. There are some airplanes for which, when they stall, no amount of nose-down elevator command will recover it. These airplanes are mostly fitted with a stick pusher, to apply forcibly a nose-down pitching moment before the airplane gets into the unrecoverable part of the stall.

Saying this, of course, brings us back to what "stall" is. It is mostly a range of aerodynamic phenomena, and what exactly happens can be different according to the range of airspeed chosen. Some people think, for example, that the point of stall can be defined as the point of maximum coefficient of lift, but this is not necessarily so according to certification criteria, which allow that sufficient buffeting can define the point of "stall", even though maximum lift may not yet have been attained. And so on.

PBL

PBL.. doesn't this help prove my point that it has more to do with the elevator and help disprove this weathercock theory? I believe the B2 flies with sophisticated electronics like any modern fighter but electronics aside, there has to be control surface manipulation for the airplane to change course and the B2 definitely doesn't have a rudder!

Flexwing microlights have no elevator. (They might bend the rear of the wing, but that is to trim, not to exercise pitch control.) How do they coordinate turns?

A related question: why do some airplanes yaw to the left during a right turn, and to the right during a left turn?

PBL

All talk
 

Weather cock effect has never been mentioned in my presence. The only sense I can see in it is a weird way to say a balanced turn or the effect of a tail fin if you allowed an a/c to slip in during a turn. As for a balanced turn without a rudder or fin even, I guess all you'd need is some nice variable drag on the inside wing.

The tiger moth was a scream for eg. as the stick got to about half way over the outside aileron (going down) started coming up again. (lessen the drag I guess.)

And practically speaking, during my constant rate medium/steep turns,
ailerons went a little opposite, rudder went about central with an opposite (up)tendency, and elevator was well back. (Regardless of theory.)

That's one way to do it.

One can also maybe think where the center of lift goes when one wing produces more lift than the other, and why that might give a rotational moment about earth-z.

So now we have three suggested mechanisms by which an airplane can rotate about an earth-z axis "into" the turn, when it rolls: one from Imbracable, one from you, and one above. Two of them are applicable to rudderless aircraft.

Which factor has the most influence? I doubt one can tell without putting specific numbers for specific airplanes in there.

PBL

PBL... can you expand on this.. how does a difference in center of lift between the two wings produce a rotational moment?

[/QUOTE]

I spoke of a center of lift. That is, where the lift may be considered for purposes of Newtonian dynamics to be punctually concentrated. There is just one of those, not more.

Just as one speaks of a center of gravity. That is, where the weight many be considered for purposes of Newtonian dynamics to be punctually concentrated. (One doesn't speak, for example, of "...a difference in center of gravity between the two wings..")

Where does the center of lift shift if, say, the left wing produces more lift than the right wing? And how does any force produce a rotational moment? (Answer: by having an arm to some point about which a body may rotate.)

PBL

PBL.. a center of lift can be in any defined section. Therefore, you could have an infinite number of center of lift vectors on a single wing. All I did was separate it into two, each wing having it's own center of lift. As you travel around a circle while in a turn, the outside wing moves faster, has more lift, but a lower AoA. It will in turn have a slightly different center of lift (a point, some distance from the leading edge of the wing).

As you mentioned, having more lift on the outside wing will cause a turning moment about the rotational axis (longitudinal axis) causing the airplane to bank further, which it will unless opposite aileron is applied or other aerodynamic forces are used to stop it.

2 questions.... 1) How does the total center of lift change when one wing produces more lift? 2) How does that give a rotational moment about the Z-axis?

italia458,

the only relevant thing I have to observe, which you also can observe, is that you didn't solve the simple problem. That would suggest that your way of thinking about the issue is not that fruitful. Rather than trying to make it complicated, maybe you would have more success if you kept it simple?

PBL

It is very clear you can't answer the two questions that require you to explain your claims. Therefore, I think you really don't know what you are talking about. I don't think I've seen one post of yours that has explained anything with proper justification. You ask questions in a developmental teaching style yet fail to answer your own questions or questions others have regarding what you have said leaving the impression that you think you are above all of us, and are teaching us all a lesson.

Thank you, but no thank you! I'll pass.

Italia, a few thoughts:

How exactly do you suppose a weather-vane or weathercock works? You kinda glossed over it's ability to turn without any control surface.

At some point in a bank, the horizontal stab/elevator is, in effect, going to become the vertical stab/rudder and vice versa. Add into that some keel effect and some vertical thrust. It's all vector analysis and it doesn't really matter what does what. Gravity is down, the rest is variable.

The tail on a plane is still a weather-vane - it's just a variable weather-vane.

Maybe you should just get a Beech 35 so you don't have to worry about this problem.

Hi italia 458

I'd say all the turning is due to the horizontal component of lift from the wing in a banked turn.

The elevator is applying a downwards force in level flight, and has an outward force away from the centre of the turn in banked flight. To say you think the majority of the turn is caused by the elevator, is like saying most of the lift in level flight is caused by the elevator.

The fin ensures the aircraft has direction stability in Yaw (your weather cock effect?), the rudder (& Yaw SAS) fine tune the slip to zero due adverse aileron drag etc.

ImbracableCrunk… on a weathervane, the rear part has the majority of the mass and the most arm from it's vertical axis so the wind acts on it, rotating it around it's axis until it lines up with the wind. If the wind sways a little sideways, it'll hit the rear part, bringing it back into line.

I think I've got a better understanding of this weathercock theory and I still contest it's use; I believe it's the wrong term for what's really happening. I've never heard of a "variable weathervane"… the whole point of a weathervane is to have it "non-variable" so that the wind will act on it, producing the change.

Looking at nature… a fish must "weathervane" through the water, which I don't think would be correct to say. An object in motion tends to stay in motion, therefore requiring another force to change it's path. the control surfaces at the tail impart that force. About 99% of the time, the aircraft will be "streamlined" with the relative airflow, only when another force, such as control surfaces, applies itself will there be a change. Fluids take the path of least resistance, so instead of piling up at the rudder like bullets, they smoothly get deflected and then impart an opposite force on the vertical stab. So what I'm saying are that these are forces "initially" imparted "by" the aircraft, not the wind. These forces are what makes the aircraft turn! A weathervane doesn't have a wing or horizontal stab or rudder… remove those from the airplane and I will fully agree that it is weathercocking as it's accelerating towards the earth!


rudderrudderrat… sorry I wasn't clear there, I meant that the elevator would be causing the nose to "come around" while in the turn, to "point" the aircraft in a circle. I fully agree the horizontal lift component is what's pulling the aircraft "into" the turn. I also think that there is a tiny component of thrust that would help turn the aircraft… like stated, there is usually a bit of downforce on the horizontal stabilizer in level flight, take that and add even more downforce required in the turn to stay level and you're adjusting your thrust angle so you have slightly more of a thrust component acting with the lift vector, perpendicular to the wings. Very small for sure, but I believe it's still there.

If while in level flight, you banked an airplane to the left 30degrees and added no up or down elevator and didn't add any power, would the plane still turn? Yes.

I think the folks here can use a good dose of Triple H Jr's Aerodynamics for naval aviators...the relevant answer is in the 'stability and control section'

don't get fooled... Hugh Harrison Hurt Jr....Makes it hurt:ouch::}

If you strictly just banked 30 degrees left, how is that changing the relative wind? For that split second you'd still have a parallel relative airflow going across the body of the airplane, then the forces imparted by the aircraft being banked 30 degrees would take over. The reduction of the vertical lift component would cause the nose to pitch down, then changing the relative airflow on the wings would cause a slip to the left which would in turn change the relative airflow and would help "push the tail around", as well as the differences in the angle of attack of each wing would create more drag on the outside wing, yawing the aircraft slightly to the right, then the nose drop would increase the airspeed, increasing the overall lift and raising the nose again, etc. Depending on the stability built into the aircraft, it will be different from aircraft to aircraft. You should go try doing that in an airplane and then note what happens. TONS happens and this can't be a justification for the weathercocking because now the aircraft is in a slip.

RE: "push the tail around"... that's because it's in a slip and not in coordinated flight. In a coordinated turn I don't believe you're going to have weathercocking whatsoever as the relative airflow is parallel to the aircraft's longitudinal axis. Even in a 30kt wind, it's the same thing. It doesn't matter what the ground below you is doing, if that ball is centred, you're in coordinated flight. In a slip or skid you will get the weathercocking effect, which would return you to coordinated flight, but to stay in the slip or skid you're actually fighting the weathercocking tendency by using the rudder.

It helps to distinguish turn (changing the direction of travel of the centre of gravity of the aircraft) from yaw (changing the heading of the aircraft, or as you put it, "pushing the tail around").

Something provides the yawing moment that "pushes the tail around". That can be the response of the fuselage and fin to a slip angle -- which is your "so-called weathercocking effect", or it can be the rudder. Or it can be some of the other effects discussed above.

Those of us who have spent too long flying aircraft with short wings and more power than necessary to stay in the air tend to bank the aircraft and wait for the slip angle to do its job and yaw us around the turn -- hey, we're just mimicking the two-axis autopilot ;). Glider pilots, on the other hand, tend to finesse the slip angle to zero with their feet.

But it would also descend because the vertical component of lift would no longer equal the weight. Up elevator till the vertical component of lift again equals the weight will now prevent the descent. However, the now increased horizontal component of lift will increase the rate of turn and because the drag has been increased and the power not increased the aircraft will slow down till the drag again equals the thrust...

Yep yep yep. Yada yada. Agreed. My point is that a plane will turn (albeit a descending turn) without any increased effort from the elevator.

I had distinguished the difference... I was talking about the so called "weathercock effect" that happens when in a slip with no rudder applied. In a turn, there is no weathercock effect because there isn't a slip or skid.

I totally agree, that's weathercocking effect. But "while established in a coordinated turn", weathercock effect is not present.

This is going on and on. Time to get more or less definitive about the original question.

italia458 asked a question. ImbracableCrunk answered it with the usual answer, found in most aerodynamics texts. italia458 wants to doubt it, in favor of some effect due to the elevator, he suggests.

Since no one seems to have taken up PA's suggestion to look at Hurt, here is an on-line aero reference for people taking up flying in T-34C's: https://www.netc.navy.mil/nascweb/ap...COM-SG-111.pdf

Let me quote from the section on "Stability". On p7 of this section one will find statements concerning the effect of the vertical stabiliser during a roll.
I don't have Hurt to hand, but I do have Shevell (more "middlebrow" to Hurt's "lowbrow" take): To complete the thought, consider: what influence does a vertical fin have vis-a-vis yaw? What influence does the elevator have? To those who may find it difficult to answer these questions, let me again quote Shevell: That should settle the original question. Please.

It doesn't settle the question, which I raised, of what happens with tailless airplanes such as flexwing microlights.

PBL

PBL... doesn't this quote you use prove exactly what I was saying?!

I completely agree with this!! I have acknowledged that in a sideslip, the airplane would weathercock if no control inputs were applied to either keep it in the sideslip or to recover from the sideslip. It seems from all the discussion and quotes used that the aircraft has to be in a sideslip before this "weathercock effect" will happen. I have not seen anything published that says that "weathercock effect" happens while in a "coordinated turn". I should also point out that the quote is, like you said, from the Stability section and is dealing with "Directional Static Stability".

Again, this quote mentions a roll and then discusses how it will progress into a sideslip and how the nose will yaw. I completely agree with this!!

I like re-examining these topics. Keeps my head in the game.

Italia, I was thinking and I'm just wondering about straight and level. The tail is back there doing it's thing, we're coordinated. But you're coordinated because it's back there maybe doing micro slips and micro skids and corrections, if you like.

As to a descending turn with no elevator:

You're banked in a turn, the plane is constantly in a "new" slip because it's always under acceleration if it's in a turn. Weathercocking effect is always in present and that's why you are coordinated and turning. The forces that caused the initial slip didn't go away, they just have a new vector.

If the turn is coordinated then, in the absence of other factors like asymmetric thrust, the slip angle will be zero. In that case, the yawing moment from the effect of the slip on the fuselage and fin has been almost entirely replaced by a moment from the rudder. In practice, there are a lot of factors contributing to the yaw budget, so coordinated isn't precisely no-slip, but it's a good approximation.

In a coordinated level turn the aircraft is yawing, under the influence of the rudder, and pitching, under the influence of the elevator.

In a climbing or descending turn the aircraft is also rolling (in a climbing turn, opposite the turn direction, in a descending turn, with the turn direction), under the influence of the ailerons and the different angle of the attack on the wings.

"Weathercocking" is an undefined, colloquial term which only really applies to questions of stability. In any case, it doesn't apply in coordinated turning.

Hi,

I thought "weather cocking" was the term used to explain why the aircraft wants to turn into wind whilst on the runway when it experiences a cross wind.

If you enter into a turn properly, there won't be any slip during entry or during the turn. If your ball is centered then you are coordinated. That's why there is an instrument called the "turn and slip indicator".

The rudder is the force that is making the tail "come around" and point the nose in the turn. it's creating a cambered surface and is essentially a wing standing on end. So as the rudder deflects left, it creates camber and lift which acts 90 degrees perpendicular to the surface, ie: to the right! This then rotates the airplane around the CofG to point the nose into the turn. It's exactly the same as applying elevator to adjust the pitch of the aircraft!! This isn't weathercocking! If it was, then the wing would fly because it's "weathercocking" which isn't true.

Yes yes yes!! the key word is "stability".

That is correct. In that case it acts like a weathervane. While in flight it doesn't matter where the wind is, only the direction of the relative airflow.

"Weathercocking" is stability. I'm simply using the OP's term. Newtonian, Bernoullian, Brobdingnagian, it doesn't matter. You've got a symmetrical airfoil at the back of the plane. Don't get caught up in semantics.

I thought Italia's original point was that it was the elevator that makes the plane turn.

The pilot makes the plane turn:}

I thought it was money! ;)

Money makes my stomach turn. :ooh:

Edit: Having brushed the dust off my PoF notes, a yawing moment is referred to with regards to the affect of the tail fin during a turn. Nothing more is said about it except that it exists.

Adverse yaw is mentioned during a turn, due to the difference in lift coefficient and hence induced drag between the outer and inner wing.

I guess the adverse yaw is greater than the yaw due to the tail fin hence the need to balance a turn.

Flexwings in turns
 

With a flexwing, be it a microlight or a hang glider, the pilot shifts his weight. If he shifts it to the left, he loads up the left wing, this increases the camber and drag on that wing relative to the right wing, and the aircraft turns left. High performance hang gliders are stiff and hard to turn, and this is usually addressed by a variable tension device (piece of cord on a pulley). Slack it off for take off, landing, and slow flight. Tighten it up for reduced drag at high speed, and accept the reduced controllability. In this condition, a weight shift will cause little effect, possibly even adverse yaw. Some foot-launched rigid microlights use differential spoilers for roll control. I seem to remember reading, probably in Flight Magazine, that the B2 is steered in the same way.

Thanks for addressing my question, 911slf. I am not sure you have answered it in aerodynamic terms.

I understand what it may mean for a dancer, say, to "shift his weight". He's got one foot on the floor; he takes it up and puts the other one down (or vice versa). However, without a floor, it is not clear to me what "shifting his weight" can mean dynamically, even though it seems to be colloquial usage amongst flexwing pilots.

I am aware that moving the control bar relative to the trike alters the roll attitude of the wing relative to the trike. That is simple geometry. What is not so clear to me is how the various force vectors change and why. Those must explain why the aircraft yaws (earth-axes), and continues to yaw into a turn.

You are suggesting that the down wing has increased drag. Yes, that is another way of saying that there is a moment created about the earth-z-axis. That must indeed happen if the aircraft is to yaw. But why that increased drag? The wing is going slower and has less lift; why should the drag have increased?

PBL