Weathercock effect in turns

[italia458]

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

[ImbracableCrunk]

Make an airplane with your right hand
Palm down
Fingers together
Bank your airplane 30 degrees to the left
Slide your airplane hand to the left due to the now horizontal component of lift
Take your left index finger and push on your vertical stab
Watch your airplane magically weathervane

[SomeGuyOnTheDeck]

Now for theory purposes, supposed you could stay level at 90 degrees of bank, all that turning action would be from the elevator IMO.
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....

[Wizofoz]

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?


YouTube - Low knife edge pass (http://www.youtube.com/watch?v=TOtjHNwcinM&feature=related)

[italia458]

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

[ImbracableCrunk]

I find the lack of basic understanding of physics displayed by some on this forum as frankly worrying.

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

[PBL]

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 ..... qualified airline pilots apparently not understanding that one can recover from a stall by pushing forward on the control column/sidestick...

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

[italia458]

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.

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!

[PBL]

[PBL: How do flying wings such as the B2 , which have no fin or rudder, coordinate turns? If you think the B2 does it with sophisticated electronics, then answer the same question for a flexwing microlight.]
PBL.. doesn't this help prove my point that it has more to do with the elevator...

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

[Air Tourer]

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.)

[PBL]

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.

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

[italia458]

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.

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

[PBL]

..... a difference in center of lift between the two wings.....[/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

[italia458]

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.

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.

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?

[PBL]

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

[italia458]

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.

[ImbracableCrunk]

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.

[rudderrudderrat]

Hi italia 458

So I'd say the actual majority of the turning is caused by elevator

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.

[italia458]

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.

[ImbracableCrunk]

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.


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.

[Pugilistic Animus]

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::}

[italia458]

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.

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.

[bookworm]

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"

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.

[petermcleland]

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.

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...

[ImbracableCrunk]

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.

[italia458]

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").

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.

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

[PBL]

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/api/NAVAVSCOLSCOM-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.
A sideslip causes the vertical stabilizer to experience an increased angle of attack. This creates a horizontal lifting force on the stabilizer
that is multiplied by the moment arm distance to the airplane’s CG (Figure 1-9-18). The moment created will swing the nose of the airplane back into the relative wind. This is identical to the way a weather-vane stays oriented into the wind.

I don't have Hurt to hand, but I do have Shevell (more "middlebrow" to Hurt's "lowbrow" take): A roll to the right.... inclines the lift vector to the right.... The lateral component of the lift vector accelerates the airplane toward the right. The sideways velocity adds to the forward velocity to produce an angle of yaw between the airplane centerline and the effective oncoming velocity.

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: Longitudinal motions occur in the plane of symmetry, which remains in its original position. Lateral motions, such as rolling, yawing, and sideslipping, displace the plane of symmetry. The technical significance of this distinction is that for normal symmetrical aircraft with small displacements these two types of motion are independent of each other.

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

[italia458]

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

A sideslip causes the vertical stabilizer to experience an increased angle of attack. This creates a horizontal lifting force on the stabilizer
that is multiplied by the moment arm distance to the airplane’s CG (Figure 1-9-18). The moment created will swing the nose of the airplane back into the relative wind. This is identical to the way a weather-vane stays oriented into the wind.

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".

A roll to the right.... inclines the lift vector to the right.... The lateral component of the lift vector accelerates the airplane toward the right. The sideways velocity adds to the forward velocity to produce an angle of yaw between the airplane centerline and the effective oncoming velocity.

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!!

[ImbracableCrunk]

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:


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

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.

[bookworm]

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.

[Checkboard]

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.

[rudderrudderrat]

Hi,

"Weathercocking" is an undefined, colloquial term

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.

[italia458]

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 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.

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

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

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.

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.

[ImbracableCrunk]

"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.

[Pugilistic Animus]

The pilot makes the plane turn:}

[ImbracableCrunk]

I thought it was money! ;)

[Checkboard]

Money makes my stomach turn. :ooh:

[Gutter Airways]

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.

[911slf]

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.

[PBL]

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

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

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

[italia458]

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

My point was that the elevator actually helps turn the aircraft. It's party due to the elevator and party due to the rudder. When the aircraft is banked, the rudder has a pitch component added and the elevator has a yaw component added, relative to the horizon and turn (assuming turning while straight and level).

[minstermineman]

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?

If you (pilot)shift yourself to the left so you hang further under the left wing, then you yourself will create more drag on that side of the aircraft than was there previously, plus you will alter the C of G to the left, so bank left.

[911slf]

I am no aerodynamicist, and my experience is on hang gliders rather than microlights, but this is my understanding of what happens on hang gliders and I believe microlights are analogous.

A hang glider pilot is in a harness rather like a climbers harness, you can sit in it and the weight would be supported on your upper thighs and buttocks. But the pilot would normally, after take off, lean forward and some of the weight would then be supported by shoulder straps - this is for drag reduction and comfort, not aerodynamically mandatory, and not done with the very early harness models.

There is a strap connected from your harness via a carabiner to a loop which is permanently connected to the glider keel. Your weight is taken on this strap (breaking strain usually two ton, and there are usually two of them):ooh:. This strap is usually near vertical, unless or until you start to pull a lot of G - not recommended. You are holding the bottom horizontal bar of an A frame rigidly connected to the glider. If you push the A frame to your right, you have effectively shifted your weight to the left. The strap is now dangling from the keel to a point a bit to the left of the centre of the horizontal bar, so no longer quite vertical, so your weight, about three feet below the centre of lift, is now a bit to the left of the centre of lift. If the glider is stiff you will have to hold this by muscle force until the wing starts to bank under the this offset load. Eventually you will turn left. If the glider is spirally neutral you will end up in a turn where the strap supporting your weight is once more perpendicular to the wing, but the wing is not parallel with the horizon, and the strap therefore, is no longer perpendicular to the ground. While many gliders are spirally neutral, some are spirally stable, some are spirally unstable, and some vary according to how you change the tension in flight. A modest degree of spiral instability (a tendency for turns to steepen), is desired for staying centred in small thermals - by hot ship pilots, not me. And spiral stability is definitely not a topic I can explain.

You ask why the increased drag on the left wing when you do this. The wing is effectively a sail, albeit somewhat stiffened by battens, the more you load it the more it will deform, and the increased deformation makes it less efficient and increases drag.

With a sailplane with a rigid wing, the greater the load, the more efficient the wing and the better the angle of glide - even though the actual descent rate increases. This is why in races they often carry water ballast.

If you put a heavier pilot on a hang glider,it will fly (and stall) faster, but at a worse angle of glide, as the wing deforms more and becomes less efficient. So if you shift your weight as above you are loading the left wing more, deforming it more, and increasing the drag more, on that side only.

Probably simpler to draw it but I am not sure how to attach a drawing.

[PBL]

italia,
My point was that the elevator actually helps turn the aircraft. It's party due to the elevator and party due to the rudder.
You are now allowing that maybe some of the lateral surface is relevant to reacting to sideslip (or yaw, depending on which axes you are measuring in). That is progress of a sort.

Let me suggest you accept what ImbracableCrunk said right at the beginning, and what aerodynamicists have said in text books (which I have quoted), and what they say when one discusses it with them. The VS is there for a purpose. Someone put it there. They know why they put it there, and it works, over decades of experience. So when they say why, and how, I imagine there would - should - be a strong inclination to believe them, no?

Minstermineman,

If you (pilot)shift yourself to the left so you hang further under the left wing, then you yourself will create more drag on that side of the aircraft than was there previously, plus you will alter the C of G to the left,

Thanks for responding to my request for explanations! Maybe you could phrase things in the usual terms of Newtonian dynamics?

There are two coordinate frames typically used when speaking about flight. One is earth-centered, in which the z axis passes through the CofG (and you, if you are hanging on a non-rigid line below a wing). So "alter[ing] the CofG to the left" makes no sense in this frame. The other is aircraft-centered, also passing through the CofG. Again, moving the CofG "to the left" makes no sense in this frame.

911slf,

Thanks also for responding to my request for explanations! I appreciate your attempt to explain what happens in discursive terms, but I think it suffers from the same issue as minstermineman, namely that you are using (let me say) hanggliderspeak which is at variance with the ways in which one expresses dynamics when talking aerodynamics.

You are hanging below some object which is not fixed to the earth. You push on a frame attached to that object. Except for very briefly (and very slightly) when you are moving the frame (equal and opposite reactions, and all that), you are hanging directly below the CofG. That's what Newton says happens when you don't want to be a pendulum (which, as you point out, is not recommended!)

I am not sure I can get much further with your explanation without trying to interpret in the usual way of balance of forces, and I am not sure I can do that as it stands.

[911slf]

PBL this

TURNING FLIGHT AND SIDESLIP IN HANG GLIDERS (Summary) (http://www.aeroexperiments.org/article/#BAR)

appears to be a fairly detailed treatment by a hang glider pilot. I have not read it all carefully, as it goes into more detail than I as a casual sporting flyer would want to read. So far as I can tell it does address your interests, and I will be interested to see what you make of it.

It does touch upon a point that has confused me in the past. Hang glider pilots are encouraged to let the nose up a bit in turns, which always seemed a bit counter intuitive to me, given we normally fly not much above the stall.

[italia458]

PBL...

You are now allowing that maybe some of the lateral surface is relevant to reacting to sideslip (or yaw, depending on which axes you are measuring in). That is progress of a sort.

Let me suggest you accept what ImbracableCrunk said right at the beginning, and what aerodynamicists have said in text books (which I have quoted), and what they say when one discusses it with them. The VS is there for a purpose. Someone put it there. They know why they put it there, and it works, over decades of experience. So when they say why, and how, I imagine there would - should - be a strong inclination to believe them, no?

What I've said from the start, in different ways, is that while in a banked attitude, the entire tail section (vertical stabilizer/rudder and horizontal stabilizer/elevator) controls yaw (and pitch) partially, either eliminating unwanted yaw (coordinated flight), or producing yaw (slip/skid). It's a function of your bank, the more bank, the more the rudder controls pitch and the more the elevator controls yaw. A V tail aircraft is a perfect example of what I mean.

I think we're talking about something else. The VS is there for stability. I'm not contesting what a VS is used for. Your second paragraph is quite confusing to me, I don't understand how it applies to the discussion on weathercocking while in a turn...

If it is correct that the aircraft is weathercocking while in a turn, then my reasons for why it does not should be false and I haven't seen anyone clearly disprove what I've been saying. I'm completely open to different ideas but I think firstly you have to clear away the false information and then start with a fresh slate to explain a concept or theory.

EDIT: PBL are you talking about this quote from IC?

Make an airplane with your right hand
Palm down
Fingers together
Bank your airplane 30 degrees to the left
Slide your airplane hand to the left due to the now horizontal component of lift
Take your left index finger and push on your vertical stab
Watch your airplane magically weathervane

I already addressed that. What he described was an aircraft in a slip and the tendency of the aircraft to "weathervane" into the wind because of the crosswind component on the tail section. I completely agree with this! But I don't see this happening while in coordinated flight!

[chris.dever]

Make an airplane with your right hand
Palm down
Fingers together
Bank your airplane 30 degrees to the left
Slide your airplane hand to the left due to the now horizontal component of lift
Take your left index finger and push on your vertical stab
Watch your airplane magically weathervane

My boss walked in .... probably to check out why I was making the noise of a radial engine ....'right hand' plane rolled onto its back and disappeared into thin air !

:*

[italia458]

Haha that's awesome!

[Thomo91]

In my understanding -

"Weathercock" is in relation to the ground (an object). {compass heading}

"Slip" is in relation to air flow (relative airflow). {unbalanced flight}

[Thomo91]

while in a banked attitude, the entire tail section (vertical stabilizer/rudder and horizontal stabilizer/elevator) controls yaw (and pitch) partially, either eliminating unwanted yaw (coordinated flight), or producing yaw (slip/skid).

Yes that's right, well kinda.

It's a function of your bank, the more bank, the more the rudder controls pitch and the more the elevator controls yaw. A V tail aircraft is a perfect example of what I mean.

I'm not much of an aerodynamicist, but in my layman way of putting it I hope I'm on the right track. :)

Relative to the airflow:

Trimmed S/L, Aileron is applied, adverse yaw is stopped by rudder input, AoA is increased to maintain height (elevator). Bank angle is maintained at, say, 45 degrees. To do that aileron is neutralized, and so is rudder. What happens if you let go of the stick to the S/L trimmed position? Yes, the nose will fall off the horizon, and everything goes yuk.

Most aircraft I've flown (which is mostly small ultralight stuff, and your average Cessna Single engine types). Only require a little of top rudder past the 45 degree mark, if you want to maintain it for a while. Depending on the day, load, and of course the speed. (note: please ensure you know what the implications of what to much top rudder will do!!)

Back to your quote; Yes the more angle of bank, the more the rudder is used to maintain a nose up position, what is generally known as top rudder.

Elevator however is still only used to increase AoA. I'll leave you to decide what the aircraft will now do.

Another way; In relation to the aircraft, the elevator isn't doing anything to the yaw, rudder still controls yaw no matter what orientation the aircraft is in. Elevator still controls AoA no matter what orientation the aircraft is in.

I reckon you're on the money but it looks like you may be mixing it with a relation to the ground, (no offense intended) you may think, that, as it's now moving along the X axis using elevator instead of rudder (if the aircraft maintained wings level - using rudder only to turn without banking) it now controls yaw. Not the case relative to airflow.

Dunno If I've just confused you all, or if that actually makes sense, I hope it's a bit of the later. :O

[PBL]

911slf,



thanks for the link to Seibel's article. I understand why you haven't read it all. It is very long (Safari wants 89pp to print it!) and to my taste somewhat meandering. However, it seems Seibel has seriously tried to experiment and can describe what he found in terms mostly familiar to an aerodynamicist. I have read about a quarter of it so far.

Here his answer to my question, from Part IV, Section "What Makes an Aircraft Turn", second paragraph, first sentence: "In hang gliders, the wingtips provide the same function as would a vertical tail". He is talking about the action of the VS during sideslip.

It does touch upon a point that has confused me in the past. Hang glider pilots are encouraged to let the nose up a bit in turns, which always seemed a bit counter intuitive to me, given we normally fly not much above the stall.

That seems to me straightforward. Your lift vector is tilted by the roll, so there is a lower component of lift in the vertical (earth-negative-z) direction and so you need to increase total lift in order to match weight with this component if you wish to stay level, and you do that by increasing AoA. That's why you need elevator in a level turn in an aircraft, and why (I imagine) you need to "let the nose up" in a hang glider.

Seibel suggests that some instructional books suggest this has something to do with yaw/sideslip, and that they are wrong. I agree with Seibel.

PBL

[Basil]

I accept that it may be flippant to say so, but I am very glad that I did not read a thread such as this before learning to fly :hmm:

[italia458]

Most aircraft I've flown (which is mostly small ultralight stuff, and your average Cessna Single engine types). Only require a little of top rudder past the 45 degree mark, if you want to maintain it for a while. Depending on the day, load, and of course the speed. (note: please ensure you know what the implications of what to much top rudder will do!!)

Back to your quote; Yes the more angle of bank, the more the rudder is used to maintain a nose up position, what is generally known as top rudder.

Please do not teach anyone this method of turning! A steep turn is probably one of the hardest manoeuvres to complete accurately. In the method you described using "top rudder" you're not flying in a coordinated turn, you're in a slipping turn, which can easily lead to a stall and possibly a spin if bank is increased enough or speed reduced enough. This is probably the most common mistake a flight instructor sees when teaching steep turns. DO NOT USE RUDDER TO KEEP THE NOSE FROM DROPPING!! Make sure all turns are coordinated. The nicely compiled graph on increase in load factor, stall speed, etc. with an increase in bank is only accurate if you are in a coordinated level turn at that bank angle. If you look at your turn coordinator while in a steep turn using "top rudder", the ball will not be centred.

Another way; In relation to the aircraft, the elevator isn't doing anything to the yaw, rudder still controls yaw no matter what orientation the aircraft is in. Elevator still controls AoA no matter what orientation the aircraft is in.

I reckon you're on the money but it looks like you may be mixing it with a relation to the ground, (no offense intended) you may think, that, as it's now moving along the X axis using elevator instead of rudder (if the aircraft maintained wings level - using rudder only to turn without banking) it now controls yaw. Not the case relative to airflow.

Yes, in relation to the aircraft that's true. I was talking in relation to the horizon, where your reference for manoeuvres is based. It's the same as your attitude indicator. You should be flying using this reference system and if you are using "top rudder" to control pitch in your turns, I know you are! So that part is good.

AoA is a different factor and yes, elevator controls AoA, but rudder also controls AoA while in a turn. If you entered a cross controlled turn, like you described using "top rudder", then the inside wing would have a higher AoA compared to doing a properly coordinated steep turn.

I'd highly recommend that if you do your steep turns using "top rudder" then go up and practice doing them properly. It'll feel weird at first but you need to break that habit, it will bite you at sometime!!

I accept that it may be flippant to say so, but I am very glad that I did not read a thread such as this before learning to fly

Good call Basil!

[italia458]

In my understanding -

"Weathercock" is in relation to the ground (an object). {compass heading}

"Slip" is in relation to air flow (relative airflow). {unbalanced flight}

Weathercocking is not relative to the ground, it's relative to the wind! If you enter a slip, there is a weathercocking tendency that is prevented by using opposite rudder to remain in the slip.

Dihedral is used to help bring the aircraft back to level flight if a slip is encountered.

[Thomo91]

Please do not teach anyone this method of turning! A steep turn is probably one of the hardest manoeuvres to complete accurately. In the method you described using "top rudder" you're not flying in a coordinated turn, you're in a slipping turn, which can easily lead to a stall and possibly a spin if bank is increased enough or speed reduced enough. This is probably the most common mistake a flight instructor sees when teaching steep turns. DO NOT USE RUDDER TO KEEP THE NOSE FROM DROPPING!! Make sure all turns are coordinated. The nicely compiled graph on increase in load factor, stall speed, etc. with an increase in bank is only accurate if you are in a coordinated level turn at that bank angle. If you look at your turn coordinator while in a steep turn using "top rudder", the ball will not be centred.

Thanks Italia458, And yes I certainly realize what you're saying (that's why I added the "note") It is more relevant in gliders I guess, but don't worry, the ball is always centered. Not much point in doing a side slip reducing your performance. Like you said some aircraft will really bite if you do it wrong. :ok: Keep the ball centered.

Weathercocking is not relative to the ground, it's relative to the wind! If you enter a slip, there is a weathercocking tendency that is prevented by using opposite rudder to remain in the slip.

Yes that's right, it has gone from one position, and 'weathercocked' into another. {compass heading} (ground wasn't a really good example, more a point on the horizon)

I'd highly recommend that if you do your steep turns using "top rudder" then go up and practice doing them properly. It'll feel weird at first but you need to break that habit, it will bite you at sometime!!

Thanks, I will certainly review how I go about it next time I'm up, as I couldn't really tell you what I do at the time as you do what ever you have to without much thought. The ball is always centered though, I make sure of that.

I accept that it may be flippant to say so, but I am very glad that I did not read a thread such as this before learning to fly

:ok:

[italia458]

It is more relevant in gliders I guess

Same goes for gliders, don't use "top rudder"! I'm a glider pilot as well and can tell you they fly just the same as any other conventional airplane. They just don't have power so you're usually descending slowly unless in a thermal.

It sounds like you're doing the turn properly if you're keeping the ball centred. I was just mostly worried about people reading that and thinking that "top rudder" is OK to be used in a steep turn. Flight instructors already encounter many common problems in training and keeping this myth circulating will just make it worse!

[Thomo91]

I was just mostly worried about people reading that and thinking that "top rudder" is OK to be used in a steep turn. Flight instructors already encounter many common problems in training and keeping this myth circulating will just make it worse!

Agree, apologies for that.

Once again, do what your instructor says, not opinions on forums! ;)

[italia458]

Agree, apologies for that.

Once again, do what your instructor says, not opinions on forums!

Cheers mate!

[Basil]

Digressing from the thread:
Like the 'top rudder' in a steep turn it is easy for a student to misunderstand a briefing or to transfer a technique which may be appropriate in one phase of flight to another where it is not.

Student Basil was told that going above or below best glide speed would be inefficient and dispose of energy. Sitting in JP with instructor, Bas decided that energy was too high and stuck the nose down and shortly afterwards applied some energy killing g to raise nose and regain original speed.
At the subsequent debriefing it was pointed out that, whilst the technique would have been acceptable as a substitute for speedbrake at 20,000ft, it was not a recommended procedure at 1000ft on the approach.

My RAF QFI sat unflinchingly through the whole event. :ok: