Weathercock effect in turns

[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

[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]

Originally Posted by ImbracableCrunk

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/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.

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):

Originally Posted by Shevell, p306

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:

Originally Posted by Shevell, p299

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.

[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]

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.

[PBL]

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

Originally Posted by 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

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