Weathervaning - what actually turns an aircraft in flight.

alfadog said:
But at some point, the nose of the airplane would be pointed back from where it started, right?
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Not necessarily, it all depends on the initial conditions. With high initial speed and small turning force (small bank) it may never complete a 360 deg turn.
The problem there is that we have not introduced any force that would cause such a effect.
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:confused: Say what, what problem? the constant turn is caused by the force perpendicular to the the speed vector. I fail to see any 'problem'.

BTW: what relevance does it all have to our Earthy Cessna?
 
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olasek said:
Say what, what problem? the constant turn is caused by the force perpendicular to the the speed vector. I fail to see any 'problem'.
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Two forces, no matter what their directions or magnitudes, acting on a single point can never cause a body attached to that point to rotate. You actually need two forces of equal magnitude operating in exactly opposite directions but acting on two different points to cause the body attached to those points to rotate. (If they are not equal and opposite they will also yield a translation.) The product of force magnitudes and distance is known as a couple in mechanics.

The sum of all forces on a body can be reduced to a force yielding pure translation and another yielding pure rotation. Any number of ways of generating rotational force in aircraft.
 
olasek said:
Not necessarily, it all depends on the initial conditions. With high initial speed and small turning force (small bank) it may never complete a 360 deg turn.
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Make it any bank angle you want, you will never turn the airplane around.

:confused: Say what, what problem? the constant turn is caused by the force perpendicular to the the speed vector. I fail to see any 'problem'.

BTW: what relevance does it all have to our Earthy Cessna?
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Because, on Earth we have the second force that will form the force couple and turn the airplane around. The pretty drawing called it "stability" but did not show it. That is OK with me as it is the thing that provides stability in yaw. But "stability" is not a placard in the cockpit, it is force acting on the airframe. If the artist were to include that force in his sketch then it would be accurate and show what really turns an airplane.
 
alfadog said:
...The pretty drawing called it "stability" but did not show it. That is OK with me as it is the thing that provides stability in yaw. But "stability" is not a placard in the cockpit, it is force acting on the airframe. If the artist were to include that force in his sketch then it would be accurate and show what really turns an airplane.
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The succession of planes were intended to illustrate the effect of the "weathervaning" forces that are acting on the airframe.

How would one best illustrate the weathervaning force related to "stability"?

Something like this?

Turns_Weathervaning_Forces.jpg


It's my intuition that "weathervaning" is referring to the fact that when the RW is offset due to the horizontal component of lift, as shown, then the rearward portion of the airplane presents more surface area to the RW than does the foreward portion, so the plane begins rotating its tail to try to make that imbalance goes away - and equivalently speaking to try to make the nose face into the RW - just like a weathervane:

rooster-weathervane-chanticleer-head-facing-back-w.jpg
 
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alfadog said:
Absolutely, it is a combination of, basically, two forces that act more or less at the same time to cause what we all consider a turn in flight. My point is that students are presented this erroneous and oversimplified concept that just one force, the horizontal component of lift, is responsible. And they will defend that misconception.
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It's probably better to think of it as correct, but incomplete.
 
alfadog said:
Make it any bank angle you want, you will never turn the airplane around.
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Yeah, based on this statement I finally know where you got it wrong.
This Lunar Cessna can certainly turn around and fly circles or curved path based on its ground track but its nose may never align with its speed vector. So what? When we say 'turn' me mean how ground track looks like and not where aircraft's nose is pointing.

So this earthy Cessna has this 'stability' like you call it because the aircraft on this planet can't really fly backwards or sideways but this Lunar Cessna certainly can and this is a good example that essence of turning really is what ground track is doing and not which way nose is pointing. So if this is supposed to be such phenomenal insight into mechanics of aircraft turns that required a separate thread and close to 90 posts - that airplanes on this planet can't fly sideways - so bid, I rest my case.

register@teamandras.com said:
It's probably better to think of it as correct, but incomplete.
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I could agree to that.
 
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SixPapaCharlie said:
OK, Cessna in space, with rockets pulling from the front but tethered, A rocket in the center of lift beneath, and 2 rockets on the ailerons.

We're all on the same page now right?

So how does an airplane turn?

999926261797.jpg
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With a rig like that, you need steering rockets that point slightly forward and then use them to align the thrust vector from the main engines in the direction you want and then ignite them to take you in the new direction.

You don't fly through space, you boost yourself around on vectored thrust.
 
alfadog said:
...My point is that students are presented this erroneous and oversimplified concept that just one force, the horizontal component of lift, is responsible. And they will defend that misconception.
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This is what I disagree with because I certainly was never taught any such concept nor am I aware of it being presented in any handbooks or other published material I've ever read.

Am I missing something here? :dunno:
 
register@teamandras.com said:
The succession of planes were intended to illustrate the effect of the "weathervaning" forces that are acting on the airframe.

How would one best illustrate the weathervaning force related to "stability"?

Something like this?

Turns_Weathervaning_Forces.jpg


It's my intuition that "weathervaning" is referring to the fact that when the RW is offset due to the horizontal component of lift, as shown, then the rearward portion of the airplane presents more surface area to the RW than does the foreward portion, so the plane begins rotating its tail to try to make that imbalance goes away - and equivalently speaking to try to make the nose face into the RW - just like a weathervane:

rooster-weathervane-chanticleer-head-facing-back-w.jpg
Click to expand...

That is really good! And yes, the original is correct but not complete.
 
Silvaire said:
This is what I disagree with because I certainly was never taught any such concept nor am I aware of it being presented in any handbooks or other published material I've ever read.

Am I missing something here? :dunno:
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I cannot speak to what you were taught but would love to see a quote from some primary training manual other that Stick and Rudder that explains the subject of turning the airplane more completely than "banking the airplane, horizontal component of lift, turn."
 
olasek said:
Yeah, based on this statement I finally know where you got it wrong.
This Lunar Cessna can certainly turn around and fly circles or curved path based on its ground track but its nose may never align with its speed vector. So what? When we say 'turn' me mean how ground track looks like and not where aircraft's nose is pointing.

So this earthy Cessna has this 'stability' like you call it because the aircraft on this planet can't really fly backwards or sideways but this Lunar Cessna certainly can and this is a good example that essence of turning really is what ground track is doing and not which way nose is pointing. So if this is supposed to be such phenomenal insight into mechanics of aircraft turns that required a separate thread and close to 90 posts - that airplanes on this planet can't fly sideways - so bid, I rest my case.


I could agree to that.
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You are getting closer but in the Lunar 172 there is no force ever acting rearward so, if you start out moving north, you will never move south no matter how much you bank.
 
alfadog said:
I cannot speak to what you were taught but would love to see a quote from some primary training manual other that Stick and Rudder that explains the subject of turning the airplane more completely than "banking the airplane, horizontal component of lift, turn."
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But that's the reality of it. When we introduce a bank, we introduce a horizontal component of lift towards the inside of the bank. Since this subtracts from the vertical component of lift, in order to maintain level flight, we have to increase the AoA of the wing, and to maintain speed, we have to add power. If we add power in a turn, then we don't have to pull back because the trim will do it for us. If we do not add power r trim, we have to pull back on the elevator to direct the correct reduction in airspeed required to transfer to additional lift.

Excess energy vectored through bank angle is what turns the plane in a normal coordinated turn.

Can someone use the rudder to push the plane through a turn on the yaw axis using longitudinal stability played off the asymmetrical slipstream? Most definitely, this is the least energy efficient way to go about it though because of the waste in friction of an inefficient surface. However at the bottom of the energy regime, it may be the last effective control. Hammerheads are kick through maneuvers with opposed ailerons to prevent spinning.
 
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Henning said:
But that's the reality of it. When we introduce a bank, we introduce a horizontal component of lift towards the inside of the bank. Since this subtracts from the vertical component of lift, in order to maintain level flight, we have to increase the AoA of the wing, and to maintain speed, we have to add power. If we add power in a turn, then we don't have to pull back because the trim will do it for us. If we do not add power r trim, we have to pull back on the elevator to direct the correct reduction in airspeed required to transfer to additional lift.

Excess energy vectored through bank angle is what turns the plane in a normal coordinated turn.

Can someone use the rudder to push the plane through a turn on the yaw axis using longitudinal stability played off the asymmetrical slipstream? Most definitely, this is the least energy efficient way to go about it though because of the waste in friction of an inefficient surface. However at the bottom of the energy regime, it may be the last effective control. Hammerheads are kick through maneuvers with opposed ailerons to prevent spinning.
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Silvaire said:
This is what I disagree with because I certainly was never taught any such concept nor am I aware of it being presented in any handbooks or other published material I've ever read.

Am I missing something here? :dunno:
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Silvaire, would you mind straightening Henning out for us?
 
alfadog said:
Silvaire, would you mind straightening Henning out for us?
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He's probably not seen it published because it is not a sensible technique to publish. You have to hold a slip to make it turn that way, otherwise the natural tendency of the plane will be to lift the outside wing with application of rudder. The plane does not want to be forced through the turn using the fuselage as the directing surface.
 
So considering that the center of lift is aft of the center of gravity, does that undermine the standard explanation that adverse yaw is caused (primarily) by increasing induced drag on the rising wing? Wouldn't an adverse couple exist so long as the lift vector is deflected throughout the turn?

Damn its been a long time since undergrad flight dynamics. Prof. K, help!
 
DJTorrente said:
So considering that the center of lift is aft of the center of gravity, does that undermine the standard explanation that adverse yaw is caused (primarily) by increasing induced drag on the rising wing? Wouldn't an adverse couple exist so long as the lift vector is deflected throughout the turn?

Damn its been a long time since undergrad flight dynamics. Prof. K, help!
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Yes. In our Lunar 172 the location of the center of mass forward of the center of lift would cause adverse yaw. In a real airplane, I do not how much of the adverse yaw would be attributable to that.
 
alfadog said:
Yes. In our Lunar 172 the location of the center of mass forward of the center of lift would cause adverse yaw. In a real airplane, I do not how much of the adverse yaw would be attributable to that.
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Maybe not. In level flight, wing lift and tail down force combine to negate any moment around the CG. When the lift vector is tilted to one direction, the tail down force vector is tilted in the opposite direction by an equal angle. The same combination of torques that maintain a stable pitch attitude in level flight should also negate any yaw when the aircraft is banked*.

*to a first-order approximation -- this does not specifically account for the typical control input in a turn being increased back elevator, namely increased tail down force, to maintain altitude. Any increased down force, being banked, would add yaw into the turn and help the aircraft turn.
 
There wouldn't be a CG in the lunar version right?
 
olasek said:
No, not the entire world but the "turn" we are discussing here. First order is enough to have a good grip what turn radius it is going to be and all you need is horizontal force. And turn R = v*v*W/g/LH (v = aircraft speed, LH = horizontal force, W = aircraft weight).
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While true, it's HIGHLY misleading.

Explain a forward slip with that. What's the LH?

First order is NOT enough. It says that a given bank angle gives you a given turn rate. And it is not so. I can place my airplane in a 15 deg bank and get ZERO turn rate. I can also place my airplane in zero bank and get a standard rate turn (at altitude, of course; I don't like skidding turns anywhere near the ground). What's the "LH" for that?

You're getting confused about net forces. That can really only be done if all the forces act on the same point. And they do not.

And one does not just assert that forces are first order. You missed some REAL important ones. A first order approximation requires ALL the first order forces.
 
MAKG1 said:
There is obvious dihedral, and quite a lot of it. Look at the bottom surface of the wing. Plus, that bird has thrust asymmetry to play with.
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There are some videos on Youtube that show the B-2 in flight and it is interesting to watch the control surfaces as it makes banking turns. Looks more like an application of asymmetrical drag to handle yaw rather than engine thrust. The engines appear to be too close inboard to give decent yaw control, though no doubt that can be done.
 
MAKG1 said:
While true, it's HIGHLY misleading.
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No, it is not, the formula is clearly for coordinated turn and has reasonable accuracy when you test in real world. You may invent a zillion of new objections but it won't invalidate this formula. Many aviation sources (including FAA's Handbook of Aviation Knowledge) show equivalent formula as a fair approximation.
 
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olasek said:
No, it is not, the formula is clearly for coordinated turn and has reasonable accuracy when you test in real world. You may invent a zillion of new objections but it won't invalidate this formula. Many aviation sources show equivalent formula as a fair approximation.
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Who said the turn was coordinated? The question was about what turns the airplane. Uncoordinated turns still turn the airplane. In the case of the boat turn, it's just rudder and thrust with some aileron applied to offset the induced roll moment and keep it level, a very different answer from what the oversimplifications say.

You can learn stuff from simplifying, but you have to be careful or you learn wrong stuff.
 
MAKG1 said:
The question was about what turns the airplane.
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And it was settled long time ago, we are clearly talking how 99.99% turns are made and not about some cute examples of yours :mad2:
 
olasek said:
And it was settled long time ago, we are clearly talking how 99.99% turns are made and not about some cute examples of yours :mad2:
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Only one turn out of 10,000 is uncoordinated?

Hot damn you must be amazing. And you must do a hell of a lot of landings to offset all the student pilots.

Don't make up statistics.

I witnessed several slipping turns last weekend to get some nadir photos. That's 45 deg bank. It's helpful not to be turning too much in order to overfly the target accurately. Sorry, the idealized world just isn't the real world.
 
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acrophile said:
There are two senses of "turning": changing the direction of flight, and yawing.

When you bank in order to turn, the direction of flight changes because of the new horizontal lift component of the airflow over the banked wings.

As the direction of flight changes, the airplane yaws due to weathervaning, affected and assisted by coordinated use of the rudder.
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:yes: This explanation is accurate and concise.
 
acrophile said:
There are two senses of "turning": changing the direction of flight, and yawing.

When you bank in order to turn, the direction of flight changes because of the new horizontal lift component of the airflow over the banked wings.

As the direction of flight changes, the airplane yaws due to weathervaning, affected and assisted by coordinated use of the rudder.
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Absolutely correct.
 
petrolero said:
:lol: at this thread.
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Yeah, this seems to have gotten out of hand, the other stayed more scientific. I think a lot of pilots when asked what turns the plane, would just say "I do." Just like when you're riding a bike, you want to go left, so you go left. But the physics are very complicated. So what turns a bicycle? *can of worms OPENED*
 
JoeFromKS said:
Yeah, this seems to have gotten out of hand, the other stayed more scientific. I think a lot of pilots when asked what turns the plane, would just say "I do." Just like when you're riding a bike, you want to go left, so you go left. But the physics are very complicated. So what turns a bicycle? *can of worms OPENED*
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The handlebars.
 
alfadog said:
I cannot speak to what you were taught but would love to see a quote from some primary training manual other that Stick and Rudder that explains the subject of turning the airplane more completely than "banking the airplane, horizontal component of lift, turn."
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Well picked out of a copy I have of the 2004 version of The "Airplane Flying Handbook FAA-H-8083-3A"

All four primary controls are used in close coordination
when making turns. Their functions are as follows.
• The ailerons bank the wings and so determine the
rate of turn at any given airspeed.
• The elevator moves the nose of the airplane up or
down in relation to the pilot, and perpendicular to
the wings. Doing that, it both sets the pitch attitude
in the turn and “pulls” the nose of the airplane
around the turn.
• The throttle provides thrust which may be used for
airspeed to tighten the turn.
• The rudder offsets any yaw effects developed by
the other controls. The rudder does not turn the airplane
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and that's pretty much what I was taught about 40 years ago. Granted the wording may not be scientific as it's not a publication meant for scientists but I believe the fundamentals are correct.
 
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