Weathervaning - what actually turns an aircraft in flight.

alfadog

alfadog

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alfadog
From another thread, the suggestion was made to start a new thread. I would like to get some feedback on my opposition to what most folks, if not all, are taught is the cause of an airplane turning in flight.

I have often taken exception with the concept that banking the airplane causes it to turn. That would caused only translational movement with the nose still pointed straight ahead if not for something else at work. Linear forces in different directions acting on the same point (the center of lift) cause a resultant translation, not rotation. Rotation requires a force couple, two opposite forces separated by a moment arm. What turns an airplane, IMO, is weathervaning, i.e. the force couple caused by the horizontal component of lift in a bank opposed by the center of sidewise resistance to the air being well behind the center of lift, this caused by the abundance of side surface area to the rear. Rich Stowell likes to say that the elevator turns the airplane but it would be more correct to say the empennage and tailfeathers turn the airplane, divided between vertical and horizontal parts of those depending on angle of bank.

:popcorn:
 
Ok, now I'm considering Euthanasia Henning.
 
alfadog said:
What turns an airplane, IMO, is weathervaning,
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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.
 
The problem with the argument on what turns an aircraft are the differing opinions on the definition of "turn." A turn is a curvature over the ground. Ailerons bank an aircraft along the longitudinal axis. The elevator causes a pitching moment around the lateral axis and the rudder a yawing moment around the vertical. The horizontal component of lift that the wing provides makes the turn. The ailerons get the wings into a position to create horizontal lift and the elevator, assisted by the rudder modulate how much horizontal lift.

The video Rich Stovell provides tries to prove the elevator turns the aircraft by showing you can bank and not turn (uncoordinated flight). Sure you can use opposing forces during the bank and not turn. Same goes for a helicopter if you were doing an Aussie slide. That still doesn't mean the elevator is the force causing the turn.

What's debatable is the primary flight control surface responsible for the turn. Well they're all responsible in a coordinated turn. I'd saying most aircraft, without the ailerons, you won't get much of a turn in an airplane.
 
Is the plane on a treadmill?






:D
 
You're arguing semantics.

The control surfaces and throttle turn the airplane. All of them together. They don't separate like steering and brakes on a car. Turn the ailerons alone? You'll turn. Turn the rudder alone? You'll turn. Adjust the elevator? That will also make you turn (up = left, down = right on a level US plane; it will adjust the turn rate if you are already turning). Adjust the throttle? You'll turn there, too.

It really doesn't matter. You need to adjust all the controls to get the attitude and turn rate you desire. Whatever it takes.
 
In college physics there is a classic problem that involves finding the radius and bank based on a certain speed of a car going around a racetrack that will cancel out all the centrifigal forces so the car is banked and the passengers wont feel any sideways force, all downforce perpendicular to the racetrack. And suprisingly, as I recall the radius doesnt matter. It cancels out. Anyway, its THAT problem. Of course it involves algebra and trig, no calculus necessary on that one as I was taught it. Might be able to bring calculus into it somehow, Im not sure. We are all up on our algebra, trigonomety and calculus amd physics right??

I suggest you get a physics book and work the problems in the back of the chapter on that one. The same principals apply to an airplane in a coordinated bank.

These attempts at "physics discussions" by people who dont really know much (the people that do know much, dont even try on a board like this) dont really lead to much understanding.

As a pilot I know what turns the plane. Ailerons will by themselves, so will rudders. Put them together for a coordinated turn. Feel the increase in downward force. Marvel at the nice bank with no swaying towards the outside of the turn (like you would in a car). Push on the rudders and feel the "yaw" and the side forces pushing you against the side of the airplane. This is as good as it gets.

As a previous physics and engineering student, I'll say that if you really want to understand it, you have to do the math. And even then there will be gaps in understanding. We really dont understand, fundamentally what gravity is. It just is. So thats a pretty big gap right there. Not suprising people dont understand problems like this completely. A COMPLETE answer isn't available--to anyone, even Einstein.

Now someone can make some obscure statement about how Im all wrong and so forth because really the elevator turns the plane. But that is my contribution.
 
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MAKG1 said:
You're arguing semantics.

The control surfaces and throttle turn the airplane. All of them together. They don't separate like steering and brakes on a car. Turn the ailerons alone? You'll turn. Turn the rudder alone? You'll turn. Adjust the elevator? That will also make you turn (up = left, down = right on a level US plane; it will adjust the turn rate if you are already turning). Adjust the throttle? You'll turn there, too.

It really doesn't matter. You need to adjust all the controls to get the attitude and turn rate you desire. Whatever it takes.
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I am pointing out a fault in what is commonly taught as the aerodynamic basis of turning, not what control inputs turn the airplane. If that were the discussion, we might as well say the autopilot turns the airplane :yesnod:
 
alfadog said:
I am pointing out a fault in what is commonly taught as the aerodynamic basis of turning, not what control inputs turn the airplane. If that were the discussion, we might as well say the autopilot turns the airplane :yesnod:
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Control inputs != control surfaces.

Even on some spam cans (e.g., Ercoupe).

It doesn't mean much to say "the wing turns the airplane" or "the elevator turns the airplane." No, they both work together, and if you try to separate them, you end up with a smoking hole in the ground.

You can adjust the direction of all four forces with ALL control surfaces.

That's the fault. Reduction of the airplane to parts that don't make sense in isolation. An airplane that can't control pitch, roll, AND yaw at a minimum, all at the same time, cannot fly for any length of time.
 
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After banking, the ailerons go neutral.

The elevator turns the plane.

Everybody knows that ... ;)
 
Velocity173 said:
What's debatable is the primary flight control surface responsible for the turn. Well they're all responsible in a coordinated turn. I'd saying most aircraft, without the ailerons, you won't get much of a turn in an airplane.
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No, I learned to fly radio control using throttle, elevator and rudder only, it flew fine but took lots of up elevator to hold constant altitude in the turn. Haven't tried it full scale as I've never seen one without ailerons.

When the plane banks, lift separates into horizontal and vertical components. Horizontal lift will cause your flight path to diverge from a straight line. Because the vector sum must remain constant, vertical lift decreases and you either lose altitude, add elevator and/or add throttle.

Elevator will "turn" the aircraft only in the vertical plane. Yes, you can fly straight while banking if you apply healthy rudder, it's called a slip.
 
Everybody here is wrong. The force that turns the airplane is money...:yes:
 
Zeldman said:
Everybody here is wrong. The force that turns the airplane is money...:yes:
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I stand corrected. :yesnod:

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The classic explanation is in Stick and Rudder, along with a great illustration of the weathervaning tendancy as the true cause of a turn. Short version is the horizontal surfaces of the tail cause the plane to turn. In straight and level cruising flight the weathervaning tendancy is at work too, but it's just off-setting the effect of gravity. If you could suddenly switch off gravity the plane would start looping (turning) in a vertical plane.

dtuuri
 
Hank S said:
No, I learned to fly radio control using throttle, elevator and rudder only, it flew fine but took lots of up elevator to hold constant altitude in the turn. Haven't tried it full scale as I've never seen one without ailerons.

When the plane banks, lift separates into horizontal and vertical components. Horizontal lift will cause your flight path to diverge from a straight line. Because the vector sum must remain constant, vertical lift decreases and you either lose altitude, add elevator and/or add throttle.

Elevator will "turn" the aircraft only in the vertical plane. Yes, you can fly straight while banking if you apply healthy rudder, it's called a slip.
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Yeah I grew up flying RC 3 channel planes as well but rudder and elevator are by no means as effective in banking an aircraft as aileron.
 
alfadog said:
Linear forces in different directions acting on the same point (the center of lift) cause a resultant translation, not rotation.
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This is bad Physics.
The easiest way to show that this is a flawed reasoning is to consider a ball on a table attached to a rope and spinning around the center, centrifugal force keeps the rope taut. The only force that causes the ball to turn is the force that keeps the rope taut, per your logic this only force would be "translation", there are here no "two forces separated by a moment arm" and yet the object turns. By the way as seen in this example you don't need the concept of "weathervaning" or which way the plane's fuselage points to explain a turn.
 
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And you guys might want to be on the lookout for our dear departed buddy CTLSi resurfacing here under a new name...given his aerodynamic expertise. I'll bet he can finally clear up this turning mystery. :lol:
 
It was figured out fairly early that you were going to have to have some form of lateral roll control to successfully fly an airplane. The Aerial Experiment Association tried to do without in order to not infringe upon the Wright Brother's wing warping patent until Alexander Graham Bell pretty much invented the aileron. Still, they knew it was possibly a patent infringement and the Wrights sued them, and specifically Glenn Curtis over it since he was the American in the group.

Anyway, if you are going to turn an airplane without ailerons it's gonna be sloppy so you can't take credit away from them. Same goes for rudder and elevator. If you are going to assign credit to any single control you end up with a crummy turn.
 
olasek said:
This is bad Physics.
The easiest way to show that this is a flawed reasoning is to consider a ball on a table attached to a rope and spinning around the center, centrifugal force keeps the rope taut. The only force that causes the ball to turn is the force that keeps the rope taut, per your logic this only force would be "translation", there are here no "two forces separated by a moment arm" and yet the object turns. By the way as seen in this example you don't need the concept of "weathervaning" or which way the plane's fuselage points to explain a turn.
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Wow, just Wow. And you managed to do that without even mentioning the term, centripetal! I'm impressed.

edit: sorry for the snark, but when you call good physics bad and follow it up with that, well...

The example you give is not relevant because the force vector of horizontal lift is not contrained to always "point" at some distant center of rotation as is the rope in the swinging weight.
 
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Ron Levy said:
Try Dr. John Denker's excellent "See How It Flies" web site, starting here and then following the links.
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Actually, I would have been quite disappointed if Dr. Denker did not agree with me on this and, wouldn't you know it, he does and makes almost the exact same point that I make.
 
alfadog said:
he does and makes almost the exact same point that I make.
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I'm lost on what that is, but maybe I'm not paying hard attention.

I did notice your mention of "weathervaning" causing a turn. Where do flying wings fit with that concept?
 
RoscoeT said:
I'm lost on what that is, but maybe I'm not paying hard attention.

I did notice your mention of "weathervaning" causing a turn. Where do flying wings fit with that concept?
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My point is that simply holding the lift vector at an angle to the horizontal can never produce curving flight where you can turn completely around. It cannot yaw or rotate the airplane. You need a second and opposing force to create rotation. As pilots, we know that instintively but folks are still taught only one half of the story as if that is the whole story.
 
Air makes the plane turn.
In space your control surfaces would do nothing.
 
alfadog said:
My point is that simply holding the lift vector at an angle to the horizontal can never produce curving flight where you can turn completely around. It cannot yaw or rotate the airplane. You need a second and opposing force to create rotation.
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Are you saying that in a left banked turn, there is airflow hitting the left side of the vertical fin "weathervaning" the airplane, helping it yaw around the turn? The elevator rotates the airplane about the center of lift, but I'm not connecting the term "weathervaning" to the elevator.
 
alfadog said:
My point is that simply holding the lift vector at an angle to the horizontal can never produce curving flight where you can turn completely around. It cannot yaw or rotate the airplane. You need a second and opposing force to create rotation. As pilots, we know that instintively but folks are still taught only one half of the story as if that is the whole story.
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Never? Are you sure about that?

In almost all aircraft, the center of lift and the center of gravity are not in the same place. That makes a torque with neutral elevator. In the correct direction if the center of lift is forward of the center of gravity.
 
SixPapaCharlie said:
Air makes the plane turn.
In space your control surfaces would do nothing.
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Very good. And the name for air making something turn by striking the side of it is called weathervaning.

You bring up a good tool for understanding this. Let us take our rocket-powered Cessna 172 to the Moon. The airframe looks exactly the same, wings, tail, etc. We have a big rocket in front for thrust (work with me) and we have another big rocket on the wing at the center of lift (on a pedestal, or whatever). We have small thrusters where the ailerons would be so we have roll control.

Now, we are flying along straight and level about 1000 feet above the surface of the Moon (yes, we have pitch control) and roll into a 30° left bank and hold that. What does our C172 do?
 
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it would roll and roll and roll and roll until you gave it opposite thrust to stop the roll
 
SixPapaCharlie said:
it would roll and roll and roll and roll until you gave it opposite thrust to stop the roll
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That is what happens on Earth, also. We stop the roll at 30° and hold it there. What does the Cessna do?
 
MAKG1 said:
Never? Are you sure about that?

In almost all aircraft, the center of lift and the center of gravity are not in the same place. That makes a torque with neutral elevator. In the correct direction if the center of lift is forward of the center of gravity.
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Ah, but the CG of the airplane is always forward of the center of lift so that can't be it.
 
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