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Author Topic: Why does an aeroplane dip one wing to turn?  (Read 7331 times)

Offline chris

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Why does an aeroplane dip one wing to turn?
« on: 29/07/2012 22:59:21 »
Why do aeroplanes dip a wing on the side facing into the turn? Is this absolutely necessary? Why not use the rudder?


 

Offline Geezer

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Re: Why does an aeroplane dip one wing to turn?
« Reply #1 on: 29/07/2012 23:41:04 »
Why do aeroplanes dip a wing on the side facing into the turn? Is this absolutely necessary? Why not use the rudder?

An aeromobile has to generate a significant amount of reactive thrust (centripetal force) from the air to make it change direction, otherwise it would tend to go straight on. Banking (tipping over) produces a large reactive force between the wing surfaces and atmospheric air. The rudder only determines the direction in which the nose of the aeromobile is pointing.

Try sticking a rudder on your bike and see if it will make you change direction.
 

Offline chris

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Re: Why does an aeroplane dip one wing to turn?
« Reply #2 on: 29/07/2012 23:56:13 »
Thanks; but the rudder will also generate a force pushing the tail away from the direction of travel; why's this not sufficient? (I don't like the bike analogy because aircraft are usually airborne, rather than driving along on their wheels!) Also, can you step through the physics of the reactive forces that you alude to above please?
 

Offline RD

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Re: Why does an aeroplane dip one wing to turn?
« Reply #3 on: 29/07/2012 23:59:53 »
When the plane yaws the fuselage will present a greater cross-section to the air flow: more drag ...



No so when it rolls ...


« Last Edit: 30/07/2012 00:10:19 by RD »
 

Offline Geezer

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Re: Why does an aeroplane dip one wing to turn?
« Reply #4 on: 30/07/2012 00:21:48 »
Thanks; but the rudder will also generate a force pushing the tail away from the direction of travel;

That's because it will only cause the aeromobile to rotate around its center of mass. That's useful for aligning with runways and the like, but it won't make the aircraft change direction. An airliner zipping along a 400 mph or so has a huge amount of momentum that will tend to make it travel in a straight line, and it takes a lot of force to cause it to deviate from that straight line.

The aircraft's wings accelerate a mass of air that is about perpendicular to the plane of the wings. In straight flight, the force produced by that acceleration balances the force of gravity to prevent the aircraft from losing altitude. When the aircraft banks, the vector of the force is no longer vertical, and the horizontal component produces the centripetal force that causes the aircraft to turn.

If the aircraft is trying to maintain altitude during a turn, the engines rev up during the turn. That's because it takes a bit more energy to maintain altitude and also generate enough lateral thrust from the wings to execute the turn.
 

Offline Don_1

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Re: Why does an aeroplane dip one wing to turn?
« Reply #5 on: 30/07/2012 11:46:19 »
Because of the way air flows over and under the wings, banking creates more or less drag as well as lift. Banking left will create less lift and more drag to the left wing and more lift and less drag to the right wing. Hence the plane will turn to the left. The more bank the tighter the turn. Altitude can be lost in steep banking, so must be compensated for with increased thrust.

The rudder is used for steering only when the plane is on the ground, to taxi to/from the runway. Otherwise, the purpose of the rudder is to control yawing. During straight line flight, the rudder will be in a fixed position to compensate for yaw.

When landing in a side wind, the pilot will bank the plane to bring the plane's flightpath in line with the runway, this can result in the plane landing with its nose pointing toward the windward side, yet its direction (flightpath) is in line with the runway. Upon touchdown, the rudder will then be used to steer the plane.
 

Offline Geezer

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Re: Why does an aeroplane dip one wing to turn?
« Reply #6 on: 30/07/2012 17:14:12 »

Banking left will create less lift and more drag to the left wing and more lift and less drag to the right wing. Hence the plane will turn to the left.


Not sure that's quite right Prof. I believe it's the difference in lift on either side that makes the plane bank rather than the other way around, if you see what I mean. The difference in lift is caused by the pilot adjusting the control surfaces on the wings.

To get the plane to turn, there has to be a lateral force continuously acting on the plane for the duration of the turn, similar to the lateral force exerted by a car's tyres on the road during a turn. The only means available for the aircraft to do this is to "push" against the air (it really has at accelerate air to generate that force) and it takes a lot of force.

In level flight, the force produced when the wings redirect air acts vertically to counteract the dreaded gravity. When the plane banks, the force produced by the wings is no longer only pointing "straight down". You can think of it as having two components. One component pointing straight down, and another component at right angles to that producing a lateral force on the aircraft. That's what produces the turn.
« Last Edit: 30/07/2012 17:17:55 by Geezer »
 

Offline Don_1

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Re: Why does an aeroplane dip one wing to turn?
« Reply #7 on: 31/07/2012 01:56:16 »

Banking left will create less lift and more drag to the left wing and more lift and less drag to the right wing. Hence the plane will turn to the left.


Not sure that's quite right Prof. I believe it's the difference in lift on either side that makes the plane bank rather than the other way around, if you see what I mean. The difference in lift is caused by the pilot adjusting the control surfaces on the wings.


I didn't make myself very clear at all there, did I?
 

Offline Geezer

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Re: Why does an aeroplane dip one wing to turn?
« Reply #8 on: 31/07/2012 06:13:14 »

I didn't make myself very clear at all there, did I?


Fear not! You were pretty damn close.

The funny thing is that this is just an example of Newton's good old F=ma, which is just as well, because that's about as far as my Physics ever went.
 

Offline Don_1

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Re: Why does an aeroplane dip one wing to turn?
« Reply #9 on: 31/07/2012 11:42:17 »

I didn't make myself very clear at all there, did I?


Fear not! You were pretty damn close.


So, let me clarify myself. Along the tail edge of the main wings are four adjustable wing sections. The Flaps work in unison with each other to increase drag and thereby reduce airspeed during landing. These are controlled by a lever in the cockpit. Then there are the Allerons, these work in opposition to each other and are controlled by the wheel on the column in the cockpit. Turning the wheel left will result in the left wing Alleron extending upwards, creating more drag and downward force on the left wing. At the same time, the Alleron on the right wing will tilt downward creating more lift. The plane will bank and turn to the left. In the main body of the wings of large aircraft is a Spoiler on the top of the wing, this creates more drag on the wing when raised, in this instance the left wing Spoiler would rise to increase the drag on the left wing.

On the tailplane section, the small wings are the horizontal stabilisers.  Along the tail edge of these are the Elevators, which work in unison to raise or lower the tail section of the aircraft. They are controlled by pulling or pushing on the steering column. Pull back on the column and the elevators will hinge up, pushing the tail down. This results in more of the underside of the main wings being presented to the airflow and the plane ascends. Push forward on the column and more of the top side of the main wing is presented to the airflow. The plane descends.

Contrary to popular depiction in films, the elevators are not used to get the plane off the ground. Pulling back on the column while thundering down the runway, would result in the tail being pushed down on to the runway. Lift is achieved when the airflow across the wings is great enough to get the plane airborne. Only after the plane is airborne and clear of the ground, can the elevators be used to put the plane into a climb. Even then, the rate of climb will be dependant on air speed and the aircraft’s capability. Going into too steep a climb, particularly at this time, can result in a drop in air speed, resulting in insufficient lift and the plane ‘stalls’ and loses height. 

Also on the tail section is the fin or vertical stabiliser. This keeps the aircraft flying in a straight line, working in much the same way as a keel on a boat. But like the keel on a boat is subject to current, the vertical stabiliser is subject to the force of cross winds. Along the tail section of the vertical stabiliser is a hinged rudder. This can be set to counter the effects of crosswinds. Under manual operation, the rudder is controlled by foot pedals, but it can also be set in a fixed position by a lever in the cockpit.

There! That’s a little better, though doubtless Geezer will point out any further errors or omissions I have made.

BTW, reading this does not equip you with knowledge to take the controls of an Airbus A380.


Yours truly and a none too impressed instructor on his first and only flying lesson. Reading the above, can you wonder???
 

Offline Lmnre

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Re: Why does an aeroplane dip one wing to turn?
« Reply #10 on: 31/07/2012 15:33:25 »
When the aircraft banks, the vector of the force is no longer vertical, and the horizontal component produces the centripetal force that causes the aircraft to turn.
Yes, and the vertical component is not so great, so it does not counter the weight vector as it did before, and the plane loses altitude. I know because I've done it.

Banking left will create less lift and more drag to the left wing and more lift and less drag to the right wing. Hence the plane will turn to the left.

I think you mean that the plane will yaw to the left. However, this analysis is incorrect. Banking left will produce "adverse yaw" to the right, which the pilot would correct with some left rudder, resulting in what's called a "coordinated turn".
 

Offline Lmnre

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Re: Why does an aeroplane dip one wing to turn?
« Reply #11 on: 31/07/2012 23:27:39 »
Contrary to popular depiction in films, the elevators are not used to get the plane off the ground. Pulling back on the column while thundering down the runway, would result in the tail being pushed down on to the runway. Lift is achieved when the airflow across the wings is great enough to get the plane airborne. Only after the plane is airborne and clear of the ground, can the elevators be used to put the plane into a climb.
I'm confused
Quote
The nose is raised to a nominal 5°–15° nose up pitch attitude to increase lift from the wings and effect liftoff. For most aircraft, attempting a takeoff without a pitch-up would require cruise speeds while still on the runway.
source

And this video shows an airliner pitching up while on the ground, the nose wheels also lifting off first.
 

Offline Geezer

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Re: Why does an aeroplane dip one wing to turn?
« Reply #12 on: 01/08/2012 03:04:39 »
Oky-doky :)

I think we have some broad agreement that a plane has to shove (a technical term) a certain amount of air sideways to make a turn.

Aeromobiles are fairly complicated bits of kit, so it might be an idea launch other questions about how they manage to do all the things they are capable of. Only sayin'.


 

Offline Don_1

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Re: Why does an aeroplane dip one wing to turn?
« Reply #13 on: 01/08/2012 10:31:39 »

Banking left will create less lift and more drag to the left wing and more lift and less drag to the right wing. Hence the plane will turn to the left.

I think you mean that the plane will yaw to the left. However, this analysis is incorrect. Banking left will produce "adverse yaw" to the right, which the pilot would correct with some left rudder, resulting in what's called a "coordinated turn".

I think you will find this only applies in extreme banking.

Contrary to popular depiction in films, the elevators are not used to get the plane off the ground. Pulling back on the column while thundering down the runway, would result in the tail being pushed down on to the runway. Lift is achieved when the airflow across the wings is great enough to get the plane airborne. Only after the plane is airborne and clear of the ground, can the elevators be used to put the plane into a climb.
I'm confused
Quote
The nose is raised to a nominal 5°–15° nose up pitch attitude to increase lift from the wings and effect liftoff. For most aircraft, attempting a takeoff without a pitch-up would require cruise speeds while still on the runway.
source

And this video shows an airliner pitching up while on the ground, the nose wheels also lifting off first.

In the case of heavy aircraft, extra lift is attained with the use of spoilers and/or flaps on the main wings not by use of the elevators. In that clip you can clearly see the flaps extended.
« Last Edit: 01/08/2012 10:33:48 by Don_1 »
 

Offline Lmnre

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Re: Why does an aeroplane dip one wing to turn?
« Reply #14 on: 01/08/2012 20:38:39 »
I think you will find [adverse yaw effect] only applies in extreme banking.

Perhaps noticeable in extreme banking, yet the opposite certainly does not occur in mild or moderate banking.

In the case of heavy aircraft, extra lift is attained with the use of spoilers and/or flaps on the main wings not by use of the elevators. In that clip you can clearly see the flaps extended.

Yes, flaps for extra lift at takeoff, yet the nose wheels clearly left the ground first, indicative of what's called "rotation" [about the pitch axis], which elevators (and not spoilers, flaps, etc) produce. This rotation (the nose wheels lifting off first) is also shown in all these takeoffs by light and ultralight planes, even in the "touch-and-go" (if observed very closely).

Heavy aircraft takeoffs:

Airliners Taking Off 1

Airliners Taking Off 2

10 Takeoffs

25 Takeoffs

Boeing Dreamliner take off

and discussions on the Internet supports this:

How Does A Plane 'take off?'

Rotation

Why Does The Nose Go Up?
 

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Re: Why does an aeroplane dip one wing to turn?
« Reply #14 on: 01/08/2012 20:38:39 »

 

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