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Home > Flying the Plane > Why airplanes use aileron, instead of rudder to turn?
Why airplanes use aileron, instead of rudder to turn?
Flying - Flying the Plane
Friday, 13 October 2006 18:24

Dear Capt Lim,

I recently attended a cadet pilot interview with SIA (Singapore Airlines). It was conducted by a human resource staff and a senior captain. I guess I shall not touch on the "soft" questions asked by the human resource interviewer as he didn*t expect a right or wrong answer - instead he expected an intelligent answer.

There were two questions asked by the captain that I couldn*t give a precise answer. Can you enlighten me on that?

First question, why the airplane has to bank towards the direction that they want to turn by using ailerons rather than the rudder?

Second question, what are the advantages of the airplane flying at a higher altitude (I told the captain that it reduces drag, he said it was a correct answer but he expected more that one advantage)?

For the benefit for those who are going for any cadet pilot interview, here are some points I wish to share with you all. Do your homework before the interview. Prepare answers to technical questions on airplane engines, how they work and the functions of their control surfaces.

Also, read up on your Geography because SIA won*t hire a pilot with no sense of directions. So good luck to you all guys...

Best Regards

Lynyrd Lim

Hi Lynyrd,

Yes, if you are on a motorboat, you use the rudder to turn to left of right. When you talk about flying, the function of the rudder is different. It is mainly used to keep the plane in a balanced flight - especially during an engine failure. If a plane is unbalanced, it will fly "sideway", like a crab and creating a lot of drag - becoming more inefficient and burning more fuel. Aerodynamically, we say, the main purpose of the rudder is to maintain lateral stability.

To turn in flight, the proper technique is to use the aileron {through the conventional control wheel of the Boeing planes or the side stick of the Airbuses (A320/330/340/380)}. In this way, the plane is turned as the aileron is banked or rolled to left or right. In smaller basic planes, you may have to use the combination of slight rudder pressure and the aileron to turn. This is to ensure that the plane turns smoothly and in well-balanced condition.

The advantages of plane flying higher are mainly that jet engines are more efficient, there is reduced drag (due to less dense air) and you, more often than not, fly above the bad weather. I have touched a bit on this in my answer in a previous
FAQ "What limits a plane to fly higher than the normal cruising altitude?"

Hope you are successful in your interview!:-)


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good answer duke!!smilies/wink.gif
witz , 03 Aug, 2012
Not really answering the question
Captain Lim, I too have been wondering why planes do not turn with the rudder. I found this through Google and although you explain that we use the ailerons for tuning, I think you fail to explain why this is a better option than using the rudder. If you use the rudder to try to change the heading of the plane, is it possible? what would be outcome
Michael , 03 Sep, 2012
Rudder or ailerons?
Rudder can turn or change the heading of a plane slightly as a secondary effect at low speed only. It is not recommended to use the rudder at high speed. An Airbus A300 crashed in New York many years back because of excessive/wrong application of the rudder in wake turbulence. The proper technique to turn an airplane is to use the ailerons.
Captain Lim , 03 Sep, 2012
Hi all, I am an aviation enthusiast but not a pilot. In answer to the original question, when a plane is in level flight, the lift forces generated by the wings are vertical only, there is no horizontal vector. When the plane banks/rolls, the lift forces now act at an angle with a reduced vertical vector but now has a horizontal vector as well. It is this horizontal vector that brings the plane around in a turn. Coincidentally, the vertical vector drops, and that is why without corrections, a plane often drops in altitude when it banks. You need to apply a bit more thrust to maintain altitude in a bank.

A rudder is used to change the attitude of a craft with respect to the direction it is travelling in. In the case of a boat, this change in attitude causes very strong forces to act on the sides of the boat, causing it to turn. This is due to the relatively low speeds a boat usually travels at, and the much higher viscosity of water as compared to air. But if you were travelling in a very fast boat and used the rudder, you would also get a degree of crabbing or going sideways before it turned.

In a plane, it is simply going too fast and the air has too low viscosity to cause it to turn based purely on forces acting on the plane body when it is yawed. You need the much more powerful lift forces from the wing to make it turn in the air.

Hope this helps for your next interview. Maybe I should go try out too!
Por , 09 Sep, 2012

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