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Imagine riding a bike on a downhill road to the north, approaching a 90-degree turn. You make a perfect right turn, and your bike now moves east. You then stop and wonder how that is possible. Your initial and final momentum is not the same; they are not even on the same axis. You didn't have any collisions, and you haven't applied any force on the pedal, yet your north momentum somehow got converted to east momentum. You then realize the same thing happens when an aircraft banks to take a turn. For the bike, at least there is contact with the ground. How is it physically possible for the aircraft to make a turn? For instance,

  • Where did the northward momentum go?
  • From where did the eastward momentum come?
  • How does turning work, be it a bike, a car, or even an aircraft?
AlphaLife
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    For anything moving fast, ordinary air puts up a tremendous amount of force. Remember what it is like standing on a hill top in a strong wind! – Andrew Steane Jun 29 '23 at 08:35

1 Answers1

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How does an aircraft turn ?

It uses the air around it. An aircraft could not turn in a vacuum.

Where did the northward momentum go ?

A mass of air is pushed northwards as the aircraft turns. This moving air accounts for the aircrafts initial northward momentum.

From where did the eastward momentum come ?

Another mass of air is pushed westwards as the aircraft turns to the east. The westward momentum of this moving air is balanced by the final eastward momentum of the aircraft.

How does turning work, be it a bike, a car, or even an aircraft ?

The vehicle must push against something to change its momentum. The aircraft pushes against the surrounding air. A boat with oars or a propeller pushes against the water. A bike or a car pushes against the ground - this requires friction, which is why it is difficult to turn on smooth ice (unless you have something to increase friction, like skates or snow chains).

A rocket needs to change its momentum in a vacuum, so in effect it pushes against its own propellant which it carries with it. Propellant ejected at high speed in one direction carries away momentum, which gives the rocket a "push" in the opposite direction.

gandalf61
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  • Perhaps for a turn using only the rudder, but an airplane is pulled through the turn mostly by angling the lift it generates. A bit like a ball on the end of a string. – David Browne - Microsoft Jun 29 '23 at 15:29
  • @DavidBrowne-Microsoft It is the same principle since lift is generated by pushing air. When a plane banks air is pushed sideways as well as downwards. – gandalf61 Jun 29 '23 at 18:14
  • But does a plane in level flight actually push air downward? I thought lift was generated by forcing air to curve over the top of the wings, causing the low pressure area that lifts the wings. – David Browne - Microsoft Jun 29 '23 at 18:31
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    @DavidBrowne-Microsoft "Does a plane in level flight actually push air downward ?" - Yes, it does. See https://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly – gandalf61 Jun 29 '23 at 20:39