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After take off, does airplane's speed include Earth's movement/speed? Do airplanes turn with Earth movement/rotation?

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Yes, airplanes keep turning with the Earth, the speed includes the Earth's rotation, and airplanes generally fly slower than the Earth rotates. – Ron Maimon Jun 14 '12 at 0:13
@RonMaimon Free body diagram please? How can gravity induce rotation with angular velocity "equal" to that of Earth's angular velocity? – Monster Truck Jun 14 '12 at 0:18
@MonsterTruck: No, no free body diagram. You draw it yourself. The airplane keeps moving with the Earth, and there is no need to "induce" rotation for an airplane any more than there is a need for you to keep pushing yourself along with the Earth's rotational motion to keep from falling behind. – Ron Maimon Jun 14 '12 at 0:19
@RonMaimon Actually that sounds correct. That's how toy helicopters stay in place. I think it should be possible to show that a rotating sphere does impart an angular momentum in its field. – Monster Truck Jun 14 '12 at 0:47
@MonsterTruck: Helicopters stay in place by pushing up, not sideways. A rotating sphere does not need to "impart" angular momentum--- the thing is already rotating, and keeps rotating along with the sphere unless you slow it down. Why is this so difficult to accept? – Ron Maimon Jun 14 '12 at 1:00
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2 Answers

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Aircraft in fact move in the medium of the air.

While they do figure both airspeed and ground speed the former matters to their flight characteristics and the latter to whether you land on time or not.

The air is--taken in bulk--moving along with the rotation of the planet, and winds are the differences between the motion of any particular element of the air and that average.

So

  • The plane flies in the air.
  • The air moves roughly with the planet.
  • From the ground the plane is seen to move at its airspeed plus-or-minus the wind speed
  • From an observation position in space nearby, but not orbiting the planet, the plane is seen to move with the planets rotation plus-or-minus its airspeed plus-or-minus the wind speed.

(All the plus-or-minus's are vector addition or subtraction, by the way.)

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Let me try to clarify what the previous posters said.

There are many way to measure an aircaft's speed. The obvious one is the speed relative to the ground. However, if the aircraft is flying into a headwind, its speed relative to the air will be greater than its goundspeed. Similarly, with a tailwind its airspeed is smaller than the groundspeed. As an example, if the plane's airspeed is 1000 km/h with a 100 km/h tailwind, its groundspeed is 1100 km/h. If it was flying into a 1000 km/h headwind, it would be standing still relative to the ground.

There are many more ways to measure the speed though. The earth rotates, which can add or subtract up to 1600 km/h to the speed, relative to the sun. The earth moves around the sun at about 30 km/s or 107,300 km/h. So, if that same plane is flying exactly in the direction that the earth is travelling, it's speed in solar orbit is 108,400 km/h. If it were going exactly opposite to the earth, the speed would be 106,200 km/h. In all other directions the aircraft's speed is between these two extremes, and I did not adjust for the earth/s rotation!

The sun also moves around the centre of the galaxy, and our galaxy moves relative to other galaxies (at different speeds for different galaxies). So we can get an infinity of different speeds for our airplane.

As you can see, there is no such thing as a "final" speed. Speed is always relative to something else. You need to specify what you are measuring the speed against, and you can get many, very different answers.

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