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How do aircraft determine their speed?

I understand that cars determine speed from wheel rotation, but how do aircraft do it? I’m interested after watching the Horizon documentary on the disappearance of ‘stardust’ in the Andes in 1946.
David Olmeda from Suffolk (Age 45-54)

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2 Responses

  1. HI David

    When you move through the air you feel the air pushing back on you – we experience this as ‘air resistance’. This is effectively an increase in the air pressure in the direction we are moving in, caused by our movement.

    Aircraft measure their speed through the air by measuring the extra air pressure generated as they move forwards. This exact details can be somewhat complicated but an aircraft’s ‘air speed indicator’ (ASI) instrument is really a kind of pressure gauge which is callibrated in units of speed rather than pressure.

    Its important to remember, when flying, that the ASI measures speed relative to the air, and that the air can be moving quite fast over the ground, so your ground speed can be quite a lot more (or less) than your airspeed. In some gliders it is actually possible to fly backwards over the ground if you’re flying into a strong headwind!

  2. A number of different methods can be used to determine the speed of an aircraft. The most accurate one in common use on commercial aircraft now is to use data from the Global Positioning System (GPS) that the aircraft carry to calculate the speed. More traditionally aircraft use a device called pitot tube to measure the pressure outside the front aircraft. This calculates what is called the stagnation pressure, that is the pressure caused by the aircraft pushing forward through the air. This pressure can be related to the speed at which the aircraft is travelling (higher speed means higher pressure) – this gives the “equivalent airspeed” of the plane. To turn this into speed across the ground (“groundspeed”), the aircraft altitude, and the wind speed and direction are also needed – from these 3 pieces of information the speed of the plane is determined.

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