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Okay, a theoretical example here which I hope will help me understand the physics better.

Conditions:

  • Altitude: 50 ft (over sea)
  • GS: 300 knts
  • CAS: 300 ktns
  • Wind: 0

If I now get a 10 knts head wind (instantly) which will keep constant at 10 knts. Will it result in GS = 290 and CAS constant at 300? Will my CAS at first jump to 310 and then go back down to 300?

What would happend if I get 10 knts direct tail wind, will it still only affect the GS by in this case increase it to 310?

Is this true for all (within reason) altitudes?

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  • $\begingroup$ @Bianfable I mean CAS. $\endgroup$ Aug 17 at 7:09
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    $\begingroup$ @Bianfable Thanks, I updated the question. $\endgroup$ Aug 17 at 7:16

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Aircraft fly relative to the surrounding air. If your aircraft is trimmed to fly at a certain airspeed, and the airstream changes due to wind or other factors, the aircraft naturally returns to its trimmed flight state and speed relative to the atmosphere. For this reason the groundspeed is the vector subtraction of your velocity vector and the wind speed.

With this in mind, you can see that your second statement is correct

Will my CAS at first jump to 310 and then go back down to 300?

Your example plays out like this: You fly at 300 knots (airspeed and groundspeed, hence wind is 0), suddenly the wind increases, and because of conservation of momentum you will not immediately slow down, therefore you will read 310 knots on your airspeed indicator. However because your airplane is trimmed to 300 knots, you will slowly decelerate until you airspeed is decreased to that value. Because of this deceleration, your groundspeed will have decreased accordingly by 10 knots to 290 knots. For me it helps to imagine that you fly by pushing against the airmass surrounding you, therefore if the whole airmass around you goes backwards, so do you (by 10 knots).

In the case of a tailwind, the whole situation is reversed. When the tailwind hits your aircraft, first you will fly only 290 knots (again because your aircraft conserves its momentum), and then your aircraft accelerates to return to its trimmed flight speed of 300 knots. At that point your groundspeed likewise accelerated to 310 knots.

This is true for all airspeeds. There is no altitude dependency, especially if you do these calculations with TAS.

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    $\begingroup$ The same principle also applies for cross winds. This is why, (contrary to generally accepted understanding), when taking off in a strong crosswind, if you apply the appropriate cross controls (downwind rudder and upwind aileron), to control aircraft heading on the runway, and release these controls immediately after liftoff, the aircraft weathervanes into the wind, and, as it accelerates, moves upwind from runway centerline, not downwind from the the wind pushing it, as is sometimes postulated. $\endgroup$ Aug 17 at 15:04
  • $\begingroup$ @CharlesBretana - alternate scenario - consider a light airplane that will not accelerate a great deal between liftoff and reaching desired climb speed, but will climb up through the wind gradient into a stronger crosswind-- $\endgroup$ Aug 18 at 15:48
  • $\begingroup$ Yes, you're right, that changes things. But perhaps not as you might surmise, (if I read your intentions properly). For the sake of simplicity let's assume that the wind gradiant changes the wind direction significantly, from a 15 knot left X/W to a 15 knot right X/W, without changing the headwind component by much. What will happen is that the nose of the aircraft will weathervane in the new relative wind, from being offset to the right of the ground track, to being offset to the left of the ground track. Conservation of Momentum dictates that the ground track will not change. $\endgroup$ Aug 18 at 20:42
  • $\begingroup$ The new, modified, "Wind" will NOT "Push" the aircraft onto a new ground track. This is because the "Wind" is not a real thing. It is just a abstract measure of the difference in velocity of two different reference frames. Once off the ground, in the air, "Wind" does not exist (except transiently, from wind shear). The only wind an aircraft feels is relative wind caused by motion through the airmass. and that (ball centered), is from straight ahead of the nose. Ball centered, and flying coordinated (without a boot full of rudder), there is no wind force pushing on the side of the aircraft. $\endgroup$ Aug 18 at 20:47

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