Aeronautical Engineering/Stall turbulence


QUESTION: If I'm flying at 60 kts, and the 15kts headwind suddenly changes its direction 180 degrees, my airspeed will be 45 or 30 kts? And when this happens, assuming either 45 or 30 is below stall speed, the plane will stall or will momentary settle until it accelerates again?


ANSWER: Initially, your airspeed is 60 kts and your ground speed is 45 kts. If the wind change is very gradual, the aircraft will adjust to the tailwind and you will accelerate to a 75 kts ground speed with the same 60 kt airspeed. If the wind change is abrupt, the aircraft will experience a gust response to the tailwind, the aircraft will experience a temporary 30 kts airspeed and the aircraft will settle to a lower altitude and accelerate to 60 kts airspeed and 75 kts ground speed. If the wind change was sharp enough, stall could occur, but drag has also gone down so the aircraft will accelerate out of stall if there is enough altitude.

The wind change also affects propeller performance, but that is a small secondary effect compared to the above.


---------- FOLLOW-UP ----------

QUESTION: What I don't understand is why is there a 30 kts airspeed loss and not 15. When the plane initially has 15 headwind it doesn't feel it, right? It is moving with the mass.

Yes, it is counter intuitive. And you are right, the aircraft is moving in the air mass and doesn't even know the wind is blowing. But, assume that you are standing on the ground and can see the aircraft and wind. The aircraft is approaching from the left at 45 kts relative to you and the wind is blowing from the right at 15 kts relative to you. The absolute sum of those velocities gives the 60 kts needed by the aircraft to fly level. Everything is steady state.

Now shut off the wind. The aircraft sees a 15 kt drop in airspeed and is flying at 45 kts airspeed (ignore stall for the moment).

Now quickly turn on the tailwind at 15 kts. The aircraft sees another 15 kts drop in airspeed and is now flying at an airspeed of 30 kts. If the thrust is unchanged, the aircraft will accelerate to the original 60 kts airspeed and (now) 75 kts ground speed.

Hope that helps.

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Paul Soderman


Aeronautics, Aerodynamics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.


38 years as research engineer at NASA


B.S. and M.S. Aeronautical Engineering - U. of Washington, Graduate work Standford U.

Awards and Honors
AIAA Associate Fellow (American Institute of Aeronautics and Astronautics)

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