Wind gusts on final

[mxalix258]

I have a question that I keep thinking about in regard to gusts on final. It confuses me when someone says they will fly a higher approach speed to hedge against a strong gust that may cause a stall - my question is why a gust would matter?

If all the wing knows is the mass of air it is flying in, then it really shouldn't matter if it's gusting or steady wind...right? Your ground speed may definitely fluctuate, but am I missing something when someone flies a faster approach speed in gusty conditions? I can also understand that you may have additional rudder/elevator authority, but still don't see a relation to stalling the airplane.

Thanks for the input!

 

[Jim Logajan]

I understand that there would be an affect on the airplane moving through the air, but don't understand how this relates to the aircraft stalling. If I'm flying my approach in a 172 at 65kts indicated, and it's calm winds, and out of nowhere I get a 10kt straight on gust...I would still be flying at 65kts but my ground speed would be 55kts.

Inertia. F=m*A.

A change in fluid velocity from V1 to V2 does not cause a body at rest or in motion relative to that fluid to instantly match the velocity change.

The danger is mostly in a tail-wind gust. For example flying in a steady head wind of 10 kts and the headwind suddenly vanishes. Not a tail wind as such but until your plane accelerates back up to the equilibrium velocity it had with the air, the wings suddenly will be 10 kts slower relative to the air.

The typical mass and drag coefficient of a C-172 are available on the net and when I find some spare minutes I may try to estimate how long it takes a C-172 to accelerate back to its equilibrium velocity after an instantaneous relative wind change. (I'll have to do an integration since the force is proportional to the relative velocity, which is changing.)