Originally Posted by
A Squared
Ian, Goldenrivet already addressed this but I wanted to add a little more detailed explanation, because this is an important concept. What you described is the very crux of why the downwind turn myth exists. The fact is, the situation you describe has no more inertia to overcome than the same airplane doing the same rate turn in still air. I know that it *seems* like it is different, because *WOW*, the airplane is going 120 knots at the end, but there is no more acceleration, and no more inertia to overcome when an airplane goes from 0 knots groundspeed in a 60 knot wind, to 120 knots downwind than there is when the same airplane goes from 60 knots East in still air to 60 knots West in still air. It *seems* like it's different, because often people think of the airplane in still air as not accelerating, because it's started out with a speed (air and ground) of 60 knots and it ended up with a speed of 60 knots. This is a fallacy, albeit an understandable one if physics isn't your strong point. The thing you have to understand is that from a physics standpoint the relevant quantity is not "speed" but "velocity", and velocity is a vector quantity. What that means is that velocity is a measure of your motion, and of the *direction* of your motion. If the direction of your motion changes, it's a change in your velocity, just like a change in your speed is a change in your velocity. So, going from 60 knots east to 60 knots west is very much a change in your velocity. This might be a little more clear if you consider two 60 knot airplanes flying past each other closely in opposite directions. When you think about that, it should be pretty obvious that the difference in their velocities is 120 knots. The same applies for an airplane flying 60 knots in one direction, then turning 180 degrees, and flying in the opposite direction 60 knots. Just like with the airplanes flying past each other the difference in velocity from before the 180 degree turn to after is 120 knots. Now going back to the airplane turning in the wind, it goes from 0 knots into the wind to 120 knots downwind, and obviously, the change in velocity is 120 knots. Which is identical to the a airplane turning 180 degrees in still air: 120 knots change in velocity. So, the change in velocity is identical. the airplanes use the same rate of turn, so the time for the velocity change is identical, so the acceleration is therefore identical, because acceleration is change in velocity divided by time. If the acceleration is identical, then too, the forces acting on the airplane must also be identical.
If you're really interested in understanding this, but you're not sure what I'm saying is correct, print this out and take it to the nearest university physics department and ask if you can have a few moments of an instructor's time to explain this. He or she will tell you that the same thing.
As far as what you witnessed, as others have said, there are a variety of explanations. It may be that the airplane did fly through a wind gradient, either a loss of windspeed (gust or shear)or a vertical (down) motion. Strong winds close to the ground are not nice and constant and uniform, even though they may seems so ot an observer.
A^2 I think you are getting carried away with the turns being air radius turns and forgetting
your basic physics the the aircraft is actually ground referenced, for acceleration and deceleration.
1, Take a 3 ton weight that is stationary that then turns along a rate 2 turn radius and accelerates to 120kts in 30sec
2. Take a 3 ton weight that is traveling at 120kts that then turns along a rate 2 turn radius and decelerates to stationary in 30sec
Your argument is neither of these weights experience any acceleration. I say that is obviously false.
My understanding is that the force in the turn to hold the weights in the turn (centripetal force
ꟺ) accelerates the weights as it is (as you say) a change in velocity that can only come with application of a force. The
ground radius described by a slow moving (compared to the wind) aircraft in an
air radius turn is uneven stretched by the wind velocity vector. In consequence the centripetal force
ꟺ to accelerate the aircraft (to change its velocity vector ) is uneven. An aircraft with a tight ground radius first being accelerated more at first while the aircraft with a wide ground radius first is accelerated less at first. These
ꟺ accelerations - changes in velocity vector - must be added to the ground speed accelerations (decelerations) These differences will only really be apparent when the wind velocity is close to or a large fraction of the aircraft velocity.