Originally Posted by
eckhard
Further to Jonkster’s post above, I humbly re-submit an earlier post of mine:
“You wake up, not sure where you are. You see that you are in a small, windowless room. Apart from your chair, the room is empty.
On the floor, you find a small model aeroplane with a battery-powered motor driving counter-rotating propellers. A simple instruction sheet tells you that when the motor is switched on and the 'plane is hand-launched, the controls are fixed so that it will fly perfect circles until a timer turns the motor off after 30 seconds.
You launch the model and watch, pleased and impressed, as it flies circles around you.
After the flight has ended, you notice that there is actually a window blind in the wall behind you. You open the blind and are amazed to find that you are in fact inside a carriage being pulled by a train along a perfectly straight, smooth track at 60mph.
You consider the fact that the model aircraft was flying in a parcel of air which was moving across the ground at 60mph. To an observer not on the train, it was, in effect, flying in a 60mph wind. This is a speed which is about five times greater than its own flight speed, yet the model did not seem to be affected at all! It did not, for example, exhibit any signs of 'stalling' or losing height when it was turning 'downwind'.
You come to the conclusion that the model was simply unaware of its location and speed relative to the tracks, as were you until you looked out of the window.”
The railway carriage effect, although a very good analogy, doesn't take into account gusts, which occur in the "real" outside world. To provide a few strong gusts, briefly open and shut the front door of the train and then see what happens.
A small, lightweight model aircraft has very low inertia and will easily be blown "downwind" during gusts, but see only relatively small variations in IAS.
A very large aircraft, such as an airliner, will see more variations in IAS but tend to follow the original flight path more closely.
Hence windshear being more of a concern on the approach in a very large aircraft.
The energy "found" during the previous video is being harvested as lift from updraughting airflow and then being converted to airspeed as the aircraft is descended on the "still air" side of the hill.