Flying faster because of decreasing winds

[Chris Scott]

Hi again, SR71,

Your minimalist and unhelpful interjections have certainly made me dust off my A-Level (1960s) Dynamics, which has been rewarding, and for which I'm most grateful. It has indeed been fun, but I suspect we are now at risk of getting a little more off topic than is really productive?

When I bravely stated that "kinetic energy" is a vector, I was thinking of the Tiger Moth with its TAS of 70kt into a headwind of 140 (see post #26). Its GS, in pilot terms, is minus 70, because aeroplane pilots think of GS in the direction of intended travel; i.e., "Track" (loosely speaking, forwards). The same must therefore apply, by definition, to its "kinetic energy"?

To illustrate my point, remember my example: when the headwind suddenly dropped to 70kt, the Tiger had to accelerate its GS from -70kt to 0kt to restore its TAS/IAS. This required an energy input to overcome its inertia (mass), i.e., its "kinetic energy" had to be increased from the point of view of the pilot.

But from the point of view of someone standing far below on the ground, who may not have been able to see which way the Tiger was trying to go, it actually appeared to decelerate from 70kt to 0kt (stationary). This implies a decrease in kinetic energy. So the extra power (or thrust, if you want to achieve the same result by a slightly different method) has, from this point of view, been negative.

Anyone confused yet? Kinetic energy, as I think Wizofoz may be already reminding me – like GS (velocity) – is purely relative to the observer. But, additionally, the direction in which it is acting is all-important. A speed is a speed (relative to a point); but a velocity is a speed and a direction/bearing (relative to a point). Similarly, the concept of kinetic energy can only be meaningful if its direction is specified. If that does not make it a vector, then please let me know what I can call it, and I'll oblige.


Quote from Wizofoz:
The amount which an aircraft needs to accelerate due to a change in the wind in order to regain it's original airspeed is equal to the change in the wind. All frames of reference relevant to the flight of the aircraft are to do with the air it's flying through.
[Unquote]

Yes, but let me remind you that the concept of wind is equally relative; it is universally defined as a velocity relative to the earth's surface. The problem for pilots arises when it changes, not when it is steady.

That's what the concept of a minimum acceptable GS on the approach is all about.
Can we get back to the subject, please?