Originally Posted by
EXDAC
Why do you introduce a discussion of wind shear when you previously were arguing the case of steady wind and I specifically stated "uniform airmass"?
How about keeping it simple to start with.
Case 1. Aircraft makes continuous circles at constant altitude with no change in thrust. The aircraft is in a completely uniform airmass that has no motion relative to the surface of the earth. What happens to indicated airspeed during the turn?
Case 2. Aircraft makes continuous circles at constant altitude with no change in thrust. The aircraft is in a completely uniform airmass that is moving with contant speed and direction relative to the surface of the earth. What happens to indicated airspeed during the turn?
Good point. Ordinarily, would assume that the cases are identical, our experience and teachings suggest the cases are identical, we would not see an observable difference, as kinetic energy is irrelevant. The kinetic energy changes considerably but the IAS would essentially remain constant. for a wind of 20kts for a C-172 the difference in kinetic energy states is equivalent to a B747 in a 200Kt jet stream going from a tailwind to a headwind. For the B747 case, the IAS change that occurs is around 6kts, which is equivalent to around 1 kt for the C-172. 6kts is ~ 0.01M and wakes up the ATR.
Is there a transfer function? I have no idea. Is the B747 case observable at all times, it is repeatable and it suggests that the assumption that we have on changes of heading in an airmass is a useful approximation rather than an absolute truth. It has no relevance at low speeds and low inertia, it is observable and recordable on aircraft in a turn in the steady state jetstream where the only change is the track of the aircraft. It is additionally reversibile, going from a tailwind to a headwind results in the opposite effect. Do we read an IAS accurately enough on a C-172? no. The B747, B777 B787 A330/A380 etc, yes.
Before the first time I saw the aircraft behave in a steady state 200Kt + jetstream, in level flight with >90 degree turn, I did not believe it would make any difference. The most spectacular case is coming into southern Japan, but it is not the only case. Discount cases where the track is traversing the core of the jet in such a manner that it results in a shear, the observable curiosity is for the case where the wind is the same entering and exiting the turn, so wind shear is removed as a factor for the change in speed and subsequent thrust required. This happens to be memorable as getting to MCT limit thrust at high altitude and with decaying airspeed is attention-getting.
Clear as mud.