Chimbu C
The aircraft cares not for GS only IAS...if the body of air in which you are flying happens to be moving back towards the airfield then that is only a help...
The aerodynamics only "care" for IAS but the point I am trying to make is that in the transient case of a sudden change of direction out of wind momentum must be gained to maintain IAS. Obviously in normal flight once this has been done then the a/c proceeds above the ground at a faster rate for a given IAS & therefore fuel consumption.
Bookworm, in the case of the balloon the energy comes from the air mass (ie it gives up a tiny proportion of its, to the balloon which sees viscous & frictional forces exerted on it until it has accelerated). However in the case of the sudden 180 deg turn our a/c needs to do, we are making a sudden change where surely we are not expecting to wait for these forces to accelerate us back up to the original IAS. (Guess the steep turn into wind would give us some of the required momentum though, but not all of it). I understand what you mean about the air mass's frame of reference and how it can do work but my problem is that when it attempts to do so by imparting momentum that initially the a/c will tend to "slip" within it (and therefore cannot quickly gain that required). If the air was a solid mass like a road surface, and moving then I would totally agree with what you've said.
FujiF
