Martin Gregorie wrote:
On 04 Jul 2004 18:48:04 GMT, ospam (Frostowits)
wrote:
Pardon this input from an uninformed intruder to this subject, but why couldn't
birds simply sense how strong the lift is by the amount of stress it puts on
their "airframe". When I pump iron, I'm all too aware of the amount of effort
required. Surely birds can do the same.

Well, the stress only occurs briefly on entry into the thermal,
when you have upward acceleration. Once established in the climb,
assuming it is more or less smooth, you no longer have acceleration
nor any acceleration generated stress forces so you need some other
means of knowing whether you are still climbing, such as rate of
pressure change, which is what the vario measures.

cheers CV