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.

Some seem to do just that.

This doesn't work, as well as the earlier mentionnned feeling of vertical
accelaration. This is because both only give just a "differential"
information, i.e. not the rate of climb, but the change in the rate of
climb. The real rate of climb can only been obtained from this information
by a (mental) integration, but every such value in any domain obtained
by integration from a raw differential input suffers a major deficiency:
errors accumulate in time during this process so that if no other
absolute information is available to determine the correct value at
regular time intervals, the integrated value becomes meaningless.
We all know that the feeling of accelaration is a good hint for
a thermal, but we also check the vario to be sure it is not due
to a high sink becoming very quickly a lower sink not deserving
any circling. And while circling we check with the vario that
the lift is not very slowly vanishing, at a rate at which no
acceleration perception would warn us, and leave the thermal if this
happens.


... They initiate a turn
with a big dab of negative in the inner tip and then control the turn
on tail tilt - the outer tail tip is raised, so you can tell that
they fly like we do with down force on the tail.
...

A raised tail (or elevator) doesn't mean there is a down force on it.
On our gliders the opposite would rather be true : modern gliders are
tuned so that that there is a zero force on the tail near the best glide
angle of attack, in order to minimize the (induced ) drag due to this
force, and as the airfoils used are unstable, at higher angles of attack
we should have an upward force and a downward force at lower angles
of attack, but the elevator is raised for higher angles of attack
and lowered for lower ones. This is not a contradiction, it just means
than without the move of the elevator, the upward or downward force
on the tail plane would even be higher, so bringing back the angle
of attack to its previous value, i.e. the tail plane fulfils its
stabilizing duty.