There is no such thing as "forward airspeed". Just Airspeed.
The speed of the air over the wing is completely independent
of ground reference points. If you want to use a ground
reference, use GROUND SPEED. Airspeed and Groundspeed
are easily confused when you try to describe things so don't take it so
hard. To use your illustration, push the nose down 100 degrees from
vertical, you are now slightly inverted and traveling backwards
over the ground. Do you now have NEGATIVE AIRSPEED?
Don't think so.............

Scott

But what happens when you cock up a chandelle and don't get the nose down
before you loose forward airspeed? Seems like negative (or backwards)
airspeed to me! 8-)

Keith