"tony roberts" wrote in message
news:nospam-51A543.18490626022005@shawnews...

On Monday I am participating in a density altitude seminar, and I have
been tasked with explaining why vx and vy converge at altitude.
The explanation needs to be simple - i.e. no algebra calculations
etc -
just stripped to the bare bones.

Here is what I have come up with.

VX utilises excess thrust.
VY utilises excess thrust hp.

As density altitude increases, VX angle of attack becomes lower,
VY angle of attack increases.
Somewhere between service ceiling and absolute ceiling they converge.

Can anyone explain it simpler (and/or more accurately in as few
words)
than that?

Thanks

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE

Nope. Vx and Vy are equal (only) AT the absolute ceiling. So the
"topological" answer to your problem -- available even to the
algebraically untarnished -- is that since Vx Vy at MSL, and Vx = Vy
at absolute ceiling, and they are both increasing functions of density
altitude, they must converge (be getting closer together) at increasing
density altitude.. Your discussion won't even need to invoke arithmetic!

For details (for those who went to high school and actually learned
something)) see the Maneuvering chapter of Performance of Light
Aircraft, AIAA, 1999.

John Lowry
Flight Physics