"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.

At the absolute ceiling of the aircraft, there's only one speed, the
minimum-power-required speed, at which the aircraft can avoid descending.
Thus just below the absolute ceiling, both Vx and Vy must be very close to
that speed. So Vx and Vy converge on that speed at the absolute ceiling.

Put another way, at lower levels, Vx is considerably less than Vy because
the excess power can be used in a way that trades slightly more power
required (and therefore slightly less climb rate) for less forward speed.
As excess power decreases, as it will at altitude, the ability to make that
trade-off reduces.

Julian Scarfe