"Matt Young" wrote in message
news
Someone correct me if I'm wrong, but changes in engine performance
wouldn't affect Vx or Vy would they?

As I mentioned before, Vx and Vy are functions of excess thrust and power,
respectively. In particular, they are the airspeeds at which excess thrust
and power are at their maximum.

If thrust were constant, Vx would happen at minimum drag airspeed regardless
of engine power. But thrust isn't constant; it varies with airspeed, and
the airspeed at which the thrust in excess of drag is at its maximum depends
on how much engine power you have.

For the same reason that thrust isn't constant (propeller efficiency changes
with airspeed), power isn't constant either, and again, the airspeed at
which the power in excess of power required for level flight is at its
maximum depends on how much engine power you have.

You can demonstrate for yourself that Vx and Vy depend on engine power, by
considering the difference between relevant airspeeds when there's no engine
power. Best glide airspeed isn't the same as Vx, even though both airspeeds
provide the best performance of the aircraft in terms of altitude change
over distance. Likewise, minimum sink airspeed isn't the same as Vy, even
though both airspeeds provide the best performance of the aircraft in terms
of altitude change over time.

Generally speaking, the differences between all of these airspeeds isn't
huge, but it's important to understand that they aren't the same.

[...] Changes in an
engines performance with age shouldn't affect these speeds, should they?

Not significantly enough for the published speeds to be unusable. If the
engine performance has changed enough to affect Vx and Vy significantly,
there's something wrong with the engine. But it's false to say that they
aren't affected at all.

Pete