[ACE-OF-ACES]

Originally Posted by camber

Obviously the pilot can choose to climb at constant IAS if aircraft capability permits. I take it, this statement should be:

Vx (IAS for best climb angle) and Vy (IAS for best rate of climb) will never be constant with altitude."

This is true enough but I know a flight instructor that had it a bit confused .

Well flight instructors are only human!

But re-reading your reply I noticed something..

But first some definitions

In pilot jargon terms (read not math terms)

Vx best angle of climb 'airspeed'
Vy best rate of climb 'airspeed'

Put another way

Vx is the 'airspeed' that produces the most altitude in the shortest ground distance
Vy is the 'airspeed' that produces the most altitude in the shortest amount of time

For example

Vx is the airspeed you would want to climb at if your goal is to clear an obstacle at the end of the runway
Vy is the airspeed you would want to climb at if your goal was to intercept the bombers

The point to notice here is both Vx and Vy are 'airspeeds', be it indicated (IAS) or true (TAS).

Which should not be confused with the Vx and Vy I used in my previous (mathematical) example on the resultant vector! Where the Vx and Vy velocity tied to the x and y axis frame of reference. These pilot jargon terms Vx and Vy are actually the resultant vector, i.e. airspeed.

Originally Posted by camber

So climb tests at constant IAS (such as that Spittie one) may not be capturing optimum rate of climb at each altitude? Interesting.

Not likely..

They were more than capable of determining the airspeed that produced the best ROC. With regards to WWII they would do several test and different airspeeds to determine which one produces the best ROC.

One thing to take note of, with regards to WWII test data, most WWII fighter aircraft didn't have TAS gauges, which explains why most WWII ROC data gives airspeeds in IAS, and most if not all of those WWII test reports that do provide TAS, TAS was calculated (post test processing) from IAS.

So most WWII references are in terms of IAS. Where as today TAS gauges are more prevalent, and therefore most modern pilot discussions on Vx and Vy are talking in terms of TAS, where most make note of how Vx (TAS) and Vy (TAS) airspeed changes with altitude.

We also know that TAS changes with altitude relative to IAS, So even with a constant IAS, TAS is changing.

In WWII they typically referred to the airspeed that produced the best ROC as the 'best climb speed', which in todays pilot jargon equates to Vy (best rate of climb airspeed).

Therefore with regards to the WWII ROC test reports jargon vs modern pilot jargon, the constant indicated airspeed (IAS) in the WWII ROC test is the same as saying Vy indicated today, convert both to TAS and this WWII story will match those of today, where Vy changes with altitude.