Climb speed refers specifically to Vx or Vy. It is not common to split hairs on it having it mean anything but Vx or Vy without specifying the condition of flight.

http://www.experimentalaircraft.info...imb-speeds.php

All aircraft can climb at a constant IAS. If you do that however, you are not at Vx or Vy.

There is no reason for a huge involved, "Gee Whiz, I wish I was smart" discussion on this topic.

It is a simple fact, an airplanes climb speeds or Vx and Vy, are not constant with altitude. No amount of tap dancing or splitting hairs will change the physics.

In practical terms for flying an airplane it is easier to remember a few speeds for Vx and Vy that get you in the ballpark or just hold it constant so you only have to remember 2 numbers.

Read your Hurricanes notes....

Optimum for what???? Is that best rate or best angle???

It is actually because of the fixed pitch propeller but lets not get ahead of ourselves and get all confused on the correct principles for climbing speeds.

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.

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.

+1

I did, it still says constant IAS to FTH and linear reduction above optimum climb.

Best rate of climb.

This is not propeller related. There are performance tests with Rotol propellers show that the same applies to Hurricanes or Spitfires with CSP.

It is a pretty simple concept to grasp. Climb speed changes with altitude.

That is it and all you need to know!!

If it does not change with altitude or is a simple linear then for practical pilotage it is simpified to get the pilot in the ballpark!

Again, very simple concept that does not require in depth discussion.

Are you talking about IAS or TAS? If you mean TAS I'm absolutly with you.

Of course it's an pratical approximation. In a theoretical discussion there is no direct relation between IAS and climb rate.

It is a very simple concept that does not require an indepth discussion.

Climb speeds change with altitude.

It does not matter if we are talking Indicated or True airspeed, they both change.

In fact, Indicated airspeed for Vy decreases with altitude and Indicated airspeed for Vx increases with altitude.

Where they meet is the aircraft's absolute ceiling.

http://aerosrv.cls.calpoly.edu/dbiez...20and%20Vy.pdf

There is a definite fixed by design relationship between airspeed and climb rate based on excess power or thrust depending on whether we are talking rate or angle of climb.