If lift and control authority derive from Indicated Air speed (IAS), why do energy-maneuvering (EM) diagrams use True Air Speed (TAS) as their x axis?

Question

Example EM diagram for P-51D and Spitfire MkIX: https://ww2aircraft.net/forum/attachments/areas-gif.594279/

Why is the X axis given in TAS rather than IAS? From my understanding, how much lift a given airframe experiences is correlated to IAS rather than TAS and lift directly correlates to turn performance (higher lift-weight ratio at a given bank angle and G, the better the turn performance). Likewise, by my understanding elevator authority is based upon IAS for compression and minimums.

So, assuming my understanding is correct, why do we use TAS?

In: Physics

Answer 1

/u/rhynod missed the point on this one.

The reason these charts use true airspeed is simple. You’re not noticing / ignoring the third axis plotted on that chart, which is a turn radius (in feet in this case).

To get a turn radius that matches what you would actually measure with a ruler or whatever, you have to use true airspeed, because you are actually trying to map the aircraft’s behavior to the air mass around it. Aircraft can turn in a much smaller dimensional turn radius at lower true airspeed, because there’s only so much of an angle they can sustain, and the size of the circle they are turning in is dictated therefore by the forward speed they have. If I can only move 10° to the right per second, for example, it takes me 36 seconds to complete a full circle. How big that circle is depends on how fast I’m moving when I begin it in real terms. The faster I’m moving, the larger the physical circle. If you care about the absolute size of that circle for some reason, then you need to use the parameter that gets you there, which is true airspeed.