# Rotor Tip Speed vs Forward Airspeed - IAS or TAS?

I created this graph to illustrate how the tip speed of the advancing and retreating blades vary with forward airspeed but am confused as to what would be the most correct way to label the x-axis:

• IAS
• TAS
• it doesn't matter
• it depends on the application

NB: Graph is based on the following data:

The y-axis is a derivative of the x-axis: $$y= 672 ± \frac{6076x}{3600}$$

eg: At $$x = 100KTS$$, $$y=672±169 FPS$$
From POH: rotor RPM @ 100% = 510RPM = 672FPS @ 151 inch rotor diameter

• It depends. What are you using the graph for? – Bianfable Mar 31 at 8:50
• Maybe I should add a it depends option in my question. I guess ultimately the different tip speeds are only meaningful because of dissymmetry of lift, in which case the answer to my question is TAS because that's what's used to calculate lift. On the other hand, I don't think that if it was labelled IAS that would be wrong either, would it? Is there any other reason to care about differing tip speeds? – jumblie Mar 31 at 8:58
• At a given angle of attack, lift is proportional to IAS, not TAS. But TAS is proportional to Mach number (at fixed temperature), which is important for the advancing blade (should stay below Mach 1). – Bianfable Mar 31 at 9:01