R22 blade tip speeds

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

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    $\begingroup$ It depends. What are you using the graph for? $\endgroup$ – Bianfable Mar 31 at 8:50
  • $\begingroup$ 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? $\endgroup$ – jumblie Mar 31 at 8:58
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    $\begingroup$ 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). $\endgroup$ – Bianfable Mar 31 at 9:01

For this graph, only TAS can be correct.

More precisely, you should have the same kind of speed on both axes. Otherwise, you would need to qualify the altitude.

Your tip speed (the vertical axis) is (presumably) the simple geometric speed, which depends only on RPM. Or in other words, TAS (with no wind). It will have simple and universal linear relationship with forward TAS. But with IAS, it will be valid only at a certain altitude.

This could be both IAS, but I don't think helicopters just increase their rotor RPM with altitude the way airplanes increase TAS. This could be true for very slow rotors, but the real ones seem to be Mach-limited, so IAS is just not very useful. But correct me if I'm wrong.

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  • $\begingroup$ Airplanes stall at the same indicated airspeed, not true airspeed. $\endgroup$ – Mark Jones Jr. Apr 1 at 4:38
  • $\begingroup$ Yes, but how is it relevant? $\endgroup$ – Zeus Apr 1 at 4:40
  • $\begingroup$ @Zeus Re: "More precisely, you should have the same kind of speed on both axes. I have updated my question to include how I created the graph (both axes use the same figures, I've just converted units). You have made me reflect to realise that linear velocity (eg:FPM) is about distance over time, and the analog of distance in the air would be TAS not IAS. To the best of my knowledge, most helicopters (if not all) use constant rotor RPM. Can you provide your rationale for "Only TAS can be correct"? $\endgroup$ – jumblie Apr 1 at 5:41
  • $\begingroup$ Well, that's exactly my rationale: if your tip speed velocity is derived from [constant] RPM, so you should use a 'compatible' linear speed, i.e. TAS. As you see, the tips vary the speed by ±169 fps of TAS (at 100 kt). You can't just add TAS to IAS, despite them having the same units. (BTW can't help but notice it would me much clearer in the metric system). Imagine the craft flies forward at 672 fps TAS: then the retarding blade stops. If it was 672 fps IAS, you couldn't conclude that: depending on altitude, the retarding blade would likely have some negative airspeed. $\endgroup$ – Zeus Apr 1 at 6:07

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