Yes, KIAS. Vx changes with density, and because of that I incorrectly assumed that the other alpha related V speeds would, too. On reflection, it seems obvious that Vy, Vbg and Vbe should not changed with density. I'll go fix the article. Good catch, thanks.Wayne,
Fantastic post! When you mention these speeds, I assume it's KIAS. Are you saying that the best glide speed (indicated) changes with air density?
Of Vx, Vy and Vbg, the only one I now lack a hard reference for is Vbg. Since it does not depend upon engine power, it stands the best chance of being invariant with density.Let’s discuss how VY depends on altitude. There are a couple of issues:
* Obviously there is no point in asking about VY(100) at an altitude where the engine is not capable of producing anywhere near 100% of its rated power. Instead we might ask about VY(ft), where “ft” stands for full throttle. For a non-turbocharged airplane, VY(ft) will decline somewhat as altitude increases, for reasons discussed in connection with figure 7.10.
* Propeller efficiency depends on TAS (not just CAS). The TAS/CAS ratio is a fairly strong function of altitude. Imagine having a fixed-pitch prop with a very fine pitch. As the altitude increases, the TAS might increase to the point where it overruns the pitch of the prop at any reasonable engine RPM. You would have to fly at a slower CAS to get any thrust at all. In this scenario, the power curve would be grossly distorted, and VY would be a declining function of altitude. Conversely, a propulsion system optimized for high-speed cruise might work better at high altitude. (For example, a turbojet – with no propeller at all – works better and better as the TAS/CAS ratio increases.)
Bottom line: In a non-turbocharged fixed-pitch light aircraft, you can expect VY(ft) to decline slightly as altitude increases. Usually you don’t need to worry about it, unless you have a special reason for flying close to the edge of the envelope (e.g. mountain flying) – in which case you might hope that the POH documents how VY(ft) depends on altitude, weight, et cetera. (Some do, but most don’t.) And POH or no POH, you should do some experiments to verify the critical performance numbers for yourself.
And a practical consequence of Eq. (7.61) is that none of these 4 V speeds - Vx, Vbg, Vmd, or Vs - considered in calibrated terms, varies one iota with density altitude.
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