And that is exactly what the Speed-To-Fly Calculator does. Of course, that requires on-the-fly graphing, because different density altitudes will produce different polars. That is, a truly useful polar-one that can be used to calculate speeds to fly-must be plotted in terms of true speeds, not equivalent speeds. The solution, it seems to me, is to go ahead and let the polar be a function of air density. This makes polars plotted using equivalent speeds essentially useless if one wants to consider the effects of density altitude on glider performance. Mechanical varios accurately report true sink rate, and electrical varios display "something like" the true sink rate times r/ r 0 (which may be considerably different from sqrt( r/ r 0).įurthermore, if wind and actual lift or sink are taken into account, those values are always in terms of true speeds. In fact, according to Welch, Welch, and Irving, Unfortunately, while the equivalent airspeed, V i, has a very practical meaning (indicated airspeed), the equivalent rate of sink, V si, has no such use. It would appear that polars are unaffected by density altitude. The performance curve of the glider applies at all altitudes provided that both the forward speed and the rate of sink are "equivalent" speeds. This leads Welch, Welch, and Irving to conclude (page 261) that The problem is that do make this work, the true rate of sink, V s, must also be replaced by the equivalent rate of sink, using Here r 0 is the "standard atmosphere" sea level air density. Equivalent airspeed is given the subscript i because it turns out to be the indicated airspeed for a "perfect" airspeed indicator. R 0 V i 2, which involves the equivalent airspeed. To remove air density from consideration, the trick is to replace r V 2 The drag equation used for calculating a polar includes in both the parasitic drag part and the induced drag part the term, " r V 2." Here r is the air density, and V is the true airspeed. ![]() I turned to Welch, Welch, and Irving for the answer to this question and was startled to (re)learn that polars are generally expressed in rather odd units. I posed the broader question: How does density altitude effect speed-to-fly calculations in general? The question Tom had was this: How does a polar change when density altitude is considered? The conditions of high altitude, high humidity, and high temperature all result in air that is especially low in density, and together these three conspire to reduce lift. conditions for which air density is such that it is like flying at high altitude) has a detrimental effect on aircraft performance. Welch, Welch, and Irving ( The Complete Soaring Pilot's Handbook, McKay: New York, 1977), is an absolutely must-read (if you can find it) for anyone interested in the subject of calculations related to soaring.Īll pilots understand that high density altitude (read high density altitude, not high density altitude, i.e. ![]() Until he mentioned it, I hadn't really thought much about it. I thank Tom Waits for getting me interested in the effect of density altitude on polar calculations. Today essay "Understanding Density Altitude" Several fine pages discuss the general definition and calculation of density ![]() Density Altitude Effects on Speeds-To-Fly Calculations
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