| RV8JD |
06-07-2020 05:50 PM |
Quote:
Originally Posted by BobTurner
(Post 1437163)
Don’t forget best glide speed varies with weight...
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The plot below illustrates what Bob said. It is for a sailplane (hence the low airspeeds and sink rates), but the idea is the same. The heavier the aircraft (i.e., higher wing loading), the lower and farther to the right the polar is for that aircraft. The Best Glide Speed is the at the line's tangent point to the polar from the origin (0,0) of the plot. The Best Glide Speed is higher at higher weights, and vice versa. The plot below is for still air.
The plot indicates that the Maximum Glide Ratio of the sailplane is approximately 21-to-1 (very low by today's sailplane standards).

Not shown on the plots, but a horizontal line drawn tangent to the polars would show the Minimum Sink Rate and the Airspeed at which it occurs (the Minimum Sink Speed). The lighter aircraft has a lower sink rate than the same aircraft at a heavier weight.
Note that the Minimum Sink Speed is less than the Best Glide Speed at a given weight.
Flying at Best Glide Speed in still air maximizes the distance flown, while flying at Minimum Sink Speed maximizes the time in the air.
The plot illustrates why sailplanes carry water ballast when the lift conditions are strong; the sailplane achieves the same glide ratio but at a higher speed and that increases their average cross country speeds. When the lift gets weak, the water ballast can be dumped and the Minimum Sink Rate is reduced so weak lift can yield a climb rate.
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