At 06:55 07 May 2014, Bruce Hoult wrote:
[1] handy formula: X knots of kinetic energy is worth (X/5)^2 feet of
gravitational potential energy. e.g. 70 knots = (70/5)^2 = 14^2 = 196 ft.
50 knots = (50/5)^2 = 10^2 = 100 ft. Less drag loses of course.
You'll never turn speed into quite that much height, and you'll need more
height than that to get speed. But the differences are large in a high
performance glider at moderate speeds.

...or you could use my rule of thumb: a change of speed of 10 knots IAS
gives
you (or costs you) the number of feet in height of the speed you arrive at.

For example:
70 knots to 60 knots : plus 60 feet
60 knots to 70 knots: minus 60 feet
70 knots to 50 knots (i.e. 70 to 60, then 60 to 50) : (60+50) = plus 110
feet

(N.B. Done without undue delay, and at sea level - more height change at
greater altitudes. Height in feet, IAS in knots, works for a change of
speed of 10 knots. Results
are realistic but approximate.)