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The removal
of thrust results in the aircraft gliding. The diagram shows this situation,
with the airspeed velocity, V, now along the glide path rather than in
the horizontal plane. This velocity can be resolved into horizontal, U, and
vertical, w, components so that the angle of steady state glide is given by:
Glide
Angle = tan-1(w/U). The resultant, K, of the lift and drag forces just balances
the weight, W. Using similar triangle arguments shows that: (D/W) = (w/V). Since W is a
fixed quantity and V is a given in the problem, the lower the drag, D, the smaller
the value of w, and the smaller the glide angle required to maintain this
power-off steady state.
The power lost to drag is (DV) and is provided by the rate of
loss of potential energy (wW) = (wmg) of the aircraft center of mass. The horizontal distance
travelled in time t is (Ut) and the height lost in this time is wt. The ratio
of these two quantities is known as the finesse F = (U/w) = (L/D). The higher the finesse,
the lower the slope of the glide path. For a human-powered aircraft a finesse
of about 20 is required. |
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