F li g h t Te s t in g Te c hn i q u e s
Figure 2
change. Similarly, engine-out glide speed depends
on airplane weight because the maximum range AOA
doesn't change. Here's why.
Let's start with the lift equation:
Where L is lift, ρ(Greek letter rho) is air density. V is
true airspeed. S is wing area. CL is lift coefficient. Recall
from last month that CL is uniquely related to AOA. In
other words, there is only one CL that corresponds to
a particular AOA. There is also only one CL that will
provide the maximum lift-to-drag (L/D) ratio, which is
the L/D ratio your airplane must be flown to achieve its
maximum range.
But thrust is zero during an engine-out glide, so D = W x
sin g. A little equation manipulation:
Max L/D
Figure 2 shows the forces acting on your airplane
during a steady descent. Notice that lift is perpendicular to both thrust and drag, but weight points
straight down. For easier force comparison, we've
shown the weight components acting parallel to lift
(W x cos γ) and parallel to thrust and drag (W x sin
γ). Cos and sin are the trigonometry functions cosine
and sine.
During a steady descent, all the forces are balanced.
From Figure 2, you can see that lift equals the weight
component perpendicular to the flight path (L = W x cos
γ), and drag equals the sum of thrust and the weight
component parallel to the flight path (D = T + W x sin γ).
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Vol.2 No.3 / March 201 3
We now have two expressions for W, so let's set them
equal to each other and do a little more manipulating.
L/D equals one over the tangent of the flight path angle.
To ensure the maximum L/D, we want the smallest tan
(γ) possible, and that means we want the smallest γ