Figure 4-8. Drag versus speed.
Lift/Drag Ratio
Drag is the price paid to obtain lift. The lift to drag ratio (L/D)
is the amount of lift generated by a wing or airfoil compared
to its drag. A ratio of L/D indicates airfoil efficiency. Aircraft
with higher L/D ratios are more efficient than those with
lower L/D ratios. In unaccelerated flight with the lift and drag
data steady, the proportions of the CL and coefficient of drag
(CD) can be calculated for specific AOA. [Figure 4-9]
The L/D ratio is determined by dividing the CL by the CD,
which is the same as dividing the lift equation by the drag
equation. All terms except coefficients cancel out.
L = Lift in pounds
D = Drag
Where L is the lift force in pounds, CL is the lift coefficient,
p is density expressed in slugs per cubic feet, V is velocity
in feet per second, q is dynamic pressure per square feet, and
S is the wing area in square feet.
CD= Ratio of drag pressure to dynamic pressure. Typically
at low angles of attack, the drag coefficient is low and small
changes in angle of attack create only slight changes in the
drag coefficient. At high angles of attack, small changes in
the angle of attack cause significant changes in drag. |
The above formulas represent the coefficient of lift (CL)
and the coefficient of drag (CD) respectively. The shape of
an airfoil and other lift producing devices (i.e., flaps) effect
the production of lift and alter with changes in the AOA. The
lift/drag ratio is used to express the relation between lift and
drag and is determined by dividing the lift coefficient by the
drag coefficient, CL/CD.
Notice in Figure 4-9 that the lift curve (red) reaches its
maximum for this particular wing section at 20° AOA, and
then rapidly decreases. 20° AOA is therefore the stalling
angle. The drag curve (yellow) increases very rapidly from
14° AOA and completely overcomes the lift curve at 21°
AOA. The lift/drag ratio (green) reaches its maximum at 6°
AOA, meaning that at this angle, the most lift is obtained for
the least amount of drag.
Note that the maximum lift/drag ratio (L/DMAX) occurs at
one specific CL and AOA. If the aircraft is operated in steady
flight at L/DMAX, the total drag is at a minimum. Any AOA
lower or higher than that for L/DMAX reduces the L/D and consequently increases the total drag for a given aircraft's
lift. Figure 4-8 depicts the L/DMAX by the lowest portion of
the orange line labeled "total drag." The configuration of an
aircraft has a great effect on the L/D consequently increases the total drag for a given aircraft's lift. Figure 4-8 depicts the L/DMAX by the lowest portion of the orange line labeled "total drag." The configuration of an aircraft has a great effect on the L/D. |