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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Forces Acting on the Aircraft

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making

Appendix

Glossary

Index

Drag versus speed.
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.

formulas represent the coef.cient of lift (CL)

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.

Lift coefficients at various angles of attack.
Figure 4-9. Lift coefficients at various angles of attack.
 

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