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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Ground Effect

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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

Ground effect changes drag and lift.
Figure 4-14. Ground effect changes drag and lift.

However, the reduction of the wingtip vortices due to ground
effect alters the spanwise lift distribution and reduces the
induced AOA and induced drag. Therefore, the wing will
require a lower AOA in ground effect to produce the same
CL. If a constant AOA is maintained, an increase in CL results.
[Figure 4-14]

Ground effect also alters the thrust required versus velocity.
Since induced drag predominates at low speeds, the reduction
of induced drag due to ground effect will cause the most
significant reduction of thrust required (parasite plus induced
drag) at low speeds.

The reduction in induced flow due to ground effect causes
a significant reduction in induced drag but causes no direct
effect on parasite drag. As a result of the reduction in induced
drag, the thrust required at low speeds will be reduced. Due
to the change in upwash, downwash, and wingtip vortices,
there may be a change in position (installation) error of
the airspeed system, associated with ground effect. In the
majority of cases, ground effect will cause an increase in
the local pressure at the static source and produce a lower
indication of airspeed and altitude. Thus, an aircraft may
be airborne at an indicated airspeed less than that normally
required.

In order for ground effect to be of significant magnitude, the
wing must be quite close to the ground. One of the direct
results of ground effect is the variation of induced drag with
wing height above the ground at a constant CL. When the
wing is at a height equal to its span, the reduction in induced
drag is only 1.4 percent. However, when the wing is at a
height equal to one-fourth its span, the reduction in induced
drag is 23.5 percent and, when the wing is at a height equal
to one-tenth its span, the reduction in induced drag is 47.6
percent. Thus, a large reduction in induced drag will take
place only when the wing is very close to the ground. Because
of this variation, ground effect is most usually recognized
during the liftoff for takeoff or just prior to touchdown when
landing.

During the takeoff phase of flight, ground effect produces
some important relationships. An aircraft leaving ground
effect after takeoff encounters just the reverse of an aircraft
entering ground effect during landing; i.e., the aircraft leaving
ground effect will:
• Require an increase in AOA to maintain the same
CL.
• Experience an increase in induced drag and thrust
required.
• Experience a decrease in stability and a nose-up
change in moment.
• Experience a reduction in static source pressure and
increase in indicated airspeed.

Ground effect must be considered during takeoffs and
landings. For example, if a pilot fails to understand the
relationship between the aircraft and ground effect during
takeoff, a hazardous situation is possible because the
recommended takeoff speed may not be achieved. Due to
the reduced drag in ground effect, the aircraft may seem
capable of takeoff well below the recommended speed. As
the aircraft rises out of ground effect with a deficiency of
speed, the greater induced drag may result in marginal initial
climb performance. In extreme conditions, such as high
gross weight, high density altitude, and high temperature, a
deficiency of airspeed during takeoff may permit the aircraft
to become airborne but be incapable of sustaining flight
out of ground effect. In this case, the aircraft may become
airborne initially with a deficiency of speed, and then settle
back to the runway.

 

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