Figure 5-5. Crosswind climb flightpath.
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GROUND EFFECT ON TAKEOFF
Ground effect is a condition of improved performance
encountered when the airplane is operating
very close to the ground. Ground effect can be
detected and measured up to an altitude equal to one
wingspan above the surface. [Figure 5-6] However,
ground effect is most significant when the airplane
(especially a low-wing airplane) is maintaining a
constant attitude at low airspeed at low altitude (for
example, during takeoff when the airplane lifts off
and accelerates to climb speed, and during the landing
flare before touchdown).
When the wing is under the influence of ground effect,
there is a reduction in upwash, downwash, and wingtip
vortices. As a result of the reduced wingtip vortices,
induced drag is reduced. When the wing is at a height
equal to one-fourth the span, the reduction in induced
drag is about 25 percent, and when the wing is at a
height equal to one-tenth the span, the reduction in
induced drag is about 50 percent. At high speeds where
parasite drag dominates, induced drag is a small part of
the total drag. Consequently, the effects of ground effect
are of greater concern during takeoff and landing.
On takeoff, the takeoff roll, lift-off, and the beginning
of the initial climb are accomplished in the ground
effect area. The ground effect causes local increases in
static pressure, which cause the airspeed indicator and
altimeter to indicate slightly less than they should, and
usually results in the vertical speed indicator indicating
a descent. As the airplane lifts off and climbs out of
the ground effect area, however, the following will
occur.
• The airplane will require an increase in angle of
attack to maintain the same lift coefficient.
• The airplane will experience an increase in
induced drag and thrust required.
• The airplane will experience a pitch-up tendency
and will require less elevator travel because of an
increase in downwash at the horizontal tail.
• The airplane will experience a reduction in static
source pressure as it leaves the ground effect area
and a corresponding increase in indicated airspeed.
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