Normal Takeoff - Ground Effect |
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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 normally occurs
up to an altitude equal to one wingspan above
the surface.
Ground effect is most significant when the airplane
maintains 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.
At high speeds where parasite drag dominates,
induced drag is a small part of the total drag.
Consequently, ground effect is a greater concern
during takeoff and landing.
At takeoff, the takeoff roll, lift-off, and
the beginning of the initial climb are accomplished
within the ground effect area.
The ground effect causes local increases in
static pressure, which cause the airspeed indicator
and altimeter to indicate slightly lower values
than they should and usually cause the vertical
speed indicator to indicate a descent. As the
airplane lifts off and climbs out of the ground
effect area, the following occurs:
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• The airplane
requires an increase in AOA to maintain lift
coefficient.
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• The airplane
experiences an increase in induced drag and
thrust required.
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• The airplane
experiences a pitch-up tendency and requires
less elevator travel because of an increase
in downwash at the horizontal tail.
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• The airplane
experiences a reduction in static source pressure
and a corresponding increase in indicated airspeed.
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Due to the reduced drag in ground effect, the
airplane may seem to be able to take off below
the recommended airspeed. However, as the airplane
climbs out of ground effect below the recommended
climb speed, initial climb performance will
be much less than at Vy or even Vx.
Under conditions of high-density altitude, high
temperature, and/or maximum gross weight, the
airplane may be able to lift off but will be
unable to climb out of ground effect. Consequently,
the airplane may not be able to clear obstructions.
Lift off before attaining recommended flight
airspeed incurs more drag, which requires more
power to overcome. Since the initial takeoff
and climb is based on maximum power, reducing
drag is the only option. To reduce drag, pitch
must be reduced which means losing altitude.
Pilots must remember that many airplanes cannot
safely takeoff at maximum gross weight at certain
altitudes and temperatures, due to lack of performance.
Therefore, under marginal conditions, it is
important that the airplane takes off at the
speed recommended for adequate initial climb
performance.
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