Wednesday, February 11, 2009

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. Takeoff in ground effect area
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

  • 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.

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 rises out of
ground effect with an insufficient airspeed, initial
climb performance may prove to be marginal because
of the increased drag. Under conditions of high-density altitude, high temperature, and/or maximum gross
weight, the airplane may be able to become airborne at
an insufficient airspeed, but unable to climb out of
ground effect. Consequently, the airplane may not be
able to clear obstructions, or may settle back on the
runway. The point to remember is that additional
power is required to compensate for increases in drag
that occur as an airplane leaves ground effect. But during an initial climb, the engine is already developing
maximum power. The only alternative is to lower pitch
attitude to gain additional airspeed, which will result in
inevitable altitude loss. Therefore, under marginal conditions, it is important that the airplane takes off at the
recommended speed that will provide adequate initial
climb performance.

Ground effect is important to normal flight operations.
If the runway is long enough, or if no obstacles exist,
ground effect can be used to an advantage by using the
reduced drag to improve initial acceleration.
Additionally, the procedure for takeoff from unsatisfactory surfaces is to take as much weight on the wings
as possible during the ground run, and to lift off with
the aid of ground effect before true flying speed is
attained. It is then necessary to reduce the angle of
attack to attain normal airspeed before attempting to
fly away from the ground effect area.

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