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initial flap deflection is because of lift increase, but the
nosedown pitching moment tends to offset the balloon.
Deflection beyond 15° produces a large increase in
drag. Drag from flap deflection is parasite drag, and
as such is proportional to the square of the speed. Also,
deflection beyond 15° produces a significant noseup
pitching moment in most high-wing airplanes because
the resulting downwash increases the airflow over the
horizontal tail.
FLAP EFFECTIVENESS
Flap effectiveness depends on a number of factors, but
the most noticeable are size and type. For the purpose
of this chapter, trailing edge flaps are classified as four
basic types: plain (hinge), split, slotted, and Fowler.
[Figure 11-2]
The plain or hinge flap is a hinged section of the wing.
The structure and function are comparable to the other
control surfaces—ailerons, rudder, and elevator. The
split flap is more complex. It is the lower or underside
portion of the wing; deflection of the flap leaves the
trailing edge of the wing undisturbed. It is, however,
more effective than the hinge flap because of greater
lift and less pitching moment, but there is more drag.
Split flaps are more useful for landing, but the partially
deflected hinge flaps have the advantage in takeoff.
The split flap has significant drag at small deflections,
whereas the hinge flap does not because airflow
remains “attached” to the flap.
The slotted flap has a gap between the wing and the
leading edge of the flap. The slot allows high
pressure airflow on the wing undersurface to energize
the lower pressure over the top, thereby delaying flow
separation. The slotted flap has greater lift than the
hinge flap but less than the split flap; but, because of
a higher lift-drag ratio, it gives better takeoff and
climb performance. Small deflections of the slotted
flap give a higher drag than the hinge flap but less
than the split. This allows the slotted flap to be used
for takeoff.
The Fowler flap deflects down and aft to increase the
wing area. This flap can be multi-slotted making it the
most complex of the trailing edge systems. This
system does, however, give the maximum lift
coefficient. Drag characteristics at small deflections
are much like the slotted flap. Because of structural
complexity and difficulty in sealing the slots, Fowler
flaps are most commonly used on larger airplanes.
OPERATIONAL PROCEDURES
It would be impossible to discuss all the many airplane
design and flap combinations. This emphasizes the
importance of the FAA-approved Airplane Flight
Manual and/or Pilot’s Operating Handbook
(AFM/POH) for a given airplane. However, while
some AFM/POHs are specific as to operational use of
flaps, many are lacking. Hence, flap operation makes
pilot judgment of critical importance. In addition, flap
operation is used for landings and takeoffs, during
which the airplane is in close proximity to the ground
where the margin for error is small.
Since the recommendations given in the AFM/POH are
based on the airplane and the flap design combination,
Plain Flap
Split Flap
Slotted Flap
Fowler Flap
Figure 11-2. Four basic types of flaps.
Ch 11.qxd 5/7/04 8:50 AM Page 11-2
11-3
the pilot must relate the manufacturer’s recommendation
to aerodynamic effects of flaps. This requires that
the pilot have a basic background knowledge of flap
aerodynamics and geometry. With this information, the
pilot must make a decision as to the degree of flap
deflection and time of deflection based on runway and
approach conditions relative to the wind conditions.
The time of flap extension and degree of deflection are
related. Large flap deflections at one single point in the
landing pattern produce large lift changes that require
significant pitch and power changes in order to
maintain airspeed and glide slope. Incremental
deflection of flaps on downwind, base, and final
approach allow smaller adjustment of pitch and power
compared to extension of full flaps all at one time. This
procedure facilitates a more stabilized approach.
Asoft- or short-field landing requires minimal speed at
touchdown. The flap deflection that results in minimal
groundspeed, therefore, should be used. If obstacle
clearance is a factor, the flap deflection that results in
the steepest angle of approach should be used. It
should be noted, however, that the flap setting that
gives the minimal speed at touchdown does not
necessarily give the steepest angle of approach;
　
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