The Aerodynamic Characteristics of Flaps

The effectiveness of the flap derives from the fact that on rotation it changes the camber of
the section and so permits of a change of circulation and therefore, of lift at a given incidence.
The change in chordwise loading produced by. a positive setting of the flap is of the type illustrated
in Fig. 2b, having a maximum near the wing leading edge and a secondary peak at the flap
hinge but falling to zero at the flap trailing edge 29.
2.2. Split Flaps.--The term spl!t flap is normally understood to mean a flap that is formed
by splitting the wing trailing edge m a roughly chordwise direction back to a hing.e,, about which
the lower portion of the Wing trailing edge can rotate, the upper portion remammg fixed (see
Fig. lb). The definition has tended to become generalised to cover any flap that is hinged on or
in contact with the wing under surface and whose operation does not alter the wing upper surface.
Thus split flaps may lie forward of the trailing-edge position, 'as is frequently required for
dive brakes or flaps on sweptback wings, or their line of contact with the wing may slide back as
they open, as for Zap-type flaps (see Fig. lb). Unless otherwise stated, however, when reference
is made to split flaps in this report, the simple trailing-edge type will be understood.
Split flaps boost the circulation when operated, by increasing the suction behind them and
therefore at the wing trailing edge, and alleviating the adverse pressure gradient on the wing
upper surface. The resulting change in the chordwise loading distribution is not unlike that for
plain flaps except for the discontinuity in pressure across the split flap and a rather greater suction
at the trailing edge.
2.3. Slotted Flaps.--When a portion of the rear of a wing can be rotated to leave a well-defined
slot between it and the rest of the wing, then it is referred to as a slotted flap. It nlay be simply
hinged just below its nose, as in the case of the Handley Page type of slotted flap (see Fig. lc)
or it may be operated with a link or track mechanism, as in the case of the N.A.C.A. (Fig. ld),
Blackburn (Fig. le) and Fowler (Fig. If, lg) types of flaps. All slotted flaps have a certain amount
of backward movement when operated.
The lift increments of slotted flaps result from three main factors. The first is the effective
change of camber produced by setting down the flap, as with plain flaps. The second is due to
the flow through the slot re-energising the boundary layer, when the slot is well designed, and so
delaying flow separation from the flap. The third is the increase of effective lifting surface due
to the rearward movement of the flap. In the case of the simple Handley Page type of flap,
the hinge is not very much below the flap nose and the rearward extension is small, the contribution
to the lift increment associated with this extension is then comparatively unimportant.
On the other hand, in the case of the Fowler flap, the rearward extension is considerable, being
about the length of the flap chord, and the corresponding contribution to the lift increment is
very important. The Blackburn type of flap is, in this respect, half way between the Handley
Page and Fowler types of flap. It moves about an effective hinge centre set well below the flap
nose, and when fully extended provides a considerable extension of lifting area.
As noted above, the effect of the slot in helping to delay flow separation from the flap and so
boosting circulation depends very critically on the design of the slot. Details of efficient slot
designs cannot be given here ; they are to a considerable extent a function of the wing section
and flap angle, and reference should be made to the separate reports giving the results of tests
of different slot shapes (see, for example, Refs. 54 to 57, 60, 62, 63, 66 to 70, 74, 76 to 78). The
more important dimensions that have been found to lead to successful slot shapes are indicated
in Fig. 1. All that can be said here is that breakaway in the slot must be avoided, and so the
slot must converge steadily from the lower to the upper surface. Further, the flow from the
slot must merge smoothly into the flow round the wing and flap ; hence, an appreciable length
of lip or shroud to the upper surface of the slot is an advantage. In general, it should be possible
to fit a smooth curve connecting the wing and flap contours.
It is important to realise that if the slot is not efficient it may be worse than no slot at all,
as the flow through it may then stimulate rather than suppress flow breakaway over the flap.
It is this tendency of a well-designed slot to suppress flow breakaway over the flap that gives the
slotted flap its characteristic of low drag at small or moderate flap angles.
5