The Aerodynamic Characteristics of Flaps

Fig. 13. This fact, as well as the close agreement shown, between the estimated and measured
values of ACr', illustrates the consistency of the results obtained with widely dissimilar flaps
when analysed on the basis of effective chord.
6.3.2. Fowler plus split flap.--The results of the R.A.E. tests of a 0.4c Fowler flap combined
with a 0.1c split flap 76 are given in Table 2 and are also included in Fig. 16. The split flap was
hinged to the lower surface of the Fowler flap in the trailing edge position. It will be seen that
with the optimum setting (35 deg for the Fowler, 45 deg for the split flap) the split flap helps
to raise the maximum lift coefficient increment from about 2.0 to 2-4 for a Surprisingly small
cost in profile drag. The ratio --A C,,,,/A CL is high and is much the same as for the single Fowler
flap.
6.3.3. Double Fowlerflap (Fig. lj).--The front flap of the double Fowler as tested at the R.A.E.
is virtually a 0.4c slotted flap of the N.A.C.A. type, and it is the rear flap which provides the
essential Fowler characteristic by extending backwards from its housing in the front flap to a
distance practically equal to its own chord length. The lift coefficient increment attained by
this combination is the highest of all the flaps examined 70, being about 2.65. The drag coefficient
increment is small; but like that of, the Fowler or Fowler plus split flap the value of
-- (A C,~,/A Cr) is high.
Again we may note that for both the Fowler plus split flap and the double Fowler the value of
--(A C,,';/A CL') is about 0-23. For a split or plain flap of the same effective chord ratio on a
wing of the same thickness this ratio would be about 0.24 (see Fig. 13).
It is interesting to note that the optimum setting is about 15 deg for the front flap and about
42 deg for the Fowler flap. It is, in fact, usual to find that for double flaps, where the rear flap
extends the chord quite considerably, the optimum front flap angle is relatively small.
6.4. N.A.C.A. Slotted Flaps.--6.4.1. N.A.C.A. single-slotted fla~s (Fig. ld).--These have
been dealt with in section 5, but for completeness some representative data have been included
in Table 2.
6.4.2. N.A.C.A. double-slotted flaps (Fig. lh, li).--Comprehensive tests have been made in
America of two sets of double-slotted flaps, a relatively small chord combination, viz., 0.26c
and 0-1c, and a large chord combination, 0.4e and 0.26c. The latter was tested on wings of NACA
23012, 23021 and 230306~ section, the former on wings of NACA 23012 section 5s. The relative
movements of the flaps when operated are given in Table 1, they are in fact the same as those
adopted for the single flaps. 'In Fig. 15b, 15c the drag coefficient increments are plotted against
the lift coefficient increments. The optimum flap angles for maximum lift increment combined
with a relatively small drag increment vary slightly with wing section and with flap chords, but
they are in the region of 30 deg to 40 deg for both flaps. With higher flap angles little lift is lost
but there is a rapid increase of drag.
The average values of A C,,,/A CL and A C,,/,/A CL' are summarised in the following table :--
A C,,,,
A C~',
A CL
A CL'
0"26c, 0"lc flap (NACA 23012) .... --0"335 --0"29
0"4c, 0"26c flap (NACA 23012) .... --0.34 --0.265
0.4c, 0.26c flap (NACA 23021) .... --0-37 --0.29
0-4c, 0.26c flap (NACA 23030) .... --0.59 --0.43
As with the Fowler flaps, the values of AC,,'~/AC;' on the 12 per cent thick wing are in fair
agreement with the corresponding values for split flaps given in Fig. 13. However, the doubleslotted
flaps show a surprising increase of A C,,'~/ACL' with wing thickness. This increase
occurs mainly at the higher values of A CL', and in fact the relation between A C,/~ andACL'
tends to depart from linearity as the wing thickness increases. Probably, the difference between
15