The Aerodynamic Characteristics of Flaps

Analysis of some experimental data has shown that the corresponding increments for plain
flaps on wings of thickness of about 12 per cent are not appreciably different from those for
split flaps on wings of that thickness. If the argument above is accepted, we may expect that the
increments tor plain flaps will not show the variation with wing thickness shown by split flaps.
Therefore, if is suggested that the curve of Fig. 6 for ;~(/3) for a wing thickness of 12 per cent
be used for plain flaps on wings of all thicknesses within the usual range.
4.1.2. Part-spa~ flaps.--From calculations of Hollingdale 16 one can derive the theoretical
ratio of the lift coefficient increment of a part-span flap of any type to the corresponding increment
for a full-span flap. This ratio is shown as a function of flap span for various taper ratios
in Fig. 9, and is denoted by Z3(bflb). An analysis of experimental data 4 has shown reasonable
agreement between experiment and these theoretical curves. Hence, for part-span flaps
ACL' = ;tl(cdc') ;t2($) Z3(bdb) . . . . . . . . . . . . . (17)
It may be noted that, where a flap has a central cut-out so that the spanwise positions of its
inboard and outboard ends are at bil/2 and bi2/2 from the centre-line, respectively, then the
part-span correction factor is
It must be emphasised that the factor 43 applies to all types of flaps.
4.2. Profile Drag Coefficient Increments.--4.2.1. Full-spa~¢ f/aps.--Proceeding on much the
same lines as in the analysls of lift coefficient increments, Young and Hufton ~ assumed that
= . . . . . . . . . . . . . . (is)
where ~ and a~ are functions that were determined from experimental data.
curves for trailing edge split flaps are shown in Fig. 10a and b.
The resulting
It will be noted that for these flaps the profile drag increment is roughly 1 o 1 sin ~/~ in terms of
the area of the flap. For a discussion of the profile drag increments of split flaps ahead of the
trailing edge (see section 11).
The data of Ref. 39 indicate that the profile drag increments of plain flaps on wings of thickness
of about 12 per cent are not quite so large as the corresponding increments of split flaps, and they
are reasonably fitted by the function for a 2(/~) for plain flaps shown on Fig. 10b. For the reasons
explained above we may expect little variation of these increments with wing thickness, and it
is suggested that the function shown be generally used for plain flaps.
For flaps of the Zap or Gouge type in their fully extended position the increment should be
readily calculable from the plain flap case if allowance is made for the chord extension.
4.2.2. Part-spanflafls.--General considerations confirmed by experimental data led Young
and Hufton 4 to conclude that the drag increment of a part-span flap of any type is proportional
to the area of the flapped part of the wing. Hence, to determine the increment for a part-span
flap we must multiply the increment for a full-span flap by the ratio of the flapped wing area to
the total wing area. The latter ratio, denoted by aa(bi/b ), is shown in Fig. 12 as a function of
flap span for wings of various taper ratios. Thus, for a part-span flap
. . . . . . . . . . . . . (19)
If there is a flap cut-out with the inboard and outboard ends given by the spanwise ordinates
biJ2 and bj~./2, respectively, then
- . . . . . . . . . (20)
4.3. Pitching Moment !ncrements.--4.3.i. Full-span flaps.--From an unpunished analysis
of the pitching moment increments of plain and split flaps by Haile the curves of Fig. 13 have
been deduced showing the ratio
ff 1 = -- A C,,,',/A Cr'
as a function of flap-chord/wing-chord (effective) for various thickness/chord ratios. A CZ is
11