The Aerodynamic Characteristics of Flaps

Blackburn Aircraft Ltd. have tested leading-edge slats on 0-4c and 0.5c chord flaps. The main
data for the two most promising arrangements are given in Table 2. For the 0-4c flap it was found
advisable to use a relatively large slat to get a marked improvement of lift. The estimated value
of d CL' in Table 2 was arrived at by treatingthe flap plus slat as a single-slotted flap with the
leading edge at the nose of tile slat, thus, the estimate includes the effect of chord and flap extension
due to the slat, but not the effect of flow improvement over the flap. A comparison of
the estimated and measured values of A CL' for the 0.4c flap shows that in this case there could
have been no appreciable flow improvement. In the case of the 0.5c flap, however, the leadingedge
slat, although small, produced a marked increase in the lift increment, a considerable portion
of which must have been due to the resulting improvement of flow over the flap. It is of interest
to note that because of this improvement tile drag increment for the optimum setting is relatively
small. The value of A C,,,~/A CL is about --0.35, but A C,,~',/A CL' is in the usual low range for
Blackburn flaps, viz., --0.19 to --0.24.
6.5.4. Blackburn flap with inset slots (Fig. lm).--An obviously related device, that was also
tested by Blackburn Aircraft Ltd., is that of a flap with fixed inset slots 69, 70. If the slots are
efficiently designed, then, like the flap leading-edge slat, they produce a general improvement
of the flow over the flap. The important data for the optimum cases tested are given in Table 2.
The cleaning up effect of the slot on the flow over the flap is indicated by the fact that in each case
the measured value of A CL' is considerably greater than the estimated value, whilst the value
of A Coo is small for the A CL obtained. The values of -- (A C,,r/A CL) are 0.29 and 0.35 for tile
wing with 0.4c and 0.5c flaps respectively, but --(A C,,/,/A CL') varies between the limits 0.225
and 0.25.
The simplicity of this device is largely offset by the fact that unless the slot or slots are sealed
when the flap is retracted, they will cause a serious increase of drag. The tests indicated that for
a slot to function properly it must be cut at a fairly small angle (about 20 deg) to the flap chord.
Hence the opening on the upper surface must occur well back, and it will be very difficult to increase
the shroud sufficiently to seal the slot.
6.5.5. Blackburn flap with deflected shroud (Fig. In).--One of the most effective devices tested
by Blackburn Aircraft Ltd. consists of a slotted flap with a slightly longer shroud than usual,
and part of the shroud is hinged and arranged to deflect downwards with the flap 69. The shroud
thus smooths over the effective change of camber involved in putting the flap down, and in fact
the combination of deflected shroud and flap acts as a double flap. The main data are given in
Table 2. The lift coefficient increment attained (2.26) is only slightly less than that attained
with either inset slots or a flap leading edge slat, and it probably presents less serious disadvantages
than these other devices.
Treating it as a double-slotted flap the estimated value of A CL' is almost in exact agreement
with the measured value. If we regard it as a plain front flap and a rear slotted flap the estimated
value of A CL' is appreciably less than the measured value. This is not surprising, since the slot
is near enough to tile shroud hinge to dominate the flow there. Tile pitching moment coefficient
increments are relatively small, A C,n,/A CL is --0.3, whilst A C,,',/A CL' is --0.18.
6.6. Venetian-blind Flaps (Fig. lo).--Venetian-blind flaps have at various times aroused a
certain amount of interest, since, if their slots function properly, their lift increments should
approach the theoretical plain flap values. The results of such tests as have been made, however,
have been rather disappointing 77,78 Representative data are included in Table 2. The main
conclusions of these tests are :--
(1) The optimum spacing of the slat components appears to be one slat-ch0rd length and
there is no advantage in using a large number of small slats rather than a small
number of large slats.
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