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Aviation History
1936
1936 - 0022.PDF
12 FLIGHT. JANUARY 2, 1936. 1 / wtm f r 1 t t t * * r'^ i t i i * j 1 / / m A t 1 ''•A * f 1 i 11^ Left, Fig. 5. Angle of incidence 16 deg., and flap angle 0. Burbling begins inboard of the wing tip near the trailing edge. The centre portion is practically unstalled, with the exception of a few tufts in the rearmost line. In Fig. 6 (right), the angle of incidence is about the same—15 deg., and the flap angle is 40 deg. Burbling spreads inboard from the tips. The two rear rows of wool tufts are completely stalled and point forward. was a blown up RAF.28 section, and the stalling of the wings seems to proceed from the trailing edge of the wing tip towards the centre of the aerofoil at an oblique angle relatively to the air flow. Whether this type of " trailing edge stall"<8) is typical for all tapered wings or only for this particular section is still an open question. wing which are already unsatisfactory, type of aileron becomes necessary. In addition a new LOAD GRADING FOR TAPERED MONOPLANE AEROFOIL WITH TIP SLOT aSLDTTED FLAP 01M5-5" MEAN KL» 0-99 NORMAL WING (TIP SLOT OPEN) CALCULATED BY \ THE LOTZ METHOD) FLAP DOWN 40 TIP SLOTS OPEN It has been suggested to extend the flap across the whole span in order to delay stalling of the tips. However, the K, distribution thus obtained will be identical with that for an ordinary tapered wing, and the improvement can only consist in reducing the stalling characteristics of the wing with partial flap to those of the ordinary tapered 4—Tapered Wing and Wing Tip Slots Having realised that tapered wings were actually worse from the stalling point than rectangular wings, the use of wing tip slots became apparent immediately. Fig. 7 shows the calculated lift distribution for a tapered wing fitted with flap and wing tip slots extending over 50 per cent, of the semi-span. Two important points are evident from this calculated distribution, namely : (a) high mean KL ot about 1.0'as a result of a very effective KL distribution across the span,* and (b) in spite of this high resulting KL there is still a margin at the tips for damping in roll due to the falling off of the K,, curve towards the tips. Fig. 10 shows similar distributions calculated for taper ratios of 3 : 1 and 2:1, and although practically the same mean Kr, is obtained over the span, it is very obvious that the reduced taper ratios are better from the point of view of damping in roll due to the steeper falling off of the K,, curves towards the tips. The K,, distribution towards the tips is comparable with that obtained for a twisted tapered wing. Flow pictures (Figs. 8 and 9) for a tapered wing fitted with a centre flap and wing tip slots indicate that there is still an orderly flow over the tips at angles where the centre portion of the wing is already completely stalled. As a lift-producing element the front slot has been criticised for the large effective angle which is connected with lift increase. This characteristic, on the other hand, makes the wing tip slot particularly valuable and applicable to the tapered wing fitted with a partial flap, because of the very big increase of effective angle at the tips. In view of this, it is therefore also possible to obtain a very * The increase of KL max. due to the tip slots only is about 20 per cent. In other words, for a given wing loading and stalling speed the area of a wing fitted with partial flaps can be reduced by about 17 per cent, when fitting tip slots. 1 J x \ 1 X 1 1 i ... \ mm \ f f • mmm A. | i V-m \ I 1 • : 4 ' ".:• / / ! IB 1 *. / / / oil «. / ! 1 1 n Slotted and flapped. In the picture on the left (Fig. 8), the angle of incidence is 14 deg. and the flap angle 40 deg. Note the orderly flow over the whole wing. In the right-hand picture (Fig. 9), with incidence increased to 16 deg. and flap angle still at 40 deg., the inner portion of the wing is completely stalled, but the outer portion, under the influence of the leading edge wing tip slot, still shows orderly flow.
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