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Aviation History
1941
1941 - 0963.PDF
APRIL 24TH, 1941. 301 'MODERN WING LOADING (Continued) accuracy of the preceding weight study. It is well to note the comparative dimensions of the airplanes involved from a point of view of hangar space required and also the sell- ing prices. Current two-engine transport sell for approxi- mately $7.30 per pound of empty weight, while the less numeious four-engine airplanes are available for about $10.00 per pound. Handling and Performance Characteristics Wing loading has little effect upon handling characteris- tics of the airplane except where maximum lift obtainable from the wing is involved. There is no reason why com- parable stability and manoeuvrability cannot be obtained equally well with the high wing loading as with the lower loadings now in use. The problems of stability can be approached in a very straightforward manner using ies already developed. There is an indirect effect of wing loading on stability that results from the use of higher power on the more modern airplanes. Many stability troubles common in the past were caused by the effect of power, and now, with relatively smaller wings and higher power, the percentage of the wing and tail span covered by the slipstream is increased. Improvements in testing technique have been of such a nature, however, that we have every right to expect very substantial improvements in control and handling characteristics of the next series of transport airplanes. Much research work has been done on these problems Due to the development of low-drag high-lift wing flaps, the problem of manoeuvrability at slow speeds and the actual landing speed problem has been kept well in hand. Most of the present types of transport airplanes are EFFECT OF WING FLAPS ON WING LOADING FOR EQUAL MANOEUVRABILITY €"-<: 4° 8" 12° 16° 20" 24° ANGLE OF INCIDENCE Fig. 2. Flaps and manoeuvrability. Point J shows reserve:it for " 2g pull-up " with 38 .'bs.sq. ft. wing loading while K ;ves angle of incidence for 120 m.p.h. with the same wingJading and 1 .'3 total flap setting. L and M are corresponding points for 27.8 lbs. sq. ft. wing loading with no flaps. J u IC l u. O0 h- Zi 1+ V? 10 •8 •6 •4 •? n # ///SI i Mi|H !1 •04- - %> /// // / •O8 DRAG • jI, B •12 16 -2O -24 COEFFICIENT CD Fig. 3. Lifl-drag curves for the four aircraft. equipped with split-type wing flaps which are not of great benefit in improving manoeuvrability at approach speed conditions. Transport airplanes now being designed will make definite use ot flaps for manoeuvring, so that it is in order to give an example of equivalent wing loadings for equal manoeuvrability. See Fig. 2, which shows the lift obtainable at various angles of attack for an airplane with flaps retracted and a 27.8 ibs./sq. ft. wing loading, com- pared to the same curve for an airplane with modern flaps set for approach manoeuvring condition. When flying at 120 m.p.h., the unflapped airplane has an angle of attack of slightly over 5°. and it has reserve lift to make a zg pull up without stalling, for the wing loading of 27.8 lbs./sq. ft. The flapped airplane flying with one-third flap exten- sion at 120 m.p.h is carrying 38 lbs. /sq. ft, of wing area and has exactly the same manoeuvrability in that it can make a zg pull-up without stalling also. A zg pull-up is equivalent to a 6o° bank. This example shows the value of the proper flap type for manoeuvring. The values of wing loading shown correspond roughly to the landing values for the four-engined airplanes having 40 and 30 lbs. / sq. ft. take-off wing loading respectively. There is often considerable comment that, when the flaps are extended, the airplane will have a very low rate of climb, but this is not true for the example shown. With one engine in- operative the four-engined airplane with the 38 lbs./sq. tt. wing loading will have a rate of climb of over 600 ft. per min. at normal (not take-off) power. The conclusion can be drawn that as far as handling characteristics and stability are concerned, higher wing loadings offer no serious problems. It might also be said that present manoeuvring speeds can still be maintained on a well-designed airplane by proper use of wing flaps, although the limiting landing speed will increase slightly over values now in use. The latter factor is not considered serious when good stall characteristics are p.vailable, as they certainly must be, and when ample power is used to give good climb in emergency. The problem dealing in the effect of wing loading on rudder controllability has been thoroughly studied and does not show that any great diffi- culties will be encountered in obtaining rudder control after engine failure. In order to evaluate the effect of wing loading on per-
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