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Aviation History
1941
1941 - 0959.PDF
APRIL 24TH, 290 MODERN WING LOADING The Desirability 0/ High. Loading in Civil Transport Design By C. L. JOHNSON* THE purpose of this paper is to investigate the effectof wing loading on transport airplane design. Indiscussing the general problem, every effort will be made to use numerous examples, keeping engineering terms and formulas to a minimum. It is hoped that by this procedure many misconceptions concerning the effects of wing loading on airplane design can be corrected in the minds of pilots, operators and designers. The wing loading of an airplane is defined as the total gross weight divided by the wing area ; in other words, it is a measure of what actual load each square foot of wing .area is carrying under normal unaccelerated flight condi- tions. Wing loading affects the flight characteristics of the airplane in that the higher the unit weight to be car- ried by each square foot of wing the higher the speed at which the airplane must fly to sustain that load or the greater must be the angle of attack at which the wing meets the air. Values of wing loading vary from 3lbs./sq. ft. in gliders and birds to 6olbs./sq. ft. in racing type aircraft. At the present time, typical transport airplane wing loadings vary from 20 to 35lbs. /sq. ft. of area. Numerous Advantages It has been unfortunate that many airplanes in the higher wing loading brackets have had poor flying charac- teristics for reasons not at all connected with wing load- ing. The most usual of these faults has concerned stalling characteristics. There is no fundamental reason why an airplane with high wing loading should have worse stall characteristics than those with low wing loading. In fact, due to the higher stalling speed actual control should be improved. A pilot can prove this to himself by merely flying a given airplane in a practically empty condition and also with full gross load, and note the difference in control obtained with the ailerons at the stall. On a typical plane it will be possible to change the wing loading by slightly over 20% in this method. It should be noted that here is a perfect example of the fact that the stall characteristics are not changed by wing loading in a given airplane. The following table shows the variation in stalling speed for wing loadings varying from 10 to 100 lb./sq. ft. It is assumed that the same type of high lift flap is used in all cases. The fact that the stalling speed varies so slowly with wing loading is of utmost importance in determining the economic value of the airplane for transport use, as ^comparatively small changes of wing loading result in a change of payload. Wing loadings may be safely TABLE I Win; Loading Pounds per square foot 1020 30 40no 80 «0 100 Stalling Speed with High Lift Flaps m.p.h. 39.5fiC.O • KS.3 70.088.5 t)fi.8 118.5 125.0 increased only when the power loading (weight per horse power) is decreased at the same time. In this way the take-off run, climb and airplane ceilings can be maintained or improved over the values now common to transport aircraft. For every condition of flight except stalling there is * Mr. C. L. Johnson is Chief Research Engineer of the Lpck- '•'i Aircraft Corporation. This paper is the result of research irk done under his direction, ••.w\ grateful acknowledgment made to him and the com;.;u:y. some combination of power and wing loading that will give a constant performance. In other words, if it were not for limitations of stalling speed or landing, winy load- ings could increase substantially to the great benefit of payload and perlormance. In a short time, power will be used for the landing and stalling speed conditions to control the airflow around the wing in such a manner that even the minimum spued factor will be a function of power. From this point onward there will be very little cause to consider limitations of wing loading such as are now considered unsafe. In view of all the discussions pertaining to wing loadings, it is well to ask the question— Who wants higher wing loadings and why? The answer is that everybody wants higher wing loadings, if safety is not sacrificed. The reasons are these: a. Added performance will be obtained. b. Smaller, lighter and cheaper airplanes will become available. c. Added safety will result due to higher speed, greater range and better low altitude rate of climb. d. Increased payload can be carried. It is also well to ask—What steps arc now being taken to make the use of higher wing loadings safe? The steps may be summarised as follows: a. Improvements in airports and airway facilities- will allow higher landing speeds. b. The use of improved landing gears which will allow faster landings with netter landing control and better braking. c. Improvements in engines and propellers are being made available to give better power-weight ratios which improve take-off and emergency safety. d. Great efforts are being directed to the study of stability, control iactors, and wing stalling. e. Improved wing flaps are being used and studies made on boundary layer control by the use of engine power. f. Serious research studies are being undertaken on icing and related problems toward a solution for keeping airplane efficiency high under all possible conditions. It is perhaps in the last field that development has been most tardy. This subject will be discussed at some length in this paper, as it is one of the critical factors affecting wing loading. Peering ahead, we might well ask what will future wing loadings be. It is almost foolish to try even to estimate such a factor. In view of the prob- able developments of boundary layer control, values of 70 lbs. /sq. ft. will very likely be common within five years. Like all other aircraft companies, the Lockheed Aircraft Corporation is vitally interested in determining the proper course to follow in future designs with regard to so impor- tant a subject as the wing loading problem. In this con- nection, a great deal of research has been carried out in- vestigating wing flaps, icing problems, stability and asso- ciate aspects of design, including all safety and economic factors affecting this decision. Factors Affecting- Wing Loading The problems related to wing loading which now face the aviation industry come under two broad groups Flight problems include: 1, handling characteristics—landing speed, stability, manoeuvrability; 2, take-off and climb performance ; 3, icing effects ; 4, pilot technique. The economic problems are". 1, payload carried; 2, per- formance gained ; 3, airplane initial cost; 4, operating costs for given speed ; 5, hangar size, servicing problems ; 6, crew required. The preceding items will be discussed and enlarged upon by means of examples which cover the present design trend for the coming series of transport airplanes and .'<lsn cover the type of equipment now being uj-e.d. Too often, deci- sions are made regarding design problems without a
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