IntroductionIn choosing the title, "The Development of All-Wing Aircraft," as the subject of my lecture I run some risk of being accused of writing a company history rather than a paper of the broad scope ordinarily presented before this time-honored institution. This is far from my intent, but being sincerely convinced that the all wing airplane is a valuable step in the development of aeronautics, and desiring to contribute a maximum amount to the available data in the limited time at my disposal, my paper must be confined largely to experience gained by our company in its work on this subject.
Outside the efforts of the Horten Brothers in Germany there has, until a comparatively recent time, been little physical accomplishment in the development of the all-wing airplane except by our company. The contemporary Horten development has been fully described in technical reports emanating from Germany since the close of the European war. In many instances the Horten conclusions were surprisingly similar to our own. Their work was not carried so far, however, and I doubt that they had the sympathetic and responsible governmental backing and the resultant opportunities for development accorded us.
In considering the development of all-wing aircraft I would like first to distinguish between all-wing and tailless airplanes. Most tailless airplanes are not all-wing by our definition. There is a tremendous background of development in tailless types, which has been fully reported by Mr. A. R. Weyl in Aircraft Engineering. These articles outlined a surprising number of reasons for building tailless aircraft which have motivated the various designers and constructors over the years. Only one of the many advantages to be gained through such development has inspired our work, namely improved efficiency of the airplane. . . . [V]irtually all our efforts have been directed toward the reduction of parasite drag and the improvement of the ratio of the maximum trimmed lift coefficient (Clmax) to the minimum drag coefficient (CDmin). It is natural, then, that we were not interested particularly in tailless airplanes as such; if we could not eliminate vertical tail surfaces, fuselages, and a substantial portion of interference drag, the gains to be made seemed not worth the effort necessary for their accomplishment.
Our work, therefore, through the years has been directed solely to all-wing aircraft, by which I mean a type of airplane in which all of the functions of a satisfactory flying machine are disposed and accommodated within the outline of the airfoil itself. Of course, we have not as yet built any pure all-wing aircraft. All have had some excrescences, such as propellers, propeller drive shaft housings, jet nozzles, gun turrets and the like. We have, however, built a number of airplanes in which the minimum parasite drag coefficient has been reduced to approximately half that ordinarily attained in the best conventional aircraft of like size and purpose, and in some of the designs completed and tested the excrescences and variations from the airfoil contour have been responsible for less than 20 percent of the minimum airplane drag.
Basic assumptionsA surprisingly large number of people, both within and without the aircraft industry, still appear to question the economic reasons for going to all the trouble to build an all-wing airplane. "Sure," they say, "after a lot of practice people can learn to walk on their hands, but it's most uncomfortable and unnatural, so why do it when nothing is gained thereby?" Actually, there are startling gains to be made in the aerodynamic and structural efficiency of an all-wing type, provided that certain basic requirements can be fulfilled by the type under question. These requirements can be simply stated as follows:
First, the airplane must be large enough so that the all-wing principle can be fully utilized. This is a matter closely related to the density of the elements comprising the weight empty and the useful load to be carried within the wing.
The dimensions of the average human body may also at times be the limiting factor but, ordinarily, in the larger types of transport or bombardment aircraft in which we are most interested, it will be found that excessive sizes are not necessary in order to secure, within a wing of reasonable thickness ratio, adequate volume for a commercial cargo or bomb load plus the necessary fuel.
The extremes explored and satisfactorily flown to date in our experience range from a "buzz" bomb having a span of 29 feet . . . to the 172-foot XB-35 long-range bomber airplane. The buzz bomb was practical because of the comparatively high specific gravity of the warhead, plus the fact that the configuration permitted almost all of the wing to be used as a fuel tank.
The XB-35, on the other hand, is considerably larger than would be necessary to provide ample space for passenger and crew comfort and ample volume for payload, be it cargo or bombs. It was designed larger than necessary because we desired to keep the wing loading comparatively low in this first large experimental venture. It has a normal gross weight of 165,000 lb., an overload gross weight of 221,300 lb., and sufficient volume within the wing envelope so that the maximum gross weight at takeoff might well be increased to over 300,000 lb., somewhat over half of which could be devoted to bombs, fuel and miscellaneous payload. It may be seen, therefore, that there is a practical range of size within which the all-wing airplane can be used. If the requirements of space and volume do not permit the full use of the all-wing principle, a rudimentary nacelle may be added without losing its economic advantages.
The second basic requirement is that the all-wing airplane be designed to have sufficient stability and controlability for practical operation as a military or commercial airplane. We believe this requirement has been fully met by hundreds of flights completed with this type, and we are fully convinced of its practicability after having built a dozen different airplanes embodying scores of different configurations incorporating the all-wing principle.
In comparing all-wing and conventional types, we may fairly assume that spans of comparative aircraft having the same gross weight are equal, and as a further simplification we may for the moment neglect compressibility effects in our comparison to the advantages of all-wing and conventional types of large bombardment or transport aircraft having maximum velocities up to approximately 500 m.p.h.
Comparison of minimum drag and maximum trimmed liftBased on these assumptions and on the following proved data on the all-wing type, a comparatively simple analysis of advantages may be made.
The ratio of the minimum parasite drag coefficient (CDmin) for all-wing airplanes to that for conventional types is approximately 1:2. Minimum drag coefficients for a number of large bomber and transport aircraft . . . average approximately .023. The minimum drag coefficients for several all-wing types have been measured both in model and full-scale configurations and vary from less than .010 to about .0113, which is the figure for the XB-35 including armament protuberances, drive shaft housings, rudimentary nacelle for gun emplacements, and so on.
The ratio of maximum trimmed lift coefficient (Clmax) for all-wing to conventional types is approximately 1.5:2.3. The latter figure is typical for a number of the large airplanes of conventional arrangement previously mentioned. The former is readily attainable in a configuration such as that of the XB-35 and may be subject to considerable improvement through the use of several types of high lift devices yet to be proved.
For comparative airplanes of the same span and gross weight the selection of the required wing area will depend either on flight conditions, including takeoff without flaps, or landing conditions. If the flight conditions govern, the ratio of required wing areas of all-wing to conventional aircraft will be 1:1 because the two wings are equally effective except under conditions of maximum lift. If landing conditions govern, the ratio will be 3:1, assuming the same landing speed in each case. If takeoff with partial flap deflection governs, the ratio will be somewhere between the above two figures.
In large all-wing bombers and transports, and a growing extent in conventional long-range transports as well, the ratio of gross weight at takeoff to landing weight will approach 2:1. Therefore flight conditions are likely to govern the selection of wing area more than landing conditions. In the following calculations both extremes are used as indicative of the range of advantage to be gained by the use of the all-wing configuration. [Calculations show] that the total minimum parasite drag of the all-wing airplane in terms of the conventional airplane will vary from 50 percent if flight conditions govern, to 77 percent if landing conditions govern. . . .
It is a well-known fact, based on the Breguet range formula, that with conventional reciprocating engines and propellers the speed for maximum range is approximately that at which parasite drag and induced drag are equal. Therefore, at the same cruising speed as the conventional airplane the all-wing type will require from 25 percent to 11 percent less power . . . and with the same amount of fuel will fly from 33 percent to 13 percent farther. . . . If the all-wing airplane is operated at its most economical speed, instead of the most economical speed of the conventional airplane, it will fly 19 percent to 7 percent faster and the range will be from 41 percent to 14 percent greater with the same amount of fuel. . . .