The All-Wing Type Airplane A Description of the New Northrop “Flying Wing” By JOHN K. NORTHROP Vice-President and Chief Engineer, Northrop Aircraft Corporation

I T HAS BEEN apparent for several years to the designer of aircraft that some radical changes in general arrangement will be necessary if any large increase in the overall efficiency of the average airplane is to be accomplished.

A steady program of development and refinement has been under way for the past twenty years; until we have at present a number of carefully designed and comparatively efficient planes embodying streamline fuselages, carefully cowled engines, and “clean” landing gears; with superfluous struts, wires and fittings suppressed to an absolute minimum. It seems quite apparent that our best designs are close to the limit of practical efficiency; yet we find that their maximum over-all L/D ratio is only about 10, whereas the L/D ratio of the active supporting surfaces of an airplane is normally double this amount.

An analysis of the items adding to parasite drag in the normal design shows that landing gear, power plant, fuselage, interference and bracing, and control surfaces are the major contributors to parasite power loss; the item of control surfaces being by far the smallest. Individual examination of the various units show that nearly all possible improvement has been made in existing designs.

Landing gear retraction offers one of the largest theoretically possible gains, but a tremendous amount of thought has been given to the problem and it still remains essentially unsolved. The low wing monoplane offers the best, and almost the only possible conventional arrangement in which a retractable landing gear can be employed, but at best the design of such a gear is exceedingly difficult due to the high loads involved and limited space available for the required mechanism.
Power plant drag is a problem that cannot be solved with a conventional arrangement unless the engine is completely housed, liquid cooled, and smooth skin wing radiators are used to dissipate the heat. Such installations have proven satisfactory for racing planes, but are far from practical for commercial or military purposes at the present time.

The resistance of the fuselage, except in a few rare instances, remains a deal loss, with no possibility of reduction except through careful fairing and streamlining; and often, due to aerodynamic necessity, the size of the fuselage is much less than would be desired from an economic standpoint.

Wing surface bracing and interference between the various parts of the airplane comprise one of the largest contributors to parasite drag and the one most difficult to determine. Even in the cleanest full cantilever monoplanes, there still remains a large increase in drag due to interference between fuselage, wings, chassis and tail surfaces.

And finally, in the control surfaces themselves, we often find it impossible to use even a fairly good aerodynamic form, or to place the surfaces where they are not subject to considerable turbulence and interference from fuselage, wings and power plant.

THE FOREGOING considerations and others of similar nature lead W. K. Jay and the author to the development of an experimental “Flying Wing” type airplane which has been built and tested during the past year and a half, and gives promise, with certain modifications, of solving many of the problems that confront the designer who seeks a large increase in aerodynamic efficiency.

The “Flying Wing” in various forms has been a well recognized dream of designers since men began to fly, but it has always been pictured in prohibitively costly and mammoth sizes. However, certain comparatively recent developments have made many of its advantages immediately applicable to planes of moderate size. Wing sections with maximum ordinates of more than 25 percent have shown excellent aerodynamic characteristics, and planform tapers with a root-to-tip ratio of more than 5 to 1 have qualities equal to or better than a rectangular airfoil. The combination of thick sections and large taper, at once makes possible a wing structure with central portions of such size as to become economically practical for the purpose of housing power plant, personnel and cargo. The structural advantages of such a design are tremendous, and the general arrangement provides ample space for the retraction of landing gear which, while somewhat unconventional, has proven entirely satisfactory during actual flight tests.

THE FIRST PLANE of this type, recently successfully flight tested, was built primarily as a flying laboratory to test out the various innovations of structure and arrangement of component parts. For this reason the plane was designed as small as practicable, while still providing ample room for a pilot and one passenger. No attempt to place this particular model in production will be made, the advantages of this type of craft becoming very marked as it is applied to somewhat larger sizes. The original plane is all-metal and employs a newly developed type of structure in which the same smooth duralumin or alclad sheets, properly reinforced, provide both covering and most of the structural strength of wings and tail surfaces. This structure is being employed in the design of a new transport monoplane of conventional aerodynamic form which will shortly be placed on the market by the Northrop Aircraft Corp.

The all-wing plane which has been flight tested, is a thick wing monoplane of high taper ratio, the taper being entirely on the leading edge, in order to balance the plane with engine and pilot in the wing. which results in a marked degree of sweepback. The wing has a span of but 30 ft., and tapers from a section of normal thickness and approximately 45 in. chord at the tip, to a chord of 100 in. at the center and a wing section with a thickness more than one-third of the chord. As may be seen from the photographs, the taper is gradual from the tips to a point about of the distance from tip to center and then increases rapidly to the thick central portion. The wings are so mounted with relation to the thickened center section as to give the effect of a center wing monoplane. The inverted air-cooled engine is carried almost entirely within the wing, a small housing projecting a short distance in front of the leading edge, and all flight tests so far conducted have been with pusher propeller arrangement, although later tests are to be conducted with the tractor set-up. Two cockpits are provided in the wing, one each side of the power plant and all control surfaces are operated by horn and cable hook-ups, with dual Deperdussin controls, one in each cockpit. The propeller shaft, approximately seven feet long, extends between the two cockpits to the propeller, which is mounted 6 in. in rear of and considerably above the trailing edge of the wing. Tail surfaces are carried on two cantilever out rigger booms, oval in section, which extend in rear of and considerably above the main wing, thus providing for ground clearance of the tail group, and also placing the tail control surfaces up above the wing wash and most of the propeller blast. The propeller turns between the two outrigger booms.

The tail surfaces consist of a rectangular stabilizer and elevator combination of unusually high aspect ratio, which terminates at each end in an elliptical vertical surface consisting of fin and rudder. The control surfaces thus form a complete structural unit supported at each end by the booms from the wing. Elevator adjustment is accomplished by rotating the entire unit in its boom mountings. These outriggers taper in section and thickness and are heavily reinforced at the point of their attachment to the wing. They are designed to carry air loads only and can therefore be made considerably smaller and lighter than though their function included carrying conventional tail skid loads. No external bracing is used in their support, and no vibration of the tail surfaces, due to their comparative flexibility, has been experienced.

The landing gear consists, essentially, of three wheels carrying approximately equal loads, and grouped beneath the center section of the wing. The two forward wheels have the unusually wide tread, for such a small plane; 9 ft., and are placed considerably farther forward, about 12 in., than is possible in a conventional design. The third wheel is located on the center-line and just forward of the trailing edge and is pivoted for easy ground handling. They are all located close to thick portions of the wing structure where there is ample room for retraction. The experimental retractable gear proved satisfactory in actual flight tests, but for the purpose of carrying on extended tests of performance and maneu verability a conventionally permanent mounting has been temporarily given to the three wheels.

Flight tests have now been conducted over a period of months and during many hours in the air the plane has shown remarkable maneuverability and performance, and although beset with most of the troubles common to new types, the plane as designed has proved eminently satisfactory. All test flights have been made by Edward A. Bellande, veteran western test pilot and superintendent of the western division of T.A.T.-Maddux Air Lines.

THE experimental plane was arranged for flight tests as a pusher and tractor in order to determine the most satisfactory and efficient location of the propeller. In the pusher arrangement a Mark III Cirrus engine, specially inverted by Menasco Motors, Inc., of Los Angeles, drives the propeller through a tubular steel drive shaft. The rear end of the shaft runs in a large ball bearing which is housed in a fin-like structure over the trailing edge of the wing. The front end of the shaft is carried directly in a flexible coupling on a light flywheel mounted on the engine crankshaft. The entire drive shaft mechanism weighs approximately 35 lb. and has proved to be free from vibration or torque whip, as now installed. In the tractor tests, a Menasco A-4 four-in-line inverted engine will drive the tractor propeller directly. In either case cooling is provided by air passing through a large tunnel extending entirely through the wing from the front opening in the engine cowling to an opening just ahead of the trailing edge on the lower side of the wing. Flight tests have proved this cooling arrangement entirely satisfactory and a material reduction in drag has been effected by enclosing the engine.

Considering the chassis and tail surface arrangement of the present experimental plane we find them to have many advantages. The tail support structure is much lighter than an equivalent fuselage. The outrigger size is so small, and the angle of intersection between wing and outriggers such that there is little possibility of appreciable interference between the two. The use of two or more supports makes possible a very efficient horizontal surface of high aspect ratio and no end loss, and its location to the rear and above the wing removes it from most turbulence due to wing or power plant, so that it has been found possible to use 30 percent less surface on the horizontal stabilizer and elevator without loss of adequate control. Therefore, the drag of the control surfaces and their support is much less than in conventional designs.

The arrangement of the landing gear concentrates the landing loads in a deep structure where they can easily he provided for; possibility of nose over is practically eliminated, and complete retraction of the whole chassis, including tail wheel, is a comparatively simple matter.

With an eye to the future possibilities of the design, it may be well at this point to compare the queer looking airplane with the best of conventional designs and to see how the items which contribute to drag have been affected. The drag caused by the chassis has been entirely and comparatively easily eliminated. The power-plant drag is not changed appreciably if we employ radial air cooled engines but in-line air cooled engines may he completely housed within the wing and cooled, as in the present model, with considerable saving. In case liquid cooled engines are used, they may he completely housed and readily accessible in flight. The type of wing structure used is easily made water tight, so that the cooling liquid may he circulated within the wings by means of a spray system with return drains to a sump from which the engine is supplied. Such a system is not subject to the faults of the ordinary wing-type radiator as the liquid is not under pressure and small leaks or even bullet holes would cause only a small loss of cooling liquid. If perfected, such a cooling system would completely eliminate any drag chargeable to power plant cooling or placing, and would also do away with much of the weight necessary in present types of liquid cooling radiators.

IN THE “flying-wing” type, the fuselage is replaced by a thick wing section. It is structurally more compact and more efficient in weight-strength ratio than the fuselage. It provides comparatively large cubic capacity, which can he increased within reason, with very little aerodynamic loss. While the profile drag of a thick section is considerably greater than that of a normal airfoil its L/D characteristics are better than the over-all L/D of the best commercial plane. Only a small proportion of the total surface is abnormally thick—the remainder being unaffected. The net aerodynamic gain is therefore large. Because of the smooth and uninterrupted taper between center and tip, the loss due to the thick section is even less than would be expected. Much of the efficiency of this design seems to be due to the fact that there is no break in the airflow across the center section of the wing, thus rendering the sharp taper more effective than if the center section were disturbed by a fuselage. Another factor of great importance is that the sharp taper and thickened center section brings most of the supporting surface, and consequently most of the air load, toward the central portion of the wing system where the structure is best arranged to carry this load, and that a relatively small portion of the total wing load is carried out toward the wing tips where the simple law of leverage renders it so difficult to care for high loadings.

The item of bracing and interference between wing, fuselage, and brace struts is completely removed, the heavily tapered wings being in the best possible form for an efficient full cantilever structure, and requiring no external bracing. There can be no landing gear-fuselage, or wing-fuselage interference. Only a small amount of drag results from the outrigger-wing intersection ; and the 90 deg. intersection between the horizontal and vertical tail surfaces is the only point in the whole machine justifying fillets or fairings of any kind. The final item of parasite drag, that due to control surfaces, which, even in our conventional designs is the smallest of all, may be still further reduced through the use of smaller and more efficient surfaces, made possible by the better placing of surfaces and the smoother air flow over the entire machine, and the elimination of fuselage-tail and all brace structure interference.

A BRIEF IDEA of what future designs may he developed upon the basis of work already done is given here more as a mark for us to “shoot at” than as a prophecy. It is probable that large single-engined mail and express planes will follow the present design quite closely, with a central engine compartment, and space for cargo and personnel on either side. In the light of tests already completed, it is to he anticipated that such a design, with conventional wing and power loadings, may have a top speed well in excess of 200 mph.

In the larger multi-engine transport class, the deepest part of the wing forms a very commodious cabin and the power plants, two or four in number, can be housed in readily accessible engine rooms on either side of the cabin. Such a design should have a cruising speed of more than 150 m.p.h. A machine of this type would provide excellent passenger vision forward, in addition to reasonable downward and sideward vision through windows in the lower portion of the bulge where the thickened center section tapers up to the wing. A small streamline compartment over the leading edge gives the pilot the best possible vision forward and downward, as well as full all-around vision upward and to the rear. Very excellent passenger protection can be provided structurally, and an unusual freedom of passenger movement would be provided and encouraged in the room shaped cabin, since planes of such design would carry their pay load so near to the c.g. that any possible variations in loading would scarcely affect the aerodynamic balance of the plane.

The placing of fuel tanks in the wings, outboard of the engine compartments, and separated from them by double fireproof walls, would greatly reduce the fire hazard, or the chance of crashed tanks throwing gasoline on the engines.
Several interesting developments will no doubt attend the use of such flying wing designs. With parasite drag suppressed to a minimum, variable lift or variable area devices become much more effective and important than at present. Planes having such a large speed range would be greatly benefited by a variable pitch propeller, or possibly some form of transmission and gearing. And lastly, hut by no means least, it would be almost essential to incorporate some form of “spoiler” so that the high efficiency so laboriously created could be “spoiled” in order that our ideal plane could reduce its normally excellent gliding range, and land at the average airport. Certainly the speed range and general performance which is possible to the flying wing or “all-wing” type airplane is most optimistic, and we hope that the experimental craft which we have developed has brought these possibilities close to realization for commercial use.

THE ORIGINAL “all-wing” plane shown in the accompanying photos was designed by W. K. Jay and the author during the latter part of 1928. Its early development and tests were carried on by the Avion Corporation, financed by George Hearst, San Francisco newspaper publisher; and Ada Wilbur, of Los Angeles.

In the fall of 1929 W. E. Boeing became interested in the development and arranged to take over the Avion Corporation as the nucleus of the present Northrop Aircraft Corporation, a division of the United Aircraft and Transport Corporation. An experimental factory and laboratory was at once erected on the United Airport, Burbank, and this is to be shortly enlarged to provide for the quantity production of all-metal aircraft of conventional type but employing the new flat plate structure which has been developed and which will be described in detail following the announcement of the first commercial plane.

A large amount of experimental work remains to be accomplished before the flying wing design can be considered commercially applicable; and it is planned by the heads of the United group that this work shall be carried on in a thorough and conservative manner before any planes of this type are put into production.
All test flights so far conducted have been made in secret from the Muroc Dry Lake bed on the Mojave desert, or more recently from the United Airport, but no public showing of the plane itself has yet been permitted and because of the many phases of flight performance being investigated it is not yet possible to announce complete flight data. However, the plane has proved to have normal flight characteristics in every respect and general performance has been more than satisfactory. A top speed of approximately 25 per cent better than any other design of like power and capacity has definitely been determined. A full determination of all flight data will be made by a long series of tests with various engine and propeller arrangements prior to actual design of a commercial model.

Some of the specifications of the experimental plane are as follows:

Span 30 ft. 6 in. Length 20 ft. Height overall 5 ft. Wing area 184 sq.ft. Maximum wing thickness 34 in. Aspect ratio 5.1—1 Landing gear tread 9 ft. Gross weight 1,600 lb. Wing loading (lb. per sq.ft.) 8.7 Power loading (lb. per hp.) 17.8