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article number 183
article date 11-15-2012
copyright 2012 by Author else SaltOfAmerica
Jack Northrop’s Flying Wing Finally Flies … 1946 XB-35
by Andrew Boone

From June, 1946 Air Trails Magazine.

The flying wing no longer is a dream. It has arrived in deadly reality—a huge 10,000-mile-range bomber capable of carrying a load much heavier than even the B-29 Superfortress can lug off the ground; and, if tests prove out theories, considerably faster.

That particular tailless airplane is the XB-35 by Northrop. Only partially are the curtains withdrawn, permitting a glimpse of this new craft. But the fragmentary view permitted by the army and the company reveal a structure whose potentialities are worthy of serious thought.

The XB-35 had not been flown as these lines were written. Flights were scheduled for late spring, the maiden voyage to be undertaken from Northrop field at Hawthorne, California—home of Northrop Aircraft, Inc. When the XB-35 first becomes airborne, those lucky enough to fringe the strip will witness John Northrop’s dream come true.

Construction of the 172 foot wingspan bomber is rolling along. It will have four engines of 3000 horsepower each.

Northrop has built more than a dozen flying wings. His first versions were not true wings, for they embodied other structures as well. But the XB-35 is a true flying wing, spreading 172 feet, from tip to tip. The wing section is 374 feet long at the center, tapering to slightly more than nine feet at the tips. The wing sweeps back from center to tips, making the plane’s over-all length slightly more than 53 feet. It stands 20 feet high, when at rest on its tricycle landing gear. The wing weighs in at about 89,000 pounds, promises to carry a design useful load of 73,000 pounds. But—its builders declare the plane is capable of operating under overloading conditions at a gross weight of 209,000 pounds, or 104½ tons. If and when the XB-35 ever takes off at that figure, it will soar into the sky bearing nearly half more in weight than did any B-29 yet built.

The XB-35 is all-metal (mostly 75S aluminum alloy, developed by Alcoa). It is powered by four Pratt and Whitney Major Wasps, one pair each of R-4360-17 and R-4360-21 series, equipped with single-stage G.E. electric turbo-superchargers delivering 3,000 horsepower each at a certain unstated altitude. Four eight-bladed Hamilton Standard super-hydromatic counter-rotating pusher propellers are used, each fifteen feet in diameter. The plane has no tail surfaces. It carries a maximum crew of fifteen, nine of whom operate the wing and its military appurtenances, as guns. Trim flaps and rudder assembly are located at the wing tips, landing flaps occupying the center section of the span.

Artist’s conception of the final product.

Several unique features help mark the XB—35 as an “unusual’ aircraft. The elevon controls, for instance. The elevons are a combination of ailerons and elevators, perfected by the Northrop group, and are included in the trailing edge with the landing flaps, trim flaps, and special rudders. The trim flaps are the split—type, opening like a clamshell. When the pilot presses left rudder, the double flap opens at the left wing tip, setting up a drag which slows the wing at that end to help turn the plane. It is use of this flap rudder that has enabled the engineers to dispense with vertical surfaces. The flap rudder creates no additional drag when not in use.

Wing slots are another. These long slots run parallel to the leading edge near the tip. Thoroughly tested on smaller models, the slots prove valuable at near-stalling speeds, when they prevent interruption of the smooth airflow which, when disturbed, causes stalling. Northrop engineers viewed these slots with considerable misgivings, knowing that at high speeds they would present a break in the dean surface of an all-wing structure. So they designed special doors which fit into the slots, popping open automatically when the wing nears stalling, closing again when high speed is attained to restore the wing’s smooth contour.

Plenum chambers, a Northrop development, are another. These are the giant lungs which “breathe” for the big ship and feed to the engines the enormous quantities of air they consume. The chambers eliminate the need for the many air scoops which otherwise would protrude from the—wing, impairing the efficiency of the airfoil.

A “feel” system of control enables the pilot to handle the big plane easily at top speed. Unaided, the pilot could not move the controls, even when cruising. To meet this difficulty, a full boost hydraulic system was installed, then coupled with the “feel” system, which places automatic hydraulic loads on the controls. Actually, in the absence of the “feel,” the pilot could move the booster controls with two fingers sufficiently to tear off the control surfaces. The “feel,” on the other hand applies the pneumatic forces with varying intensity at different speeds to hold control movement within safe limits.

Northrop has been experimenting with modified wings since 1923, when he and a friend, Anthony Stadl, started to build an all—wing glider, which soon was abandoned when they ran short of funds. Five years later his first real opportunity arose when he and W. K. Jay formed the Avion Corp. and built a wing-like plane in which the fuselage was eliminated and the tail surfaces were carried on two tubular booms. Simplification of form resulted in a substantial reduction of parasite drag. The pilot and 90-hp engine were housed in the wing and the landing gear was designed for retraction. This plane had a span of 30 feet, 6 inches, and was 20 feet long. It made many flights, in 1929 and 1930, which afforded valuable research data.

Jack Northrop’s 1929 design. 30 foot, 6 inch wing span.

Financial conditions forced Northrop to set his ideas aside until 1939, when the present Northrop Aircraft, Inc. was formed, and he was once more able to pursue his experiments. In August, 1939, Northrop started work on the first of his true flying wing designs, a completely tailless aircraft, containing all the controls and stability for safe operation. He was not alone in the work. From the California Institute of Technology he recruited the consulting services of Dr. Theodore von Karman, director of the Daniel Guggenheim School of Aeronautics and Dr. William R. Sears, von Karman’s assistant, who now is Northrop’s chief of aerodynamics. W. J. Cerny, a well-known designer associated with Northrop since 1928 became supervisor of the project.

The first flying wing was known as the N1M, a craft of 38-foot span and 17-foot length. One major problem in design and construction of the N1M was how to obtain directional stability and how to obtain the effects of fins and vertical surfaces without actually having them protrude from the wing surfaces. Northrop and his associates found the answer while weekending at the ranch home of Moye Stephens, then the company’s secretary. Unable to take a planned hike in the rain, the group experimented with paper airplanes. First of these paper models were in the conventional schoolboy’s pattern, but later were folded into a shape much like that of the flying wing. Northrop bent the wing tips up and down, and after each change flew the plane. With these frail models, he found that a downward bend of 35 degrees gave the best performance. Later in wind tunnel and flight tests, the desired effect was obtained by drooping the N1M’s tips.

N1M in flight. Note drooped wing tips.

Even before the tunnel tests were undertaken, the N1M had been taken to the Muroc Army Air Base on the California desert, where Vance Breese began experimental runs. The first flight actually resulted from an accident. While taxiing at high speed, the N1M struck a rough spot, bounced ten feet into the air and made a satisfactory flight several hundred yards in extent. The little wing landed safe and sound. Following more than 200 successful flights, this historic wing went to the Army’s Aviation Museum at Wright Field. As development of the N1M continued, Northrop found that straight wing tips could be substituted for the drooping version without loss of stability. This change was made, and the resulting configuration virtually duplicates that of the giant XB-35.

Next step in progress toward the XB-35 appeared in September, 1941, following a visit by a group of War Department officials to the Northrop plant. Satisfied the NIM design offered promise of unusual performance, they awarded Northrop a contract for the big bomber. Among the enthusiasts were General H. H. Arnold, Assistant Secretary of War Robert Lovett, Major General Oliver P. Echols, and leaders of various groups at Wright Field.

Partial group of design variations which led to the final XB-35.

Immediately, Northrop began building wing tunnel models in 8 and 16-foot sizes. Mockup of the XB-35 was built, including full-size crew quarters and a portion of the left wing. To provide flight research data, four 60-foot scale models were constructed and designated the N9M series. The first three were powered with two Menasco engines of 275 hp each, so located as to duplicate as nearly as possible the thrust lines of two of the XB-35’s Wasp Majors. The first N9M made about 50 flights, and was followed by a second N9M, and N9M-A and N9M-B. The last named was powered with 300-hp Franklin engines. Testing on all model continued at Muroc with Harry Crosby, John Myers, and Alex Papana at the controls.

N9M contrasts with near final drawings of the 4 engine XB-35 bomber.

These wings, whose controls and general flying characteristics duplicate those now used on the XB-35, except for size, flew many hundreds of hours. About one-third the size of the XB-35, each N9M weighs only about 7,100 pounds. In increasing each dimension three times to obtain the XB-35 plan, weight was increased to 162,000 pounds, more than 23 times the weight of the scale model.

If one were to ask Northrop to name the single outstanding feature of the XB-35, and of tailless aircraft in general, he would reply: “low drag and high lift.” To amplify this statement, one must consider the relative merits, in this respect, of flying wings and conventional airplanes. In both fields, the designer strives to produce maximum lift, minimum drag. For the best orthodox commercial transports or bombers, this ratio is in the order of 90 or 100 to one. If a conventional plane has a maximum lifting capacity of 100 tons, it has a minimum drag of at least one ton, which the engines must overcome. On the flying wing, Northrop estimates this ratio as being between 140 and 200 to one. Thus a flying wing having a drag of one ton would have a lifting capacity of 70 to 100 tons as compared to 50 tons of the conventional plane.

Unhappily, estimated performance of the XB-35, in terms of speed, has not been revealed. If, however, it proves as successful as anticipated, it is not unreasonable to expect resulting radical changes in military aircraft designs. This in turn would hasten development of commercial airliners on the flying wing principle.

Top view of XB-35 under construction.

Flight trials with the smaller wings turned up some problems, none serious, which now have been overcome. The first versions possessed flight characteristics somewhat different from those of a conventional plane, because of the elimination of the fuselage and tail surfaces. Considerable effort went into making the wing fly like a conventional plane, and later models proved no more difficult to fly than standard aircraft. Yet, according to Northrop, not one was seriously unstable, and all were flyable-—from the beginning.

In Jack Northrop, the principle of the flying wing is so sound he has dreaded for years the possibility of his morning newspaper’s announcing the development of another one by some other designer. Others have tried. Burgess Dunn constructed a tailless biplane in 1917. Lippisch in Germany and Hill in England tried out their versions of the idea. But these, and others, incorporated vertical tail surfaces, short fuselages, exposed engine nacelles, or other drag-producing elements. Following the recent occupation of Germany, our air experts learned of a Gotha P60, a flying wing design offering considerable promise. Horten flying wing sailplanes and powered planes likewise attained a considerable efficiency. But Northrop’s design probably is the first we can term a true flying wing. At this moment, it promises a revolution in design for both military and civilian aircraft.

Northrop Grumman B-2 Stealth Bomber … first flight, 1997.
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