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Development of All-Wing Aircraft (part 3)

continued from part 2

Elevons and rudders

From the inception of the work, longitudinal and lateral controls were combined in the "elevon," which word was coined to designate the trailing edge control surface members which operate together for pitch control and differentially for roll control. At no time during early tests did control about the pitch or roll axes give any appreciable difficulty. The control which was least expected to cause difficulty gave the most, namely the rudder.

Early in the test program it was found that the airplane had quite satisfactory two-control characteristics that is, a normal turn resulted from a normal bank without the use of rudder controls and as a result, throughout the program we have often considered the elimination of rudder controls entirely. It was indeed fortunate that the first airplane developed such docile characteristics, for many of the rudder configurations tried proved to be ineffective--or worse, affected the flight characteristics of the airplane adversely.

From the start it was determined to eliminate, to the greatest extent possible, vertical fin and rudder surfaces; first, because they violated the all-wing principle and added drag to the basic airfoil; second, because with the moderate sweepback employed in our early designs the moment arm of a conventional rudder about the C.G. was small, and an excessively large vertical surface would have resulted had we tried to achieve conventional yaw control moments. The rudder development was therefore concentrated on finding a type of drag-producing device at the wing tips which would give adequate yawing forces without affecting pitch or roll. To this end we tried 25 or 30 different configurations in flight which were first tested in the wind tunnel. As a result of this experience it was concluded that dynamic reactions were likely to be very different from static reactions; some of the configurations which looked best in the wind tunnel proved to be quite unsatisfactory in flight.

The best and most practical rudder found was one of the simplest in concept and one of the first to be flown, namely a plain split flap at the wing tip which could be opened to produce the desired drag. This flap was later combined with the trimming surface needed to counteract the diving moment of the landing flaps, forming the movable control surfaces at the wing tip of the XB-35.

Among the many flights accomplished with the first experimental airplane were several in tow of other aircraft where the distance to be covered, or the altitude to be gained, made it impractical to depend solely on the airplane's own engines. After a few minutes of acquaintanceship with the slight differences brought about by the presence of the tow cable, the airplane behaved well in tow and several comparatively high altitude flights were made to investigate the spin characteristics. These appeared to be quite normal, based on preliminary tests of this airplane. Later experience, however, indicated that the spin characteristics of tailless types vary from one design to another, in the same fashion as may be expected in conventional types, and that no broad generalization as to spin behavior can be made with safety.

N-9M flying mockup for bomber

Northrop N-9M scale model The N-1M was first flown in July 1940 and for about a year was consumed in a combination of aerodynamic tests and attempts to solve engine cooling problems. As soon as good sustained flight demonstrations could be made on schedule the Army Air Forces took active interest in the program and top-flight officers, including General H. H. Arnold and Major General Oliver P. Echols, encouraged us to investigate the application of the all-wing principle to large bomber aircraft. To this end it was decided to construct four scale models of a larger airplane. These were designated N-9M (Northrop Model 9 Mockup) and they duplicated, except for the power plant and propeller arrangement, the aerodynamic configuration of the proposed XB-35 airplane.

The first of these aircraft was completed and test flown on December 27, 1942, and had completed about 30 hours of test flying with pilot (and sometimes an observer) when it crashed, killing the pilot. The machine had been on a routine test flight across the desert away from its base, and was out of sight of technically qualified observers at the time of the accident. However, all evidence pointed to a spin, and the attitude of the airplane on the ground indisputably indicated autorotation at the time of impact.

This loss was a serious setback and work was started immediately to recheck the spin characteristics of the airplane in a spin tunnel. It was later determined, both in the tunnel and in flight, that recovery was good, although a bit unconventional (requiring aileron rather than elevator action), but that the spin parachutes which had been attached to the airplane for the low-speed stalling and stability tests then in progress were ineffective as to size and improperly located.

Spinning and tumbling characteristics

Subsequent models, over hundreds of flights, gave no trouble. The low-speed stall and spin tests with rear C.G. positions were accomplished without further difficulty and the N-9M proved an invaluable test bed in which various control configurations could be proved in detail. A large number of additional rudder configurations were developed and tested on the N-9Ms; likewise different types of mechanical and aerodynamic boost for the control surfaces were investigated, as well as the general behavior of the airplane in all types of air, and with different C.G. positions.

In connection with the model spin tests of this airplane, an investigation of the tumbling characteristics of the type was made in the spin tunnel. These tests showed that if the model was catapulted into the airstream with an imposed high velocity about the pitch axis in either direction, it would continue to tumble or come out of the maneuver, depending on comparatively minor differences in elevon and C.G. position. In other words, under circumstances of induced rotation about the pitch axis the recovery was marginal. However, it would never tumble from any normal flight condition, such as a stall, spin, or any other to-be-expected maneuver. In some configurations, if dropped vertically trailing edge down into the wind stream, a tumbling action would be induced which might or might not damp out. This was not judged a serious matter in view of the fact that a vertical tail slide is hardly a maneuver to be courted, even by a fighter airplane, let alone a 100-ton bomber.

The three remaining N-9Ms have been flown almost continuously since their completion dates to the present. Only recently have all desirable test programs been completed and the airplanes relegated to a semi-retired status from which they are withdrawn only for the benefit of curious pilots. . . .

XB-35 long-range bomber

Northrop XB-35 Flying Wing bomber During all this development and testing . . . . the design and construction of the big ship had been under way. N-9M airplanes had proved the practicability of the design. They closely approached the XB-35 configuration with the exception that they mounted only two pusher engines, located at positions corresponding to points midway between engines 1 and 2, and engines 3 and 4.

The problem of control-surface actuation on the big bomber involved the development and testing of a complete hydraulic control system, as none of the aerodynamic boosts or balances developed and tested in the N-9M models had proved satisfactory. The system used in the XB-35 employs small valves which are sensitive to comparatively minute movements of the control cable and which, when displaced, permit large quantities of oil to flow into the actuating cylinders. This arrangement eliminates any pilot "feel" of the load on the control surfaces unless a deliberate arrangement for force feedback is made. Rather than undertake this later step, a comparatively simple force mechanism, which is sensitive to accelerations and airspeed, was developed. This device gives the pilot a synthetic feel of the airplane which can be adjusted in intensity to anything he likes, and which has proved satisfactory in flight. For reasons to be outlined shortly, a synthetic feel was much more satisfactory than the feedback of actual control surface loads, particularly at high angles of attack.

The XB-35 was first flown from Northrop Field to the Muroc Army Test Base in June 1946. The first several flights indicated no difficulties whatsoever with the airframe configuration. Indications of trouble with propeller governing mechanisms were discerned at an early date and it was shortly discovered that flights of any substantial duration could not be accomplished because of oil leakage in the hydraulic propeller governing system. On the last flight difficulty with both propellers on one side caused a landing with asymmetrical power, which was accomplished without trouble.

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