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Development of All-Wing Aircraft (part 2)

continued from part 1

Advantages of low parasite drag

Under high-speed conditions with any type of power plant the parasite drag becomes a much larger percentage of the total drag than for cruising conditions with reciprocating engines. At high speed the parasite drag may account for 80 percent or more of the total, while the induced drag drops to 20 percent or less. Using an assumed figure of 80 percent parasite drag . . . the power required to drive the all-wing airplane at the same speed as the conventional airplane will be from 40 percent to 18-1/2 percent less . . . and the range, at the high speed of the conventional airplane, will be from 66 percent to 22 percent greater. . . . As turbojet and turboprop power plants both operate at relatively high speed for best fuel economy, the advantages of the all-wing configuration, when used in combination with these power plants, will closely approach the above figures for maximum range as well as high speed.

These advantages are all based on the simple aerodynamic values obtained with all-wing airplanes; namely, that; CDmin equals 50 percent of conventional CLmax equals 65 percent of conventional. The probabilities are that the minimum parasite drag can, within a comparatively short time, be reduced, at least for commercial types, to about 40 percent of the conventional figure and that the maximum trimmed lift coefficient (CLmax) may, within a similar short time, be increased to at least 75 percent of conventional values.

Methods for increasing maximum trimmed lift

One of the most interesting devices for increasing maximum lift is, of course, the judicious use of boundary layer control in conjunction with turbojets or gas turbines. Another involves the development of a better combination of low pitching moment flaps and trimming devices which will permit of "lifting ourselves by our boot straps" in a more successful manner than we have achieved to date. Model configurations tested up to this time, employing such methods, have shown improvements of .1 or .2 CL over the figure now used of 1.5.

A third possibility of rather unconventional nature remains to be proved in the all-wing airplane. This consists of placing the C.G. behind the aerodynamic center of the wing, eliminating inherent longitudinal stability by so doing and replacing this characteristic, which heretofore we have always considered as an essential to satisfactory aircraft design by highly reliable (and perhaps duplicate) automatic pilots which take over the function of stability from the airframe and may perhaps do a better job of maintaining the proper attitude than the present classical method. While unconventional and possibly a bit horrifying to those unaccustomed to the idea, it may have practical application to very large aircraft where the pilot's skill and strength are largely supplanted by mechanical means of one sort or another, and wherein the pilot controls the mechanism which in turn places the airplane at the desired attitude. If the C.G. is located aft of the aerodynamic center the airplane will trim at a high angle of attack with the flaps or elevator surfaces deflected downward rather than upward from their normal position, thereby increasing the camber and rendering the whole airfoil surface a high-lift device. It is possible that trimmed lift coefficients in the order of 2.0 may be achieved by this method, and experiments completed to date with such a device on conventional aircraft show that the C.G. may be displaced at least 10 percent of the mean aerodynamic chord aft of a normal position without any uncomfortable results in the flying characteristics of the airplane.

When these improvements in CLmax and CDmin can be realized, further startling gains in performance will accrue, as will be outlined later. It would seem, however, that the present accomplishments offer sufficient incentive to warrant all they have cost in time, effort and money, and that the question, "Why bother with an all-wing airplane?" is already well-answered.

Other major advantages

There are other major advantages of the all-wing type which cannot be so definitely evaluated but which can and do contribute appreciably to improvement in efficiency and range. Two of these, namely the elimination of jet-tail surface interference, and the possible elimination of wing-tail surface shock wave interference, have already been mentioned. The third, and the most immediately applicable to designs of the near future, is the improved adaptability of all-wing types to the distribution of major items of weight empty and useful load over the span of the wing. While such distribution can be made to a limited extent in conventional airplanes, it can be much more fully accomplished in the all-wing type. Such weight distribution results in substantial savings in structural weight which have important effects on the ratio of gross weight at takeoff to landing weight. An analysis of the range formula indicates that this ratio is one of the most important range parameters. Competent authority has shown that distribution of fuel in the wings instead of the fuselage of a large conventional modern transport would allow an increase in gross weight of 16 percent without increase to weight empty, with a corresponding increase in range up to 30 percent.

It is fairly obvious that the all-wing airplane provides comparative structural simplicity, plus the possibility of structural material distribution in a most effective way at maximum distances from the neutral axis, plus an opportunity to stow power plant, fuel and payload at desirable intervals along the span of the wing, which cannot be equaled in conventional types. These matters are rather intangible and difficult to illustrate by numerical relationships. They depend to a large extent on the type and size of the airplane, what it is designed to carry, and what the desired high speed may be.

Problems involved in all-wing design

Having demonstrated, perhaps, that the advantages of the all-wing type are fully worth striving for, let us consider the problems involved and their solution. Based on our present experience these difficulties do not appear now of surpassing magnitude, but in 1939 several of them seemed so serious as to discourage the most hardy optimist.

To one testing a swept-back airfoil having a desirable root thickness, taper ratio and symmetrical section, together with reasonable washout at the tips such as might be designed from the then available data, the first results were a bit terrifying. The elevator effect [on the model] was erratic, changed in sign with varying deflections, and was entirely unsuitable for the control of an airplane. It was also seen that the degree of static longitudinal stability indicated by the average slope of the pitching moment curves was less than that considered desirable in a conventional airplane. Experiments involving visual observation of tufts on the model indicated a separation along the training edge of the airfoil which was apparently due to the planform configuration, and which was responsible for the erratic curves. In early experiments a simple addition of 10 percent to the chord length with a straight line contour from approximately the 70 percent chord point to the new 110 percent chord point, almost completely eliminated the difficulty.

First full-scale airplane

It was soon determined that data applicable to conventional wings with little or no sweep were completely unreliable for the degree of sweepback required in practical all-wing designs, and that a whole new technique had to be developed to determine the limits within which taper ratio, sweepback and thickness ratio could be combined for satisfactory results. All these variables were explored in a series of wind tunnel models, and when a reasonably satisfactory group of configurations had been determined it was decided to build our first piloted flying wing, the N-1M (Northrop Model 1 Mockup).

Because of the many erratic answers and unpredictable flow patterns which seemed to be associated with the use of sweepback, it was decided to try to explore most of these variables full scale, and the N-1M provided for changes in planform, sweepback, dihedral, tip configuration, C.G. location, and control surface arrangement. Most of these adjustments were made on the ground between flights; some, such as C.G. location, were undertaken by the shift of ballast during flight. The variations to which this first airplane was subjected involved two extremes of arrangement in which the airplane was found to be quite satisfactory in flight.

It is an interesting commentary on the comparative ease with which the basic problems of controlled flight were solved to note that no serious difficulties were experienced in any flight attempt, or with any of the various configurations used. Some "felt" better to the pilot than others, but at no time was the airplane uncontrollable or unduly difficult to fly. The principal early troubles were related to the cooling of the small "pancake" type air-cooled engines which were buried completely within the wing, and because of the pusher arrangement did not have the benefit of slipstream cooling in taxiing, takeoff and climb. Engine-cooling problems seriously handicapped the early flights but later, somewhat larger engines were installed and the design of the cooling baffles was sufficiently improved so that repetitive sustained flights were accomplished easily.

The first flight was more or less an accident in that, while taxiing at comparatively high speed over the normally smooth surface of the dry desert lake bed used as a testing field, the pilot struck an uneven spot. He was bounced into the air and made a good controlled flight of several hundred yards before returning to earth. Altogether, this first airplane was used in over 200 [sic] flights of substantial duration, during which numerous configurations were tested and a great deal of work was done in the determination of the best types of control surface and surface control mechanism.

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