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Development of All-Wing Aircraft (part 4)

continued from part 3

The next six months, from August to March, were spent in a vain attempt to eliminate these difficulties, plus those caused by a series of engine reduction gear failures. To date the XB-35 has not had sufficient time in the air to fully demonstrate its ability to meet its design performance guarantees. However, large-scale model tests in numerous tunnels have indicated the low-drag figures presented earlier in this paper, and preliminary speed versus power tests completed early this month have given gratifying confirmation of our original expectations. Flights accomplished to date have included all maneuvers necessary for large bombardment airplanes. So far, however, violent maneuvers have not been attempted and no exact evaluation of stability and control parameters has been possible.

YB-49 Flying Wing over the Mojave Desert Two turbojet powered all-wing airplanes, having the same basic shape and size as the XB-35 are virtually complete at this time and will be flying late this summer. They are powered by eight jets having a sea level static thrust of 4,000 lb. apiece. They incorporate small vertical fins to provide the same aerodynamic effect as the propeller shaft housings and propellers of the XB-35.

Let us now turn to considerations of stability and control of the all-wing airplane. They are quite different from those of conventional types and, unless reasonably well understood, may lead to discouragement at an early date concerning projects well worth further evaluation.

Static longitudinal stability

In any airplane the primary parameter determining the static longitudinal stability is the position of the center of gravity with respect to the center of lift or the neutral point. Obviously, the neutral point may be shifted aft by adding a tail or by sweeping the wing, or the C.G. may be shifted forward by proper weight distribution, so that from the standpoint of static stability no particular configuration has any special advantage except as it affects the possibilities of proper balance. In an all-wing airplane the elimination of the tail makes the problem of balance somewhat more critical but not excessively so. Unfortunately, for any given airplane the neutral point does not ordinarily remain fixed with variations of power, flap-setting or even lift coefficient, so that the aft C.G. limit for stability is often prescribed by some single flight condition has always occurred for power-off flight at angles of attack approaching the stall.

Characteristics at high lift

The pitching instability of a swept wing at high lift coefficients is by now a somewhat familiar phenomenon. The complete mechanisms involved, however, are still somewhat obscure. There are apparently two opposing effects which are of prime importance. They are the tendency for sweepback to increase the relative tip loading and also (by creating a span-wise pressure gradient) to promote boundary layer flow toward the tip. On a plain swept-back wing the latter effect apparently nullifies the former, so that there occurs in the tip portion of the wing a gradual decrease in effective section lift-curve slope with a resulting progressive decrease in stability. The tip, under these circumstances, never completely stalls, as evidenced by the stable pitching moments occurring at the maximum lift coefficient. On the other hand the addition of end plates will prevent to a large extent the effects of span-wise flow, thereby straightening the pitching moment curve but producing the normally expected tip stall, as evidenced by the strongly unstable moments in the vicinity of the maximum lift coefficient. Thus, any modification to the basic wing which affects the span-wise flow will have a noticeable effect on the pitching behavior at high lift coefficients.

In the case of the XB-35 the propeller shaft housings act to inhibit span-wise flow and straighten out the moment curve below the stall as in the case of the end plate; but in order to obtain stability at the stall, a tip-slot is provided to increase the stalling angle of the tip sections. By raising the trim flap in the outer 25 percent span and lowering the main flap in the inner 35 percent span, the stability characteristics are noticeably affected, presumably because of a decrease in spanwise pressure gradient and therefore in boundary layer flow.

Recent investigations have indicated that the problem of static longitudinal instability near the stall for plain swept-back wings depends not only on sweep but also on aspect ratio and it now appears that for a given sweepback the magnitude of the unstable break in the moment curve decreases with decreasing aspect ratio, eventually vanishing.

The possibility of controlling the stalled portions of the wing, as outlined, means that trailing edge flap controls can be laid out to maintain their effectiveness at very high angles of attack. Since a certain portion of this flap must be used to provide high lift and roll control, the amount available for longitudinal trim is limited, so that for the XB-35, for example, the total available nose-up pitching moment coefficient is .15 as compared to .30 for a conventional airplane. This limited control plus the fact that the main wing flaps apparently cannot be made self-trimming and impose a diving moment in the landing condition reduces the available C.G. range . . . as compared with conventional airplanes. The XB-35 has a C.G. range of only 5 percent or 6 percent as compared with conventional values in the order of 10 percent or 12 percent. This comparison is somewhat misleading, however, because the all-wing airplane may have a greater comparative [mean aerodynamic chord] in view of its somewhat lighter wing loading. It is also much easier to arrange weight empty and useful load items spanwise within close m.a.c. limits than in conventional types.

Where manual control of the elevator is employed the stick-free stability and control of all-wing aircraft are impaired by separation of the flow from the upper surface of the wing near the trailing edge, causing up-floating tendencies at higher lift coefficients. If not corrected these up-floating tendencies lead to stick-free instability and, in some cases, to serious control-force reversal at high lift coefficient. Aerodynamic design refinements devised and tested by us to date have not provided a satisfactory solution to the up-floating tendency. For small airplanes these undesirable forces can sometimes be tolerated, but for large aircraft the only solution found so far has been the employment of irreversible full power driven control surfaces.

Lateral stability derivatives

It is when considering the lateral stability and control factors that the difference between the all-wing and conventional airplanes becomes most apparent. It is reassuring to state that despite the large differences apparent between the XB-35 and conventional aircraft, the dynamic lateral behavior of the XB-35 type is quite satisfactory, as will be discussed later.

Definite requirements for the weathercock stability . . . depend to a large extent on the airplane's purpose, but positive weathercock stability is always required. The swept-back wing has inherent directional stability which increases with increasing lift coefficient; but this is not considered sufficient for satisfactory flight characteristics under all circumstances and must be supplemented by some additional device. The wingtip fin has been favored by some since it gives the largest yawing lever arm and provides a suitable rudder location. However, as previously pointed out, wingtip fins may be unsatisfactory at the stall. For the XB-35 configuration, effective fin area is provided in large measure by the side force derivative of the pusher propellers. . . .

Effective dihedral

Considering now the effective dihedral . . . it is apparent that sweepback is the essential difference between the all-wing and conventional airplanes--a difference that will disappear as flight speeds increase and it becomes necessary to employ the desirable high-speed characteristics of swept wings in conventional tailed configurations. . . . Flight ease may indicate that a slightly positive effective dihedral is desirable while dynamic considerations point toward a slightly negative dihedral. Our practice has been to retain positive effective dihedral over the complete flight range.

Roll control

The rolling control for all-wing airplanes is essentially normal. When elevons are used rather than separated aileron and elevator control, certain variations from conventional craft appear, in that, with the upward elevator deflection required for longitudinal trim, the adverse yaw ordinarily due to aileron deflection disappears. On the other hand, if large up-deflections are required for longitudinal trim, the up-going elevon used as aileron loses effectiveness rapidly, thus reducing the available roll control at high lift coefficients. This is particularly undesirable when considering the increased dihedral effects of swept wings at high lift coefficient.

Side force effects

All-wing airplanes, particularly those without fins, have a very low crosswind derivative; thus a low side force results from sideslipping motion. Some crosswind force is probably important for precision flight, such as tight formation flying, bombing runs, gun training maneuvers, or pursuit. This importance arises because with low side force it becomes difficult to judge when sideslip is taking place, as the angle of bank necessary to sustain a steady sideslipping motion is small. This lack of side forces has been one of the first objections of pilots and others when viewing the XB-35. After flying in the N-9M or XB-35 the objection is removed, except for some of the specific cases mentioned above. For the correction of the lack of sideslip sense, a sideslip meter may be provided for the pilot or automatic pilot, and for very long-range aircraft there is a valuable compensating advantage in being able to fly under conditions of asymmetrical power without appreciable increase in drag.

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