35th WILBUR WRIGHT MEMORIAL LECTURE
The Development of All-Wing Aircraft
John K. Northrop
The President: They had now reached the highlight of the 1946-47 session, the Wilbur Wright Memorial Lecture. It seemed incredible to think that these memorial lectures had been going on for 35 years, no sooner was one over than another one came round. The Lecture was usually given in alternate years by an Englishman and an American, and this year they were fortunate in having as their lecturer that distinguished American, Mr. John K. Northrop.
Before introducing Mr. Northrop he had another duty to perform. As was customary on the occasion of the Wilbur Wright Memorial Lecture, as President, he had cabled Orville Wright as follows:--
"On May 29th 1947 your distinguished countryman John K. Northrop will read the 35th Wilbur Wright Memorial Lecture on The Development of All-Wing Aircraft. The reading of this annual lecture is a constant reminder of those early years of this century when you and your brother laid down so clearly, yet so simply, the firm foundations of the art of flying upon which must ultimately be built that structure for world peace which can never be destroyed." He had received the following reply:--
Heartiest greetings to the Society and to all assembled to hear my esteemed fellow countryman Northrop deliver the 35th Wilbur Wright Lecture. I believe Mr. Northrop will bring to you a good message on matters useful in peace.
He had also sent the following cablegram to the Institute of the Aeronautical Sciences:
On behalf of the Council and Members of the Royal Aeronautical Society I send you our greetings on the occasion of the reading of the 35th Wilbur Wright Memorial Lecture by your distinguished member John K. Northrop on The
Development of All Wing Aircraft.
They had cabled in reply:--
On the occasion of the reading of the Wilbur Wright Memorial Lecture by our distinguished American contemporary John Northrop the Officers and Council of the Institute of the Aeronautical Sciences extend heartiest greetings to the
Royal Aeronautical Society. We are all looking forward to a period of greater collaboration between the Societies on either side of the Atlantic.
He now had great pleasure in introducing Mr. John K. Northrop. Mr. Northrop was well-known on both sides of the Atlantic for his work on all wing aircraft. Indeed, he was one of the great pioneers in that field and probably knew more about this particular type of airplane than anyone. He was chief designer, President and everything else--in fact, he was the Northrop Aviation Company. He had been designing and developing the all-wing type since about 1923. This evening they were to have the pleasure and privilege of hearing from Mr. Northrop something of the difficulties and successes connected with that development.
He had pleasure in calling on Mr. Northrop to deliver his lecture.
One cannot undertake the presentation of one of the long series of Wilbur Wright Memorial Lectures without a deep sense of appreciation of the tremendous contributions made by the illustrious group of scientists and engineers who have given such great distinction to this event. The happy precedent of inviting individuals from without the United Kingdom to make this presentation in alternate years has gone far in the past toward improving the understanding, cooperative effort and fellowship of the English-speaking peoples, and I am deeply honored to have been among those chosen to further this very worthy cause.
Outside the efforts of the Horten Brothers in Germany there has, until a comparatively recent time, been little physical accomplishment in the development of the all-wing airplane except by our company. The contemporary Horten development has been fully described in technical reports emanating from Germany since the close of the European war. In many instances the Horten conclusions were surprisingly similar to our own. Their work was not carried so far, however, and I doubt that they had the sympathetic and responsible governmental backing and the resultant opportunities for development accorded us.
In considering the development of all-wing aircraft I would like first to distinguish between all-wing and tailless airplanes. Most tailless airplanes are not all-wing by our definition. There is a tremendous background of development in tailless types, which has been fully reported by Mr. A. R. Weyl in Aircraft Engineering. These articles outlined a surprising number of reasons for building tailless aircraft which have motivated the various designers and constructors over the years. Only one of the many advantages to be gained through such development has inspired our work, namely improved efficiency of the airplane.
More recently, through the rapid development of turbojet power plants, a second advantage has arisen, which is the elimination of design difficulties attendant upon the impinging of high speed high-temperature jets on tail surfaces. Still more recently a third possible advantage has appeared, this being the (as yet unproved) probability that problems of stability in the transonic and supersonic ranges may be somewhat more simple of solution in the tailless type than in the older and more conventional arrangements.
Only the first of these basic advantages, namely that of improved efficiency, has been readily apparent over a number of years and, as a result, virtually all our efforts have been directed toward the reduction of parasite drag and the improvement of the ratio of the maximum trimmed lift coefficient (Clmax) to the minimum drag coefficient (CDmin). It is natural, then that we were not interested particularly in tailless airplanes as such; if we could not eliminate vertical tail surfaces, fuselages, and a substantial portion of interference drag, the gains to be made seemed not worth the effort necessary for their accomplishment.
Our work, therefore, through the years has been directed solely to all-wing aircraft, by which I mean a type of airplane in which all of the functions of a satisfactory flying machine are disposed and accommodated within the outline of the aerofoil itself. Of course, we have not as yet built any pure all-wing aircraft. All have had some excrescences, such as propellers, propeller drive shaft housings, jet nozzles, gun turrets and the like. We have, however, built a number of airplanes in which the minimum parasite drag coefficient has been reduced to approximately half that ordinarily attained in the best conventional aircraft of like size and purpose, and in some of the designs completed and tested the excrescences and variations from the aerofoil contour have been responsible for less than 20 percent of the minimum airplane drag.
First, the airplane must be large enough so that the all-wing principle can be fully utilized. This is a matter closely related to the density of the elements comprising the weight empty and the useful load to be carried within the wing.
The dimensions of the average human body may also at times be the limiting factor but, ordinarily, in the larger types of transport or bombardment aircraft in which we are most interested, it will be found that excessive sizes are not necessary in order to secure, within a wing of reasonable thickness ratio, adequate volume for a commercial cargo or bomb load plus the necessary fuel.
The extremes explored and satisfactorily flown to date in our experience range from a "buzz" bomb having a span of 29 feet, in which the warhead was cast as a portion of the aerofoil to the 172-foot XB-35 long-range bomber airplane. The buzz bomb was practical because of the comparatively high specific gravity of the warhead, plus the fact that the configuration permitted almost all of the wing to be used as a fuel tank.
The XB-35, on the other hand, is considerably larger than would be necessary to provide ample space for passenger and crew comfort and ample volume for payload, be it cargo or bombs. It was designed larger than necessary because we desired to keep the wing loading comparatively low in this first large experimental venture. It has a normal gross weight of 165,000 lb., an overload gross weight of 221,300 lb., and sufficient volume within the wing envelope so that the maximum gross weight at takeoff might well be increased to over 300,000 lb., somewhat over half of which could be devoted to bombs, fuel and miscellaneous payload. It may be seen, therefore, that there is a practical range of size within which the all-wing airplane can be used. If the requirements of space and volume do not permit the full use of the all-wing principle, a rudimentary nacelle may be added without losing its economic advantages.
The second basic requirement is that the all-wing airplane be designed to have sufficient stability and controllability for practical operation as a military or commercial airplane. We believe this requirement has been fully met by hundreds of flights completed with this type, and we are fully convinced of its practicability after having built a dozen different airplanes embodying scores of different configurations incorporating the all-wing principle.
In comparing all-wing and conventional types, we may fairly assume that spans of comparative aircraft having the same gross weight are equal, and as a further simplification we may for the moment neglect compressibility effects in our comparison to the advantages of all-wing and conventional types of large bombardment or transport aircraft having maximum velocities up to approximately 500 m.p.h.
COMPARISON OF MINIMUM DRAG AND MAXIMUM TRIMMED LIFT
The ratio of the minimum parasite drag coefficient (CDmin) for all-wing airplanes to that for conventional types is approximately 1:2. Minimum drag coefficients for a number of large bomber and transport aircraft such as the B-29, B-24, C~4 and others average approximately .023. The minimum drag coefficients for several all-wing types have been measured both in model and full-scale configurations and vary from less than .010 to about .0113, which is the figure for the XB-35 including armament protuberances, drive shaft housings, rudimentary nacelle for gun emplacements, and so on.
The ratio of maximum trimmed lift coefficient (Clmax) for all-wing to conventional types is approximately 1.5:2.3. The latter figure is typical for a number of the large airplanes of conventional arrangement previously mentioned. The former is readily attainable in a configuration such as that of the XB-35 and may be subject to considerable improvement through the use of several types of high lift devices yet to be proved.
For comparative airplanes of the same span and gross weight the selection of the required wing area will depend either on flight conditions, including takeoff without flaps, or landing conditions. If the flight conditions govern, the ratio of required wing areas of all-wing to conventional aircraft will be 1:1 because the two wings are equally effective except under conditions of maximum lift. If landing conditions govern, the ratio will be 21 3:1, assuming the same landing speed in each case. If takeoff with partial flap deflection governs, the ratio will be somewhere between the above two figures. In large all-wing bombers and transports, and a gr
It is a well-known fact, based on the Breguet range formula, that with conventional reciprocating engines and propellers the speed for maximum range is approximately that at which parasite drag and induced drag are equal. Therefore, at the same cruising speed as the conventional airplane the all-wing type will require from 25 percent to 11 percent less power, as shown in equation (2), and with the same amount of fuel will fly from 33 percent to 13 percent farther, as indicated by equation (3). In these equations P represents power required, and D total drag. V is airplane velocity and
ADVANTAGES OF LOW PARASITE DRAG
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
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 aerofoil 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
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
To one testing a swept-back aerofoil 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 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 aerofoil 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
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 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.
ELEVONS AND RUDDERS
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
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
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 eleven 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.
XP-79, ROCKET-POWERED AIRPLANE
As a second expedient, detachable dollies were built from which the airplane was expected to take off at flight speeds. Minor crack-ups occurred with this configuration and it was finally decided to compromise the aerodynamic cleanness of these first test airplanes in order to provide a rugged permanent and dependable landing gear for experimental purposes. The unusually large fin used here was required to stabilize the fixed landing gear, a substantial portion of which extended ahead of the C.G. After this gear was installed, and with another airplane as the towing medium, the takeoff difficulties were eliminated and a number of successful glider flights were made.
These airplanes were flown both with and without wingtip slots and slats which were tested for the purpose of eliminating tip-stall difficulties, as will be described later. They were also flown with a wide variation in vertical fin area, to determine the amount necessary or desirable for various flight conditions.
In one memorable test during which the airplane was equipped with a fixed slat, a rather peculiar accident occurred. The pilot, as mentioned before, lay prone within the wing contour. Two escape hatches were located approximately opposite the center of his body, one on the upper surface, the other on the lower surface. The handle which released the escape hatches was located close to the handle which released the towing cable from the tug airplane. At the start of this particular flight, after a successful climb to 10,000 ft., the pilot inadvertently released the escape hatches at the time of his release from tow, and as a result partially fell out of the airplane. The instinctive grasp on the control mechanism resulted in an indescribable wing-over maneuver. When things calmed down the pilot found himself in a steady, uniform glide with the airplane upside down. Minor movement of the controls seemed to produce little effect and the much shaken individual crawled out of the airplane, sat on the leading edge of the center section while he checked his parachute harness, and then slid off to make a perfectly normal parachute descent. The airplane, undisturbed by the change in C. G., continued a long circling flight of the test area and finally landed in a normal continuation of its upside down glide, a short distance from the takeoff point. It was rather seriously damaged but not so much so as to prevent repair. A later check in the wind tunnel indicated that there was a very stable region in inverted flight with this particular slat combination. Later the slats were abandoned as unnecessary and perhaps undesirable.
The airframe was considered suitable for the purpose intended long before the rocket motors had been developed to a degree of reliability considered safe for use, but finally a small motor having about five minutes' duration, was installed and a number of rocket-powered flights were accomplished. The first powered flight occurred in July 1944.
Although the first concept of the XP-79 as this fighter was designated, was as a rocket-powered vehicle (similar in basic idea to the Messerschmitt ME-163), it soon became apparent that the completion of the rocket motors would be far behind schedule and that serious difficulties were attendant to this development. One of the basic concepts for the full-size motor was that the fuel pumps would be driven by rotation of the combustion chambers, which were set at a slight angle to the thrust axis in order to develop torque. It was not foreseen that the rotation of the combustion chambers would have a serious effect on the combustion therein, and this difficulty, never completely solved, caused the abandonment of the particular engine which was being developed for the project.
XP-79B TURBOJET AIRPLANE
After about 15 minutes of flying, the airplane entered what appeared to be a normal slow roll, from which it did not recover. As the rotation about the longitudinal axis continued the nose gradually dropped, and at the time of impact the airplane appeared to be in a steep vertical spin. The pilot endeavored to leave the aircraft but the speed was so high that he was unable to clear it successfully. Unfortunately, there was insufficient evidence to fully determine the cause of the disaster. However, in view of his prone position, a powerful, electrically controlled trim tab had been installed in the lateral controls to relieve the pilot of excessive loads. It is believed that a deliberate slow roll may have been attempted (as the pilot had previously slow rolled and looped other flying-wing aircraft developed by the company) and that during this maneuver something failed in the lateral controls in such a way that the pilot was overpowered by the electrical trim mechanism.
ALL-WING BUZZ BOMBS
The first of these buzz bombs was tested as a pilot-controlled glider with good success. It was very small and incorporated a number of extra bumps which were originally conceived to be the best way to carry standard 2,000 lb. demolition bombs. In spite of its peculiar configuration, which departed appreciably from the all-wing ideal, it had quite good flight characteristics, was flown on a number of occasions (the airplane was successfully slow-rolled) and demonstrated the suitability of the type for the purpose intended.
The one difficulty experienced in this series of tests is worthy of note. The piloted version of the buzz bomb naturally required some type of landing gear for takeoff and landing, and in this case we employed tiny, low-pressure air wheels, rigidly mounted in the airframe structure and extending only a few inches below the contour of the aerofoil or, more specifically, the bomb-shaped bumps thereon. Landing on this gear involved bringing the airplane in at an altitude of approximately 15 percent to 20 percent of the mean aerodynamic chord just prior to contact, and no amount of practice on the part of the pilot produced a technique satisfactory for this purpose. In every case a change in airflow appeared to develop as the airplane approached within a quarter-chord length of the ground. The drag was apparently reduced, the lift increased and the airplane rose, in spite of anything the pilot could do, to a height of 8 or 10 ft. above the ground, at which point it stalled and flopped down out of control. This maneuver resulted in a number of rough landings but no damage to either the pilot or the airplane. It was later found that the only way to make any sort of smooth landing was to bring the airplane in at comparatively high speed and actually fly it onto the ground. This difficulty was not experienced in airplanes having normal landing height above the ground, such as the N-9M and XB-35.
XB-35, LONG-RANGE BOMBER
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.
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.
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
CHARACTERISTICS AT HIGH LIFT
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 in percent of the m .a.c. 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 m.a.c. 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
Definite requirements for the weathercock stability CAB, 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.
The use of pure drag rudders is feasible on the all-wing type because it is not necessary from a performance standpoint to fly at zero yaw. Thus in the case of an engine failure equilibrium conditions involving a yaw angle and the resultant corrective yawing moment do not involve appreciable side forces and associated bank angles, nor noticeable drag increases. Thus, the rudder is used only rarely for trim and its drag is therefore unimportant.
Of the many types of drag rudder investigated, a simple double-split trailing edge flap at the wing tip has been found to have the most satisfactory all-round characteristics. This arrangement permits the simplest construction and allows combination of trim flap and rudder in the same portion of the trailing edge. One disadvantage of this type is its comparatively low effectiveness at low angles of rudder deflection, which may be remedied by the employment of a nonlinear pedal-to-rudder linkage in the case of power-operated rudders.
SIDE FORCE EFFECTS
DYNAMIC LONGITUDINAL STABILITY
The second mode of longitudinal motion is a long-period oscillation commonly called the phugoid. This is a lightly damped motion even for conventional airplanes, and seems slightly less damped for all-wing airplanes, because of the fact that they have relatively low drag, and drag is the chief means of energy absorption in this mode. N -9M tests indicate that calculation is slightly optimistic in this matter, but still this phugoid motion is sufficiently damped so as to give no serious difficulties.. Being a slow motion, it is easily controlled.
To date the criteria for the description of airplane dynamic stabilities are vague. In the past it has been thought that consideration of damping rates and periods of oscillatory motion were adequate, but it has become evident that some further criteria are necessary. Consideration of the angular response of airplanes to various unit disturbances may supply this need.
DYNAMIC LONGITUDINAL RESPONSE
The response of the all-wing airplane to elevator deflection seems entirely adequate. It errs, if at all, on the side of over-sensitivity because of low Cmq and low moment of inertia in pitch. An abrupt control movement giving the same final change in trim speed for a conventional and a comparable all-wing airplane results in a larger initial swing in pitch for the all-wing.
DYNAMIC LATERAL STABILITY
The second mode, the "Dutch Roll" oscillation, is more critical for all-wing airplanes, particularly at low speed, high weight and high altitude. All-wing airplanes seem comparatively bad in this respect because of the combination of relatively large effective dihedral and low weathercock stability and, for the conditions noted above as critical, are likely to approach neutral damping in the Dutch Roll mode. However, analytical determinations of this motion, using calculated damping derivatives, indicated less satisfactory characteristics than were obtained in actual flight tests. Because of a relatively low weathercock stability, the Dutch Roll is of a rather long period, in the order of ten seconds for the XB-35. It is usually assumed that for periods of such length, it is not important to have a high rate of damping since control would seem easily "inside" the motion. However, there may be particular instances where this is not true. For instance, in an all-wing airplane in which the rudder is particularly weak, the time of response to rudder control may be of the same order as the period of Dutch Roll motion. This would make directional control extremely difficult in a condition, such as landing, where the roll controls are not usable for changing heading. It is notable that for the very low weathercock stability commonly encountered in all wing airplanes, the conventional solution of increasing weathercock stability to offset increased dihedral does not hold. Increasing Cur leaves the damping essentially untouched, but reduces the period and increases the number of cycles required to damp.
Another factor contributing to the relative lack of damping of all-wing airplanes in Dutch Roll motion is the low value of the damping coefficient in yaw, Cur This appears to be inherent in all-wing designs, particularly if the use of fins is abandoned. For special occasions, when particular airplane steadiness is required (such as a bombing run), it is probable that the equivalence of such damping in yaw may be supplied by an automatic pilot, or by temporarily increasing the drag at the wing tips. This latter effect can be accomplished on the XB-35 by simultaneously opening both rudders and gives deadbeat damping in yaw.
DYNAMIC LATERAL RESPONSE
Some data from the free-flight tunnel of the National Advisory Committee for Aeronautics indicate that increasing weathercock stability, even for all-wing airplanes, materially helps the "flyability" of the airplane. Another bit of evidence that is of interest in this connection has to do with the magnitude of the side force derivative, Cy.B. Increase of this parameter improves Dutch Roll damping very materially but has virtually no effect on amplitude of response to gusts, according to calculations. Free-flight wind tunnel data again give tentative support to the investigations of response as a criterion by showing little improvement of flight qualities of models with increase of Cy'B.
Flight tests of the all-wing glider in which the vertical fin, located aft on the ship's center line, was varied in size from approximately 2 to 7 percent of the wing area, left the pilot somewhat undecided as to fin requirements except that the larger fin seemed somewhat easier to fly. Presumably, this was, in the light of the foregoing discussion, primarily because of the increased CnD, the coincidental increase in Cy'0 not being effective.
AUTOMATIC PILOT CONTROL
PROBLEMS OF CONFIGURATION--SWEPT vs. NON-SWEPT WINGS
An all-wing configuration embodying a straight, or non-swept wing, has been proposed and flown successfully in model sizes. It offers the serious disadvantage that suitable distribution of weight empty and useful load items is difficult and, if proper balance is to be accomplished, most of the structural weight and useful load must be included in the forward 30 percent or 40 percent of the wing, leaving a large volume of space within the wing unusable. Such a configuration results in an unnecessarily large airplane to accomplish a given job and for this reason has not been considered seriously.
The swept-back arrangement exemplified by the various airplanes previously illustrated and described seems to offer the best configuration for a materialization of our all-wing ideal. It can be balanced satisfactorily within quite wide ranges of sweepback, utilizing almost all available volume within the wing for storage of useful load items. It seems to fly satisfactorily in many different configurations and the arrangement is such that large payloads can be carried virtually over the C.G., with the weight empty items so distributed as to cause little variation in C.G. position between the fully loaded and empty conditions.
In the case of the XB-35, the useful load, consisting largely of bombs and fuel, can be readily disposed in suitable position about the C.G. While some fuel is located well forward and other fuel well aft of the desired C.G. location, under normal operating conditions the proper balance is readily maintained. In case of failure of one or more engines, it is necessary to pump the fuel from unused tanks to those supplying the remaining engines, but a simple manifolding system provides this facility.
Based on a great many studies of various types and applications of the all-wing principle, some practical limitations may be approximately defined. Where very dense (high specific gravity) payloads are contemplated, such as warheads or similar munitions, quite small units are practical, as demonstrated by the all-wing buzz bombs to which reference has been made. Medium-sized units having a span of perhaps 100 ft. and a gross weight of 50,000 to 60,000 lb., appear entirely practical for medium bombers and freighters. Here again the density of the useful load, both in payload and fuel, is comparatively high.
Airplanes designed to carry people need the largest volume of all. Even individual reclining chair accommodations require a minimum space of perhaps 40 cubic ft. per passenger, which is a density of only about 5 lb. per cubic ft. This is one-half to one-quarter the density of typical air cargo, and only 4 percent or 5 percent of the density of a warhead.
IMMEDIATE APPLICATIONS--ALL-WING AIRCRAFT
An airplane of the XB-35 configuration and size can carry 50 passengers in comfort in the existing aerofoil envelope with adequate headroom for all, and with vision forward through the floor, and upward if desired. Passenger vision in a flying wing may be more satisfactory than in conventional types if we get used to the idea of forward vision rather than that provided by side windows. The really interesting views are likely to be forward and downward rather than to the side. An airplane like the XB-35 will have cargo space for 40,000 to 50,000 lb. of air freight at a density of 10 to 15 lb. per cubic ft., in addition to the necessary crew and space for 50 passengers.
A maximum trimmed lift coefficient 1.9 for the all-wing configuration seems attainable by methods already suggested and possibly may be further increased by judicious use of boundary layer control in combination with turbojet power plants. It is our opinion that the ratio of C1max to Cd min may be increased to a value of 235 within the not -too-distant future from our present actual achievement of about 130. In contrast, the years of intensive development of the conventional types already passed promise an improvement of less magnitude within a comparable time. In our judgment a trimmed maximum lift of 2.8 vs. a minimum drag of .020 seems reasonable to expect for large, long-range transport and bombardment aircraft of conventional type.
These estimates are, of course, completely arbitrary and controversial. However, if one cares to assume their validity, the following conclusions may be reached, based on methods and calculations used in the early part of this paper. The total minimum profile drag of the all-wing airplane in terms of the conventional will be from 40 percent to 59 percent. The power required by the all wing to maintain the same cruising speed as the conventional will be from 70 percent to 80 percent and, conversely, the maximum range of the all-wing, at the cruising speed of the conventional airplane, will be 143 percent to 125 percent. The maximum range of the all-wing airplane at its best cruising speed will be 158 percent to 130 percent of the conventional, and the most economic speed will be from 125 percent to 115 percent faster.
Under high speed conditions corresponding to full power of reciprocating, turboprop or turbojet engines, where the induced drag is assumed to be 20 percent and the parasite drag 80 percent of the total, the power required to drive the all-wing airplane at the speed of the conventional airplane will be 52 percent to 67 percent and, conversely, the range will be 192 percent to 149 percent of the conventional airplane. The maximum speed of the all-wing airplane at comparable powers will be 124 percent to 114 percent of its conventional counterpart.
Different assumptions of comparative maximum lift and minimum drag values can be made to suit individual opinion, but it is believed that any reasonable assumptions will always result in an advantage to the all-wing configuration of such magnitude as to fully warrant whatever trials and tribulations may be associated with its development.
POSSIBLE SUPERSONIC APPLICATIONS
So far in this discussion we have purposely avoided transonic and supersonic considerations. The neglect is possibly a reasonable one when discussing commercial ventures, in view of the cost of higher and higher speeds. A reasonable degree of sweepback, such as is required in the type of aircraft under consideration, will permit speeds up to about 500 m.p.h. without involving great compressibility drag increases. For military aircraft, however, we cannot ignore the sonic "barrier" and its implications, and it is a reasonable assumption that sooner or later improved fuels will permit higher and higher operational speeds, even in commercial aircraft.
Based on present knowledge of supersonic flight, it will always be more difficult to carry a given payload for a given range at supersonic speed because of the additional wave drag encountered at these speeds. At transonic or comparatively low supersonic speeds, a plain swept-back wing appears to be one of the best possible configurations, provided that sufficient is available within the wing. Since the flow normal to the leading edge is subsonic over almost the entire wing surface, subsonic airfoils with reasonably good subsonic flight characteristics can be used at these speeds. The all-wing design eliminates wing-fuselage interference as well as adverse interference between the tail surfaces and wing or body.
At higher supersonic speeds the problem or providing adequate volume is more difficult because of the fact that more and more fuel is required for a given range and the percentage of thickness of airfoils suitable for such use is much less than that satisfactory for subsonic flight. Save for one compensating factor, this problem of volume and size might well rule out the all-wing airplane for supersonic use, and certainly does limit its usefulness for low altitude flight. However, an attractive field of operation exists at very high altitude where air densities are low and therefore wing areas must be comparably great if suitable lift coefficients are to be maintained. If we design a frankly supersonic airplane to fly at, say, a Mach number of 1.6, with supersonic diamond-section airfoils, the maximum cruising lift coefficient will probably be no greater than .15, and the corresponding loading must be held to 40 lb. per sq. ft.
The above figures are based on assumed operation at 60,000 ft. and an air density ratio of .094. Such an airplane might likewise be suitable for landing and takeoff at low altitude, in view of its comparatively light wing loading, which would eliminate the necessity of high-life devices. The practicability of the design depends on the relative density of the air at the altitude selected for cruising operation. If a sufficiently high altitude is chosen it seems quite possible that adequate volume can be secured in the wing, in spite of its small thickness ratio, by using low aspect ratio planforms approaching the triangular.
We can compare data on two wings having the same physical depth at the root, and identical wing areas. The conventional wing is of a type already proved practical for all-wing airplanes. The delta wing has thickness ratios suitable for supersonic flight, identical thickness and only slightly reduced volume. It should be quite suitable for all -wing aircraft of reasonable size. From the aerodynamic point of view it appears that with the delta wing it is possible to eliminate a substantial portion of the wave resistance and thus realize fairly favorable lift-drag ratios at supersonic speeds.
It is gratifying to those of us who have been working on all-wing projects for years to recognize the increased interest in the type evidenced in Germany toward the end of the war, and more particularly in England and Canada in recent years. For many years we received scant encouragement and often seriously questioned our own judgment, as well as our ability to achieve a successful solution to the many problems involved in the development of this type. The goals and rewards have always seemed well worth attainment, however, and I believe accomplishments to date have justified the effort required.
I hope this discussion may provide encouragement and incentive to those in Great Britain who have pioneered all-wing airplanes and that these projects, both here and in the United States, may profit by each other's mistakes and successes, thus bringing the two countries to the forefront in this important phase in the development of air transport.