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Many thanks to Al Bowers for this document!
7/18/46 C. G. D.-468 Restricted Engineering department Title Paper presented by the Horten Brothers, Bonn
Ten Years Development of the Flying Wing High-Speed Fighter By Horten Brothers, Bonn Report No. LGB 164
After three years of flying activities which were used to prepare the before the glider examinations, and aside from workshop practice for the theoretical basis of airplane construction, we decided in 1928 to go into the construction of all-wing airplanes. The development stage of the model lasted five years, until enough confidence was gained to assume the responsibility for the construction, to study the flight characteristics of all-wing full-scale airplanes. The construction of models, which was supplemented by continued designs of large airplanes, after systematic study, gave us the experience for this full-scale construction. The results of the model construction very soon showed, the necessity for observing Reynolds' laws of similarity as they are presented in the recently published book by F. W. Schmitz, "Aerodynamics of the flying model" (publisher Volkmann). After these relationships had been cleared up, systematic work, particularly in the field of flight characteristics, could follow. It was shown at that time that the apparently problematic stability about the lateral axis of tailless self-stabilizing wings does not give rise any particular need for attention. Furthermore, it was shown the center of gravity location was possible in front of the C-point (see the work "Stability consideration in swept back wings" in this report) upon which the calculations of the times were based. The models were usually balanced at the C-point. A rear location of the center of gravity had the accompanying typical characteristics ("falling off"), a forward location of even 10 percent to 15 percent of the measure of sweep back had objectionless flight characteristics. For this reason considered the problem of stability about the lateral axis as solved, and turned our complete attention in the construction of models to the moments about the vertical axis. Even today it is our view that the essential problem in the construction of airplanes is the stabilizing about the vertical axis. Of course, the model flights were made only with glider models which, in order to fill the Reynolds' similarity laws, had spans of 3.5 to 4 meters and weights of 10-15 kg. Flights were tied to hilly country. After the start of such a model, absolute adherence to the course was necessary, and a turning meant contact with the ground against the direction of suspension at a large angle, hence fracture. For this purpose models have to be built in such a way that turning does not take place. In contrast to the construction to full-scale models, the sailplane model had to be neutral in its moments about the vertical axis in yawed flow. The full-scale airplane, when in yawed flow, demands all weather-cocking effect of the side surfaces which lie behind the center gravity. The sailplane model in contrast, cannot, as already stated, turn like a weather vane, but must use the yawed flow to raise the depressed area. In other words it must possess a positive rolling moment due yaw, but no yawing moment. In 1933 the real development began with the construction of the sailplane of 12 m span at the parental residence. This model "Horten I" was intentionally so designed in its side areas that in yawed flow no moments resulted about the vertical axis. Directional control was already then, as in all later types, accomplished through braking of one wing. The flap arrangements of the lateral control services, which when not actuated, form the surface of the wing, have been developed in various ways in the course of time. This depends and the control forces available to produce the drag required in each case, as well as on the effect of other disturbances on wing, which they evoke. On all models arranged in such a way that they created neither lift nor negative lift at the wing tips; that is, flaps on the underside of the wing, depending on the method of attachment, were over-dimensioned by 20 percent to 50 percent compared to the top side flaps. Moreover, it was possible to produce the aileron behavior in the same way as is done in the conventional airplane, that is, the maneuver and action are the same as usual. At that time we naturally discussed the advisability of such a steering about the vertical axis, and found that exists; that is, we discuss the question whether the product of flight time x drag is smaller with this form of control than with the conventional arrangement. It is clear that the rudders in the normal arrangement cause a relatively small drag when displaced and that the lift component perpendicular to the surface, amounting to from ten to twenty times the drag, acts on the lever of the fuselage in the conventional airplane and thus produces the yawing moment. The added drag caused by the movement of the aileron in the conventional airplane surely is smaller, in general, and that other braking flaps at the wing tips, which appear in our models, where it is exactly this drag, acting on the lever of the semi-span, which must produce the yawing moment. When they are not displaced, however, these flaps do not produce any additional drag since they form part of the contour of the wing. In discussing the practicability of this form of control the question now arises concerning the frequency of their use, in order to determine from this the product of drag x time. According to our test and counts in 1934, one achieves a reduction in drag with his form of control of 50% to 70% of that of the surface controls on sailplanes in thermal flights. According to later experiences with powered airplanes this reduction increases to 95%. In these comparisons the shortened high-speed flight condition is not taken into consideration; instead drag for the whole flight is considered. For the purely high-speed flight conditions, however, the saving is 100%. In contrast to the aileron development, the model "Horten I" showed an incomplete vertical and lateral swing. The advantage of the flying wing does not stand out so much in the case of the sailplane as in the development direction of the high-speed fighter. In order to supply advance work for this, profiles were developed whose mean camber lines mathematically is approximately the "constant center of pressure" type. Profiles of this type which were tested, brought out all their weaknesses; be it in the distribution of thickness, which makes the statics unpleasant; be it in the form which causes too high local velocities, or, as is particularly the case with American wing sections, where too little emphasis is placed on the flow juncture at the trailing edge. Thus there came into being after many estimates, a mean camber line with a mathematically constant center of pressure, which obtains its camber for the particular application through a change in the scale of the ordinates. The "teardrop" which is placed on the mean camber line unfortunately could not be taken over either, since the profile forms from which a good flow juncture can be expected, approximately the Joukowsky-forms, become so sharp at their trailing edge that their incorporation does not appear possible. The other forms, however, are right away so blunt, that the characteristic of the mean camber line is lost through the bad flow juncture at the trailing edge which is then to be expected. Thus here too, satisfactory combinations had to be found through suitable compromises. Much more preliminary work was necessary order to make a project of flying-wing high-speed fighter (see a lecture on the aspect ratio considerations of this problem in Report A34/1 of the Lilienthal Society, p. 65). The design resulting from these considerations, the model "Horten V" was completed in 1935. All attempts to interest the aircraft industry, or other national specialists, in it, remained unsuccessful with the exception of the Dynamit Co. Troisdorf, which is not an aircraft construction firm. If we wanted to stay in the country, and this we considered necessary for national reasons, we had to accept this offer. With it we took over as a further problem, the use of fabric in airplane control surface arrangements. As a result, in spite of good flight stability, bad controllability was encountered, so that we would not assume responsibility for flights by other pilots. After the participation at the Rhoen sailplane contest it was dismantled in order to make way for new constructions. The flight characteristics themselves were so good, that even then an instrument flight of 3/4 of an hour was made through clouds. After the Rhoen sailplane competition of 1934 - at that time the "Stability Considerations in Sweptback Wings" was completed – it was both logical to prove the independence of the lift slope distribution of the platform, and its dependence on the center of gravity location in sweptback wings, by incorporating an extreme taper ratio of 1 to 10. For this reason alone the model "Horten II" was built, again in the parental house, since it was not possible to obtain any kind of help or workshop facilities for this problem. After more than a year's construction time, the proof could be attempted, and the hundreds of pilots and several thousand hours on this model always confirmed the correctness of these principles. The elevator and aileron arrangements of this airplane, according to experience, were built in such a way to every airplane pilot and sailplane pilot even with the least flying experience (1 to 2 hours in the air) could fly it without re-schooling, so that soaring on instruments, and cross-country flights up to 240 kilometers (150 miles), followed repeatedly. With this the aerodynamic problem was in itself solved. In order to decrease the flight expense the airplane is equipped with a 60 horsepower Hirth motor, with which it flew about 80 hours. The work was then interrupted by the draft. Before this, all variations of the sailplane and powered airplane were worked out as projects, and even the prone pilot position was treated. The resulting designs of the models "Horten III and IV" were set back, even though they yielded advantages in performance over the comparison sailplanes. The double problem and the draft made construction very difficult, so that the twin-engined model "Horten V" could not be broken in until 1-1/2 years later in May of 1937. The flight results were satisfactory, even if (we were working with an all-fabric airplane) difficulties arose the gluing of materials which would have been shortened the lifespan, and which further strengthened us in our search for a shop were simple wood construction could be undertaken. The airplane itself was quickly evaluated, and in a landing received damage which made it's reconstruction seem impractical. We succeeded in the winter of 1937-1938 in constructing a shop in which the airplane could be created anew in the mixed wood-steel tubing construction. This new craft has proved itself by many hours in the air with various pilots, and is extensively described in magazines. The real project, the fighter airplane, could not be constructed with such simple means. For this reason we busied ourselves constructively in the field of sailplanes. Thus in 1938, the old project "Horten 3", sitting the pilot in the flying wing, was studied further. This model was reproduced in about 20 specimens by the various glider clubs, and is well enough known. At the participation in the Rhoen sailplane competition of 1938-1939, the advantages of the flying wings were shown, and repeated altitude flights over 6,000 meters (20,000 feet), and distance flights over 300 kilometers (185 miles) were made. The results showed that such flight characteristics had been achieved that it was called the appropriate tool for instrument flight. This was partly achieved by constructing the elevators and rudders in a particular way. Each one has two flaps, of which the outer forms the function of climbing control and the inner performs the function of diving control. By means of the activation then, the function other elevators divided into its main actions, that is, with this arrangement at deflections of the diving controls the total incidence in the wing is maintained, and thus transcends the neutral stability which necessarily appears high-speed and diving flight with wing flap controls. At the deflections of the climb controls, however, only the outside flap deflects upward (the inside flap only accompanies it three or four degrees) so that the wing elements with large cords are not equipped with controls and consequently can maintain their full lift coefficients. Conversely the wing tips, which because of the control deflections work at lower lift coefficients, form a smaller fraction of the total area. Consequently, the maximum lift of the whole wing for this control arrangement is high, although a "falling-off" because of the ca decrease at the wing tips, is not to be feared. At the deflections of the ailerons a differential action is effected by this arrangement. The aileron deflection itself is, as we know, composed of diving control deflection on one side and of climbing control deflection on the other side, where the control flaps must be so dimensioned that a moment about the lateral axis is not produced. In the case of single-flap control with differential transmission, this action cannot be obtained, and the pilot will counteract the pitching moments resulting from the aileron deflection by a pushing of the control column, and thus will eliminate the differential action of the arrangement. The only possibility of realizing differential controls in tailless airplanes lies in the two-or-more flap system. But this arrangement also brings with it to further advantage, especially for high-speed flight. Through the method of actuation the transmission can be selected in such a way that it is small in the vicinity of the control stick neutral position; that it is, relatively large stick movements are accompanied by small flap deflections, while with larger deflections the transmission ratio increases of its own accord. This transmission characteristic, which we have called "progressive controls" for short, will be presupposed for the realization of high-speed aircraft. Through this and other examples there arose the opportunity to bring construction to such maturity that the responsibilities for the building of high-speed combat aircraft from constructive points of view could be assumed at anytime. Other examples of construction of all-wing sailplanes, for instance, a Parabola for special purposes, transport sailplanes for 17 persons with 24m span, arose incidentally. In the Fall of 1938 we again submitted a design for a twin-engined airplane which was first to be powered by two As 10c motors. We were referred to several industrial firms who, although they were agreeable to a personal entry into their factories, could not be dealt with regarding the carrying out of the flying-wing project in the form which would be necessary for the realization of such a problem. Thus this project, the "Horten VII", also had to be set back. In 1939 then, the opportunity of working in the main direction of development was not open to us, so we decided to build a high-performance sailplane with prone pilot's position, the "Horten VI". Through this arrangement for the pilot it was possible to hold the wing thickness, and with it the chord of the root, so that with a 20m span, an aspect ratio appropriate to soaring was obtained. Here for the first time the flying wing had opportunity to prove that it brings with it the performance advantages for even this problem. Comparison flights with corresponding conventional aircraft prove the correctness of this point of view. The work was again interrupted by the war until 1941, when in the Fall by virtue of the announcement of a similar aircraft by the Northrop Co., USA, a repair contract for our old twin-engined plane, "Horten V", was received and was added to our experiences. This airplane and shown interesting results in the increase of the maxim lift, particularly through the arrangement of it's landing aids. It was possible, by the use of cambered and split flaps, to surpass a total ca of 1.5 without slats, with and angle of sweepback of 42 degrees. The use of landing flaps on sweptback wings, and their action, yields the basis for performance comparisons with conventional aircraft. The landing aids of the model "Horten V", however were not yet sufficiently developed that they could be used for final comparisons, and parallel comparisons on sailplanes with cambered flaps and retractable center slats (similar to the Duck) permit measurements of maxim lift of 1.8, even though these experiments were unfortunately carried out the wings with only 24 degrees of sweepback. By skillful use of these landing aid combinations may be assumed that even with 40 degrees of sweepback a maximum lift for the whole airplane of 2.0 will be attained, a value which today is present only a few conventional airplanes. In this connection the flight experiences with the model "Horten 5" may be interesting, which is stable even with full landing flap deflection and makes possible by dive-like approaches to the point of landing, and after pulling out, spot-landing the airplane in the narrowest of spaces. The control characteristics with depressed flaps, were hereby shown to be adequate and the moment about the lateral axis is not disturbed. It should be mentioned that almost all or airplanes are equipped with tricycle landing gears and that the nose wheel in all cases is free-swiveling. Through this arrangement of the wheels, for which the all-wing airplane is well predisposed, the ground characteristics will be appreciably improved over conventional aircraft. Many individual experiences have occurred in operation of airplanes during the course of the years. Thus, to give a few examples, the flight of a sailplane without pilot's canopy, through which a larger area of separation was created on the upper surfaces. On the flight under consideration, it was possible to land the aircraft without a trouble, even though the landing speed was about 20 kilometers per hour higher. In another case a piece of cowling from the topside of the airplane flew into the propeller, in the takeoff of "Horten V" and forced the pilot to stop one engine. The pilot continued to climb on one engine without cowling and concluded his local flight, and even for lack of flying experience, turned towards the dead engine and landed smoothly. In another case an airplane iced up heavily in clouds. The pilot could not notice this until he left the cloud. According to his report, the aircraft had at the leading edge a 5 cm layer of ice. The sole result was an increased rate of sink, but there was known nose heaviness, as is usual with straight wings. The loosening of the layers of ice on the sweptback wing in such cases also leads one to expect simplified methods of deicing. The studies undertaken so far aim at the development of the flying-wing fighter. They assume that the power plants are available which can be built into the wing, and that consequently in the nacelles will not be necessary. Even though the fighter airplane with propeller and reciprocating engine has not yet been created in this form, the problem can nevertheless be considered as solved by the experiments and flight characteristics studies. This line of development will surely come when it becomes necessary to fly economically, and this is only the case with long-range airplanes in time of war. In order to obtain criteria for the comparison of performance with conventional airplanes one must select a reference carefully and determine the other quantities from it. The landing speed is frequently taken as this reference quantity; that is, the wing loading is chosen depending on the maximum lift attainable, so that the airplanes to be compared have equal landing speeds. This type of comparison for the fighter problem, or even the long-range fighter airplane, is not quite right, for the disposable loads amount to 50 percent of the flying weight, and it is no problem to negotiate a landing with an airplane thus lightened. The conditions are different, however, at the takeoff. For this, aids have been created in the form of concrete strips, takeoff rockets, catapults, etc., which facilitate the takeoff. It is, therefore, more purposeful to introduce the speed of takeoff as the reference quantity; that is, to select a wing loading dependent on the lift coefficients attainable at takeoff. The landing flaps and then are extended in takeoff position, and there is no reason why in this configuration the conventional airplane should obtain higher lift coefficient than the sweptback wing. Refinements, such as "increase in dynamic head through the slipstream of the conventional aircraft" or better "operating efficiency of pusher propellers", particularly at the takeoff of flying-wings, are not to be considered here. Based in these assumptions one can easily estimate the advantage in performance of the all wing construction over the conventional airplane. Under similar conditions it manifests itself in a gain of speed of 8 to 10 percent, or by an increase in useful load of about 20 percent. To name other advantages of this type of construction the pilot's visibility is quoted, which corresponds to that of a pulpit, but without the disturbance of the usual engine nacelles. Further the possibility of using braking propellers should be pointed out, which could be actuated without tail blanketing, and it should be also mentioned that a sweptback wing without protrusions is suited toward warding off cable barriers, balloon barrages, etc. This development, and its effects on armament, can now be surveyed down to its smallest consequences, and shows that the advantages of the flying wing compared to the conventional construction, in this arrangement lies with the limits given. A different status obtains for the development of a high-speed fighter or fighter-bomber with jet propulsion, which now impends, because of the development of corresponding tools. Even more than in the previous problems, the reference quantity must here be taken as the speed of the takeoff. The performance of jet power plants at the start and at low speeds is bad. Disposable loads are large. The landing speed here really no longer offers a reference quantity. Because the performance of the power plants increases with the increasing speed, and on the other hand the specific fuel consumption decreases with increase in speed, the aerodynamic development of the nacelle is imperative as a presupposition for the economy of jet propulsion. Since studies by Busemann show that the sweptback wing has advantages at high Mach numbers, and it will be even possible to obtain speeds with the sweptback wings which the straight wing cannot achieve because the local appearance of sonic speeds, sweepback becomes necessary for high-speed flight. The conventional airplane as a sweptback wing airplane shows poor flight characteristics in low speed flight, since been here the sweptback wing is not "free" but "bound" (see "Stability Considerations and Sweptback Wings" in this report). Moreover, the conventional airplane would have to show nacelle bodies on the wing, which, as can be seen from the above mentioned study by Busemann, does away with the advantages of the sweptback wing. The flying wing, therefore, has to be presupposed here. The working out of a corresponding project shows that the single seat fighter with two TL-powerplants and 1000 kg of ammunition results a 40 to 45 m2 wing area is taken at the smallest construction. The installation of the power plants and fuel as well as armament, landing gear, and pilot in the wing alone, presupposes a 13 percent profile thickness at the root, were the outer panel can, of course, become thinner. The space for a maximum of 4-obm of fuel per power plant would be created in the outer wing, of which, because of takeoff weight, one would only use half at first. With this the objective would become: 1000 kg bombs - 1000 kph (625 mph) maximum speed and 1000 km radius of action. In summary it may be said that the form of the high-speed airplane cannot be determined from the point of view of giving the optimum shape to a given space, as for instance in a spindle-like fuselage. This fuselage, it is true, has the least drag, but it does not produce the lift necessary for flight and must be equipped with wings and control surfaces. The necessary space, therefore, is to be shaped in such a way that it has an adequate L/D ratio for all possible flight conditions. If, in addition, the shape has the quality of avoiding as nearly as possible the local velocity of sound at high Mach numbers, a direction of development is obtained which must, for practical reasons, be created.
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