The Horten Tailless Aircraft
by K.G. Wilkinson, B.Sc. D.I.C.

Horten X, H XI, H XII, H XIII, H XIV, Parabola & Jet Bomber

3.12  Horten X

       The H X was a high speed arrow shaped flying wing (Fig. 20, ed. – not reproducible) inspired by Busemann’s statement in 1936 of the beneficial effect of sweepback on delay of the shock stall.  This apparently cheered up the Horten brothers and gave them new proof that they were working on the right lines.
      Initial work on the H X consisted of experiments with flying models of 10’ length weighing about 8-10 kg.  From these they deduced the CG position needed for satisfactory flight with low aspect ratio and high sweepback, and found that they got good results with 4° dihedral and no fin area.
      The next step was a man carrying glider model weighing 400 kg, with the overall dimensions of Fig. 25.  The wing section was a symmetrical D.V.L. low drag type with maximum thickness at 45% chord.  Wing washout was 12° (?), dihedral 4°.  Small Frise nose elevons were fitted but no flaps; a trimmed CLmax of 0.8 was expected with a stalling incidence of 20° - 25°.  Rudder control was to be by wing tip “trafficators”.  The undercarriage was of tricycle layout giving zero ground incidence but clearance for a 15° nose up attitude at takeoff; the front wheel was to be retractable but the rear wheels fixed.
        Work on the glider H X was in progress at Hersfeld.  When the writer visited the works on June 4, 1945 it was being used as an M.T. servicing depot and all aircraft components had been dumped in a basement.  Only one wing rib and the main spar could be found.  All drawings and calculations had gone.
       After an exploration of low speed control problems on the glider, the next step was to have been a power version with an Argus AS 10C pusher engine.  The final development was envisioned as a jet propelled aircraft, with the same general dimensions, weighing 6-7,000 kg.  A single H 11 jet engine was proposed and a top speed of 1,200 kph was expected with 1,300 kg thrust; the thrust was to be improved to 1,500 kg.
      Initial tests on control effectiveness with high sweep were carried out on the H XIII to guide the control design for the H X.  No ideas for controls on the final version had emerged but Horten said he intended to stick to Frise nose balance as long as it would work.
      In general appearance the  H X bears resemblance to the Lippisch designs for high speed and supersonic aircraft, particularly the P 13.  Horten said he had not heard of Lippisch’s work in Vienna until he came to London.  The main difference in design is that Hortens think a fin unnecessary whereas Lippisch favors a very large one.

3.13 Horten XI

      This was an aerobatic sailplane of 8 meters span.  It was built at Hersfeld and had no features of special interest.

3.14  Horten XII

        The H XII was a light side by side two seater with a 100 hp engine.  It was intended as a private owners coupe but R.L.M. were interested in it as a trainer.  The first of the type was built and flown at Gottingen (where it was found destroyed in June this year) as a glider; work was also in progress at Kirtorf (sp.) where a mock up of the power center section was found, badly damaged by fire.

General dimensions were as follows:

Span                                                   10 m
Aspect Ratio                                        8
Wing Area                                         32 sp./m (345 sp.ft.)
Leading Edge Sweepback             30°
Weight                                             700 kg (1,550 lb.)
Wing Loading                                2.19 kg/sq.in. (4.5 lb/sq.ft.)

     The wing used a Mustang section at the root graded to a symmetrical section with Mustang fairing shape at the tip.  Washout was 3° - 31/2°.
      Elevon controls were of H VII type with a 20% Frise nose.  Plain flaps were fitted with a H IX center section spoiler and “trafficator” drag rudders.
     The undercarriage was unusual in having two wheels forward and one main wheel aft taking 60% of the weight.  All three wheels were retractable.
     Little flying had been done, but it was found that the same troubles were arising as on the H IVb.  The laminar flow sections were causing bad tip stalling and loss of control effectiveness at the stall.

3.15  Horten XIII

        As part of their program on high sweepback, the Hortens built the . . (ed. – next few words missing) . . to give flight test results on control . . (ed. – next printed line missing) . . section giving a leading edge sweep of 60°.  This increase of sweep reduced the aspect ratio to four.  Control deflections were all doubled to compensate for the obliquity of the hinge line and the ends of the control flaps trimmed to run parallel to the new aircraft center line.
      The pilot had to be carried in an underslung nacelle to give him a reasonable view; the control column was inverted and hung from the roof of the nacelle.
      Undercarriage consisted of a main wheel inset in the nacelle and a fixed nosewheel mounted on a welded steel tube fork (Fig. 24, ed - not reproducible).
      Flying the H XIII totaled about 10 hours.  Trials were interrupted in the middle by Scheidhauer landing in a barbed wire fence.
     CLmax was found to be 0.9, with the stick right back.  In this condition the incidence was larger than expected – about 20° – 25° instead of 15°, but Horten thought that the induced drag was not correspondingly increased.  It was thought that the wing was not completely stalled with the stick hard back because the CG was too far forward.
      Control was moderately satisfactory, but in spite of the increased elevon travel was inclined to be sluggish.  Elevator control particularly was much less sensitive than on other Horten aircraft.

3.16  Horten XIV

       This was a sports sailplane designed to conform to the 1939 Olympic Games specification.  It was designed for simplicity and ease of production.  The first aircraft was built from sketches and proper drawings for the production type were made later with slight modifications.  Construction was carried out at Hersfeld.
        Span was 15 m and aspect ratio 16.2. The wing had 23° leading edge sweep and 8.6° total twist.  Sections were 4% camber and 17% thickness at the root with 10% thick symmetrical tips.  Empty weight was 120 kg giving a flying weight of 225 kg.
        Controls consisted of one Frise nose elevon per side with rudders and dive brakes of H IV design.  The glider trimmed at CL=1.0 with elevons neutral.
        The pilot was put in a prone position as on the H IV and the undercarriage arrangement was also similar but for the replacement of the rear skid by a wheel.

      Performance was stated to be:
                Maximum Gliding Ratio                  1:30 at 70 kph
                Minimum Sinking Speed                 0.62 m/sec at 55 kph

3.17  Horten Parabola

      According to Reimar Horten this planform is theoretically the most efficient and has the minimum induced drag.  The validity of the theory is not known.  The glider was constructed in a burst of enthusiasm but proved rather awkward to make because of its curves and was damaged during transport and never flown.

3.18  Projected Jet Bomber

       In February 1945 a committee under Professor Bock with representatives from Junkers, Messerschmitt and Horten, deliberated over the optimum design for a 4 jet engined bomber.  Designs by Junkers (Ju 287. A swept forward tailed aircraft), Messerschmitt (Project 1107 as swept back tailed aircraft), and Horten (swept back tailless) were considered, and a joint report issued giving the committee’s opinion on the best estimate for relative performance.  Junkers published the report.
        The specification to be noted was for 900 kph at 10 km height and a range of 3,000 km using four H 11 jets.  According to Horten the committee decided that his machine, given the same top speed as the others would have more range and less landing speed.  (125 kph against the 175 kph for the others.)  Alternatively he could carry 8 tons (metric) of bombs against the 4 by his competitors for the same range.
        The dimensions of the aircraft were roughly as follows:

                                                         Ju and Me                            Horten
           Span                                     17-18 m                                30 m
          Aspect Ratio                             5.5                                      5.5
          Wing Loading                  500-600 kg/sq,in.                220 kg/sq.in.

       Horten aid the agreed CDo for this aircraft was 0.0078 excluding Mach number correction.
       In the structural design he reckoned to save 6% of the all up weight (spar and rib weight) compared with the conventional type.  He thought the committee a bit unfair because they insisted on increasing his estimate of structure weight by about a ton.
      All the above figures were remembered by Horten, who used them as a rough illustration.  They are not accurate.

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