VIII & IX
This was to have been a flying model of a proposed six-engined
trans-Atlantic passenger transport weighing 100,000 kg. The span was to be
40 m with an aspect ratio of 10 and sweepback of 28°. Power units were six
Argus AS 10 C engines.
To make the aircraft attractive to R.L.M. and thus get backing for the
project, the Hortens added a rear loading cargo carrying body with an
internal space approximately 14’ x 10’ x 6’; this was not part of the
design for the full size aircraft. With construction under way, another
modification was made (but not disclosed to R.L.M.). This consisted of
removing the nose of the cargo body, replacing the nose wheel by wheels on
either side of the body and putting a large venturi tube with a 2m x 2.7m
throat inside to form a flying wind tunnel. They expected to get about 500
mph airspeed in the throat combined with low turbulence – this they
proposed to check by the sphere drag method. Later they hoped to be able
to test models of their aircraft which could be made of wood because of
the absence of dust in the air stream.
Construction was proceeding at Gottingen and was 50% complete at the
cessation of hostilities. The steel tube framework for the venturi centre
section was finished.
Estimated Weight and Performance Figures
Max. all up weight as a wind tunnel 9,000 kg
Max. all up weight as a cargo carrier
Without takeoff assistance 15,000 kg
With rocket assisted takeoff 20,000 kg
At 23,000 kg the sea level rate of climb at full power would be zero.
At 9,000 kg rate of climb at 180 kph was expected to be 6.5 – 7 m/sec.
Estimated trimmed CLmax’s were
No Flaps 1.4
With Flaps 1.6
CL for Takeoff 1.1
The design of the wing and controls was similar to that of the Horten IV.
Washout was large, 7°, to give trim without elevator deflection at
cruising CL. Elevons were the three stage type with 35% Frise nose on the
outer flap, and 22% on the middle and inner flaps. Compensating geared
tabs which could also be used a longitudinal trimmers were fitted to the
inner flaps. Maximum control deflections were a follows:
(Note: All figures in degrees) .. ------- PORT ------- --------- STARBOARD
CASE OUTER CENTRE INNER INNER CENTRE OUTER
Stick fwd. & central 5 12 15 15 12 5
Stick back & central -10 -18 -10 or -15 -10 or -15 -18 -10
Stick central & to port -30 -15 -8 12 10 5
Stick central & to stbd. 5 10 +12 -8 -15 -30
Trailing edge split flaps with a constant chord of 80 cm were to be fitted
between the engines.
Drag rudders were of the H VII “trafficator” type with vent hole balance
plus spring centring. Projection was about 1 meter.
Wing sections are shown in
18. Root thickness is about 16%, with the usual reflexed centre-line,
graded to an 8% symmetrical tip section.
Wing structure was in seven parts; a welded steel centre section with
pilot and co-pilots seat and three outer wooden wing panels per side. The
wooden structure was of single spar D-tube form with subsidiary trailing
At the factory in Gottingen the centre section was found in a
semi-complete state, D-noses for the inboard wing panels were finished and
spars and ply noses for the outer panels were under construction. Much of
the work on components such as engine bearers, petrol systems,
undercarriage etc., had been completed and the six engines were in crates
at the works, with one spare. Unfortunately all drawings had been taken
and many of them seem to have been buried by Horten employees near
Kilenburg, in the Russian sector.
The fixed main wheels were arranged in tandem pairs on either side of the
fuselage and took 85% of the static weight of the aircraft. The castoring
nose wheel was retractable on the cargo version and had to be mounted on a
stalky strut because of the high wing layout. Static ground incidence was
The H IX was a single seat fighter bomber of 16 m span with twin jet
engines, being a further development of the H V and H VII designs.
Fig. 19 is a general
arrangement drawing made from a wooden model found at Gottingen, where the
first two of the type were built.
Four aircraft of the H IX type were started, designated V.1 to V.4. V.1
was the prototype, designed as a single seater with twin B.M.W. 003 jets,
which were not ready when the airframe was finished. It was accordingly
completed as a glider (Fig. 20) (not reproducible) and extensively test
flown. D.V.L. instrumented it for special directional damping tests to
determine its suitability as a gun platform. V.2 was completed (also at
Gottingen) with two Juno 004 units and did 2-hours flying before crashing
during a single engine landing. The pilot (Ziller) apparently landed short
after misjudging his approach. V.3 was being built by Gotha at
Friedrichsrodal as a prototype of the series production version. V.4 did
not get beyond the project stage but was to be a two-seater night fighter
with an extended nose to house the extra man.
In shape, the H IX was a pure wing with increased chord at the centre to
give sufficient thickness to house the pilot and the jet units, which were
placed close together on either side.
The H IX started as a private venture and the Hortens were very anxious to
avoid failure so they avoided aerodynamic experiments wherever possible. A
lower sweepback was used than on the H V and H VII and laminar flow wing
sections were avoided as a potential source of trouble. Wing section at
the junction with the centre sections was 14% thick with maximum thickness
at 30% and 1.8% zero Cmo camber line. At the centreline thickness was
increased locally to 16% to house the crew. The tip section was
symmetrical and 8% thick. Horten also believed that since the
compressibility cosine correction to drag was based on the sweepback of
the maximum thickness line, the ordinary section would show little
Wing twist was fixed by consideration of the critical Mach number of the
underside of the tip section at top speed. This gave a maximum washout of
1.8°. Having fixed this, the CG was located to give trim at CL = 0.3 with
elevons neutral. In deciding twist for high speed aircraft, CD values were
considered in relation to local CL at operational top speed and altitude
(10 km in the case of the H IX). Twist was arranged to give minimum
overall drag consistent with trim requirements. The wing planform was
designed to give a stall commencing at 0.3 to 0.4 of the semi-span.
Wing structure comprised a main spar and one auxiliary spar or wooden
construction with ply covering. The centre section was built up from
welded steel tube. Wing tips were all metal. The undercarriage was
completely retractable and of tricycle type the front wheel folding
backwards and the main wheels inwards. The nose wheel was castoring and
centred with a roller cam. When resting on the ground, wing incidence was
7° and the nose wheel took about 40% of the total weight.
The jet engines were installed at -2° to the root chord and exhausted on
the upper surface of the wing at 70% back from the nose (Fig.
22b). To protect the wings
the surface was covered with metal plates aft of the jet pipe and cold air
bled from the lower surface of the wing by a forward facing duct and
introduced between the jet and the wing surface. The installation angle
was such that in high speed flight the jest were parallel to the direction
Lateral and longitudinal control was by single stage elevon control flap
with 25% Frise nose and compensating geared tap balance. (This system was
also used on the H VI) The pilots control column was fitted with a
variable hinge point gadget, and by shifting the whole stick up about 2”
the mechanical advantage could be doubled on the elevons for high-speed
Directional control was by drag rudders. These were in two sections,
slight movements of the rudder bar opening the small (outboard) section
and giving sufficient control for high speed. At low speeds when courser
control was necessary the large movement also opened the second spoiler,
which started moving when the small one was fully open. By pressing both
feet at once, both sets of spoilers could be operated simultaneously; this
was stated to be a good method of steadying the aircraft on a target when
The Hortens stated that the spoilers caused no buffeting and
claimed an operating force of 1 kg for full rudder, with very little
variation in speed. The operating mechanism is illustrated in
Fig. 28. A change was made
from the original H VII parallel link system to improve the control force
characteristics. With the new system, aerodynamic forces could be closely
balanced by correct venting of the spoiler web, leading the main control
load to be supplied by a spring. The cover plate of the spoilers was
spring loaded (Fig.
form an effective seal with the rudders closed; this device was used on
most Horten spoiler and dive brake designs.
On further models of the H IX it was proposed to fit the “trafficator”
type rudder tried experimentally on the H VII.
Landing flaps consisted of plain trailing edge flaps (in four sections) on
the wings, with a 3% chord lower surface spoiler running right across the
centre section, which functioned as a glide path control. The outer pair
of plain flaps lowered 27° and the inner pair 30° – 35° on the glider
version V.1. On V.2 mechanical trouble prevented the inner pair operating
and all flying was done with the outer pair only. The centre section
spoiler could be used as a high speed brake and gave 1/3 g at 950 kph. No
dive recovery flap was considered necessary.
Proper performance tests were not done on V.2 before its crash and top
speed figures were calculated values, checked by Messerschmitts. The
following figures were remembered by Reimar Horten:
All Up Weight, Including Ammunition and Armour 8,500 kg (18,700 lbs.)
All Up Weight, Excluding Ammunition and Armour 7,500 kg
Wing Area 52 sq.m (566 sq.ft.)
Wing Loading 33 lb./sq.ft.
Fuel (I2 Crude Oil) 2,000 kg (4,400 lbs.)
Performance at 7,500 kg (16,500 lbs.)
Takeoff Run 500 m
Takeoff Speed (10° Flap) 150 kph (95 mph)
(Note: This corresponds to a CL of 1.30 which is the stated stalling CL of
Top Speed (at Sea Level) 950 kph (590 mph)
(CDo estimated to be 0.011)
Calculated ceiling was 16 km (52,000’). Engines would not work above 12 km
as the burners went out.
Rate of Climb at Sea Level 22 m/sec (4,300 ft/min)
(Note: This has been checked roughly by observation.)
In tests against the Me 262 speeds of 650-700 kph (400-430 mph) were
obtained on about 2/3 throttle opening. This appears to be the only flight
test figure available.
Messerschmitt sent performance calculators to the Horten works to check
their estimates. The method suggested by D.V.L. for getting the sweepback
correction to compressibility drag was to take an area of 0.3 x the root
chord squared at the centre section as having no correction applied, and
then apply full cosine correction over the outer wing. Sweepback angle was
defined as that of the quarter chord locus. Test data was available for
CDv. for zero sweepback.
The Messerschmitt method was to base sweepback on the max t/c locus and to
scale Mach number by the square root cos Ø.
Stability and Control
The H IX V.1 was flown by Walter Horten, Scheidhauer and Ziller.
Scheidhauer did most of the flying (30 hours) at Oranienberg, Horten and
Ziller flew for about 10 hours.
D.V.L. instrumented the aircraft for drag and directional stability
measurements. No drag results were obtained because of trouble with the
instrument installation – apparently an incidence measuring pole was
fitted which could be lowered in flight and glide path angle was obtained
from the difference between attitude and incidence measurements. One day
they landed without retracting the pole. Directional oscillation tests
were completed successfully and an advance report was issued (10 pages of
typescript) by Pinsker and Lugner fo D.V.L.
The essence of the results was that the lateral oscillation was of
abnormally long period – about 8 sec. At 250 kph and damped out in about 5
cycles. At low speeds the oscillation was of “dutch roll” type but at high
speed very little banking occurred. Many fierce arguments took place at
D.V.L. on desirable directional stability characteristics , the Hortens
naturally joining the “long period” school of thought. They claimed that
the long period would enable the pilot to damp out any directional swing
with rudder and keep perfectly steady for shooting. It was found that by
using both drag rudders simultaneously when aiming, the aircraft could be
kept very steady with high damping of any residual oscillation.
Lateral control was apparently quite good with very little adverse yaw.
Longitudinal control and stability was more like a conventional aircraft
than any of the preceding Horten types and there was complete absence of
the longitudinal "wiggle" usually produced by flying through gusts. Tuft
tests were done to check the stall but the photographs were not good
enough for much to be learned. Handling was said to be good at the stall,
the aircraft sinking on an even keel. There seems to be some doubt,
however, as to whether a full stall had ever taken place since full tests
with varying CG and yaw had not been done. Although the stick was pulled
hard back, the CG may have been too far forward to give a genuine stall.
Directional stability was said by Scheidhauer to be very good, as good as
a normal aircraft. He did not discuss this statement in detail as he was
obviously very hazy about what he meant by good stability and could give
very little precise information about the type and period of the motion
compared with normal aircraft.
Scheidhauer had flown the Me 163 as a glider and was obviously very
impressed with it; he was confident enough to do rolls and loops on his
first flight. We asked him how the H IX V.1 compared with the 163; he was
reluctant to give an answer and said the two were not comparable because
of the difference in size. He finally admitted that he preferred the 163
which was more manoeuvrable, and a delight to fly (he called it “spielzeug”).
The H IX V.2 with jet engines was flown only by Ziller and completed about
2 hours flying before its crash. This occurred after an engine failure –
the pilot undershot, tried to stretch the glide and stalled. One wing must
have dropped, for the aircraft went in sideways and Ziller was killed.
Before the crash a demonstration had been given against an Me 262; Horten
said the H IX proved faster and more manoeuvrable, with a steeper and
In spite of the crash, Horten thought the single engine performance
satisfactory and said the close spacing of the jets made single engined
flying relatively simple.