IV and H IVb
The H IV represented the
Hortens mature thoughts on sailplane design. The span was the same as that
of the H III but aspect ratio was increased from 10.7 to 21.1, and the
control system further developed. In order to retain their finless wing
layout and get the maximum aerodynamic efficiency, the pilot was put in a
prone position with his body in a 27% thickness ratio egg and his knees
and legs in a small leg well, which also supported the rear skid ( or
wheel in the case of the H IVb).
A specimen of HIV was found at Göttingen in good condition and was brought
back to R.A.E. for test flying. It has completed 500 hours flying since
its construction in 1942, including a cloud flight of 1-hour on
instruments; such a flight demonstrates that stability and control and the
comfort of the prone position must be satisfactory.
The three stage control flaps were all geared to the spectacle type
control wheel and operated on the same general principle as the earlier
two flap control on the H III. The following table gives the (measured)
flap movements corresponding to full control by the pilot.
It will be seen that the outer flap works principally
as up going aileron whereas the “climbing elevator” action comes mainly
from the middle flap and “diving elevator” action from the inner flap.
Down going aileron, needed to neutralize pitching moments, comes from the
inner and middle flaps together.
The centre and inner flaps were unbalanced, with round noses, the tip
flaps Friese balanced with a skew hinge giving 39% balance at the inboard
end and zero balance at the tip. This scheme gave the required aileron
yawing moments without making the control flap at the tip vulnerable when
a wing tip scraped the ground.
Drag rudders were of the upper and lower surface spoiler type placed
immediately ahead of the outer control flap; the upper surface spoiler had
a vented web. To open the rudders the pilot had to press with his toes,
moving the foot from the ankle against a spring loading on the pedals
which gave "feel" to the control. By pressing both feet together he could
open both rudders simultaneously, thus giving extra drag for glide
control. Rudder operation was said to cause no buffeting of the control
flaps. The movement transmission from the pilot’s pedal included a cam
plate (Fig. 11) cut to give
no rudder movement for negative movement of the pilot’s foot (i.e.,
pressure on the opposite pedal) and an approximately linear relationship
between pedal movement and rudder projection for positive movement (i.e.,
pressure on the pedal).
All controls were operated by push rods, the inner and central flaps and
teh drag rudders being moved by skew-hinge cantilevers; the system is
illustrated in Fig. 11. In the IVb the skew hinge principle was extended
to the outer flap operation also. The method of operating the control
flaps was simple to construct and eliminated all external control horns.
Longitudinal trim was obtained by an internal bungee “spring” which can be
adjusted to take any out-of-balance aerodynamic loads on the elevator
There were no landing flaps by large spoiler type dive brakes were
provided, which could be used to give variable drag for glide path
The H IV used reflexed cambered sections (zero Cmo) of R.A.F. 34 type,
changing to asymmetrical section at the wing tip. Sections at four
stations on the wing are given in Fig. 9 and tables of ordinates in Table
II. The Horten method of deriving wing sections is described later.
also shows the
measured washout distribution; this was such that leading and trailing
edges were approximately straight (second power distribution) but this was
fortuitous as the actual design formula was more complicated.
The large wing dihedral of 5 percent was used to give adequate wing tip
clearance. Reimar Horten considered that aerodynamically this might be on
the large side but advisable for practical reasons. It should be
remembered that both the H III and H IV have an abnormally low value for
the lateral relative density Uso that unusual values of Lr and Nv would be
permissible without dynamic instability resulting.
Performance was measured by flying the H IV against the D 30, a
conventional high performance glider which had been carefully performance
tested by D.V.L. to form a “standard”. The essence of the method was to
two both aircraft up together and let them glide down from about 10,000’
at a series of speeds, measuring the relative height photographically, at
intervals. From these tests the best gliding angle of the H IV was found
to be 1 to 37 and the minimum sinking speed 1.7 ft/sec. Minimum sinking
speed was slightly less than the D 30, but at high speeds the d 30 was
Scheidhauer, Horten’s chief test pilot, has done the majority of the
flying in Horten IV’s (about 1000 hrs) and his comments are worth
recording. H is a strong advocate of the prone position - in his own words
“it has nothing but advantages.” All H IV controls he described as very
light, he flew the glider with “two fingers”. The elevator was apparently
rather sensitive compared with the aileron but not unpleasantly so.
Aileron application produced no adverse yaw - a definite improvement after
the II and III - and could reverse a 45 degree banked turn in 5 secs. at
70-90 mph, which is better than the average sailplane.
stability he thought satisfactory but he commented on a “wiggle” which was
produced by flying through gusts; this is apparently a sharper pitch
response than for a conventional sailplane, but well damped, quite
harmless and requiring no corrective action by the pilot. A true stall
could not be produced with normal elevon adjustment because of increasing
static stick fixed stability at the stall, which used up available
elevator power before the wing tips were stalled. Spins could only be
produced by applying full aileron and rudder with the stick hard back;
recovery was easy.
Stability and controllability on tow were excellent. Scheidhauer described
a competition in which a number of sailplanes were aero-towed form Grunau
through the very turbulent air in the “standing wave” from a nearby
mountain; the rough air had to be negotiated on tow to get to the area of
rising currents. All the instructors from the school at Grunau were flying
conventional sailplanes and broke their two lines without exception.
Scheidhauer in his H IV managed to get through and soar in the standing
wave. He attributed his success partly to his own skill and partly to the
good controls of the H IV plus his ability to use the tip rudders together
to check surging in the tow rope.
Take-off seems to present some problems to a pilot new to the aircraft. It
seems that the short undercarriage base, responsive elevator and small
wing tip clearance can produce a very erratic take-off if the pilot is not
smooth and precise in his control movements.
Construction followed the normal Horten practice, but the wing panels were
made with detachable tips of sheet clektron. This was necessary because
the narrow chord at the tip made accurate construction in wood very
difficult. The centre section was of welded steel tube, with Perspex (sp.)
nose and a large jettisonable access cover behind the main spar (Fig. 6)
The front skid was retractable and fitted with a wheel which automatically
dropped off as the skid retracted.
The pilot’s harness was modified from the original version shown in Fig.
6, being a single broad strap passing under the buttocks. This was
released by the same handle that jettisoned the access cover. The pilot’s
parachute was stowed in a pocket on the cover and connected to the pilots
harness by short straps. In this was the pilot was relieved of the weight
of the pack, which would otherwise have caused some discomfort on a long
Flying instruments included a low reading A.S.I. driven by a venturi,
electrical turn and bank indicator, sensitive variometer, high reading
variometer, altimeter and clock.
Oxygen equipment comprised two bottles, pressure gauges, reducing valve
and economizer, and provision was made for electrically heated clothing.
Ventilation was under the pilot’s control.
Prone Position Bed
(ed. - Several lines were missing here) . . . . well could be adjusted for
varying pilot size and a chin rest with adjustment for height was
The pilot was prevented from sliding forward by shoulder rests and the
reaction of his thighs against the knee well.
Comfort appeared to be satisfactory when we tried the bed but elbow and
shoulder movement was restricted which constrained one to stay in the same
position all the time.
Superficially the IVb resembles the IV very closely but the
aerodynamic changes were a
fundamental experiment. The Hortens intended to produce a laminar flow
sailplane with superlative high speed performance - in this they were
partially successful but they sacrificed too much on the stability and
control to make the venture a real success. Production had been started,
prematurely, at the rate of about two a month.
Wing sections were derived from the Mustang section which had been
measured by D.V.L. for captured aircraft and tunnel tested. The Hortens
were excited by the low tunnel drag figures and designed the H IVb to
exploit them. The root section was the original Mustang profile, changing
to an uncambered section with the same fairing shape but reduced thickness
at the tip. Wing twist was reduced (compared with the IV) to 5.6 degrees
to get the greatest spanwise extent of laminar flow, and sweepback reduced
to 2 degrees to get the CG farther back relative to the mean chord (this
was necessary because the aerodynamic centre of the basic wing section was
farther aft). It is interesting that although Cmo was not zero for the
root section, the high aspect ratio enabled the glider to be designed to
trim, elevons neutral, at the required top speed (140 mph) without needing
The wing structure ahead of the main spar was a ply sandwich monocoque
with Tronal filling. Tronal was an expanded work with specific gravity 0.1
to 0.09, invented by a Dr. Barschfeld of Dynamit A.G., Troisdorf (near
Cologne). The sandwich was made up on moulds, with outer ply 1 mm thick
and inner ply .8 mm; the filling was 20mm at the root tapering to 5 mm at
the tip. The nose sections were stuck onto the front of the main spar with
supporting ribs every 2 meters. Between the main and rear spars normal ply
covering was used, insufficient Tronal being unavailable for sandwich
construction all over.
Waviness in a chordwise direction was not controlled or measured. Sag (spanwise)
between ribs had been measured on the IV and eliminated on the IVb.
Special care was taken to kep dust off the wings; wing dust covers were
made and all handling was done with gloves on.
Control circuit mechanism remained the same except for the outer flaps
which were also operated by a skew hinge lever on the IVb. The dive brakes
were moved back to the rear spar to suit the revised wing structure
No transition measurements were made on the IVb, but it was flown against
a calibrated IV and the following relative sinking speeds measured.
up to 80 kph no difference
at 100 kph IV 1 m/sec. IVb 0.85 to 0.9 m/sec.
120 kph IV 1.40 m/sec. IVb 1.20 m/sec.
The change of section raised the stalling speed from 45 kph on the IV and
to 60 kph on the IVb.
These were very unsatisfactory. A wing tip stall occurred followed by wing
dropping and spinning. The first aircraft crashed for this reason after
the pilot got into trouble in a cloud. An attempt was made to improve
matters on the second glider by clipping the span from 20.25 meters to
18.5 meters but results were disappointing. As a cure on the final design
a reversion to the old H IV tip section was proposed, the theory being
that section stalling characteristics were bad due to the sharp nose
radius. Partial breakaway behind the maximum thickness point was
suggested, aggravated by spanwise boundary layer drift which rendered the
elevon ineffective. Horten thought the small wing tip Reynolds number made
the use of low drag sections inadvisable.