aspects of the Horten designs
Stability and Control in Unstalled Flight
This seems generally to have been satisfactory. It is clear that,
particularly on their light aircraft, there was a difference in
longitudinal gust response (and probably also in control response) which
seems to have been more sudden than on a conventional type but this was
less marked in the H IX which had more normal wing loading. The Horten
view was that with correct CG positions there was little important
difference. When questioned about static stick free instability when
approaching the stall they thought it might possibly be present but were
sure that stick force reversal did not occur.
On the H IV a peculiar form of longitudinal instability was mentioned by
Reimar Horten. Apparently a short period oscillation (period about 1 sec.)
would be produced with damping varying with speed, being about zero at 160
kph. At higher speeds it showed signs of building up spontaneously (-ve
damping). A theory was that it might be due to a coupling with flexure and
torsion of the wing but no proof had been obtained.
All their aircraft appear to have had less damping of the lateral
oscillation than normal but also a longer period which made it easier for
the pilot to damp out disturbances with rudder. This feature is of
interest in view of the widely held view in Germany that the period of the
lateral oscillation on a high speed fighter (particularly jet propelled)
is too short for good gunnery and should be increased if possible to 4
Directional stability would be expected to be indifferent because of the
absence of fins. Northrop has found on his aircraft that the low values of
nv and yv made it possible to fly with appreciable yaw without the pilot
knowing it, and this led to unpleasant characteristics in rough air.
Horten, is satisfied with the behaviour of his aircraft without fins and
Stuper could not find anything to criticize in this dimension of the H V.
After about 1 hour’s flying in the H IV, the impression is that there is
definitely something unusual in directional stability and control. It
seems possible to fly with considerable yaw and the response to drag
rudder is quite different form that of the conventional aircraft to a
normal rudder. As yet it is too early to give a full report on these
Behaviour at the Stall and Recovery From the Spin
Besides doing normal stall tests, Horten did a certain amount of research
with wool tufts to gain insight into the flow changes at the stall. On the
H II glider surface tufts were used and photographed by a camera in the
pilots head fairing. On the H III more extensive tests were done using
surface tufts on one wing and tufts on 2” masts on the other, the stall
being photographed from a Storch flying immediately above. To assist in
the interpretation of the photographs the glider was fitted with a
sideslip vane and an A.S.I. which registered on the upper surface of the
wing and appeared in the pictures. Chordwise lines were painted on the
wing to show readily any yaw of the tufts. This technique proved
difficult, particularly for the Storch pilot, and the photographs were
apparently not very good.
They did confirm, however, that the stall started at the middle of the
semi span, and showed the spanwise drift of the boundary layer. The H V
and H IX were also tufted but no pictures were taken. Horten said that the
tufts again showed that the wing tips did not stall and in the case of the
H V the stall was sketched as spreading to the root whilst leaving the
outer 30% of the semi span unstalled.
In general, Horten thought that a stall with CG back would be worst
because the upward elevon angle would be reduced, producing reduced
pressure on the upper surface of the wing and increasing the spanwise
The picture of designed and available CL distribution was as shown. This
actually gives slightly better results with CG aft, for, if the extra up
elevon required for trim with CG forward decreases the available tip CLmax
by Delta CL, then the obtained CL at the tip decreased only by 0.9CL.
However, the spanwise flow effect was thought by Horten to be of
Tests on the Influence of CG Position on Flying Characteristic
Tests on the stalling and spinning of a H III glider were done at the
Hornberg with varying CG positions. To start with, weight was added at the
back of the center section until the glider was only just flyable. This
required 35 kg of ballast (the normal CG position was 2.1 meters behind
the center section leading edge, corresponding to 4% pfeilmass static
margin). A tube 2 m. long was then put under the center section with a 10
kg sliding weight which was kept forward for takeoff and landing and moved
aft for tests at height, giving a static margin of 0 to – ½% pfeilmass.
Fight characteristics were as follows:
(a) CG Forward – (10% pfeilmass ahead of neutral point) Stalling and
spinning were impossible.
(b) CG Normal (4% of pfeilmass ahead of neutral point) Stalling was
possible but spinning difficult. Spinning attitude was steep and normal
recovery procedure resulted in a steep dive with little sideslip.
(c) CG Aft (2% pfeilmass) Normal flying characteristics began to be
unpleasant. Longitudinal control became very touchy.
(d) Extreme Aft CG (0 to – ½% pfeilmass) With this CG position it was only
just possible to fly the aircraft because with the stick hard forward it
was very near to the stall. Scheidhauer refused to do these tests. The
spin was entered with full aileron and rudder, and recovery after two
turns was by centralizing aileron and rudder and pushing the stick
forward. After one turn the aircraft slid sideways out of the spin with
about 60° sideslip and went into a dive. General flying characteristics
were most unpleasant.
Note: On the H III, the dimension of “pfeilmass” is about 10% greater than
the mean chord, so that static margins can be taken as % mean chord for
Tests on Laminar Flow
In the course of his work on laminar flow sections Horten carried out some
observations on a 2-seater H III in which transition was detected by a
creeping total head tube connected to a stethoscope, worn by the
passenger. Transition from laminar to turbulent flow was accompanied by a
roaring noise in the earpieces.
Results of the H III experiments were quoted as follows:
CL Transition Point % C Back from Nose
0.1 to 0.2 30% Upper 60% Lower Surface
1.2 to 1.4 10% Upper 80% Lower Surface
Chord at the test section was about 3 m. and forward speeds about 10
m/sec. at high CL and 30 m/sec. at low CL.
None of the Horten designs claimed very high CLmax values. The following
table summarizes the maxima stated for various aircraft.
Type CLmax Comments
H III 1.3 (no flap) Measured
H IV 1.4 Measured
H VI 1.5 Estimated
H VIII 1.4 (no flap) 1.6 (flap) Estimated
H IX 1.3 (flap) Measured on V.1
CLmax measurements were made with a swivelling pitot static held below the
aircraft on a 4 meter pole which could be retracted for takeoff and
Waggle Tip Control
This device was first tested, unsuccessfully on the first H V and, later
more successfully on a special H III (Fig.
27). The final objective was a “stockbrokers” aeroplane with a
throttle and rudder bar as the only controls.
Fig. 26 shows the principle
of operation and Fig. 27 the four shots of the damaged wing of the H III
(found at Gottingen) with the waggle tip in its extreme and mean position.
The wing tips were mounted on a skew hinge so that forward and backward
sweep was accomplished respectively by increase and decrease of incidence.
In the form used on the H III the wing tip was geared to the stick, but
finally it was proposed to have the tips freely floating but damped. The
diagrams of Fig. 26 describe the automatic stabilization as propounded by
the Hortens. Control in the free floating case was to have been by
spoilers on the wing tips, operated by the rudder bar. Opening of one
spoiler would drag back the wing tip and apply bank and yaw in the correct
sense for a truly banked turn.
Preliminary flights with the H III were apparently not very successful
because of the large control inertia.