The Horten Tailless Aircraft by K.G. Wilkinson, B.Sc. D.I.C. Horten IV & H IVb
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General 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).
Control The three stage control flaps were all geared to the spectable 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.
Wing Design The H IV used reflexed
cambered sections (zero Cmo) of R.A.F. 34 type, changing to
assymetrical 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 (para. 4.5). Fig. 9 also shows the meaured 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 ( para. 4.2.1).
Flying Qualities 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 better.
Structural Features 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
center section was of welded steel tube, with perspex (sp.) nose
and a large jettisonable access cover behind the main spar (Fig. 6) (not
reproducible).
Equipment 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.
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 provided.
3.6 Horten IVb General 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. Aerodynamic Design 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 center of the basic wing section was farther aft). It is interesting that although Cmo was not zero for the root section, the high aspec ratio enabled the glider to be designed to trim, elevons neutral, at the required top speed (140 mph) without needing excessive twist. Structure The wing structure ahead
of the main spar was a ply sandwich monocoque with Tronal filling.
Tronal was an expanded wook 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 molds, 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.
Performance 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
The change of section raised the stalling speed from 45 kph on the IV and to 60 kph on the IVb. Handling Characteristics 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. ..PREVIOUS SECTION................................NEXT SECTION |
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