FLIGHT, 3 October 1958
556
ECONOMY AND PERFORMANCE COMBINED
IN KIRBYMOORSIDE'S LATEST SAILPLANE
Slingsby Swallow
FABRIC-COVB
AILERON
25
T
HE growing popularity of gliding over the last few years
has resulted in an increased number of pilots wishing to
own their own aircraft, and this has opened up markets for
new types of low-priced sailplanes. Most private gliders are owned
by syndicates of up to four people and it is usual for costs to be
kept to a minimum by the owners carrying out most of the maintenance and repair work on their own aircraft. An increasing
number of owners, particularly abroad, are in addition assembling
their own sailplanes from kits, thus saving the cost of assembly
and finishing and, in the case of exports, a major part of the
shipping charges.
It is with this market in mind that Slingsby Sailplanes, Ltd.,
of Kirbymoorside, Yorks, have introduced the Swallow, an aircraft
they describe as a small high-performance sailplane. Although in
most ways a straightforward and conventional aircraft, the Swallow
contains noteworthy features. The prime object in the design has
been to produce a sailplane with a performance sufficiently high
to appeal to experienced pilots, yet with a structure that can be
assembled and maintained by persons of no great skill.
Aerodynamically the Swallow is a follow-on from Slingsby's
recent family of laminar-flow sailplanes such as the Skylarks and
the Eagle. The aerofoil sections used for the wings are the same
throughout this series and consist of an NACA 6 series section
at the root and one of the older 44 series at the tip. Low-drag
characteristics are obtained by building the forward 40 per cent
of the wing chord to a high standard of accuracy which allows
a low degree of chordwise waviness in the profile.
The original philosophy behind the design called for a laminarflow wing of small span and low aspect-ratio. The reduction in
profile drag as a result of laminar flow was to compensate for
the higher induced drag caused by the low aspect-ratio. The large
wing-area, a further result of low aspect-ratio, would give a low
wing-loading to provide suitable slow-flying characteristics and,
at the same time, would also provide a large base on which to
carry the parasite drag of the somewhat rugged fuselage. The
success of this philosophy, as always, depended on the compromises
made between the various conflicting requirements.
The idea of small sailplanes is not new; in recent years
considerable interest has been shown in them in America and
Australia. Suggestions have been put forward for machines as
small as 30ft in span but it became obvious during preliminary
work on the Swallow design that a span of 12 m (39.5ft) would
be required to provide an acceptable performance. The Swallow
first flew in October 1957 with a span of 12 m but, after the early
tests, it was decided to increase it to 13.2 m and so obtain additional performance. Full performance tests have yet to be carried
out but it is clear from the flying done on the prototype that the
best gliding angle is around 1 in 25. This particular aircraft, as
a result of modifications and repairs, lacks the finish that can be
expected on the production machines; and it is not equipped with
tip fairings on wings or tail unit or a canvas fairing on the main
skid. It therefore seems probable that owners willing to clean up
their aircraft can expect L / D ratios in excess of 1 in 26.
For an aircraft to become popular, performance is not the only
consideration; it must also have good handling qualities and be
pleasant to fly. The Swallow, being a small machine, is very crisp
"Flight" photograph
The four detail sketches which
complement the main cutaway
drawing show (top centre) wingrigging; (top right) trim-tab
damper, which provides friction
to prevent vibration of tab in
case of control slackening during flight; (lower right) tailplane rigging; and (below) the
cockpit, with seat and floor
panels removed. Key to instrument panel in cockpit sketch:
1, Airspeed indicator; 2, turnand-bank; 3, artificial horizon;
4, Horn variometer; 5, Cosim
variometer; 6, altimeter; 7, Cook
compass.
CABOON PLY
TO MAIN SPAR
HOLLOWED -OUT
L E BLOCK
FOR
COCKPIT
SEE
SEPARATE DETAIL
TOW-HOOK
RELEASE
DIVE-BRAKE
CONTROL
CLASS-FIBRE NC
AND FORWARD D
\
:LEVATOR
TRIM
CONTROL
BATTERY
STOWAGE
UNOCR SEAT
on the controls. In bank the movement of the wings can be said
almost to follow the angle of the stick, while in pitch the speed
picks up rapidly as soon as the controls are moved forward. Considering the snappy controls the stall is surprisingly gentle—in
fact it is doubtful if a true static stall can be obtained, as the
machine can be flown continuously at any e.g. position with the
stick fully back. In this condition th'ere is a marked buffet and
the aircraft wallows and has a comparatively high rate of sink,
but full control is still available in all planes. The stalling characteristics make it virtually impossible to spin the machine inadvertently and several attempts are often required before it can be
made to spin intentionally.
For pilots with a taste for aerobatics the Swallow will have a
special appeal as it is cleared for more manoeuvres than is any
other British sailplane. Its repertoire includes stalls, loops, spins,
tight turns (3ig), stall turns, chandelles, rolls, half loop and roll