Once I had more or less finalised the
layout, I built a 1/4 scale radio controlled model and
put it through a fairly rigorous program. The results
were very encouraging indeed. The stall was a non-event,
no adverse yaw could be seen and, flying slow at tick-over
RPM then applying full power produced no negative pitching
(the thrust line of the engine is aligned with the wing
C.of G. position). The landing flare was perfect. I
tried different strakes, but the results were inconclusive.
The lack of downward vision always
seemed to be a problem with wings so the pilot had to
be out in front, therefore the engine was put in the
correct place as a pusher. I had a spare Rotax engine
(a 447) so that was the one I used.
NACA 23012 section was chosen for its nearly neutral
pitching moment, and more importantly, I already had
the co-ordinates and some of the figures for such a
section. The extended centre section has a reflex from
about 80% chord with the T/E level with the section
height. The ramp effect between the extended fins should
help with aero effect. The ailerons are large with small
movements and are differentialled 1:1.5. For the first
flights they will be set at about +3°. The rudders
act "out only" and have tip plates to help
with rudder authority at low speeds since they are outwith
As you can see from the photo in the
field (above, right), we were taxiing without the outer
wing panels to check out the undercarriage. After this
test I molded a new, stiffer undercarriage and a new
nosewheel steering system.
As mentioned, the Sgian Dubh is constructed
from extruded polystyrene foam with 290t glass cloth
and West System epoxy. All awkward contours, leading
edge D's etc. were hot wired. The main spar is built
up plywood to take the mortice and tennon wing joints.
The strakes round the lower edge of the cockpit extend
the "pitch platform" and are set at +1.5°
covered with carbon fibre whilst also adding structural
rigidity without sacrificing cockpit space.
The tank is situated on the C.of G.
Home designed and made prop and undercarriage. The all-up-weight
so far, with 86 kg pilot is around 266 kg. The side
stick is a slider type and the same push rod operates
both elevators and ailerons.