| Robin Andrew of Birmingham
has been a PSS enthusiast for many years and has built several
own designs. Recently I received 3 separate letters from
him on the subject of his design ideas - the following are a
precis of his views.
For a relaxed flying model choose
designs with a small amount of dihedral. This helps to
provide built-in stability. In the case of more
aerobatic models, zero dihedral is to be preferred. A
substantial amount of fin area is required to keep the model
straight whilst manoeuvring. Thickening the fin section
also helps to achieve this. A built up fin may be the
easiest way to provide this and should help to keep the tail
end light.
Tip stalling can be prevented by
thickening the tip section rib depth over the root section -
i.e. 10% root 11% tip.
A low wing design with a high set
tailplane needs some 3º positive incidence to keep the
fuselage level in flight. T-tail designs still fly with
a smaller tail area than conventionally mounted designs, being
more efficient by flying in cleaner air.
For aerobatics when using normal wing
sections the CofG will be best at more than 33% back from the
Leading Edge. If wing and tail are in line then CofGs of
up to 45% can be used with consequent improvement in
performance by increasing the lift - i.e. the wing lift has
less 'nose heaviness' to overcome.
Having a preference for models capable
of stunting, Robin aims to produce a PSS model with both good
looks and an aerobatic performance. This can be
accomplished by choosing a design with wings and tail in line
- i.e. zero/zero incidence. By employing a 10% Eppler
205 wing section one can fly in lower wind speeds. This
section, coupled with the use of ailerons and the minimum aspect
ratio of 8 seems about right. An All-Up Weight (AUW) of
2lbs 6oz maximum for a 6ft span model is to be aimed for.
So long as wing sweep back does not
exceed 15º measured at the Trailing Edge, no loss of lift
should occur. A thickening of the wing section will help
but only at the expense of speed.
One problem when scaling up model plans
from 1/72 size is a small sized tail plane. It will be
found that small sized tails will work providing a lifting
wing section such as Clark Y or better still R.A.F. 32 is used
for tail sections. The thickness needs to be at least
10% coupled with a low entry angle radius. This usually
means having to use a built up tail structure, but the bonus
here is a lighter tail end and less lead in the nose.
In the next newsletter our Scottish
friends will be providing material for the 'Design Trends'
column. In the meantime, keep up the design work - let's
see more new designs in the air and above all have fun POWER
SCALE SOARING.
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