Just to tag on the ending of the DG article:

'The C/G definitely influences the flight performance
and efficiency.
A position in the forward half of the C/G envelope
produces negative results and should be avoided.
A good compromise is a C/G position about 30 to 40
% forward of the aftmost position.
Flying with the C/G at the rearmost position is endangering
your life.
Check your C/G and do a weight/balance of your sailplane,
and weigh yourself.'

John Galloway


At 13:18 24 October 2003, Mark Stevens wrote:
this has been referenced before from the DG web site,
but it's not long so I'll post it here..



The Optimum C/G of Sailplanes

A Caution from Wilhelm Dirks

In the 'classic' aerodynamic theory of airplanes the
wings generate lift and the tail plane generates stability.
Because curved wing profiles are used, the aerodynamic
moment generated by the wings, which tries to push
the nose down and has to be countered by the tail plane.


For this the tail plane has to produce downward force
dependent on the airspeed and C/G. The higher the airspeed
and the more forward the C/G, a higher downward force
is produced. In a rearward C/G the tail plane can even
produce a lifting force.

Most contest pilots trim their sailplanes to the most
rearward permissible position. In theory this improves
performance, especially in circling flight one does
not have to 'pull' on the stick as much. At the 'German
Soaring Symposium' in Stuttgart a paper was presented
and discussed which showed these well known facts in
graphic detail for several sailplane types.

In the first phase it was researched just how much
the performance was affected by forward or rearward
C/G positions. The permissible C/G values are determined
by the designer. A forward C/G determines the size
of the tail plane and elevator, i.e. that sufficient
force is available in the elevator to make circling
flight possible. The aft C/G is determined so that
the airplane has satisfactory pitch stability and is
able recover from a spin. Generally the calculations
for all types showed a performance difference of 1.5
to 2 points between the foremost and rear most C/G
positions. That is a significant difference! Is it
therefore correct to fly with the rearmost C/G? Let's
think about it.


Performance Factors
The L/D of a sailplane is calculated by the ratio of
total lift to total drag. If the tail plane produces
downward force the wings have to generate more lift,
and that causes increased induced and profile drag,
reducing the L/D. In spite of this the rearmost C/G
does not necessarily produce better results. The tail
plane is not designed to produce much lift. It normally
has an almost symmetrical profile.
The wing profile is designed to produce lift and is
much better at this task. In addition the tail plane
produces a disproportionate amount of induced drag
because of it's low aspect ratio. The optimum condition
would be one in which the tail plane in slow flight
produces just enough lift to compensate for the loss
of lift of the wings in the fuselage area. That would
minimize the induced drag of the sailplane.
This condition is obtained, more or less, depending
on the design, by the distance of the tail plane from
the most rearward C/G position.


Flight Characteristics and Safety
At aft C/G stability is minimal, especially in the
roll axis, and the sailplane must be 'flown' at all
times to avoid air speed variations usually encountered
when thermaling. Depending on experience and skill,
the pilot may tire faster and his concentration can
diminish, so that the theoretical advantage is greatly
reduced. At the rearmost C/G the sailplane will enter
a spin much easier at less than the minimum airspeed
then at forward C/G, where spinning in many instances
is not even possible. This can have deadly results,
especially close to the ground. Different sailplanes
react differently, but the tendency is clear. Even
an experienced contest pilot should be very wary of
choosing the extreme rear most C/G position.