The reason is quite simple. Even if it doesn't seen logical, water in the wings
increase the stresses in the spar of the wing. This because the water is
situated at the root of the wing. The effect is smaller that an increase in the
non lifting parts. The original wing of the discus was desinged to be a pure
glider. To make the turbo they had to reduce the ammount of water in the wing
by so much that the stresses in the wing would not exceed those of the pure
glider. Therefore they reduced the ammount of water you are allowed to carry by
reducing the MTOW. If you remove the turbo from the glider the MTOW goes back
to 525 kg., indicating that the wing for a Discus T is the same as the pure
glider. New glider are desinged from the start to carry turbo's because
everybody want's them in their new expensive glider. Therefore the wings of the
pure glider may be (do not read : are !) stronger then necessary. MTOW is
determined by the some factors as stall speed and landing gear forces. For
example the new DG 808B ompetition required a new landing gear to be allowed to
carry the new 600 kg MTOW. see
http://www.dg-flugzeugbau.de/dg-808-...ml#competition for more
info on that aspect. Also you can read there that they had to move the ballast
bags further out in the wing, to reduce the stresses on the wing.

Roelant

Eric Greenwell wrote:

Roelant van der Bos wrote:

I simplified it a bit too much, perhaps. It would come out of the
"non-lifting parts" limit (basically the fuselage and everything in it).
Generally, the effect would be to reduce the cockpit allowed load, but
not always, depending on the exact weight of fuselage and installed
equipment. The amount of water ballast allowed would not likely change,
since it is carried by the wing (a lifting part), not the fuselage.

A glider designed to carry a motor will have a higher "non-lifting
parts" limit (perhaps from more structure, stronger lift pins, etc) than
a similar non-motorized one, in order to preserve the cockpit load.

Not really. For allmost every turbo equipped glider the max.
total weight is the same as the one for the pure glider, and in some
cases the turbo has one even lower. The discus 1 for example has a
MTOW of 525 kg without the engine, but with the turbo it reduced to
450 kg.

An increased "non-lifting parts" limit would not necessarily increase
the total allowed weight, which depends on several factors (landing gear
strength and tow hook mounting come to mind as possibilities).

I don't know why the Discus 1 turbo has such a lower MTOW, but I'd guess
the major effect is you would fly it with a lot less water ballast -
about 100 liters less! I don't think this reduction has anything to do
with the "non-lifting parts" limit, but might be related to required
climb rates or other regulation.

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Eric Greenwell
Washington State
USA