Max Gross Weight Question

Our glider has two max takeoff weights, one with water ballast and one with only pilot (134 lb difference). I just weighed the glider and the empty weight subtracted from the max takeoff weight (no water ballast) limits the useful load to 225. There's still some room to play with for the forward CG limit but it's the max weight with no water ballast that caps the pilot weight at 225. If the empty weight was lower and the CG was still within the limits, the glider manual references a max pilot weight of 245. Why is the glider legal with increased water weight but not increased pilot weight (assuming CG is still within limits)? I could see the rationale for an increased water weight gross if there was a requirement to jettison the water for landing but I could find no such restriction. So as far as I could tell, you're legal landing with water at a higher weight but not if the extra weight is pilot body mass.

On Friday, March 1, 2013 4:13:22 PM UTC-6, 150flivver wrote:
Our glider has two max takeoff weights, one with water ballast and one with only pilot (134 lb difference). I just weighed the glider and the empty weight subtracted from the max takeoff weight (no water ballast) limits the useful load to 225. There's still some room to play with for the forward CG limit but it's the max weight with no water ballast that caps the pilot weight at 225. If the empty weight was lower and the CG was still within the limits, the glider manual references a max pilot weight of 245. Why is the glider legal with increased water weight but not increased pilot weight (assuming CG is still within limits)? I could see the rationale for an increased water weight gross if there was a requirement to jettison the water for landing but I could find no such restriction. So as far as I could tell, you're legal landing with water at a higher weight but not if the extra weight is pilot body mass.

because the water is in the wings and reduces the bending moment on the spar instead of weight in the cockpit which increases the moment.

cue to Holinghaus in Zulu Romeo: Good Start saying that 'these fiberglass gliders are so strong it is almost impossible to break them, in fact the heavier they are the safer they are'

i don't think i would take that too seriously. obey what is in the book

Tony has it right, it usually has to do with the Bending loads carried through the wing attach fitting to the fuselage.

What you are really looking for is what the Maximum Design Fuselage Weight is for the glider. This is what controls the Maximum Pilot Weight. If the wings have an extra coat of paint and filler on them you can count this extra weight as Ballast weight. Unfortunately not all manufacturer actually publish the Maximum Fuselage Weight.

Brian

True, in most sailplanes the critical factor is maximum bending moment
of the wing structure; the critical location is usually at side of
body or thereabouts. However, depending on the type of wing attach
used, the attachment of the wing to the fuselage might not be
particularly critical.

At issue is that in most modern sailplanes, the wing main spars of the
right and left wings connect to each other with no direct connection
to the fuselage. All bending loads are reacted by the spars
independently of the fuselage. The connections between the wings and
the fuselage, usually located near the leading edge of each wing and
also near where the flap or flaperon hinge axis meets the fuselage,
react only lift loads through shear, and drag and thrust loads through
compression.

The term that is usually used for the weight of everything but the
wings is "non-lifting weight" or "non-lifting load." This includes the
fuselage and pilot, the tailplane, and all equipment carried on board.
This is basically all of the stuff that contributes to wing bending
under flight loads.

Thanks, Bob K.



On Mar 1, 3:53*pm, Brian wrote:
Tony has it right, it usually has to do with the Bending loads carried through the wing attach fitting to the fuselage.

What you are really looking for is what the Maximum Design Fuselage Weight is for the glider. This is what controls the Maximum Pilot Weight. If the wings have an extra coat of paint and filler on them you can count this extra weight as Ballast weight. Unfortunately not all manufacturer actually publish the Maximum Fuselage Weight.

Brian