On Tuesday, March 9, 2021 at 3:14:10 PM UTC-8, Derry Belcher wrote:


Food for thought: I cannot find a definitive answer to the question of why certain manufacturers state both a maximum all up weight (MAUW) and maximum non-lifting load. It seems more prevalent with German designs to state these two limitations and even the design book I have (Design of the Aeroplane by Darrol Stinton) has no reference to this...

From a sailplane designer's perspective:

I cannot speak for my European colleagues, but I specify a maximum mass of non-lifting components (which most certainly _does_ include the horizontal tail) because it drives the most critical structural metric in my glider--the wing bending moment. That in turn drives the tensile and compressive stresses in the wing main spar upper and lower caps, and also the shear developed in the main spar shear web.

A simplified way of looking at it is to understand that the wing basically carries itself. So if you make the wing uniformly heavier, it does not increase the bending moment incurred under high-g maneuvering. Let's say for whatever reason you add 100 kg of paint and filler to the wings. Because the mass distribution follows the area distribution, and the area distribution is about elliptical, the stresses in the wing spar are not increased. Except of course for the landing case where the wings are supported by the fuselage and landing gear. But those are areas where adding extra margin has a much more modest cost than adding extra bending strength and stiffness to the wing spar.

For this reason, I am fairly open to increasing the maximum gross weight of my gliders by adding batteries and water ballast inside the wings, but there are definite limits to increasing the mass of the non-lifting parts, and there I hold the line.

--Bob K.