At 22:44 30 November 2017, Bob Kuykendall wrote:
On Thursday, November 30, 2017 at 9:00:07 AM UTC-8, Kevin Neave wrote:

So what does the main pin(s) count as?

Very incisive question! That's a good demonstration of how the mass of
non-=
lifting is a reasonable metric for the approximation of wing main spar
maxi=
mum bending moment, but is still just an approximation.

At issue is that using a maximum mass of non-lifting parts to limit the
win=
g spar bending moment is only valid when any mass added to the wings is
dis=
tributed spanwise according to the wing lift distribution. So if for
whatev=
er reason you add a bunch of mass to the inboard ends of the wings
(deplete=
d uranium root ribs maybe?), you can be within the allowable non-lifting
ma=
ss and within the load factor envelope, but still develop excessive wing
sp=
ar bending moment.

Bottom line: Respect mass limits to the spirit and the letter of the
publis=
hed values. And if you want a glider with lots of non-lifting mass
margin,
=
build an HP-24. The one we finished last summer has an empty non-lifting
ma=
ss of ~220 lbs (ready to fly with batteries, instruments, 24 ft^2 O2
bottle=
, etc) and a maximum of ~605 lbs, yielding a payload of ~385 lbs for
pilot,=
parachute, FES, electric self-launch, BRS, etc.

--Bob K.

Ever wondered why commercial jets now all hang the engines from the wings?
Lighter spars and the added roll inertia in the wing component reduces the
effect of turbulence on the passengers.