Max Weight of Non Lift Producing Components

On Friday, December 8, 2017 at 1:45:06 PM UTC-8, James Thomson wrote:
At 01:59 08 December 2017, Dan Daly wrote:
On Thursday, December 7, 2017 at 7:27:02 PM UTC-5, Jim wrote:
I admit I am baffled by "max weight of non-lifting parts". I
understand
=
the issue of spar bending moment limits, and I think I understand that
weig=
hing a glider without its wings will give the current non-lifting (i.e.,
no=
n-wing) weight, but I have a feeling there are subtle things here that I
do=
not understand.

I've rigged an ASK-21 and its wings are HEAVY! I haven't weighed
them
so=
I don't know just how heavy they are though. Certainly felt like more
tha=
n 100 lbs each. Ask me how I know. I'll guess (I know, don't guess)
each
=
wing weighs 150 lbs. Likely more. If I subtract 300 lbs from 780 and
then=
subtract that result from 1320 I'm still short of 902 - which I suppose
is=
a good thing.
=20

Glider cockpit load limits are set by the lowest value determined by:
Max AUW
Seat strength limits
Max wt of non-lifting parts
Forward CG limit

For a ASK21 for which I have the actual weighing results:
Max AUW 1320 lbs
Empty weight, fully equipped, 884 lbs
Therefore disposable load is 436 lbs
Seat strength limit is 242 lbs, so with a max weight pilot the other pilot

cannot exceed 194 lbs

Each wing weighs 219 lbs, max weight of non-lifting parts is 904 lbs.
Fuselage, tailplane, etc wt is 884 - 438 = 446 lbs. Hence max load
considering only non-lifting parts limit is 904 - 446 = 458 lbs. The AUW

limit takes precedence.

On this aircraft forward CG limit did not determine max cockpit load.
Min cockpit load is set by aft CG limit.

This whole issue has to do with the design limits of the airfoil design. The non-lifting parts impart a bending moment at the wing root. This is, of course, anticipated by the designer, but has a design limit imposed upon it. Hence, the limit on the non-lifting components.

Tom