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 weighing a glider without its wings will give the current non-lifting (i.e., non-wing) weight, but I have a feeling there are subtle things here that I do not understand.

For example, the ASK-21 POH lists the following weights:

Empty weight: approx 780 lbs
Max all-up weight: 1320 lbs
Max weight on non
lift producing members: 902 lbs

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 than 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.

Oh well. This all just makes my head hurt. Maybe what I should be doing is adding the weight of pilots, water, parachutes, etc. to 780 to ensure that the total is not beyond 1320. I suppose I already do this when I use Foreflight to calculate W&B.

TIme for a nap.

1320-904=416 lbs. I'd guess closer to 180 per wing when you consider an ASG-29 wing weighs 140. Also, the horizontal stab and elevator are lifting. I don't know if you count the 22 lbs per wing root baggage compartment or not... I think that's lifting too. So, 360 lbs of wing, 44 lbs of baggage (worst case) which leaves about 12 lbs for tail bits.