On Monday, 11 December 2017 at 15:45:27 UTC+13, 2G wrote:
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


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.
The only explanation I can come up with is that the designer has to have a maximum load figure for designing the fittings that take the shear loads from the wings to the fuselage in flight and in heavy landing loads. On fibreglass gliders these are usually four metal pins that are either part of the wings or the fuselage that automatically fit into the applicable mating receptacles on the other component. The big rigging pin you put into place usually only holds the wings together to stop them coming apart and does not take the flight shear loads of wing to fuselage. In the case of older gliders such as the Schleicher Ka6, two big main pins are inserted that join the two wings and carry the wing bending loads from the upper and lower spar caps and there are still fixed shear pins that take the wing to fuselage loads in flight as well as forward drag-spar pins for each wing.
My rationale for the two limits is that from a design point of view, the fittings are capable of taking the maximum non-lifting load figure multiplied by the G load design requirements multiplied by the normal design safety factor of 1.5 (which could be what the BGA 3% is eating into for their overweight allowance for aging gliders unless limiting the max G loading - prohibiting aerobatics is one way but only goes part way to satisfying the Vn load factors associated with gust loads to airspeed) but during certification the gliders were only tested up to a MAUW figure determined to be sufficient at the time, hence when we reweigh a glider, we are limited to the certificated AUW first and foremost, but must not exceed the non-lifting figure which includes the payload (pilot etc). The dilemma we have is that the wings on a glider represent a major part of the structure and seem to get heavier with age for whatever reason, that is sometimes hard to understand, yet the wings alone do not essentially impart any detrimental bending loads to the spars or fittings in flight but technically speaking we are bound to adhere to the MAUW even when the maximum non-lifting figure says we could carry a heavier pilot from a structural standpoint.
DB