Max Weight of Non Lift Producing Components

In addition to max gross weight, some glider manuals specify a max weight for non lift producing components. But I haven't found much discussion of the significance of exceeding the max or information that would allow one to calculate the weight of non-lifting components (like a breakout showing the weights of the wings, etc.)

Would appreciate some discussion by those in the know.

Tom

the only way to know is to weigh the wings separately. That is what we did the last time we did a W&B on the Cirrus. That was part of incorporating the Tech Note from Schemmp Hirth to increase the weight of non lifting parts.

Some airplanes, especially twin engine airplanes, have a maximum zero fuel weight which is a similar concept.

On Wednesday, November 29, 2017 at 10:13:28 PM UTC-6, Tony wrote:
the only way to know is to weigh the wings separately. That is what we did the last time we did a W&B on the Cirrus. That was part of incorporating the Tech Note from Schemmp Hirth to increase the weight of non lifting parts.


Or, if you want to know how much the fuselage and tail weigh, you can weigh them rather than weigh the wings and subtract that from the empty weight. The former gives you the weight you really want to know. The latter assumes the previous empty weight is still accurate, and there there have been no changes made to anything in the fuselage or wings.

Steve Leonard

We weighed everything together and then the wings. Felt that was easier than trying to hold the fuselage with tail attached upright and on the scales.

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

KN

At 16:07 30 November 2017, Tony wrote:
We weighed everything together and then the wings. Felt that was easier
than trying to hold the fuselage with tail attached upright and on the
scales.

On 11/30/2017 9:53 AM, Kevin Neave wrote:
So what does the main pin(s) count as?

KN

At 16:07 30 November 2017, Tony wrote:
We weighed everything together and then the wings. Felt that was easier
than trying to hold the fuselage with tail attached upright and on the
scales.

Try flying without it/them in place and get back to the group how it went!

More seriously, and structurally, it/they are part(s) of the wing.

Bob W.

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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 maximum bending moment, but is still just an approximation.

At issue is that using a maximum mass of non-lifting parts to limit the wing spar bending moment is only valid when any mass added to the wings is distributed spanwise according to the wing lift distribution. So if for whatever reason you add a bunch of mass to the inboard ends of the wings (depleted uranium root ribs maybe?), you can be within the allowable non-lifting mass and within the load factor envelope, but still develop excessive wing spar bending moment.

Bottom line: Respect mass limits to the spirit and the letter of the published 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 mass 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.

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.

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.

On Thursday, December 7, 2017 at 5:59:55 PM UTC-8, Dan Daly wrote:
...Also, the horizontal stab and elevator are lifting.

I don't think that's the generally accepted interpretation. It is definitely not the interpretation I apply when I publish W&B data.

Yes, they produce lift, but in most (but not all) flight regimes it's downward, not up.

--Bob K.