Cargo tie down with load limiters

[Aeroing]

Hello!

I'm working on a tie down concept for cargo in a helicopter cabin.
I have been reading about load limiters on the tie down straps which allow having less straps or reducing the ultimate load capacity of anchor points on the cabin floor. This also means weight reduction.

My problem with this is the following:
I don't know how to use it in operation.
An army load master explained to me that for calculating a tie down they take the FAR29 crash accelerations (16g FWD, 8g SWD, 4g UPWD, 2g AFT). With these g-load factors and the weight of cargo they calculate the total forces to be absorbed in each direction, e.g. FWD with 1000 kg:
F = 16g * 1000 kg = 156960 N

Afterwards they make a tie down and calculate the load each belt can take in each direction by esimating the angle of each strap in relation to the longitudinal aircraft axis and the horizonal plane and considering the ultimate load capability of the anchor point it is attached to, e.g. anchor point with U ultimate load and angles ALPHA and BETA:
FORCE_FWD = U * COS(ALPHA) * COS(BETA)

Finally they sum up all forces of each strap in each direction and compare to the required force F. If SUM>F the tie down pattern withstands crash loads.

The problem with load limiters is that for this method I don't know which g-load factor to be used.
I calculated some specific cases and I obtain the g-load factor to be used out of a cargo displacement vs. g-load diagram. But as soon as I change the shape, position or weight of the load or the tie down pattern, the g-load factor also changes.
In operation this is a problem, because the load master can't calculate the g-load factor to be used each time he has to put cargo in the helicopter. It is too complicated and there is not enought time for it in operation.

My question:
Does anybody know about a practicable method for tie down calculation of cargo tie down patterns with load limiters?

I really appreciate your help.

Thank you.

 

[rb1957]

"which g factor to use" ... all of them ... the strap arrangement has to withstand each of the load cases, so you need the load in each strap for each load case (and possibly for combinations) ... you might start with a unit load in each direction then factor and superimpose.

but why load limiters ? the load is limited by the FMS restrictions and/or placards.

 

[IRstuff]

A load limiter ostensibly stretches the shock pulse energy in time, thereby reducing the peak shock amplitude, since the energy remains constant.

You need to look at the FAR shock requirement, which should include some sort of time duration, e.g., 11-ms half-sine, and see how the load limiter will modify the shock load to the anchor for differing cargo loads.

TTFN

FAQ731-376

 

[bf109g]

As I understand your question, you want to know how to incorporate load limiters in your calculations for strap tiedowns. The short answer is you don't.

The g factors don't change, load stays the same (ie you can't exceed the floor capability of the aircraft), straps stay the same and the tiedown fitting capability is the same. Tying down a load on an aircraft floor is a static problem just as your Army loadmaster described.

You don't want straps to stretch and have the possibility of a load shift. Pilots don't like that much and passengers need to be able to get to the exit without it being blocked by a pallet.