×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Cargo tie down with load limiters

Cargo tie down with load limiters

Cargo tie down with load limiters

(OP)
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.
 

RE: Cargo tie down with load limiters

"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.

RE: Cargo tie down with load limiters

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: Eng-Tips.com Forum Policies

RE: Cargo tie down with load limiters

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources