Dear PeterKo,

adding a harness induces restraints (on the passenger), hence, this constraint force has to be taken by some medium. Therefore, the floor will be experiencing this additional load.

Ramesh75,
Please correct me if I'm wrong, but how can adding a harness to a seat add additional force to the floor structure, isn't all of the force taken out in the floor structure anyway?  

i.e. Assuming that the harness is attached to the seat, I think all it will do is possibly change the seat track reactions on the floor structure (i.e. LHS seat track takes more loading instead of RHS). I don't see how adding a harness will induce "additional load" into the floor structure.  Maybe it induces an alternate load path, yet equal loadpath?

Thanks,
'Nert

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'Nert

without a shoulder harness, the seat occupant's upper body would move forward, dissipating some inertia.  whereas with the shoulder restraints the force would be exerted to the restraint attachments.  the mass of the passenger remains constant.  so if we said the passenger was an in elastic solid then the resulting addition of the shoulder restraints would only cause a change in the resulting forces locaiton on the seat, not an overall amount of change.  however some force would turn in a moment and create a higher stress on the floor.  i do think though that since the passenger is elastic, how he restained effects the amount of force exerted on the floor.  but then again i could be totally wrong.

Yes jdswell,
That makes sense - if the pax upper body is restrained such that movement forward is not possible, then it seems to me that the line of action of forward momentum may be higher (relative to the pax with only a lap-belt on).  

If the reaction is to be taken out via the seat tracks, then I would think that magnituded of the loading per seat leg may be increased.  

I guess the only thing left is to figure out where the usual lap-belt-body cg is vs where it is when the whole body is restrained.

Good deal.
Thanks!

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Nert

You all seem to missing the point.  The question is does the addition of a shoulder to a passenger seat have any effect on the seat leg reaction.  The answer is yes, and here is why.  Under a decelleration the mass of the seat and passenger cause an overturning moment on the seat.  The magnitude  of theis moment is equal to the combined mass of the passenger and the seat x the height of the c of g above the ground.  When you are dealing with a seat equiped only with a lap strap the c of g of the entire body is acting through a single point which is located 10.5 inches above a compressed seat cushion (FAA standard).  If you now add a shoulder harness you must now proportion the passengers mass between the lap strap and the shoulder harness. FAA guidelines suggest a 60/40 lap / shoulder is acceptable.  The overturning moment of the seat has now increased dur the the heigher application of load ie. the reaction between the shoulder harness and the seat back.  It the overturning moment increases, so does the seat leg reaction and the seat track/floor reaction.  This reaction moves through the floor beams, beltframes and is finaly reacted in the skin of the aircraft.  The effect of adding shoulder harnesses is considerable.

Nigel Waterhouse
n_a_waterhouse@hotmail.com
 
A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996.  Currenty,living in British Columbia,Canada, working  as a design engineer responsible for aircraft mods and STC's.

Nigel,
Essentially jdsewell was addressing my first question, not PeterKo's question.

I figure, though, that all of my questions could have been answered if I would have taken the time to draw up a simple free body diagram.


Cheers,



 

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Nert

Modern aircraft seats use designs that are "energy absorbing" or "load limiting."  This means, basically, that there is an element in the seat that deforms plastically (i.e., with little elastic rebound)under some defined transient impact load.  When the loading tries to exceed that load, the seat deforms, absorbs some of the impact energy, and limits the force transmitted through the structure.  Displacement of the seat is the penalty for limiting the force, but the displacement is usually very small (under an inch or so) for the transient forces of a crash.  This is the technology that has enabled "16 g" seats to be installed in "9 g" transport aircraft without overloading the aircraft floor.  If you use load limiting, the loads from the restraint to the attachment of the seat to the aircraft can be controlled, and need not increase.

Excellent info and insight Crash Engineer.
Please sign up with the forum!  I did, and it was painless :)

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Nert