hook and sway brace loads

[midsidenode]

hello all,

I've calculated a set of inertia load values at a store C.G. (military, wing mounted store) - Does anybody have (or know where to go to get) a good technique or tool for calculating bombrack hook and swaybrace loads independent of aero loads?

thanks

 

[rb1957]

there's a mil-spec for that ... try the "assist" site

 

[49again]

What rb said.....I think it's MIL-STD-8591 or some such....

Steve R.

 

[midsidenode]

Yes, I'm pretty familiar with that STD. Nevertheless, it can only take you so far as to getting the store intertial loads @ the CG. It's not capable of providing info w/r to how those net loads work their way up into the bombrack hook and sway brace interface for specific flight maneuvers and different bombrack hook and sway brace geometry. I know certain companies use different tools or calculation methods to generate these interface loads. 'Was wondering if anybody was familiar with any of these methods.

 

[rb1957]

won't basic statics (and reasonable assumptions) get you there ?

i mean, you've got maybe 6 loads applied to the CG, and two reaction places

 

[KENAT]

Basic statics as rb1957 says.

http://www.dtbtest.com/PDFs/MIL-STD-8591.pdf

has a lot of the info you'd need in it.

Get yourself the set of AEII (Aircraft Equipment Installation Information) standards that give the relevant dimensional information for standard NATO stores and racks.

I think they may also have the dimensional information for standard bail lugs and possibly even the more exotic lug patterns such as MACE for Tornado or the T lug from I think it was Douglas.

There is also a standard for the bail lugs themselves if you need it.

You may also want to consider how you handle the G loads. I had always used them as quasi static loads, however at least one stress guy I worked with argued that the higher value G loads should be treated as an impulse or something like that as they weren't really sustained for long, the details escape me.

As I recall I did partial analysis based on just what 8591H has and knowing the C of G & mass of my store.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[midsidenode]

rb,
typical bombrack is 2 hooks and 4 swaybraces. yeah, you could try basic statics but i think you'd get into a big trial and error loop cause you just don't know in advance which sway brace pads would be in compression during the flight maneuver - and you certainly can't make a pad effective in tension.

 

[KENAT]

On most racks the sway braces are 'pre tensioned' when the store is loaded. You'd probably need the store AP/manual to know how tight, even then I don't recall torque renches being used, they're just done till it feels right I believe.

Based on doing a vector analysis of the G load I'd think you could get a good approximation of what load the sway braces are reacting.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[rb1957]

in fact, isn't that the way to determine how much pre-load to apply? ... assume that both arms of the sway bridge are effective and so determine the "tension" load. clearly, the arm can't react tension (as you state) but if it is pre-loaded to a higher level it reacts the "tension" load by relaxing the compression load. remember the scatter involved with a torque wench, about 1/3. this should result in a "standard" torque for the rack; if this is too big (unreasonable), then you're into the more involved sums of saying the arm is effective up to x% of the applied load, and the remainder is reacted with the arm ineffective.

really, it's quite like a pre-loaded tension bolt analysis ...

 

[KENAT]

I suppose the more correct term is the the sway braces are 'pre compressed'.

I don't think a torque wrench is usually used, but I could be wrong, like I said you'd have to check the loading manual for the relevant store/rack combination.

I'd basically assume the sway braces are tightened up till there is no slack but aren't under significant compression. So they will react additional compressive loads but aren't 'unloading' any in the other direction.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?