Interface loads for LRU

Yes, the reaction forces should always be derived from statics. The starting point, however, are "prescribed" load factors. The source of those factors is, fundamentally, the FAA design and certification requirements. Somewhat equivalent to those are test standards such as DO-160, but I will put that aside for now because most of those tests are dynamic - and I don't think that's what you're asking about.

Back to the static loads: the monitor is destined to be installed in an aircraft. Items of mass, no matter what they are, must be restrained in a manner to prevent them from coming loose and posing a threat to the occupants of the aircraft under all conditions, including maneuvers, gusts, emergency landings, and so on. Obviously, the video monitor must not break loose! The FAA has defined what load factors apply to each situation, some subject to analysis by the aircraft designer, but in all cases you can use a "safe" value that will ensure you have a structure strong enough. The load factor will be multiplied by the weight of the unit. That will be the load, in a specific direction. Now the reaction forces can be derived from simple statics, and stress analysis will give you margin of safety, provided you know the strength of the mounting tray and hardware. If you don't know the strength of the hardware, then you're using the wrong hardware. Or maybe you skip detailed stress analysis by carrying out load tests (which is a good option if done systematically and observed carefully).

It's not really that simple, but that's where you have to start. If you know that your equipment is going to be installed in one specific aircraft, and no others, then you may want to research the aircraft's type design data, which will tell you its certification basis, and look up the design requirements from that section of the FAR's. There you will find the load factors I mentioned. Maybe that sounds easy, but it actually contains a large number of non-obvious steps to be taken. So I should step back a bit.

It sounds like this research process has already been done by your colleagues, they just haven't taken the time to show you yet. Hopefully what I've explained is enough to help you find what you need in the previous test plans. In time you will become more familiar with the terminology which will help a lot. If not, you're welcome to ask more questions.

There is a much more comprehensive explanation in FAA Advisory Circular 43.13-2b, should you care to look that up. It can be downloaded from the FAA website. Even if you are not in the USA, the process is the same in any other jurisdiction I know of (Europe, Canada, Australia, etc.).


STF

this is likely to be a pretty light piece of equipment, so the crash loads (25.561) will probably not be that high. so to make a robust structure, consider ground handling/abuse loads ... say 200 lbs anywhere of the piece (that's a defined load, but usually company's have some load they like)

Quando Omni Flunkus Moritati

Thank you Sparweb.

I know that the interface load on the unit will be 9.0 Forward, 8.6 Down, 5.4 Up, 4.0 Side (+/-), and 1.6 Aft per Boeing requirements with a fitting factor of 1.33 and a 15% growth factor.

The LRU max. weight is 3.2 lbs.

No abuse loads are required by Boeing due to the size and the way LRU is installed on A/C.

The LRU has four attachments on each corner (some have six or more) but I do not know how to calculate the single vector loads or combine vector loads with out the provided excel macro and I want to understand how this is performed. I do not want to relied on excel macro.

Can some one guide me? I have asked our principle engineer but he just says, "it is basic statics".

Calculating moment arm does not give me the same results as excel macro.

Thank you.

...per Boeing requirements ...

Click to expand...

Boeing refers to the FAR 25 for all of the factors you referred to, except the growth factor (but that's a good idea when you are in a product development lab).
Abuse loads are worth investigating - but you and RB1957 see this from very different perspectives. You are sending a part out which has to be strong enough to survive in the world - probably with a warranty that doesn't cover abuse.

Just do the basic sum of moments and sum of forces static analysis, double-check it, and then stand by it. The macro is probably correct but it could be wrong under certain conditions; it sounds like you can't see "inside the black box" that does the calculations. Give the principal engineer a clear and thorough analysis, with FBD's, showing your work and conclusions, but without the macro. If he rejects it he'll have to give his reasons.

If you don't know how to do a static analysis without Excel, then that's a completely different problem.


STF

I have completed the Moment arms and Forces with a FBD for all four attachments.

Thank you SparWeb.