static loads and 3Sigma Loads

[midsidenode]

Have you ever seen someone add static load results to 3Sigma loads? In other words….say I exposed a structure to some aerodynamic and inertial loads and then obtained resulting fastener loads. Then I exposed the same structure to a power spectral density curve and obtained resulting fastener loads from that multiplied by 3. Is it (or is it not) appropriate to add those loads together?

 

[rb1957]

sounds super conservative. static limit loads define the complete loads to be applied, limit load is meant to be a once in the lifetime of the airframe. "PSD" implies dynamic (gust?) loads, which could pose the question what manoeuvre at the moment of the gust ?

but ... often gust curves are superimposed on flight envelopes so "someone" could interpret this as saying simultaneous, but I don't think that was meant at the time. I think it means this gust intensity at this speed, separate from manoeuvre loads; something like at Vc you have 50 fps gusts which may give you something like your static manoeuvre loads.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[midsidenode]

Thanks for the reply rb1957.
The PSD is not associated with gust - it's vibration. The static loads established from aero and inertia were ultimate loads.

 

[rb1957]

like a helicopter ? if a helicopter (well, you might have mentioned that) then I'd consult someone (or some text) who does helicopter loads

if vibration is causing significant internal loads for a typical plane, then I think you have other problems.

why "fastener loads" ??

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

MIL or Civil helos may very different criteria.

Regards, Wil Taylor
o Trust - But Verify!
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o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
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[Ng2020]

To me it sounds like the 3 sigma psd load is intended as an equivalent static criteria load. Sometimes this is done to simplify design/abalysis in applications where vibration dominates, rather than the flight/ground static loads.
If that's the case here then it's probably not appropriate to combine. Caveat- it's hard to say for certain without knowing the full story.