Spectrum Data 2

I assume you are looking at a fatigue spectrum for a wing box structural detail?
As far as I know there are different ways of obtaining such a spectrum.
One way goes like this:
First a number of Fatigue-flights (or missions) are defined. For example a short flight, medium flight and long flight mission.
Each of these are subdivided in a number of segments, say towing, taxiing, take-off, ascent, cruise, descent, landing, taxiing.
Again each of these segments are subdivided into a number of intervals. Each interval defines a flight or ground condition as an average load case plus a possible unit delta load case (fluctuation) plus the spectrum of fluctuation that goes with it. For example an interval of the cruise segment could be defined as a 1g cruise case at 20000 ft plus a 0g unit (10ft/s) lateral gust case plus the spectrum defining the exceedances of a certain lateral gust level (x ft/s) as function of altitude and distance traveled.
Per interval, the load cases translate directly (linearity of the structure) into average stresses and exceedance curves of fluctuating stress levels at the considered structural detail, in your case a detail at the wing box.

Another way goes like this:
First a number of Fatigue-flights (or missions) are defined. For example a short flight, medium flight and long flight mission.
Each of these are subdivided into a number of segments, say towing, taxiing, take-off, ascent, cruise, descent, landing, taxiing.
Again each of these segments are subdivided into a number of intervals. Each interval defines a flight or ground condition as an average load case plus a Power Spectral Density function of the fluctuating load case (This is found as the (Transfer Function )^2 * Power Spectral Density function of the excitation.)
The average load cases and the PSD of the fluctuating load cases again translate directly (linearity of the structure) into average stresses and PSD of fluctuating stresses at the considered structural detail. The fluctuating stress PSD’s can be integrated to get the Variance(RMS^2) stresses or RMS stresses. From these, Stress Exceedance curves of fluctuating stress levels can be found.

Regards

Onemorechance

Of course there is lots more to say on this topic. Because I was not at all complete in my previous post. I only commented on how you build up a mission spectrum for one flight.
This one flight being a long, medium or short mission.
Often a real flight is seen az a mix of these 3 missions, and the fatigue damage resulting from this one flight will determine how many flights are achievable until failure of the detail. Etc…

Onemorechance