Aircraft - Fatigue Load Spectrum 2

[lLouie]

Hi,
I will need to determine a fatigue spectrum based on known or conservatively estimated load exceedances for the aircraft and its flight missions.
Taxi - Climbing - Cruise - Maneuver - Descent - Taxi
After the CFD results, I can know the limit and ultimate load of aircraft. I can calculate the MTOW for aircraft.
However, there is no any flight, but I want to change some design, if the fatigue occurs on design.
How can I perform a fatigue analysis for the GAG profile using both MTOW and limit/ultimate loads?

 

[lLouie]

If it's a fighter UAV it's not surviving 8000 hours. If it can last that long it weighs too much.

You are right, it's weigh is too much. Almost 5.5 tonne.

 

[lLouie]

Is this UAV considered...

1-use expendable
limited-re-use, expendable
unlimited re-use, recoverable/expendable
unlimited re-use, recoverable
unlimited re-use, optionally manned
optional ISR or ordnance payload
small [hand held/launched]
medium [crew-served, catapult launch or landing gear takeoff]
large [enough to carry an occupant]
giant [transport Acft or helo]

Also Related...

ASTM F3600 STANDARD GUIDE FOR UNMANNED AIRCRAFT SYSTEM (UAS) MAINTENANCE TECHNICIAN QUALIFICATION

AC107-2 SMALL UNMANNED AIRCRAFT SYSTEMS (SUAS)

ICAO CIRCULAR 328 UNMANNED AIRCRAFT SYSTEMS (UAS)

It will be unlimited re-use, recoverable.
Thanks sharing for the source.

 

[lLouie]

'MIL-spec-6688' caught me off guard. Head scratch... Oh, yeah...Perhaps You are referring to...
MIL-A-8866 Airplane Strength and Rigidity Reliability Requirements, Repeated Loads, Fatigue and Damage Tolerance ???

The following might also be useful...
ASTM F3178 Standard Practice for Operational Risk Assessment of Unmanned Aircraft Systems (UAS)
ASTM F3298 Standard Specification for Design and Construction of Lightweight Unmanned Aircraft Systems (UAS)
ASTM F3478 Standard Practice for Development of a Durability and Reliability Flight Demonstration Program for Low-Risk Unmanned Aircraft Systems (UAS) under FAA Oversight
ASTM F3657 Standard Specification for Verification of Lightweight Unmanned Aircraft Systems (UAS)
ASTM F3686 Standard Practice for Production Approval of Unmanned Aircraft Systems (UAS)

Thank you, I will look at

 

[lLouie]

Yeah, there is really nothing to tell you specifically what to do, which loads to use. You can define these yourself ...
what is a lifetime ? hours and or flights
what is a flight ? typically a spectrum of flights, varying in length and intensity.
you might say a lifetime is 1000 flights. (nothing to tell what this is, only your definition)
there many be 100 flights of type 1 (1 hr duration, maximum manoeuvre 6g)
you could define flight type 2 ... 1 hr duration with 10 6g manoeuvres. This will be 1 GAG (6g to 0, on ground, = flight type 1) and 9 cycles 6g to 1g. This shouldn't be much different to flight type 1, but if it is you've learnt something.
and 900 flights of type 3 (2hr duration (so a lifetime is 1900 hrs) with 1 4g manoeuvre).
then check the reality of these assumptions ... 1000 flights ... 10 flights a day for 3 months ... that sounds like a lot. You might say this is my factored lifetime, ie you advertise 100 flights (so a safe life factor of 10, high but reasonable with all the guesswork). You may be able to get some statistics from an ongoing active environment !! You may say "I don't need a safe life factor ... if the UAV fails in fatigue it is a disappointment, not a loss-of-life accident".

What fatigue assumptions are other UAV companies using ?
Company XYZ may say "our UAV has a fatigue life of 100,000 flights" so you may be pressured to say something similar (or better). Define a flight to achieve this !?
But then you may say "I want something real" ... so define a severe spectrum (in addition to the benign marketing one).
Ukraine folks may be able to tell you "our UAVs are lucky to last 10 flights ... fatigue is not really an issue for us."
UAV covers a large field of aircraft, from quad-copters to Global Hawks and Predators ... so compare to something similar to yours.
I would strongly recommend putting loads monitoring onto these and seeing what the real world can tell you.

Does maneuver time have any significance for fatigue life? For example, will it make a difference in damage if an aircraft moving at 5g cruise reaches 6g in 10 seconds or 5 seconds? After all, it will have completed 1 cycle from 5g to 6g.

 

[rb1957]

yes, maneouvre loads have some impact on fatigue life, but typically manoeuvre cycles are much smaller than the GAG cycle and their impact on fatigue life is small ... smaller IMHO than the guesswork you've doing to assume a spectrum.

but like I said the spectrum is yours to define. you could define the manourve as +6g to -6g ... in my experience (commercial transports) manoeuvres are +ve to 1g flight and 1g flight to -ve (as a separate cycle) ... with a bunch of nuance I don't want to go into here. You can go simply (I'd recommend at this early stage) and just +6g to -6g ( the -ve of +6g), or with more complication you could go (1g flight +6g) to (1g flight -6g). Linking the +ve and -ve manoeuvres is very conservative, and greatly simplifies the problem.

 

[GregLocock]

6g is similar to the seen forces when a car rips its suspension out in a deep pothole. That seems a bit over the top. Presumably with a drone you are very much limited to the thrust/mass ratio in flight.