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?

 

[SWComposites]

fatigue in what part of the a/p? wing? fuselage? (pressurized?) empennage? control surface? all have different internal loading levels.
CFD just gives external loads on the a/p. do you have a structural FEM of the airframe?
what type of aircraft?

 

[LiftDivergence]

Well there is quite a bit of data out there regarding gust and maneuver exceedance curves for commercial aircraft.

Just try searching: "dot/faa statistical loads data for commercial aircraft"

For example: https://apps.dtic.mil/sti/tr/pdf/ADA378308.pdf

Theoretically if you have a CFD that gives you external loads on the airframe for some baseline or unit values of gust or maneuver you could use the statistical loads data to scale your CFD loads and develop a load exceedance curve which you could arrange into a spectrum that could be cycle counted etc.

I wouldn't be doing fatigue analysis with limit or ultimate loads (except for residual strength requirements).

Is this a modification you've made to a commercial aircraft?

 

[WKTaylor]

About fatigue spectrum development, use, analysis, etc MIL-Acft...

MIL-HDBK-530-1 AIRCRAFT USAGE AND SERVICE LOADS STATISTICS, VOLUME 1, CRITERIA AND METHODOLOGY AMSC

ASD-TR-80-5037 DEVELOPMENT OF AN AIRCRAFT MANEUVER LOAD SPECTRUM BASED ON VGH DATA

Prior E-T thread on this subject with other references... https://www.eng-tips.com/threads/fatigue-load-spectrum.491489/

 

[SWComposites]

ILouie - you posted a bunch of similar questions back in August. Why are you starting over again with a vague question? This is a student project, correct?

 

[lLouie]

fatigue in what part of the a/p? wing? fuselage? (pressurized?) empennage? control surface? all have different internal loading levels.
CFD just gives external loads on the a/p. do you have a structural FEM of the airframe?
what type of aircraft?

There is a metal connection connecting the wing to the body. Yes I do, I did the static analysis before the fatigue, however I do not have any flight data.

 

[lLouie]

Well there is quite a bit of data out there regarding gust and maneuver exceedance curves for commercial aircraft.

Just try searching: "dot/faa statistical loads data for commercial aircraft"

For example: https://apps.dtic.mil/sti/tr/pdf/ADA378308.pdfI need to update the design according to the g forces that other fighter planes are exposed to

Theoretically if you have a CFD that gives you external loads on the airframe for some baseline or unit values of gust or maneuver you could use the statistical loads data to scale your CFD loads and develop a load exceedance curve which you could arrange into a spectrum that could be cycle counted etc.

I wouldn't be doing fatigue analysis with limit or ultimate loads (except for residual strength requirements).

Is this a modification you've made to a commercial aircraft?

I have done fatigue analysis before for UAV. Now, there is a fighter UAV which is different other, because it can do maneuver. Since there is not any flight data, I need to update the design according to the g forces that other fighter aircraft are exposed to

 

[lLouie]

About fatigue spectrum development, use, analysis, etc MIL-Acft...

MIL-HDBK-530-1 AIRCRAFT USAGE AND SERVICE LOADS STATISTICS, VOLUME 1, CRITERIA AND METHODOLOGY AMSC

ASD-TR-80-5037 DEVELOPMENT OF AN AIRCRAFT MANEUVER LOAD SPECTRUM BASED ON VGH DATA

Prior E-T thread on this subject with other references... https://www.eng-tips.com/threads/fatigue-load-spectrum.491489/

Thank you so much

 

[lLouie]

ILouie - you posted a bunch of similar questions back in August. Why are you starting over again with a vague question? This is a student project, correct?

I think this is not vague question.
Although I posted two times different topics (vibration) in student forum, noone answered the question.
Additionally, There was flight data on the previous plane and my job was easy, but now there is no flight data. There is only the design. When I look at many articles, yes, there were those who shared the loads, but it only remained on the graph.

 

[LiftDivergence]

I have done fatigue analysis before for UAV. Now, there is a fighter UAV which is different other, because it can do maneuver. Since there is not any flight data, I need to update the design according to the g forces that other fighter aircraft are exposed to

In that case the reference data I mentioned is not going to help you.

It sounds like this is not really a question of how to do fatigue analysis, but rather what you're asking is effectively:

"How do I develop an operational loads envelope and spectrum for this UAV?"

Usually this would be accomplished by instrumenting a test article / test flight. Or by making analytical estimates of the mission.

You need to:

• understand the mission profile and divide it into nominal segments similar to what you wrote above
• understand the loads / free bodies for those segments under nominal conditions. What are the load magnitudes you expect to occur every mission? And how to those loads resolve as internal stress?
• For things like taxi, roll, landing, etc. what type of runways are you on? If there is no instrumentation based measurements you are going to have to make estimates of the G level exceedances you can expect.
• For gust and maneuver, again you need to define the mission... where are you flying, what's the environment, how long is a flight, etc. These are not questions we can answer

Depending on the product there is usually a whole team of engineers acting as a loads group to do this.

 

[3DDave]

Since this is for a weapon of war, the fatigue is going to be low cycle fatigue where the failure is at 50% to 100% of the static loads.

The majority of information is for aircraft that will see multiple 10,000s of hours; yours will see a few hundred at most. Double the worst loads from maneuvering and landing loads and call it a day.

 

[rb1957]

I get (as I suspect the others do to) what you are asking ... you want to develop a flight profile for a military UAV. There is a lot (and I mean a lot !) of data out there from MIL-SPEC (something like MIL-SPEC 6688 and a host of similar) that are used to define fatigue spectra in design development. But this is for aircraft, and your UAV could be very different, like much higher g manoeuvres, so use at your risk. so what to do ? Make a conservative estimate for now ...

how long is a flight ?
how many limit loads per hour ? guess 1, 10?
Don't worry too much about the global affect of different flight phases. The GAG is about all you can guess now. Everything, and I mean everything, is a guess (at this stage).
For each component figure out the GAG cycle, to say limit load. Forget the gust impact on the wing, look at limit manoeuvre. For flaps, use the limit landing loads.
If you think a limit manoeuvre 1/flt (1/hr?, 10/hr?) is too severe the 50% limit ... all is just guesswork (guesses that you agree with your contracting agency).

Ok, now you'll say "we don't have a contract, we're just designing a UAV and want brochure numbers"

This is not the thing to be discussed in any depth here. We know little of what you're doing (in detail). The best approach is to approach a local university or manufacturer or possibly a freelance designer/company and take them under contract. But all will be basically guesswork (since there is no data, reliable data, to work with).

So what are you to do ... make reasonable guesses now, do some instrumented load surveys, fly an instrumented UAV for some time, get some real data, and then you can make some sensible calculation. I agree with 3DD, as a military "fighter" the fatigue life is likely to be cut short be some intended or unintended mechanical disassembly event.

 

[lLouie]

Since this is for a weapon of war, the fatigue is going to be low cycle fatigue where the failure is at 50% to 100% of the static loads.

The majority of information is for aircraft that will see multiple 10,000s of hours; yours will see a few hundred at most. Double the worst loads from maneuvering and landing loads and call it a day.

Thank you so much.
Actually, although it is not like an F22 or F35, it's fatigue life can be +8000 hours after the design change.

 

[lLouie]

I get (as I suspect the others do to) what you are asking ... you want to develop a flight profile for a military UAV. There is a lot (and I mean a lot !) of data out there from MIL-SPEC (something like MIL-SPEC 6688 and a host of similar) that are used to define fatigue spectra in design development. But this is for aircraft, and your UAV could be very different, like much higher g manoeuvres, so use at your risk. so what to do ? Make a conservative estimate for now ...

how long is a flight ?
how many limit loads per hour ? guess 1, 10?
Don't worry too much about the global affect of different flight phases. The GAG is about all you can guess now. Everything, and I mean everything, is a guess (at this stage).
For each component figure out the GAG cycle, to say limit load. Forget the gust impact on the wing, look at limit manoeuvre. For flaps, use the limit landing loads.
If you think a limit manoeuvre 1/flt (1/hr?, 10/hr?) is too severe the 50% limit ... all is just guesswork (guesses that you agree with your contracting agency).

Ok, now you'll say "we don't have a contract, we're just designing a UAV and want brochure numbers"

This is not the thing to be discussed in any depth here. We know little of what you're doing (in detail). The best approach is to approach a local university or manufacturer or possibly a freelance designer/company and take them under contract. But all will be basically guesswork (since there is no data, reliable data, to work with).

So what are you to do ... make reasonable guesses now, do some instrumented load surveys, fly an instrumented UAV for some time, get some real data, and then you can make some sensible calculation. I agree with 3DD, as a military "fighter" the fatigue life is likely to be cut short be some intended or unintended mechanical disassembly event.

Yes, I you want to develop a flight profile for a military UAV. I will look at the MIL-SPEC 6688.
Maneuver loads are different than others, because much more load is created and the duration of the maneuver is also important.

Actually, I want to find out, for a fighter UAV with a limit of 6g, it can do the maneuver in 3 seconds and 100 times in its lifetime. It can see 5g 200 times while going at cruising speed etc.

I will work for these condition.
Thank you so much.

 

[rb1957]

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.

 

[WKTaylor]

'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)

 

[rb1957]

yes, that`s it ... some decades since I used it ... there`s a whole series of reports on various loads for preliminary design decades ago. But I have suspicions about how applicable they are to a UAV. You`ve pulled a series more directly applicable.

``The Garden of Eden was just outside Moscow. A very nice place.`` Chekov, on Star Trek ...

 

[3DDave]

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

 

[WKTaylor]

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)

 

[lLouie]

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

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.

Thank you very much.
It is totally up to me.
I will continue to search.