Limit or Ultimate Load 4

[airmail]

Hi collegues!

Here goes my question:

Is there any criterion (in order to prove a structure) about using limit or ultimate loads when the load case is a "fail safe case", it means, when a structural element is broken?

Thanks in advance.

 

[aerodog]

There is no simple answer.

Check FAR 25.571
Damage-tolerance and fatigue evaluation of structure.

 

[vorwald]

I'm not sure on the question, but limit loads are the maximum loads expected in service (FAR 23.301(a)), and ultimated loads are limit times 1.5 (FAR 23.303). Then you can have an ultimate and limit margin of safety.

I tend to use ultimate allowables when doing crash analyses.

 

[airmail]

Vorwald, you are right in the ultimate and limit loads definitions. And it is true that you can have several margin of safety depending on the kind of loads analyzed, but it is common to take the lower as margin of safety of a structural item.

The thing is that ultimate loads are not "true" loads, because are obtained from limit ones (maximum in the aircraft life) times 1.5. Fail safe cases are also "extreme" loads (the fail of a structural item), so the question is that if it is necessary (or required by authorities) to prove the structure combining both kind of loads.

 

[Sparweb]

Ultimate loads are not "fake".

http://aviation-safety.net/database/record.php?id=20011112-0

Margins of safety have a purpose: insurance. Insurance against abuse, insurance against aging, insurance against the unknown. Engineers trade this insurance for performance or utility. Aero engineers have been given the liberty to shave the insurance down very thinly: 1.5. Many other industries expect MS > 4. Some > 10.

"Fail safe" is just another way of trading insurance for utility. Now, however you assume something HAS failed, and deal with the consequences.

Sec. 23.571 Metallic pressurized cabin structures.

For normal, utility, and acrobatic category airplanes, the strength, detail
design, and fabrication of the metallic structure of the pressure cabin must
be evaluated under one of the following:
(a) A fatigue strength investigation in which the structure is shown by
tests, or by analysis supported by test evidence, to be able to withstand the
repeated loads of variable magnitude expected in service; or
(b) A fail safe strength investigation, in which it is shown by analysis,
tests, or both that catastrophic failure of the structure is not probable
after fatigue failure, or obvious partial failure, of a principal structural
element, and that the remaining structures are able to withstand a static
ultimate load factor of 75 percent of the limit load factor at VC,
considering the combined effects of normal operating pressures, expected
external aerodynamic pressures, and flight loads. These loads must be
multiplied by a factor of 1.15 unless the dynamic effects of failure under
static load are otherwise considered.

The analysis requires understanding the process of the failure. It deserves an analysis of deflections and energy absorbtion, not just the beginning and final static states of being.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[airmail]

SparWeb, thank you for your necessary remark.

I didn't know the probable cause of the accident of AA's A300. The question that comes to me now is what "American Airlines Advanced Aircraft Maneuvering Program" mean.

Because I suppose that Airbus had designed their vertical tail plane able to support any rudder pedal input (at limit load and, afterwards, at ultimate load...). I mean, I don't understand why a rudder pedal input involve loads higher than ultimate ones...

Thank you.

 

[aerodog]

SparWeb,

Ah are you talking about Margin of Safety
or Factor of Safety???

 

[airmail]

aerodog, as far as I know:

(Margin of Safety)= (Reserve Factor) - 1

Being Reserve Factor==> RF=Pallowable/Papplied


I suppose that what you call Factor of Safety is the same as Reserve Factor, isn't it?

So, at the end, we are talking about the same thing.

 

[Sparweb]

Yes, aerodog, I occasionally use the term MS when SF would be more appropriate. Adding 1 to a safety margin gives you safety factor, as airmail points out.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[rb1957]

there's also the "factor of safety" in FAR25.303 ... what we usually call the ultimate factor

 

[harrisj]

For clarification between MS and SF - does that mean that where Sparweb stated that engineers can shave the margin down to 1.5, he actually means 0.5 for margin? (ie safety factor is 1.5).

I'm not trying to be picky, but I'm used to much bigger factors, where adding or subtracting 1 doesn't make a lot of difference. On lifting cables, a factor (reserve) of 12 is quite usual.

John

 

[rb1957]

to harrisj

yes in aerospace we design to the load. The FARs and/or company guidelines define the airplane loads (like manoeuvers), the aero. guys tell us structures guys the external loads applied to the plane, and we calculate the internal loads. Limit load is the maximum load expected in service, and the structure is be designed to ultimate load (1.5*, "factor of safety"). Then the RF (reserve factor) is how much the allowable load exceeds the applied load, and this can be 1.00 or even 0.99.

What this does mean is that we typically have LOTS of cases, covering all the conceivable situations, and lots of "ambulance-chasing" requirements !

 

[Sparweb]

Just looking at the bigger picture...
HarrisJ, you have understood me. The safety margins (and safety factors for that matter) are very thin in the aircraft business, when compared to what are used in most other industries. But there are several points to consider when comparing aircraft to other machines:

1) The loads on aircraft structures are applied under strictly controlled operating conditions (pilots must be conscientious),
2) Aircraft are very weight sensitive. To design an aircraft with the safety factors used by automotive engineers would make it incapable of flight (and cars are weight sensitive, too),
3) Aircraft wings and fuselages penetrate through air, a very forgiving medium, unlike, for example, a bulldozer through gravel. Most other machines are designed for loads applied when they contact other solid surfaces/structures.

Each industry has developed standards that suit their normal practices. Hoisting cables, for example, must be able to absorb shocks from dropped loads, friction on sheaves, sharp bends, etc, and all with a covering of rust from the salty sea air... no wonder a safety factor of 12 is imposed.

By the way, RB1957, I've always thought of the FAR 25.303 definition of Factor of Safety to be worded awkwardly. Those who find the differences murky could start with reading Bruhn, Chapter A4, and then going on to machine design textbooks like Shigley's and reading what they have to say about safety factors. The assumptions that go into selecting appropriate safety factors are different.


Steven Fahey, CET

 

[GregLocock]

In cars we use 'safety factors' of about two, often, on worst case loads, which in themselves are about double what any reasonable user would see. (That is, normal peak load say 3 kN, worst measured 6, design for 12). You can't afford to do that on all parts, just those that would be immediately dangerous if they failed.

In comparison, I was once working on a marine project where the loads were well understood, and there was no danger to life, and we were able to simulate the main life-limiting load very accurately. The weak point in the system was proof tested on every part. So we ended up with a safety factor of 1.05, as we had overdesigned the critical joint by 5% to allow for variation.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[debodine]

A star to both SparWeb and GregLocock for giving good examples of why engineering experience and judgment are bedrock, and upon this bedrock rests the realization of man's creative dreams.

 

[israelkk]

MIL-A-85311H AIRBORNE STORES, SUSPENSION EQUIPMENT AND AIRCRAFT-STORE INTERFACE (CARRIAGE PHASE); GENERAL DESIGN CRITERIA FOR. Gives specific criteria for design.
1. The structure should not yield in the maximum expected load (including extreme temperatures, environmental conditions accelerations, aerodynamic loads etc)
2. At 1.15 of the maximum expected load the structure is allowed to yield as long as there is effect on the system performance. For example, if a missile fin will yield and will have a permanent 0.5 degree deflection it probably will not affect the missile performance because it is with the manufacturing tolerance.
3. At 1.5 of the maximum expected load the structure should not break. Above it if is allowed to break

 

[Sparweb]

The same numbers keep coming up whether you talk about civilian or military aircraft, european or north american designs, and even when looking at the piloted aircraft versus missiles/UAV's. There are exceptions, and other finer details like "A"-basis/"B"-basis allowable stresses in materials, and "good practices" for lugs that Bruhn gets into in Chapter D1, to name but a few examples.

But my knowledge is fuzzy about the origins of some of these factors. Most factors appear arbitrary - reasonable, easy to remember, but still arbitrary, and I've not been able to track down how they came about. NACA Reports dodn't give more than hints about them.

Specifically, I am always eager to know how the Fitting Factor (25.625) come about. (15% extra margin on parts substantiated by analysis without tests.)
Also, the seat factor (25.785) of 1.33?
The Normal Category manouvering load factor of 3.8? (Or the formula in 25.337 that calculates it, for that matter)
There are many more, but let's not get bogged down.

Better understanding of how these factors came about would help me and all others practice them.



Steven Fahey, CET

 

[Sparweb]

A friend dug up an old copy of Kitplanes Magazine for me, where David Thurston (some of you might recognize the name from Grumman) wrote an article about the meaning of V_A. Worth a read (Kitplanes, April 1987). This info might also be in one of his books (Design for Flying / Design for Safety, TAB Books).
There's no historical background in the article, so I'm still looking...


Steven Fahey, CET

 

[rb1957]

yahoo'd "va +airspeed" ...

an excerpt for one of the hits ...

As student pilots, we all learn Manoeuvring Speed (Va); it is one of the three speeds that must be memorised for the Private Pilot flight test. We learn that Va is the “…maximum speed [at a particular weight] at which full deflection of the controls can be made without exceeding the design limit load factor and damaging the airplane’s primary structure”(3). We also learn that Va is the maximum recommended speed for turbulent air penetration. This is good information and will, most probably, help us earn pass marks on the private pilot flight test.

As we grow and develop as pilots, we begin to take on new challenges. Developing a deeper understanding of Va becomes essential, particularly if we start thinking about doing any mountain flying or extending our range to distant points from which we will not be able to immediately “return to base” if the weather begins to deteriorate.

The bottom line: Va is the speed below which our aircraft will stall rather than bend or break when we impose or have imposed on us—as in the event of a vertical gust—an increased load. But, as with most things, the devil is in the details.

 

[Sparweb]

Before everyone goes away nodding in agreement with RB1957, please read the following article from Flying Magazine.

http://www.flyingmag.com/article.as...n_id=12&article_id=527&page_number=1&preview=

and/or this NTSB report:

http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20011130X02321&ntsbno=DCA02MA001&akey=1

V_A is not iron-clad protection against breaking your airplane.



Steven Fahey, CET