Primary Structures Stress or Cabin Interiors Stress or Nacelle Stress?

[stressebookllc]

Some people think that aircraft cabin interiors stress engineering is dumbed down furniture stuff and low tech. But primary structure and nacelle stuff is high tech. What is your opinion?

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[rb1957]

personally I'd put nacelle stress with "dumbed down" interiors stress, and systems stress ...

but then I'm an elitist primary structure analyst ...

another day in paradise, or is paradise one day closer ?

 

[stressebookllc]

The nacelle is the only part of the aircraft where high temp, dynamics, fatigue, sonic fatigue, hitech composites, systems, FBO, TR and everything else is acting in a relatively tiny envelope.

I think it is much more complex than primary structure, but then that is just my personal opinion.

rb I'd love to discuss something with you, do you think you can email me at stressebookllc at gmail?

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[tbuelna]

I would not put most commercial aircraft nacelle structures in the same category as wing structures, the only exception being engine pylons. Nacelle thrust reverser components might see some significant loading for short periods during landing, but failure of a thrust reverser structure would not be catastrophic like in-flight failure of a wing spar would. As for the rest of the nacelle components, it's not uncommon for commercial aircraft to lose large sections of engine cowl and continue to fly safely.

I would disagree with rb1957's comment about "dumbed down" stress environment of some mechanical systems like landing gear or primary flight control actuators. These systems have some very highly stressed, fracture critical components.

As for the single component on a commercial turbofan aircraft that is most challenging to engineer in terms of stress, dynamics, fatigue, heat transfer, metallurgy, reliability, aerodynamics, etc, I would say it's the engine HP turbine blades. In comparison, everything else is a cake walk.

 

[rb1957]

I agree that an engine (turbine, etc) is a wonder of engineering ... but a nacelle isn't an engine. a nacelle is IMHO a simple structure supporting the engine, mostly a fairing, keeping the wind out. sure there a nuances in the design but then there is designing lavatories too, and if you're a lavatory designer they mean a lot to you (if you're not, you probably just hope that the guy did a good job).

another day in paradise, or is paradise one day closer ?

 

[KENAT]

Actually, optimizing the aerodynamics & acoustic effects of nacelles is really quite a wonder in itself.

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[rb1957]

if you're a nacelle designer, then I'd expect you think the issues are significant. I'm sure there are tricky design problems to solve.

for me, I can't help but remember, when the talk turns to nacelles, about my P.Eng board interview. The local professional engineer association wanted me to do a bunch of exams 'cause I had the nerve to get my degree in a foreign country. I said (surprisingly nicely) "no way, I've been working for something like 20 years ..." so they said "ok, we'll do a board interview". so I sat down with three of their guys to talk over the work I'd done. one topic was (yes, you guessed it) nacelles. one interviewer, not familiar with the subject matter, unwisely asked "I'm not familiar with the term". I replied (equally unwisely) "they're where the engines are, like on Star Trek". surprisingly, they accepted me into their association ...

another day in paradise, or is paradise one day closer ?

 

[tbuelna]

if you're a nacelle designer, then I'd expect you think the issues are significant. I'm sure there are tricky design problems to solve.

A fan blade off condition might fit that description. During a fan blade off failure most of the heavy work is performed by the engine containment structure around the fan case. But the important nacelle/pylon structures must still survive mostly intact (bent but not broken) the period of very high dynamic forces produced by the unbalanced rotating assemblies. A few years back I worked on the design of an EMAD housing for a R-R commercial turbofan engine. The housing sustained a partial fracture next to one of the mounting lugs during the fan blade off test, but the EMAD remained attached to the engine. However, I still had to modify my housing design since the test acceptance criteria did not allow a failure that would create a fire hazard, such as leakage of lube oil from a fracture in the EMAD housing. The EMAD housing was sand cast magnesium, and I also recall that it required quite a few design/analysis iterations to show it capable of meeting the (20 minute?) fire rating.

Or what about the fuse pins used to attach the engine to the pylon on some Boeing commercial aircraft models? There have been a few instances where engines have separated from the aircraft after failure of the fuse pins. Here's a quote from the article linked: "In March, an Evergreen International Airlines jumbo jet lost its left inboard engine minutes after taking off from Anchorage International Airport. The jet returned safely to the airport. The 9,000-pound engine fell into the parking lot of a densely populated residential area. No one was injured."

 

[stressebookllc]

What you are talking about are the engine mounts and pins I believe, typically part of the nacelle product package. At least that is how it was at UTAS.
They are super critical and have redundant load paths built into the design and analysis. They have en entire team dedicated to them.
Engine mount failure is considered catastrophic to flight maneuverability as far as I know.
You are correct in that FBO is one of the most critical load cases analyzed in the nacelle structure. The analysis is done on the nacelle is done based on the mass and size of the escaping body and needs to be contained within the nacelle structure even if it penetrates through the fan casing, per regulations.

I also think that the nacelle TR actuation system (similar although not as complex as the landing gear) is quite complex and drives a lot of critical load cases for the entire nacelle structure, RTO, Jammed Track just to name a few.

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[Kwan]

Do not forget that a rub strip from a nacelle brought down the concorde. Also, most of the same methods are used on all the structure that have been mentioned.

 

[stressebookllc]

That's correct, most checks are the same. Except interiors is not FCS (fatigue critical structure) so rarely any fatigue stuff there.

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[tbuelna]

Things like engine containment structures, and even the location of the engine itself on a pylon, are designed to minimize collateral damage to the wing/fuselage in the event of an explosive failure of high-energy rotating engine assemblies. It is also established design practice to provide physical separation between critical systems so that any single failure event will not disable back-ups. This is why you don't see turbine engines mounted side-by-side.

But you can't always foresee every possible situation. Here's one of the most unusual failures I can recall. It involved one engine, both nacelles and the fuselage. During a ground run-up, the HP turbine disc of the left engine on a 767 separated from the spool, exited thru the inboard side of the nacelle, sliced thru the lower fuselage, and then finally embedded itself in the right side nacelle.

http://www.iasa-intl.com/folders/belfast/AA763EngineFire-3.htm

 

[KENAT]

"This is why you don't see turbine engines mounted side-by-side." - on civil A/C anyway, B47 & B52 had/have twin pods on some or all engines, and most twin 'fast jets' have them next to each other in the fuselage.

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[tbuelna]

After hearing of this fatal incident I have never sat in the rear cabin section of any aircraft with a similar engine configuration. The pictures of damage to the nacelle and adjacent fuselage are scary. The un-contained fan failure cut the front half of the engine and cowling clean off and left them laying on the runway.

The NTSB report indicated the failure resulted from a cracked fan hub, which should have been detected at some point well before it failed. Since this failure occurred in spite of QA/maintenance procedures being in place to prevent it, it is conceivable the same situation could happen again. When I look at the incredible amount of damage that resulted, I can't imagine it would be practical to design a nacelle capable of safely containing this type of failure without adding excessive weight and bulk. So I think the best approach for commercial jets is to mount the engines on pylons below the wing, and well away from the cabin.

Unless of course some very clever nacelle designer were to come up with an advanced, lightweight, compact containment structure.