Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!

Which load case is the design case for any location in the aircraft 1

Status
Not open for further replies.
ykantor1

ykantor1

Aerospace
Jul 2, 2011
14
0
0
IL
Which load case is the design case for any location in the aircraft? preferably as an image , such as page 11 in this file.

I have looked at loads analysis books ( e.g. Lomax and Howe) but they have some textual description only. e.g. The lateral gust on the vertical tail is the critical design case for XXX).

Any ideas?
thanks
Ykantor1
 
Replies continue below

Recommended for you

Sort by date Sort by votes
I don't think there are universal cases, they may not be even be important either. It may not even apply to the different parts of the aircraft. All the different load cases shown in that slide are specific worst cases for those parts of the aircraft. For the pressurized cabin on the other hand, you do have universal cases for ultimate crash landing. Refer to 14 CFR Par 25.561 for example. All interiors components must be certified to these or higher load factors depending on the aircraft, customer etc.

Stressing Stresslessly!
 
Upvote 0 Downvote
what don't you get about pg11 ?

it lists which types of cases are generally critical for different pieces of structure. i'm surprised that dynamic landing, dynamic gust are not more prevalent, maybe a strict intrepretation of "static" ... for me "static" includes dynamic "one-time" loads, and the great divide is between static and fatigue loads.

pg 12 gives you a more detailled view ... it isn't simple !

another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
to Stressebookllc : apparently those are universal load cases, albeit with some minor modifications evolving with other materials ( e.g. composites) and software that limits pilot's mistakes. e.g. Lomax, who accumulated 40 years of experience as structural loads engineer at Boeing, writes: "The structure affected by the vertical tail loads are...the fuselage center section (overwing) structure".

to Rb1957 : The page 11 slide is a good source, but It might be more useful to divide the A/C to more regions, and to watch the critical load cases for different directions. e.g. The design cases for the stringers are not necessarily the same like the Frames' critical load cases.

Page 12 slide is indeed more detailed, but for the wings only.

ykantor
 
Upvote 0 Downvote
this is a little futile, though reasonable background knowledge.

fuselage stringers are typically critical in compression, so lower fuselage is critical for down bending, upper fuselage up bending, side stringers lateral bending.

fuselage skin panels are typically critical in buckling ... shear and compression ... diagonal tension; which also loads up the stringers.

the futility is that you analyze the complete structure for all loads, then pick the max/min internal loads. If you're pushed for computing space you might run a simple "stick" model to get the overall fuselage bending moments and shears, and do local detail analysis; or you might pick obviously conservative cases as critical.

You then need to worry about the non-static cases ... dynamic and fatigue (fatigue typically limits tension stresses and is often the design case ... eg fuselage skin is typically designed by hoop stress from fatigue pressure loads).

another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
Take an engine nacelle for example. Just within the thrust reverser, there may be many different engine related load cases, actuation system related load cases, like a jammed actuator, even ground cases or gust cases and so on.

That is the point here, you may have a rejected take off case that may be critical for one part, or maybe some other maneuver case for some other part. There maybe 300 different load cases, and some cases may be critical for quite a few parts, and some parts may have a different critical load case for each part. Fatigue is an issue also especially coupled with high temperatures and vibration related amplification..

Stressing Stresslessly!
 
Upvote 0 Downvote

to Stressebookllc : Yes, sometimes there are few alternative critical loads , depending in the situation, but as the image (the page 11 above) presents, there are typical critical loads, at least for some of the regions.

to Rb1957 : Thank you for the loads information, but I need a sourced data ( such as Lomax and Howe books) in order to use the data for a stress report of installations in FAR 25 aircraft. The installations are adding loads and cable holes to the primary structure (e.g. skin). The installations might be bumped up in order to justify them, but it seems preferable to use the real loads and to size it right.

ykantor
 
Upvote 0 Downvote
there's no "source" that'll say "the critical case for this piece of structure is XYZ."

if you're adding something to the outside of the aircraft, you'll need to consider aerodynamic effects. if it is low profile, then you may state "negligible aerodynamic loads". if you're adding anything on the outside of the aircraft you'll have to remember 251 (freedom from buffet) ... FAA were very difficult on this in the past.

you then need to consider inertia loads ... 561 crash cases are a good envelop (even if they don't strictly apply). you'll need to consider inertia loads from the airplane manoeuvres ... if no data 9g ult (up/down and side/side) should be reasonably conservative.

most important will be fatigue (and DTA) since you've got "holes in the fuselage".

if you want "real loads" then you'll need to ask the OEM. conservative loads should have negligible impact on the weight of the installation.

another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
Some indications for critical load cases:

Lomax, book:
p. 195: "In general, the forward body becomes critical for vertical gust conditions and the aft body for lateral gust conditions.

p. 195: may become critical for the aft body... is the oblique gusts.

p. 196: Dynamic landing...may be critical on the vertical bending structure aft of the wing body rear spar bulkhead.

p. 198: Fig 11.9 indicates...vertical gust are critical in the extreme aft end of the body.

Howe's book:
p. 264: fuselage... nearer to the rear wing attachments the main landing gear loads may prove to be more critical... loads transmitted from the wing and possibly, the one wheel landing case are major factors in the loading of the center fuselage.


rb1957 :"if no data 9g ult (up/down and side/side) should be reasonably conservative." Is there a source for applying 9g for these directions? It will help me in possible arguments with the designers.

thanks
ykantor
 
Upvote 0 Downvote
sure "typically" certain cases are crtical for certain structures (like your original post), but you asked for more certainty (referenceable) and more detail (detail structures, not just "fuselage" but "stringers" and more specific cases ?).

i gave you three typical types of loads, and there are many "special" cases ... decompression, ditching, ...

you should have someone at work to assist you better than we can.

another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
thinking about your comment "It will help me in possible arguments with the designers.". i took "referencable" to mean a reference you can write in a report, rather than a reference you can call upon in an "agrument"; clearly discussing something with design is a lower standard than a report reference.

there should be someone there to help. for example, who'd "design" appeal to if they thought your analysis was too conservative ?
the loadcase is really on a part (a small part) in the analysis, we can use conservative analysis to make our life easier (the whole job is about making your life easier and shifting the work onto someone else ... really).

there are a whole bunch of requirements, the problem is very different if you have access to OEM data or not. we get by a lot of installations with 9g inertia loads and 200 lbs ground handling loads, and pretty large margins as well.


another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
sure wieght is a big deal for OEMs ... i work in a "chop-shop", "pimp-your-plane", when it's more important to make it easy (for the rock apes downstairs) and to use standard sizes (sheets, extrusions) and avoid complex operations (heat treat forming).

for us a pound of aluminium isn't worth a tonne of analysis.

another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
"twins" maybe ... but no, I work mostly on primary structure mods ... not "monuments" ;)

another day in paradise, or is paradise one day closer ?
 
Upvote 0 Downvote
Rb1957: yours- " we get by a lot of installations with 9g inertia loads and 200 lbs ground handling loads, and pretty large margins as well."

Will it be possible for you to indicate a source for using sideways or up/ downwards 9g load case. I can't use it without a reference. thanks
Ykantor
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

DELBZ
  • Question
WHAT IS THE CORRECT WAY OF PAVING MULITPLE LAYERS OF ASPHALT? 1
Replies
2
Views
78
WKTaylor
WKTaylor
KirbyWan
  • Locked
  • Question
Is the flow correction needed?
Replies
0
Views
19
KirbyWan
KirbyWan
dcascap
  • Locked
  • Question
Plate stress evaluation in the middle
Replies
14
Views
110
rb1957
rb1957
KirbyWan
  • Locked
  • Question
Is there a source for beaded flanges per AS5131? 4
Replies
8
Views
76
Sparweb
Sparweb
rb1957
  • Locked
  • Question
now that's putting faith in the guys ...
Replies
0
Views
21
rb1957
rb1957
Back
Top