Fuselage Cutout

[gutboy17]

I need some assistance on where I could find an analysis method for cutouts in a fuselage. I need to add a small opening in the side of an Aluminum semi-monocoque fuselage. I'm not cutting any frames or stringers, just the fuselage skin boxed within the frames and stringers. I don't have any loads data for the airplane. I was thinking about using shear-flow to compare the loss in skin structure to the added doubler and or internal frame. I've looked over the internet and in some of the classics (Bruhn, Niu, etc.), but I haven't been able to find a method that would work in my application. Does anyone have any directions or pointers? Thank you.

 

[Andries]

Hi Gutboy,

Niu's two books (airframe design and airframe strsss analysis) deal with such cut-outs, as does Bruhn. Peery (1950) also has some material on cut-outs.

Andries

 

[rb1957]

a pressurised fuselage ?

a part 25 a/c ?

usually you'll need a dblr to reinforce the skin. can be external or internal. same thk as skin, riveted to the skin (two rows better than one) this is like a repair patch, research that for some further clues.

if you're mounting a "large" blade antenna (like a VHF), then you'll need to add a U-channel, attached to the adjacent frames, to make a stiff installation.

be aware, the FAA is taking a lot of interest in buffet/flutter, and may require flight test to Vd. we usually argue them back to Vmo.

 

[gutboy17]

Not pressurized.

No antenna.

It is Part 25

It is a small opening for a sensor that will not be extending into the airstream. No loads are transmitted from the sensor to the opening. I "just" need to show that my opening and the external doubler I'm putting around the opening returns the local strength back to original or greater.

I started down the path of of shear flow, q=VQ/I, and tried to set the q(no cutout)=q(with cutout). Then I started getting a little lost after that.

I'll do some further research on the repair patch.

Thanks.

 

[rb1957]

so for your plane the fuselage skin is just a shear panel. adding a hole in the middle of a shear panel really doesn't degrade the strength significantly.

the easiest approach is to show you've added area at least equal to the area removed ... a ring doubler, width = the hole radius or wide enough for two rows of rivets. same thickness as the skin. i think Bruhn has an analysis for a ring doubler (maybe Niu as well), but it's not Vq/It.

as FAR25, you'll need to consider the damage tolerance impact ... if you have an external dblr it's more involved (new inspection technique ?) an internal doubler could have no impact on the plane's ICAs.

 

[Sparweb]

How about getting a copy of Falbel's "Practical Stress Analysis". The 2nd volume in particular would do a pretty good job of showing you what you need to do for that cut-out. Especially for your first time out on the subject. It's one of those things that "looks easy" but actually there are a lot of potential snags, like the DTA or the inspectability issues that could get you in trouble. Detail issues, like radius of the corner, edge distances, rivet pitch and spacing, can get you into trouble if the analysis says one thing but "standard practice" says another.

Mass of the sensor, if it's significant, cannot be ignored. Aerodynamic loads due to its shape and size, (again, assuming it's fairly large) are a factor, too.


STF

 

[gutboy17]

Thanks rb1957 and SparWeb for the info. It helps out a lot. I have all of those references and I'm looking through them for an appropriate substantiation. The sensor does not extend past the OML of the fuselage, so I can't see any aerodynamic loading affects. The sensor will also have no structural interface with the fuselage skin nor surrounding stringers nor frames. The sensor mounts to the cargo floor. The fuselage is non-pressurized. Thanks again.

 

[rb1957]

i'd just use a ring dblr to reinforce the hole, internal would be nicer than external but which ever is easier given the a/c structure.

as part 25 you'll need to think about DTA, but unpressurised the loads'll be very low (i'd think).

 

[lisa247]

How are you going to stress it if you have no loads?

 

[rb1957]

you worry about the airplane loads, as opposed to the loads from the local structure. a small hole in the middle of a skin panel, pressure would be critical; so 2 deltaP.

 

[lisa247]

Ah I see, I just thought he was saying that he had no loads at all.

 

[WackoJ]

There is a program called RapidC which you might find helpful in this instance. Although not currently endorsed by the FAA (was in the past), its a nice tool to double check hand calcs.

Info on RapidC can be found here:

http://www.eng-tips.com/viewthread.cfm?qid=284439