Floating vs Integral Frames

[cyt4]

Good day, fuselage masters,

I have seen couple of really interesting frame designs on few aircraft structural skeletons.I have been thinking about certain fuselage structural elements. What are some of the benefits and cons of integral frames(inner flange attached to skin) vs floating frames(ie frame shear clipped to stringer and/or attached to skin with shear ties)? Which is more weight efficient? Would the answer change if frame is composite or metallic? Pressurized fuselage vs non-pressurized? Lightly loaded vs heavily loaded? Any wisdom is much appreciated.

 

[SWComposites]

Think damage tolerance, FAR 25.571. Crack arrestment. Discrete source damage. There is ni simple answer.

 

[rb1957]

Heavily loaded frames (wing attach frames, U/C attach frames, door frames) are typically machined. Sometimes machined segments with s/m segments (to minimise the machining cost). Lightly loaded frames are usually sheet metal. Floating with shear ties vs "integral" … a matter of choice, most airplanes I've worked on have "integral" frames. Floating frames without shear ties (a la B707) is old fashioned. Choice dependent on …
1) assembly time/cost … adding shear ties to floating frames vs "integral" frames (with less assembly time)
2) pre-processing time/cost … "integral" frames will have more time before assembling in the plane … more difficult forming and heat-treat

I think it's arguable which is lighter … I think floating frames could be lighter.


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

 

[Countryman]

Certainly locations where the skin needs all the support it can get, such as at door cutouts, you will see integral frames. This comes at weight cost however, as your moment of inertia is reduced due to having the frame closer to the skin. Likewise, shimming becomes an issue with integral frames, whereas tolerances can be taken up at the frame-to-clip attachments on floating frames. Composite frames certainly changes things, as you want to reduce the number of fasteners going through composite as much as possible, so floating isn't necessarily preferred. That said, there are cost-effective design/manufacturing options with composite that you don't have with metallic.

As for pressure, since a frame's primary purpose is to provide hoop strength, you might prefer a monocoque construction on an un-pressurized aircraft? Not sure though, this isn't my area of expertise.

 

[WKTaylor]

There is a practical element RE using shear clips to tie the frame to the surrounding structure.

With modern NC machining, tolerances can be held VERY tightly... as they need-to-be for automated assembly. NO Need for peripheral clips: Position/drill/fasten.

HOWEVER older designs often relied on the shear clips for precise outer-contour fit/adjustment due to looser/variations in tolerances... which minimized fit-up issues and secondary-parts... such as shims and fillers. Install the frame into the fixture... then install the clips in position... which 'floats to natural contact with surrounding structure and the frame. Position-the-frame/fit-adjust-the-shear-clips/drill/fasten... add/adjust with shims/fillers... ONLY as-required.

Regards, Wil Taylor
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