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LARGE CUTOUT ON FUSELAGE SKIN
4

LARGE CUTOUT ON FUSELAGE SKIN

LARGE CUTOUT ON FUSELAGE SKIN

(OP)
I have to create a Large cutout on FAR 23 Airplane Fuselage (Unpressurised ) see attached figure.

Since it is a large out of plane loading, decided to frame it up. (design load)

* A Doubler, wedge between the skin ,frame and stringers

* A STIFFENING ANGLE between the frame to Transfer the loads to the frames

What else details do i need to consider..Clet, Intercostals.. etc..

* Do I need to consider any panel buckling ? ( No access to AC shear flow at these location. )


Many Thanks

RE: LARGE CUTOUT ON FUSELAGE SKIN

Your cut-out is "big" but not IMO "large" ... large would be several frame bays.

there's lot's more to be analyzed ... refer to Bruhn as a start. Yes, you need to understand the impact on shears; note adding teh doubler increases the skin thickness (increasing the allowable shear, delaying diagonal tension loading) and adding "panel breakers" also increases skin panel allowables.

I would strongly suggest not "wedging" the doubler between the skin and the internal structure. I know this has been done in the past, but I wouldn't do it. Either an external doubler (easier but a different set of issues) or an internal doubler, cutting and splicing the internal structure.

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

RE: LARGE CUTOUT ON FUSELAGE SKIN

Just wondering...

Is there a door fitted over the cutout?

Is this panel loaded ONLY in compression... or does it also see tension and shear?

WHY does the cutout HAVE TO be a rounded-corner rectangle... why not a slightly larger ellipse or oval?

CAUTION: Corner Radii with this configuration should be maximized... skin/double stress concentrations can be very high with sharp corners.

Also why does the cutout HAVE TO cross/remove the stiffener segment?!? I noticed that the cutout is 6.5 X 9.0... and that the stringer center-to center distance is 6.0... making open skin bay ~5.0??? So why not allow the cutout to decrease and place it between stiffeners; then reinforce both the skin [bear-strap doubler, external] and adjacent stiffeners [add stub-stiffener within the stiffener passing beyond both frames shown]??

Depending on these answers, I can see some intriguing/creative ways to design reinforcing structure.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: LARGE CUTOUT ON FUSELAGE SKIN

Can you tell us about the object that is mounted to, or through, the cut-out? It will impose loads on the skin, stringers, and frames that could have a significant effect on their design.

Since stringer splicing is unavoidable, I can see some creative ways to splice the stringers that will permit them to box around a 6.5" cut-out, basically because one of your stringers can be flipped over and fastened back-to-back. Splicing of the stringers can also be done in a way that accommodates the "step" on the edge of the internal doubler.

Big cut-outs can be difficult to seal around, and some early consideration of how moisture will be kept out will pay off later. Depends a lot upon what's installed in the hole, of course.

STF

RE: LARGE CUTOUT ON FUSELAGE SKIN

(OP)
The cut out need to be remain same. It is a flare dispenser equipment exerts a out of plane loading ( equivalent Blast/statc load) of 8kN (1800 lbf) and that is the main driver.
The load should pass from equipment casing 4 steel bolt ( 2 each shorter side of the cutout) to the skin / stringer/frames .

I have to cut the stringer and as suggested go for a external doubler to reinforce the skin. But main worry which is the best way to Frame it up. 1800 lbf is a big load..

The skin is a floating type design.

RE: LARGE CUTOUT ON FUSELAGE SKIN

I have an issue with individuals who attempt to 'reinvent' the wheel / or in this case start from ultra-elementary concepts. On critical items such as fuselage structure - some prior experience should be mandatory. First place was to review the aircraft structural repair manual - looks like a standard repair - should be there. Otherwise look at manuals at similar aircraft - they are more or less all the same. If your design has some issues that are not addressed in these manuals - these issues should be discussed - not the basic concept of a "how-to" Doubler repair.
2/ What was the original damage? Is this a pressurized aircraft? Are the bays machine milled? Original skin thickness? Is DTA involved? ... since we are talking about ultra-basics ..

RE: LARGE CUTOUT ON FUSELAGE SKIN

you're right, the SRM is a good first look, but a part 23 SRM is usually pretty light on details !

I'm not sure damage is involved, not stated but I assumed the cut-out was for some equipment. If to repair damage ... much easier (rebuild what you're removed, take care with the interface).

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

RE: LARGE CUTOUT ON FUSELAGE SKIN

Yes, there seems to be an interpretation here that may be incorrect.

So, OP, can you clarify? Is this:

1. A cutout made in the skin to gain access for some equipment installation which then needs to be repaired, or
2. A cutout made in the skin which must remain open or has some removable or non-structural access panel or something.

What is the intent of this?

Keep em' Flying
//Fight Corrosion!

RE: LARGE CUTOUT ON FUSELAGE SKIN

LD,
The OP has told us it's a flare dispenser, and said that the loading is "out of plane". No pun intended.
I'm assuming the flare will be propelled out of a chute with a rocket, otherwise there wouldn't be any load.
The dispenser may hold many flares, hence the rectangular shape.
Something that doesn't make sense is the OP's load (8kN). It sounds like they are launching Sparrow missiles, not flares.
Or maybe ALL of the flares will be launched at the same time...

Payyan,
This is not a trivial modification. Forgive me if I am jumping to the wrong conclusions, but your initial and follow-up questions make me think you have never done a project like this before and have no place to turn for guidance (except us on the internet, which may turn out to be worse than helpful). The best I can offer is that you should get a copy of Bruhn, Analysis and Design of Flight Vehicle Structures, Flabel, Practical Stress Analysis, and Niu, Airframe Structural Design. And to see how cut-outs of that size should be done, look at the windows in your very own aircraft. They will be "framed-up" as you put it, because cut-outs always interrupt load paths in terrible ways.

STF

RE: LARGE CUTOUT ON FUSELAGE SKIN

Ah, thank you SparWeb, I missed that tidbit.

Payyan,
My next question would be what is the plan for certification & approval?

Keep em' Flying
//Fight Corrosion!

RE: LARGE CUTOUT ON FUSELAGE SKIN

Who is going to approve this mod ? Their input is crucial, they should at least tell you the analysis they're expecting, they may be helpful and suggest reinforcement. Bruhn and Niu are good references. But this (your "obvious" inexperience) does worry me.

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

RE: LARGE CUTOUT ON FUSELAGE SKIN

(OP)
Thanks for all replies. I had no access to the SRM, But now access to Bhrun and Niu and referring it.

The repair will be endorsed by a Part 21J EASA design approval.

RE: LARGE CUTOUT ON FUSELAGE SKIN

1800 lbf point load? My first thought would be crippling of the frames and stringers.

For reference, the Cessna 152 is ~1000 lbf. Consider that you have to design your cut-out to be beefy enough to land a light aircraft on.

Quick calc of F=ma (est 0.5kg mass):
F = m * a
8 kN = 0.5 kg * a
a = 8e3 / 0.5 = 16,000 m/s^2
a/9.81 => 1630 'g'

Double check (1 lbm ~ 0.5kg) :
1 lbf = 1 lbm * g fps^2 (g =32.2)
1800lbf ==> 1800 'g' for 1 lbm.


velocity of flare:
v = a * t (assuming t ~ 0.05 secs)
= 1800 * 32.2 * 0.05 = 2898 fps

double check:
= 16000 * 0.05 = 800 m/s (~ mach 2.4)

(assuming t ~ 0.01 seconds)
v = 800/5 = 160 m/s (OK, that is still quite quick)

... are you sure about the applied loads?

[NOTE: I don't know about flare weights or muzzle velocities]

RE: LARGE CUTOUT ON FUSELAGE SKIN

surprised you don't have the SRM that would normally come with the plane.

1800 lbs from firing the flare is a load not to be taken lightly (no pun intended).

There are also many other loads to be considered (dynamic landing, ditching, ...). Talk early to whoever is approving the mod, else you're in for a world of hurt.

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

RE: LARGE CUTOUT ON FUSELAGE SKIN

(OP)
Thanks Guys,

I could get hold of the Niu Third edition, which is a fantastic reference.

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