Split Nose Cowl Mod?

[Steve Pierce]

So if you are going to cut your metal nose bowl in two would that be concidered a major alteration or repair. I would concider it a fairing. AC43.13 has sheet metal repair data.

[stevesaircraft(Bri)]

So if you are going to cut your metal nose bowl in two would that be concidered a major alteration or repair. I would concider it a fairing. AC43.13 has sheet metal repair data.

FAA considers it a major modification as it is a main part to the cowling system... They are afraid that if the connection fails you can loose the cowling in flight... For my STC project utilizing the already approved method I did for the engine conversion they want me to pressure test the nose bowl... That is my stopping point as I have not figured out how I can mount the bowl to a sealed surface to accomplish the test... Having a nose bowl that is already modified and flying with several hundred hours is not good enough for them...

Brian.

[Steve Pierce]

FAA considers it a major modification as it is a main part to the cowling system... They are afraid that if the connection fails you can loose the cowling in flight... For my STC project utilizing the already approved method I did for the engine conversion they want me to pressure test the nose bowl... That is my stopping point as I have not figured out how I can mount the bowl to a sealed surface to accomplish the test... Having a nose bowl that is already modified and flying with several hundred hours is not good enough for them...

Brian.

That makes sense.

[Gilbert Pierce]

Clamp the back of the nose bowl to your table. Stick a ballon inside and inflate it to the total pressure plus a safety factor seen when flying.

A monometer with one port inside the ballon and the other port outside the nose bowl will give you the differential pressure on the nose bowl.

Determine the necessary differential pressure with the momomeetr hooked up with the ports the same way on your flying airplane. Fly at low level at VNE. Then increase the ballon pressure you found by flying to some safety factor the FAA engineers determine necessary and see if nose bowl comes apart.

You may need to determine the velocity pressure and static pressure both when flying to determine the total pressure.

[andya]

I'm thinking they would like to see the pressure on the front side of the nose bowl vs on the backside that the balloon would provide, just thinking anyway.

[Gilbert Pierce]

My thought was the pressure inside had to greater inside due to static regain. There is static pressure re-gain in the engine compartment caused by the dynamic pressure if you have adequate baffles.
Putting one leg of the manometer inside the cowling and the other input outside the nosebowl when flying would resolve that. Velocity pressure outside the cowl very well could make it want to blow in.

At 160 mph the velocity pressure on the nose bowl is about .54 psi. (15 in-H2O)Assume the nosebow has a flat plate area of 2 sq ft (288 sq in.) the pressure on the out side of the cowl would be about 155 lbs. What is the static pressure inside cowl? Less than 15” the nose bowl is in fact being pushed in as Andy surmised.
If that is the case instead of the ballon inside vacuum bag the outside like fiberglass guys do and pressurize it.

[andya]

Be curious to find out how much pressure the FAA would want to see if you developed a method like Gilbert suggests.
A friend of mine had split cowls made for the twin comanche (fiberglassed the vertical split) and cut an horizontal
split that was cut down below the air inlets. Had DER look over his design and wrote up the paperwork to give him
approved data for a field approval. Be interesting to know what he examined to to insure it stayed together (especially
at the speeds it could see)

[ysifly2]

Makes one wonder, would all of this required for a new airplane certification?
Analysis paralysis :-/


Bryan


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[ysifly2]

As an alternate approach... the cowling material and design itself is not (should not) be what is in question, but rather the strength integrity of the new joint line right?
I.e. couldn’t (shouldn’t) a “simple” pull tensile test of two mated pieces of equivalent material and thicknesses with the same number and pitch of rivets and screw fasteners as the split mod would entail be what is under test?

And therefore, what you are showing is that the “new” fastener joint is “equivalent” to the base material

Edit: actually, shouldn’t you only have to show that the split line joint is at least as strong as the top piano hinge line ... would be the same pressure trying to blow the top cowl off and out.?


Bryan


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[Steve Pierce]

Edit: actually, shouldn’t you only have to show that the split line joint is at least as strong as the top piano hinge line ... would be the same pressure trying to blow the top cowl off and out.?


Bryan


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I agree and should be pretty easy to do espcially figuring how soft the nose bowl is.