Is there a good reason for bypasses to be made from carbon fibre. The ones I have seen don't look like they would contain a blade failure any better than glasscloth. Is it simply fashion?

Harry

Hi there Harry,
I have made bypass's out of carbon cloth and while wet a layer of wing skinning cloth, which makes it strong and the outer very smooth. Do people use carbon because it is more rigid once set? As far as blade retainment goes you could use a carbon/kevlar mix but kevlar impregnated with resin is not very resistant to being cut than carbon is so there may be little advantage.

Jason

I like the clear ones, you can keep an eye on the colour of the turbine exhaust for heat stress and check the compressor blades for FOD damage.

No reason to use carbon fiber but you can charge more for it if you used CF.
Darryl

Hi Harry:

I think it is a multiple of reasons. Weight, strength, fashion, moderate heat resistance, thin side walls and stiffness. There is no way that CF will retain a blade, even the Kevlar ring that BV used to make supposedly did not hold one many years ago (I did not see it, but I was told so)...

I have had not one, but two turbine wheel failures caused by bearing failure way back when, with RAM 750 PLUS engines.
In one case, the exhaust nozzle was blown off the engine, and in the other, it managed to stay on. In both cases, the turbine wheel shed multiple blades when the wheel contacted the NGV, puncturing the engine case and nozzle.
The BVM bypass tube contained all of the fragments, (except the departed nozzle), exhibiting no signs of internal damage. The RAM engine has a stainless steel containment ring around the wheel which is supposed to slow down or stop fragments to minimize the damage as the carbon bypass tube isn't designed to contain a turbine wheel explosion.

The airframe was not damaged in either case.....I guess the containment ring did it's job. BTW, it had the exterior BVM Kevlar containment ring installed also.

Darryl, here's the reason:
It's not for fashion; The BVM bypass tube is made from a high temperature epoxy resin and carbon cloth in order to have more stiffness and higher heat resistance and than any comparable thickness E-glass or S-glass low temperature epoxy resin laminate can offer. It will retain it's shape at the operating temperatures that heat soak it, especially during ground opeations.
The high temperature resin and the carbon cloth will also allow you a few more seconds to get a fire put out, which I view as insurance in an expensive airframe.
There is a laminate of 2 oz. glass skin to help "compact" the carbon cloth and make the exterior smoother, (Many of us like to paint them). After the wet layup and room temperature cure is complete, high temperature resins receive a "post cure" (about 450 F oven) which cures the resin further, allowing the resin system to withstand much higher temperatures than it will hopefully ever see.
Normal epoxy resins (low temp) soften with temperature increases. That's why Burt Rutan (and other knowledgeable composite engineers) always recommends painting composite airplanes white.

Bypass tubes made from low temp resins and boat cloth are much cheaper to produce, but are very old tech.
But, as you have so eliquently stated, you can charge more for a carbon bypass tube. I'll bet that you would have to, as basic market economics would dictate.

Harley Condra
BVM REP
JetCat REP