flyrotary@lancaironline.net Mailing List Archive

I thought it was only 5 years ago that one of the rotary guys died from using plastic fuel flow. It melted and spewed fuel in engine compartment.

Very decent of you to help Mark. In automotive design, we know that using materials that are affected by temperature, vibration, etc are a leading cause for failure. If you compare the two materials, huge difference in safety just by using a robust material.

A great example is the plastic brake lines on original canards. Failures due to hot brakes, age of plastic, sun exposure, install error. We've had a few planes burn to ground due to this. No failures of guys who use ss lines.

-Al Wick

-------- Original message --------

From: Mark Steitle

Date:10/29/2014 10:30 AM (GMT-08:00)

To: Rotary motors in aircraft

Subject: [FlyRotary] Re: electric fuel selector valves

Ernest,

The latest plastic part failure that comes to mind is the plastic overflow tank on my 2007 Mazda 3. It is under something like 18-20 psi. It developed stress cracks near a seam.

I also have seen failures in plastic radiator end tanks. Of course, this could be the fault of a ham-handed mechanic, or not. Then again, I have also seen brass radiator tanks with cracks.

You made a good point about AN fittings. Hoses need to be supported in some fashion other than hanging by the fitting.

Mark

On Wed, Oct 29, 2014 at 8:32 AM, Ernest Christley <flyrotary@lancaironline.net> wrote:

On Tuesday, October 28, 2014 9:26 PM, Mark Steitle <flyrotary@lancaironline.net> wrote:

>Six PSI is one thing, 40 psi is much different. I've experienced too many cracked plastic parts on cars, trucks and
>RV's to feel comfortable putting me and my family inside a cabin with a plastic RV fuel valve. A small crack under
>40 psi can cause a very real fire hazard. To me, this is a no-brainer.

I have to give you that. I'm not sure I would want to put a plastic valve under pressure and expect it to operate reliably either. 40psi really isn't all THAT much, but I'd still want to design the system so that plastic parts came before the pumps.

Did the cracks you experienced have a common theme (barbs breaking off? body of the valve cracking open? mostly located in the engine compartment where heat would prematurely age them?) I ask so that I can avoid a hidden snake, if at all possible.

>If its the cost of a quality aircraft quality valve, then get a part time job and save up your lunch money. That's my reasoning.

Cost is one thing.
Defining "quality" is another. That goes back to replacing the 1000lb rope with a 100,000lb chain. The rope is sufficient, but the chain is "better", because it is stronger, even though it causes a host of its own problems. It is heavy. It doesn't roll smoothly through a pulley. It will rust. In my case, the only fuel leak problems I've had were at the AN's metal to metal fittings.
What really grates though is design mismatches. Everyone is using stiff, steel braided hoses to connect a stationary firewall to a vibrating carb, the vibrational stress being absorbed by an aluminum fitting. We go to extreme lengths to avoid NPT fittings on the ends of tubes and hoses, mainly by screwing NPT-AN adapters into all the NPT holes. We use steel braided hose to protect against "abrasion", when everyone knows a properly laid out system should not have hoses rubbing (and what is done to protect what it was abrading against?). We use steel braided hose to protect the hose from fire, as if a porous metal screen will somehow slow the heat down. Now Steve relates that his father was convinced by a racing guy that a steel braid will somehow protect the rubber inside from ethanol incompatibility. At some point the lack of logic assaults the senses and you just have to stop and say, "WTF, guys?! What were we building again?"

>Oh, one more thing... my kit included a 6-way Andair valve. So, the decision was easy.

hehe. Yeah, that does make a difference 8*)