Repairing Broken Section of Flex Pipe in Header

[JimD]

While towing at the recent MRCA Aero Tow in Ann Arbor, Michigan, my DA-120 powered Decathlon suddenly got very loud. I knew right away what had happened because I have had a similar thing happen on a different model...the header pipe had broken.

Pre-made Header Pipes typically have a section of flex pipe built into them so minor adjustments can be made to lineup the header pipe with the muffler.This flex section is typically wrapped with layers of white Teflon pipe thread tape.

After considering various options, I decided to try repairing the break with a sleeve made from a piece of pipe I had ordered when making the custom header for my DA-85 Porter. The DA-85 header pipe is larger in diameter than the pipe used for the DA-120 headers. After some grinding and filing, I got a section of this pipe to fit over the broken area while keeping the shape and length of my header the same. The section was silver soldered in place and wrapped with new Teflon tape. I found Teflon tape made for gasoline pipe joints...not sure if this will make a difference, but we will give it a try.





sorry this one is out of focus, but this is how the joint looked after scrubbing it with a Brillo pad.

[ChrisPratt]

It must be really out of focus or I've got to get to the ophthalmologist right now! :-)
When you say silver solder do you mean silver braze? Normal silver solder will melt with the heat from an exhaust pipe, I know from personal experience by losing a muffler in mid air.

By the way, which Decathlon do you have (i.e. what scale, manufacturer) I have 1/3 scale Hacker Decathlon I want to put a 3W 120 on, but have been advised against it by a friend that has the same model.

Thanks for all your hard work with the web site and hobby.

Chris

[JimD]

Chris, it is the silver "solder" that DA sells for welding these header pipes together.

The Decathlon is the 36% scale Hangar 9 model that Horizon Hobby sells. Horizon designed it for a 100-120cc engine.

[Mosquito]

The 3W 120 is a great motor, I own three of them but the motor is huge in comparison to the DA 120. It's not only larger but heavier. I was at one time considering replacing a 3W 120 with a DA 120 but the added work to make it work with such a smaller and lighter engine wasn't worth the effort. Both engines are very respectable power plants.

Tim

[JimD]

So far so good...I have put four hours of flight time on the Decathlon since the header repair and it seems to be doing fine. The failure was caused when the engine mounting bolts loosened, and this was secondary to the natural compression of the plywood firewall over time. This allowed slight movement of the engine which then resulted in metal fatigue in this flex pipe. The mounting bolts were removed, Loctite added, and reinstalled. The plywood compression has likely reached its max, I hope, and should not result in further issues.

Lesson learned here...routinely open the cowling and inspect the engine mounting bolts and other items to see that all is well (just like full-scale). Also, listen to your plane. If something sounds different, check out the reason before there is catastrophic failure. This change in sound is how I discovered the problem. The engine just did not sound as quiet and an inspection revealed the reason to be the broken header which in turn was caused by the loose engine mounting bolts.

[JimD]

Update:

Hour number 5 was the limit, the header pipe broke again! This time it broke in the flex pipe section right behind the repaired area.

The pipe has to make a short "S" bend after the 90 degree section in order to line up with the canister. We used two small 90 sections that were cut off to make 45 degree turns. Here is the finished product, now we will see if it holds up in regular use.