Exhaust

Modify Exhaust System

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The stock exhaust on my airplane was about as attractive as Muppet Gonzo’s nose. The stock exhaust provided by Hirth looks fine on most aircraft with the muffler hanging dead center of the airplane. But the steering bar for my nose wheel would run right through the muffler so that’s not an option. I managed to make it fit but it was ugly.

Gonzo nose exhaust system
Gonzo nose exhaust system

My engine was purchased from Recreational Power Engineering so I contacted them to find out the proper way to go about this modification. Matt said the total length from engine to muffler needs to be 21″ and the pipe diameter should stay the same size throughout but could be slightly larger just never smaller. Bends are the tricky part, a regular pipe bender will slightly crush the pipe so that’s not an option. RPE sells pre-bent pieces that you can use to build your custom exhaust or you they can build it for you. I managed to make things work without needing any additional bends
The first problem I had was the 90° pipe coming off the engine does not match the angle of the fuselage bringing it too close to the fiberglass and wood. I cut the straight part at the rear of the 90° off and then cut a little off of the bend, you can see the little notch in the left picture​ below to see the small section I removed. The result is a 65° degree bend. The straight part was also rotated so the springs will be top/bottom instead of side to side. Again this was effort to get more clearance between the fuselage.

Front header 90° modified to 65°
Front header 90° modified to 65°

The muffler having a 90° elbow welded to it is not going to work so I cut it off.

Removed 90° pipe from muffler
Removed 90° pipe from muffler

I don’t have a welder here at my house so I made a template of the of the fuselage shape, marked some key locations and took this to my brothers automotive shop to weld the parts. 

Fuselage shape template made from scrap
Fuselage shape template made from scrap

He tack welded all the parts so I could take them home and verify fit, if anything needs tweaked the tack welds can be easily cut.

Only small tack welds until fit was verified
Only small tack welds until fit was verified

Back home I checked the fit and was satisfied so I created a bracket to hold the muffler onto the fuselage. The bracket was made from some 3/4″ wide steel that I shaped to match the curve of the muffler.

Fabricated muffler bracket with grommets
Fabricated muffler bracket with grommets
Muffler bracket
Muffler bracket

I used a couple large zip ties to hold the bracket in place so I could check the fit. Once satisfied I marked the locations where the new spring hooks need to be welded. Back to the shop for the final weld.

Zip ties to hold bracket to muffler until it is welded
Zip ties to hold bracket to muffler until it is welded

The result is a much better look than the stock system that came from Hirth. The only downside is when it comes time to replace it, all this customization will need recreated again. 

Hirth F-33 modified exhaust
Hirth F-33 modified exhaust
Hirth F-33 modified exhaust
Hirth F-33 modified exhaust
Hirth F-33 modified exhaust
Hirth F-33 modified exhaust

I also checked that it flexes properly when the engine moves.

I’m undecided on how I want to paint the exhaust. Black is obvious, maybe I could use red and white to match the fuselage color scheme. Maybe I’ll decide to wrap it. Need some time to think about this.

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