Friday, May 20, 2016

Fine Tuning The Exhaust System Instillation

Having the muffler offset to the pilot side if the airplane has been bugging me and while placing safety wire on the springs I realized a correction had to be made … as one of the springs for the #3 cylinder’s exhaust pipe was perilously close to a coolant hose.
One can see in this photo how close the spring for cylinder #3’s exhaust pipe is tot close to the black coolant hose.

Once the faux pas was spotted, the nuts on the exhaust flanges were loosened and the muffler was moved back towards center (where it should be) and I just accepted the fact the slot in the lower cowling required more trimming to accommodate the muffler’s exhaust pipe. This only took five minutes, if that, by using a Dremel tool outfitted with a coarse sanding drum.
After doing the rough cut, this is the first test fit … looking good, just need to tweak the clearances around the exhaust pipe to at least 1/4" and then round off the edges a bit.
Finished shape of the cutout for the muffler’s exhaust pipe. I probably did not need to get quite this aggressive and could have left the muffler offset more to the left …  but do not want the spring on the spring on cylinder #3’s exhaust pipe (or the pipe) anywhere near the coolant hose.

The following photo clearly shows the improvement in clearance between the cylinder #3’s spring and the coolant hose by extending the cutout in the lower cowl.
Compared to the first photo of the interference between the spring and the coolant hose, one can see there is now plenty of clearance in the area of the spring. The angle of the camera gives the illusion the exhaust pipe is very close to the water line … actually, the hose is swinging away from the exhaust pipe but in the photo it appears to be much closer than it really is.

At this point, the exhaust flange nuts were torqued (yet again) and work continued by way of safety wiring the muffler springs. Using safety wire on the springs is not spelled out in Van’s instructions (although it is mentioned in one of the Rotax manuals) nevertheless, it is a very good idea. These springs have been known to go missing because they can and do break … which means they end up either flopping around inside the engine compartment (unlikely for any extended period of time), laying on a runway somewhere for another airplane to run over or falling out of the sky possibly on people below. Any of the three aforementioned possibilities is unacceptable in my opinion, so the springs on the DOG Aviation RV-12 will be safety wired.

Rotax mechanics suggest passing .041 safety wire through the center of the spring … much the same idea as the safety wire used inside a garage door spring. When adding the safety wire, the idea is to only contain the spring if it breaks so the wire need not be drawn tight to the spring and should be left a little lose on the loops the springs attach onto. The safety wire will be encapsulated in the silicone at the loops (and hopefully inside the spring as well) to prevent it from wearing from vibrations. I used 9 inch pieces of wire folded in half which seemed to work out about right. When twisting the wire, a drill chuck was slipped into the loop to prevent the loop from closing while the wire was twisted.

Using the safety wire pliers to twist the safety wire on one of the springs on cylinder’s #2 exhaust pipe spring. Notice the other springs have been completed and the safety wire has a lose fit as can be determined by looking at the visible loops. Looking closely one can see the drill chuck inserted in the loop to insure there is play after the safety wire is twisted..

The safety wire was trimmed along with curling the ends ... next, high temperature silicone needs to be placed onto the springs and safety wire. Due to an engagement, I did not have the time to finish off those tasks, so will do so during the next work session.

Return from the future: Decided to amend this post to include the last step of the instillation which involves placing high temperature silicone inside the exhaust system springs and on the spring hooks. There are two schools of thought here either fill-up the entire spring with high temperature silicone as Van’s suggests, or use a method that many Rotax mechanics have adopted which is to only lay down a thick bead along the sides of the spring and leave most of the inside of the springs open so the spring can dissipate heat .… I’m going with the latter plus the additional step of placing .041 safety wire through the center. The main reason for the use of the silicone with the springs is twofold … it changes the resonant frequency of the springs so they don't vibrate as much making them last longer and placing a little silicone on the hook prevents vibrations from moving the hooks and prematurely wearing through the hooks on the springs.

Admittedly, even with the use of silicone the springs do wear and do break but the life span is increased greatly if the coils are potted in the silicone along with the hooks. I placed high temperature silicone on my finger and worked it through the openings between the coils until there was a large bead inside the springs. Next I built up silicone on the outside of the springs then took pieces of small wire and pulled the safety wires into the silicone so they are secured in the silicone as well.
The muffler springs after the application of the high temperature silicone … the safety wire that is running through the springs has been pulled into the silicone bead inside the spring.

This completes the assembly of the RV-12’s exhaust system with the exception of two temperature probes that need to be inserted into tiny holes on the number 3 & 4 cylinder exhaust pipes (the tiny hole can be seen on the bend of the #4 exhaust pipe in the photo with the safety wire pliers). That will be covered in a separate post because of a modification that will be made to the connectors.