A great way to add fun and excitement to your RC flights is to add a smoke system. Do a loop or a roll and then add a dense, white smoke trail and you have an airshow! And it’s easy to save some bucks by modifying your stock muffler instead of buying a commercial smoke muffler. You can then take that money and apply it to the cost of the smoke pump and required hardware.
Here’s how I did it with my Zenoah G-20 powered 1/4-scale J-3 Piper Cub from Hangar 9.
Step 1: Here’s the stock muffler on the Piper Cub’s engine. Remove it and clean it up with some solvent to really degrease it. I used a new single-edged razor blade to remove the old exhaust gasket. Here you see the new gasket I will replace it with.
Step 2: To plumb the smoke oil tank, I used gas grade Tygon fuel line. Make sure to use a gas rated stopper as well.
Step 3: To modify the muffler all we need is a large brass tube that fits into the main exhaust pipe to be used as a baffle tube, and a thin brass or copper for the oil preheat injection tube. I use Du-Bro and K&S tools to cleanly bend the tubing. For the smoke oil tank, I use a sullivan 10-ounce tank set up with a standard two-tube setup. Be sure to use tie wraps to secure your tubing as well.
Step 4: The first thing to do is to cut, bend and prepare the end of the Injector tube. To make a fine atomized spray of oil into the muffler, I use a wire cutter to snip off the end of the copper tube. This seals the end and shapes it so it resembles a fish tail. I then use my Dremel and a thin cut-off disk to nick the middle of the fish tail. This produces a very small opening about a quarter of the area of the tubing end. The tube is then bent so it’s long enough to enter the side of the muffler, pass through to the other side, and then bend 180 degrees to re-enter the muffler.
Step 5: Now drill three holes in the side of the muffler. For mine, the single entry hole is drilled in the muffler’s aft side, and the two others holes are drilled in the forward side. Drill the holes slightly larger than your injection tube’s diameter.
Step 6: Use a drum sander or grinding bit with your Dremel and remove the black finish around the holes. This will provide a clean surface for the JB Weld metal epoxy to stick to.
Step 6: Here you see the injection tube slid into place and ready to be sealed with JB Weld. Be sure to clean the injection tube as well as the surface of the muffler with solvent.
Step 7: Mix the JB Weld together until it is a uniform gray color then build up fillets around the tubes/holes areas. Smooth the fillets with a wet finger and then set the muffler aside for 24 hours to fully cure.
S tep 6: To provide a baffle tube, a large brass tube is slipped into the exhaust pipe, it should be a snug. Slip it into place, bottom it out and then mark the length, and then use a K&S Tubing cutter and cut it to length.
Step 7: To determine the internal length of the exhaust tube, bent piece of wire or welding rod, slide it into the muffler and slip it over the end of the tube,use your thumbnail as a length guide.
Step 8: Now use the wire to transfer the muffler exhaust tube length to the brass tube.
Step 9: The area at the end of the brass tube will be the baffle section and will need to have several holes drilled into it! Drill several holes in the end and then sand smooth. Slip the baffle tube into the muffler until it bottoms out and is flush with the exhaust pipe.
Step 10: Now slip the brass tube back into the muffler, and drill a hole in the side of the exhaust pipe and through the tube. Secure the brass baffle tube in place with a pan-head sheet metal screw.
Step 11: Here is the finished smoke muffler reinstalled on the Piper Cub’s engine. Make sure your smoke oil line does not chaff against your engine cowling.
Step 12: There are several good smoke pumps available and I have had excellent results with the SkyWriter from Sullican Products.
Step 13: When connecting the smoke muffler to the pump, I use a fuel filters for a clean oil flow, a check valve to prevent muffler pressure from pushing oil back into the pump tubing, and most importantly, a remote needle valve assembly from an old glow engine to regulate the oil flow. If you don’t meter the flow properly, the oil can cool off the muffler and reduce the amount of smoke it generates.
That’s it, you now have a complete ready to go smoke system. Be sure to first do a ground run to check the quality of your smoke. If there smoke is weak, check to see if there is residue on the bottom of your model. Too much smoke oil can cool the muffler down and prevent a full burn of the oil. Use the needle valve and reduce the oil flow a half turn at a time.
Check out the smoke density! This is what it looks like with the oil flow almost completely shut down (only one turn open on the needle valve). This setting gives a lot of “smoke-on time” and my10-ounce tank lasts more than 10 minutes. The system works great and when you switch the smoke off, it takes a few seconds for the smoke to stop completely.