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Installing Canister
Mufflers In 2002 IMAC instituted a new sound limit rule. To review the criteria visit the IMAC web site at http://www.mini-iac.com and go to the Sound Task Force pages.
We have found that to meet these requirements the best methods we have today are to install quieter propellers. Three bladed props being the quietest, or two bladed ones that load the motor more and reduce the RPM. The second method to noise reduction is through the installation of canister muffler systems. These systems, although not seeming by some to be of much benefit, actually do lower the noise level and / or alter the exhaust "sound" to a more tolerable type. For those in doubt that your plane will suffer from these "add on" methods, a picture is worth a thousand words. The plane above is a typical 40% Edge 540,.mainly stock. The flying weight is about 40 pounds dry. It has a DA-150 motor, an Air Models 3 bladed prop and Pefa canister mufflers. Watching Don torque roll we don’t think we need to convince you much about the performance from this typical "quiet setup" well below 96 DB over grass Installing or retrofitting canister mufflers in our giant scale models is not the "Herculean" job many would be led to believe. The obvious question is of course "Do I have enough room up front to install the pipes and cans?" In applications such as the Extra 330S or Edge 540 there is plenty of room in front of the wing tube for cans, even with rear outlets. In the Cap 232 models the wing tube is generally more forward and lower than the Extra or Edge and you will probably find it necessary to install cans with front outlets. It is a bit of a tight fit, but it is definitely feasible to install cans in the Cap. You will need to build a "muffler tunnel" into your motor box area thus dividing your motor box into an upper half and a lower half. The installation of a sub floor accomplishes this task. Run a stringer from the front of the motor box to the rear of the motor box. You can then glue the sub floor box divider to this stringer.
The front of the motor box, which is usually closed will need to be opened up for the pipes to pass through. This will also let the cooling air pass through the tunnel
We have found most muffler cans in the 150cc class are about three inches in diameter and 12 inches long. Pipes are generally recommended to be "about" ten inches long. Refer to your engine distributor for more detail on exact length for best performance It is typically recommended to have at least 3/8" "free air" space around each of your cans to provide proper cooling in the tunnel. The more free air" you can provide the better. The length of your tunnel obviously needs to accommodate the can. Take into consideration whether or not you will use a rear outlet or front outlet on your canisters.
Provide an adequate size opening at the rear of the pipe tunnel for cooling air to escape.
Don’t forget to support and secure the cans well. One method we have been using with very little maintenance and few problems is a "cradle system". Some argue this is a "hard mount" system and could cause the pipes to crack. On the contrary we have had good success with it so far, with over 300 flights on my own 40% Edge 540.
Attach the cradle to the inside of the landing gear plate, a removable plate behind the gear plate, or maybe even the motor box sub floor. Your choice here. The height of the cradle is such that you do not force the cans into an unnatural position. Let the cans find their own place in the motor box and fabricate the height of the cradle to fit the can. The cradle is lined with Aerotrend (gray hi-temp) silicone tube. Cut the tube to length and slice it in half. Silicone it to the cradle with high temp silicone.
The cans are secured in the cradle with a ty wrap and another piece of half-silicone tube. Although the pics shows the ty wrap installed aft of the can center joint, we suggest installing the cradle and ty wraps at the joint where the can is the strongest and stiffest. This willpermit you to snug the ty wrap down well without distorting the can. Be prepared to change these ty wraps after each flying season or even after a couple hundred flights, as they will get brittle from the heat of the can. An alternative might be to use an aluminum or stainless steel strap.
Another variation of the cradle / ty wrap system . It is an ounce or two lighter than the "full silicone" method. This method has not been "field tested" yet but looks good. It is true that these canister systems, with the added weight of the cans and mounting systems, may add some extra weight to your system over the weight of stock mufflers. If you are "creative" in your construction methods you will probably be able to save a pound or two and end up with a "weight neutral" setup (comes out at suggested manufacturers weight!). The added benefit of canisters is a quieter and more "mellow" sound to your exhaust as well as an "instant" increase of about 200 to 300 RPM. (on the same prop). This increase in power let’s you "prop up" a bit so your overall performance just about stays the same. Enjoy this project and treat it as "another challenge" in the whole scheme of competition. After all, that’s what this game is about, overcoming all the little challenges and doing the best we can. If we can make are miniature aircraft that "little bit quieter" we, and our neighbors, will enjoy our sport that much more. Thanks to John Mitchell, Don McLellan and Bob Hudson for contributing photos Bob Hudson, IMAC 1865 |
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