Muffler Housing
Housing uses space created by moving battery & fuel selectors.
Housing fits perfectly between rudder pedals
I decided early on that I wanted to have a very quiet airplane, both in the cockpit and from the ground. The Mazda rotary engine really needs a muffler as the exhaust gasses are traveling at supersonic speeds when they exit the engine. I've been told that an unmuffled rotary can kill small animals at a close distance! I will discuss this further in the engine section. Even had I chosen any other engine I would still have installed a muffler as I feel that if we address noise issues now, on our own, then there will be less likelihood of having noise restrictions imposed upon us, as has happened in Europe.
Recently there has been issues raised about the noise generated by GA aircraft. When you consider that no other motorized vehicle is allowed to be operated without a muffler, you can understand some of these concerns. There are groups out there that are against damn near everything, and want to have everything banned. General aviation is one of their targets and the noise generated by these unmuffled planes is great ammunition for them. If we don't try to quiet our engines on our own we may end up with legislation which may be very difficult to comply with.
Putting a muffler on an airplane usually means either installing it under the cowling, or hanging under the belly of the plane. Putting a muffler under the cowling will limit the size of muffler, with a corresponding limit on it's effectiveness. It will also produce a large amount of radiant heat, which will lead to cooling problems regardless of which type of engine that you use. It will also move your center of gravity forward.
Hanging a muffler under the belly will eliminate all of these concerns but adds another: Drag. Lots of it. Flat mufflers like the Spintech seem to be in the boundary layer enough to minimize this (more on the Spintech on the engines page), but I wanted to eliminate this, so I decided to build an enclosed housing that would accommodate a reasonably sized muffler out of the airstream and out of the engine compartment. Of course there is no free lunch, and to achieve this I had to take a small weigh penalty (4 lbs - not including the muffler), and more importantly I had to give up some of the floor space. However I don't feel this to be an issue since your feet are supposed to be on the pedals anyways.
This will accommodate approximately a 24x6x6 muffler with possibly an augmenter on the exit to help pull cowling air through the rads, through the engine compartment and out through the housing, reducing heat transfer from the muffler.
The RV 7/9 have a center strip of 8 inches wide between the peddles that is freed up due to the battery now being located on the front of the firewall. The wing spar happens to be 8 inches high, so this allows for a convenient 8x8x28 housing to be located between the spar and the firewall. The fuel selector must be relocated from it's normal location in front of the spar. While it would be possible to leave the belly skin in one piece and install the muffler from the firewall side, replacement of the muffler would have been very difficult, so I opted to have an 8x28 access panel from below. This must be cut before building of the floor to allow a shear to be used to cut out this panel. Cutting it any other way wouldn't do an acceptable job. I chose to use .020 SS for the housing as it is in reality a continuation of the firewall. The floor stiffeners are doubled up here to provide a mounting flange for the attachment of the access panel with 76 screws/nutplates. Early in the design I had reservations about cutting the floor here due to concerns with structural strength. However it is far more than compensated by the installation of the 8x8 "beam" that is formed by the housing. I plan on insulating this housing against sound and heat with firewall insulation, however I'm not sure if this will really be necessary.
This muffler is now complete and installed in the housing and I'm very pleased with all aspects of it so far. I will report more after flight testing.
May 31, 2005
Housing is made from .020 SS as it is exposed to FWF.
Extra stiffeners are used to provide a mounting flange for 76 nutplates to attach the access panel
1 extra piece of .062 angle is used to frame in front of muffler housing
2 pieces of .062 angle are used in place of fuel selector brackets to frame in rear of housing.
