Exhaust throttling
02-24-2004, 12:17 AM
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Exhaust throttling
Having seen just how well the LiteMachines throttle worked on their heli engine, I decided to try and duplicate that performance but in a form more suitable for fixed wing aircraft. The LightMachines unit is basically a muffler with a brass bullet that intercepts the exhaust.
Several designs were tried, some with and some without mufflers as the first two pics show. None of the pure baffles worked very well. Of the muffled versions, the only one that worked well enough was the one that had the "bullet" installed after the muffler as shown in the last two pictures. However, while low end is steady and solid, the engine loads up with fuel after prolonged idling, causing poor transition.
Muffler pressure to the tank sometimes helps but if you restrict the exhaust, you're going to get way too much pressure at idle. So, a pressure tap was installed in the outlet tube with a bevel facing the exhaust stream. This gives good pressure to the tank at full bore but at idle the effect diminishes dramatically, causing the engine to go lean and quit. As it turned out, what made this experiment successful, was the use of a Nelson plug. Prior to that, Cox, Norvel, stock and even Glowbees were tried. The Nelson plug worked, even without any kind of pressure to the tank, because that plug required a considerably leaner setting of the main needle. This in turn leaned the idle sufficiently to allow a good idle and clean transition.
Several designs were tried, some with and some without mufflers as the first two pics show. None of the pure baffles worked very well. Of the muffled versions, the only one that worked well enough was the one that had the "bullet" installed after the muffler as shown in the last two pictures. However, while low end is steady and solid, the engine loads up with fuel after prolonged idling, causing poor transition.
Muffler pressure to the tank sometimes helps but if you restrict the exhaust, you're going to get way too much pressure at idle. So, a pressure tap was installed in the outlet tube with a bevel facing the exhaust stream. This gives good pressure to the tank at full bore but at idle the effect diminishes dramatically, causing the engine to go lean and quit. As it turned out, what made this experiment successful, was the use of a Nelson plug. Prior to that, Cox, Norvel, stock and even Glowbees were tried. The Nelson plug worked, even without any kind of pressure to the tank, because that plug required a considerably leaner setting of the main needle. This in turn leaned the idle sufficiently to allow a good idle and clean transition.
