This article is intended to describe a throttle coupled spark adjustment system that I developed for a Zenoah 38 and to discuss the merit of the concept. Although the drawing could be used to guide construction for Zenoah or other engine, the many details required for construction are precluded in an introductory article. Various schemes for automatic spark advance have been used since the days of the manual spark advance of the "T" Model Ford. The scheme for advancing the spark discussed here may be somewhat unique in that the transducer moves on a straight track rather than following an angular position. This approach seems to offer some advantages in construction simplicity and does not suffer any disadvantages in operational capability based upon about thirty five hours of flight testing.

This installation uses Bill Carpenter's (C.H. Electronics) breakerless capacitor discharge ignition system. Basically, the C.D. ignition system consists of a 500 ma. hr. four cell battery pack, an electronic box, and a Hall effect transducer together weighing about a half pound. About a pound and a half of parts is removed from the Zenoah consisting mainly of flywheel and magneto. The C.D. ignition equipped Zenoah is almost a pound lighter than the factory version. This fact alone is worth a lot in the performance of your airplane. Weight distribution may be another advantage since the engine weighs less and battery and electronics units are behind the engine. There are a few qualitative advantages to a spark advance ignition system also. The ignition timing is adjustable for best start-idle and full throttle settings. I am presently using 3.5 to 32 degrees before top dead center. The engine will fly my airplane with a spark setting of 5 degrees after top dead center. I discovered this last year while experimenting with different settings. I had incorrectly timed the engine for running backwards. Since it was a beautiful day, I flew anyway with full throttle for starting and flying at 5 degrees after T.D.C. The power developed was equivalent to about one-fourth normal throttle power. I could fly with this power since the FAR-CAM (see Fall 85 HIGH-FLIGHT, page 67) is relatively light at 14% pounds. Another qualitative advantage of a spark advance ignition is the ease of starting and engine running smoothness. The unmodified engine has a fixed spark setting which is a compromise between ease of starting and development of full power. The 3.5 degrees before T.D.C. makes hand starting easy with no kick-back and allows a very low idle speed that is exceptionally smooth. Engine running smoothness results from gradually advancing the spark as the throttle is increased from idle to full power. I have used a couple of non-linear spark advance arrangements but have settled on a linear spark advance with throttle because of no obvious disadvantages and because the linear linkages are easier to maintain in calibration.

The bottom part of the drawing shows the throttle coupled spark advance linkages set at mid throttle and spark. I rotated the carburetor about 40 degrees clockwise to simplify throttle linkage. However, this required a somewhat complex adapter as shown on the drawing made from 1/8 inch fiberglass plus two gaskets. The larger holes use flat head bolts that screw into the heat isolator. Linkage length and effective quadrant length are changed for calibration of spark advance. The track assembly allows about 40 degrees of spark advance at its radius from crankshaft centerline. This assembly is made of 1/16 inch piano wire, two pieces of telescoping brass tubing, a piece of 1/4 x 3/8 inch brass and a pair of aluminum clamps. The clamps are made from one piece of aluminum drilled with 1/16 inch holes, then carefully razor sawed into two pieces. The brass tubing is soldered to the other brass piece by assembling the parts in place on the rails and jiggling for alignment. I used silver bearing solder but regular solder may be strong enough. The ball is made to press fit into the trolley and is soldered also.

The idler crank assembly that couples throttle motion to the spark advance trolley is made from aluminum, 3/32 piano wire, brass wheel collars, and uses nylon bearings made from the adjustable type aileron horns. The ear, where the celvis would go, is cut off close to the 1/4 inch diameter nylon with the 3/32 inch hole.

The timing disc and timing plate are made from 1/8 aluminum. The timing disc has three 3/16 holes, one for the timing magnet and two for a home made spanner wrench to tighten the prop hub against the disc. I use about 25 ft. lbs. to torque the hub. Two screws in the timing plate go into engine holes vacated by the magneto. A steel stabilizing bracket is secured to an engine crankcase bolt.

The aluminum taper collar will probably require some lathe work for accuracy and fit of the taper. This piece is highly loaded and must be accurate. It replaces the flywheel but is not keyed to allow adjustment of spark timing. When torqued down after timing, I have never experienced any slippage. I have made two of these units to date. The first required many hours to design and build. The second unit required about 16 hours to build. If you have the time, the extra engine smoothness and airplane performance gain are worth the effort. The extra battery pack in the airplane requires additional maintenance but is quite adequate for my flying sessions of one to one and a half hours. As with any of our engines, careless flapping of the prop is risky. With the ignition switch on, this unit will fire the spark plug at any propeller speed since the sensor switches when the magnet leaves its edge.

My modified Zenoah produces enough power to fly the FAR-CAM airplane vertically as long as you wish after a twenty foot ground roll on takeoff. You may roll the airplane on the way up. Flying an overpowered giant scale airplane is fun. The full size CAM Special (July 1984 Sport Aviation) has a power loading of 4.8 pounds per horsepower and I believe the FAR-CAM to be about the same.