Experimenter

April 2013

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

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S c o t t Ehn i 's Tur b in e - Po w e r e d Z e n i t h C H 701 S P engine-driven accessories every time the starter button is engaged. But there are changes that need to be made to this little turbine starter motor for it to spin a prop, and there are some features and details that need to be removed. The permanent magnet generator (PMG) is designed to power all the aircraft instruments and accessories while the engine is being started. Removing the PMG eliminates about 15 hp worth of drag from the little engine, in addition to its excess weight. The original jet-fuel starter used some electric solenoids to control the fuel flow, and even though they are high-quality mil spec and Scott has never had one fail, A simple but very useful instrument panel, including an Apple iPad. Sharp eyes might pick up the USB pen drive used for downloading fight data. he didn't want to have to supply full-time electric power to the engine to keep it running. So he replaced $700 to $800 worth of electric solenoids with a single $70, highpressure-brass, three-way valve rated at 3,000 pounds per square inch. The high side of the fuel pressure system only generates about 10 percent of that, so it should work for a long time. The igniter that comes with the engine is designed to run off the PMG, and it requires some ridiculously high voltage and amperage. But to make the engine more compatible with traditional aircraft electrical systems, Scott opted to use a 24-volt igniter, wired into the start solenoid, so that it's only "igniting" when that solenoid is activated during the starting procedure. Once started the engine keeps running on its own flame front. An elegant feature of the starting system is the 50-percent switch. Upon hitting the starter switch, the engine begins to rotate. Fuel is then added and begins to burn, but once the turbine reaches 50 percent of its design rpm, it automatically shuts off the starter. The idea here is that the pilot can simply leave the starter switch in the on position, and in the event of a flameout, it will begin to restart on its own. Scott opts to switch the starter to the off position after the turbine is running, and has yet to regret it, even in light rain. They have no desire to fly in heavy rain. Since this is merely a starter motor for a much larger turbine engine, the designers were more concerned with size and weight than they were with fuel burn efficiency. So the stock exhaust system is inherently inefficient. A new system was in order. The difficulty with this is that it had to be built and installed to exacting tolerances, being concentric within 0.001 inch. Welding 321 stainless can be difficult as it tends to "move around" according to Scott. So the process they chose to use was to fabricate it to the best of their abilities, heat it as an assembly to 1,400°F (the normal operating temperature of the part) over and over again to "season" it. Then once they were happy that the part had become stable, they machined it to fit the required tolerances. When you combine these major changes, which also include Teflon-coating all the inlets, they were able to get more than 150 hp from this little turbine that's rated from the factory at only 90, with an 80-percent safety factor on the gearbox. But they limit the engine to 120 hp. The stock planetary gearbox that came with the engine, which converts the 72,500 turbine rpm to 3,000 at the prop, is really limited to 120 hp by design, and Scott doesn't want to exceed the recommended limits. The 120 hp is only used for extreme climb-out and is capable of taking the 701 beyond its redline at straight and level 20 Vol.2 No.4 / April 2013 Photography by Pat Panzera

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