My Desultory Blog

Sonex jet runs engine for the first time

December 23, 2009 — The SubSonex, a jet-powered homebuilt aircraft being developed by Sonex Aircraft, LLC, completed its first engine run last week at the company’s Oshkosh, Wisconsin, headquarters causing warm smiles and melted snow. Sonex Founder John Monnett said the aircraft features a new Czech-built engine that has shortened the development time frame of the aircraft, which led to the wintertime test.

“We just wanted to see what the flame front out the back end and make sure we weren’t having any problems of melting the tail off the airplane,” said Monnett, whose main focus as of late has been the jet project. “We did run it up to 100 percent after a few starts, we were cautious, and we melted a lot of snow.”

The PBS engine (První Brnenská Strojírna Velká Bíteš, a.s. TJ-100) was delivered a week ago. It’s bigger and more powerful than the original Heward engine that was seen when the aircraft was unveiled at EAA AirVenture 2009. The Heward was designed for radio-controlled applications and currently lacks the engine controls systems that the PBS has since it’s a “legitimate production engine,” according to Monnett. PBS is certifying the engine to Czech civil aviation standards.

The TJ-100 is used in such applications such as Auxiliary Power Units, (APU) and on drone aircraft. On a SubSonex, it will produce as much as 240 pounds of thrust and is the same engine used by the Super Salto powered sailplane that has performed at AirVenture in the past. To use the new engine Monnett said they had to reconfigure the entire aircraft, including new engine mounts, engine controls, and boost the electrical system to 24 volts.

The successful engine run may lead to a test flight soon once minor tweaks and FAA certification is completed. Monnett says the SubSonex is basically ready to fly and the first flight will likely come as soon as Wisconsin winter weather and runway conditions are optimal.

Starting a turbine engine
For the un-initiated, starting a jet engine takes several steps. Depending on the engine, those steps are done automatically or with some input from the pilot. On larger, multi-stage turbine engines the starter turns the core of the engine first and the igniters fire immediately. After the core has reached a certain RPM, fuel is introduced and the engine “lights off” and accelerates, becoming self sustaining much like a propane torch. Since the TJ-100 is a one-stage turbine, its start sequence is slightly different, saving the fuel and ignition for last once the Electronic Control Unit (ECU) has determined that engine systems are functioning normally. Monnett says that the engine may be small, but it still has the unmistakable sound of a jet.