Progress on the Sonex Highwing prototype continues with a major milestone reached on Friday, June 6: the aircraft’s first engine start. The 130-hp UL Power UL350iS fired up smoothly, followed by successful brake checks and a short taxi test—marking a significant step toward first flight.
The Sonex team reports that only a few tasks remain before inspection and registration are complete, and they’re aiming for first flight ahead of EAA AirVenture Oshkosh 2025. With the FAA registration in progress and an inspector lined up, the final stretch is in sight.
Engine Start and Taxi Test
The UL350iS engine ran without issue on its first start, confirming that systems are operating as expected. A short taxi run and brake test followed, with all results nominal.
Sonex Highwing Progress Since the Last Update from the Factory
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Windshield & Doors: The new windshield was successfully formed using heated flat-wrap tooling, yielding a nearly flawless fit. Machined hinges arrived and the doors are now installed. The original lanyard-style interior release was replaced with a compact, lightweight handle for better functionality and safety.
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Wings & Control Surfaces: Final skinning of the outboard wing panels and assembly of ailerons and flaps were completed in May. Simple metal flap root fairings are in development to aid preflight inspections. Aerobatic endplates will be used for initial flights, with mold prep underway for full-span fiberglass tips. The Highwing supports modular tip configurations for either a 23′ 1.3″ or 26′ 2.4″ span.
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Control System: A CNC-machined lower aileron torque tube bellcrank completed the linkage between the unique center-mounted control stick frame and elevator pushrod. The all-pushrod system enables responsive handling and flexibility between dual-stick, pilot-only, and center-stick setups.
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Cockpit Wiring & Headsets: Headset jacks have been installed in the center tunnel, with 3D-printed insulating caps ensuring clean, isolated installs. Control stick wiring is complete, supporting trim and push-to-talk on the pilot stick. Receptacles allow optional dual trim/flap control with an appropriate electronic controller.
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Throttle Quadrant: A single throttle quadrant has been installed on the pilot’s side with enhanced cable travel to suit the UL engine. A dual-throttle configuration is in development and will be evaluated during flight testing.
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Brakes: Revised brake pedal designs have improved strength, with the current setup featuring pilot-side brakes only. Co-pilot braking systems will be evaluated during the test program.
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Fuel System: The aircraft’s primary fuel system is ready for inspection and first flight, although wing tanks are still pending delivery. Plumbing and venting for the wing tanks is already installed.
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Cowling Modifications: The standard B-Model cowling is being modified with a fiberglass scoop to accommodate the UL engine’s oil cooler. This graft-on part will be offered to other builders using UL Power engines.
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Seating Adjustments: Adjustable seat back supports are now installed, providing three backrest angles and two hinge points for improved pilot comfort across a range of body sizes.
More information: sonexaircraft.com
