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What Our Members Are Building/Restoring: Twinjet Sailplane - Wisconsin HP-18J

From September EAA Sport Aviation Issue

By Chris Esselstyn, EAA 245088; Waukesha, Wisconsin

Powered flight and experimental aircraft building have been among my primary passions for the last 40 years. As a youth, I was bitten by the flying and building bug through my father. There wasn’t a time while I was growing up that we didn’t have a car restoration, ice boat, dune buggy, or model airplane father-son project in the works. When I was 13 my dad built his first aircraft, an RS-15 kit sailplane, which provided my first insight into soaring. Going full circle some 40 years later, I have followed in his footsteps.

Over the last four decades I have constructed a number of experimental composite plans-built aircraft. My first homebuilt was a single-seat Quickie, followed by a three-place Cozy, and then a Cozy Mark IV with a Lycoming 540 and fully retractable gear. After 3,000 hours in powered homebuilt aircraft, I was looking for a new project with a new challenge. Soaring and sailplanes have always captured my interest with their long sleek slender wings and aerodynamic efficiency. After researching on the Internet, I ran across the Schreder Sailplane Designs page, and the HP-18 became my clear choice.

The HP-18 sailplane kits were offered in the ’70s, and I was able to locate and purchase a kit in Alabama that only had the tail surfaces drilled and Clecoed. All of the components were in the original boxes, and the parts list was complete. I purchased a one-way commercial airline ticket to Huntsville, Alabama, and drove back to Wisconsin in a rental moving truck carrying all of the aircraft kit components. With my previous building experience, I decided to incorporate a number of changes to optimize the performance and comfort. These modifications included a modified airfoil design, a higher custom canopy, a new composite nose and composite turtleback, winglets, a center control stick, and retractable twin micro turbines.

The wings were the first items to build with the new Udo 15.3 airfoil geometry. The new wing profile was created using CAD to fit the existing per the plans’ spar height and wing chord. The templates were laser-jet cut out of mild steel for the construction of the new wing ribs using 3-foot stations from the wing root to the tip. A total of 396 new leading edge and trailing edge PVC foam wing ribs were made from scratch for the new airfoil geometry.

The fuselage was the next item. A raised canopy and turtleback was needed to fit the twin turbine installation. The old canopy, canopy frame, and turtleback were replaced. A new nose was also added to the fuselage for the new canopy geometry. When you change a couple of items it creates a ripple effect that adds a tremendous amount of extra work and building time.

Bob Carlton (Sonex jet test pilot and air show performer) provided valuable input and needed advice from his previous experience with the micro turbines. By building on Bob’s experience I engineered the installation of the retractable twin JetCat P300 turbines.

My goal was to keep the turbine installation as simple as possible. Instead of pivoting the turbines on a fulcrum like the traditional auxiliary-powered sailplanes do with a piston engine and prop, I chose to extend and retract the turbines with a vertical telescoping mast. A 12-volt linear actuator was installed to move the mast up and down with the turtleback turbine cover. The electrical wiring and fuel lines were routed to the turbines with a flexible moving cable guide. Fuel pumps and air traps were installed with the engine control units and cockpit turbine monitoring support units.

Why install twin turbines into a sailplane? By providing the ability to self-launch and restart during flight you no longer have to use a towplane to get airborne. Plus when the lift unexpectedly dies and you get low, a restart of one or both turbines will keep you out of the farmer’s field.

The turbines are compact in size (5.25 inches by 14.5 inches) and lightweight. The installation has less complexity than a piston engine with a prop and folding mast mechanism. One great advantage with this installation is that it has little effect on the aircraft’s center of gravity and pitching moment. A homebuilt composite 7-gallon fuselage header fuel tank filled with Jet-A, kerosene, or gas station diesel mixed with 5 percent turbine oil provides enough fuel for a self-launch and climb to 3,000 feet AGL. This quantity also provides enough fuel remaining for 25 minutes of sustained flight, with the use of one engine at reduced throttle settings. Each wing is constructed with a 3.5-gallon fuel cell for another 7 gallons of capacity if needed. The engines lack complexity, have few moving parts, and are virtually vibration-free. So far they have proven to be reliable and easy to start. Extending the engines has a similar drag frontal area as the retractable landing gear. This is quite different than a conventional auxiliary-powered sailplane folding mast with its piston engine and propeller.  

The turbine start sequence is fully automated. First extend the turbine mast and flip the turbine master switch “on.” Then move the throttle trim levers to the “up” position. Next move the throttles up to initiate the automated start sequence that brings the rpm to idle. At this point the throttles are active, and the turbines are ready for use. The shutdown procedure is as follows: Reduce the throttle to idle, move the trim levers to the down or “off” position, and let the turbines cool down and retract. Total time for mast extension and start up is around 1 minute 15 seconds.

I would like to thank all of the members in our EAA Chapter 18 and our soaring club (Wisconsin Soaring Society) for their support and flight-test assistance. Most importantly I would like to thank my wife for her continued support and assistance. 

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