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John Thompson's Kitfox IV-1200

By John P. Moyle
Photos by Patrick Panzera

John Thompson

John Thompson
The PRSU used was designed and built for a pusher application and gave John some problems. Therefore he does not recommend it for this application.

John Thompson
Among many other details in this photo, one can see the creative geometry applied to the engine mount.

John Thompson
The compact yet effective radiator is located just aft of (and below) the firewall, and neatly cowled. The exhaust stacks had to be lengthened to keep the exhaust heat from adversely affecting the cooling ability of the radiator.

John Thompson
The standard Subaru water pump is employed and performs its function more than adequately. A firewall-mounted expansion tank completes the cooling system.

John Thompson

John Thompson

John Thompson

John Thompson

John Thompson

John O. Thompson has led an interesting life centered on an aviation-based career path. At an early age he was hanging around the airfields in eastern Missouri hoping to score a ride. He chose to enlist in the U.S. Army Air Force in 1947 and served as a flight crew member onboard C-47s before and during the Korean War. Later, he served onboard B-25s.

Upon his release from active military duty he returned to the St. Louis area and began working on aircraft in the civilian arena. He and a partner took over the aircraft maintenance facility at a small county airport and were fortunate enough to garner some special project work from nearby aviation giant McDonnell Aircraft. These development contracts allowed the business to flourish, but more importantly, they exposed John Thompson to the corporation. His talents were apparently well-appreciated, because from these humble beginnings came a lifetime career. He traveled the globe as a customer relations manager, finally retiring after 38 years with McDonnell Douglas. His last duty assignment was as the F/A-18 West Coast customer service manager at Naval Air Station Lemoore in California.

Retirement has given John the time to pursue (and fulfill) his dream of building and flying his own aircraft. John investigated many popular designs and was interested in an airframe that would allow an automotive engine conversion to be installed without too much difficulty. He and a friend visited a builder whose Kitfox IV-1200 project never proceeded far along the way toward completion, but when John’s friend decided not to pursue the purchase of this otherwise neglected project, John decided to make an offer himself! The deal was struck, and the components were relocated to a local communal hangar. The purchase consisted of only the welded steel fuselage structure, the Cub-style bungee spring landing gear, plus the wing and empennage pieces. Kit construction proceeded following the manufacturer’s instructions until John got to the engine installation.

The Engine
His preferred powerplant was a horizontally opposed water-cooled Subaru engine, which has become a favorite among experimental aircraft builders. John chose the EA-81, which is not a model installed in Subaru automobiles sold in the United States, but which does arrive here after being previously operated in Asian countries. This particular engine was purchased from a local engine importer as a used engine and was adapted by John for use in his Kitfox while it was still under construction.

This plane has never flown with any other type of engine installed. John had no desire to tempt the fates with even the finest two-stroke technology that the factory recommends. Fortunately there are a lot of Subaru engines, in a wide variety of displacements and performance, readily available to the experimental aviation community. There have been so many Subaru conversions installed in experimental aircraft that it’s hardly considered “groundbreaking” any more, although in truth, each conversion is unique in some way. Most importantly, several commercial entities offer propeller speed reduction units (PSRU) for the Subaru, so that part of the equation is merely a matter of selecting a suitable unit. In this case, John feels he made a mistake in matching this PSRU with his application.

John chose the RFI-brand PSRU Don Parham built in Oklahoma, but this particular model was intended for gyroplanes where the engine is typically installed in a pusher configuration. John found it tricky to get the 80-mm Gates-type cog belt to track correctly with his tractor installation. He was eventually able to sort out the issues, but he categorically states that for this particular application, he would not recommend this redrive installation to others; it’s best to get the proper PSRU rather than monkey with adapting a pusher PSRU to a tractor one. The PSRU features a ratio of 2.1-to-1, which gives the 72-inch diameter, three-blade Warp Drive propeller the ideal speed of 2380 rpm when the Subaru is making its best torque at around 5000 rpm.

Engine Mount
John selected a motor mount manufactured by Western Power Products of Bakersfield, California. The mount was intended for this model of aircraft, but it had to be modified to facilitate the Subaru installation in the Kitfox, which was accomplished with relative ease. (The Bakersfield company no longer offers this product.)

Cooling System
One of the unique challenges of adapting a water-cooled engine to an airframe is choosing the correct radiator size and location, both of these items being critical to the success of the installation. The science involved in arriving at the correct dimensions sometimes results in a radiator so large as to render it impossible to mount unobtrusively. It’s also possible to end up with a compact unit that works fine on the ground, yet fails to provide adequate cooling capacity for the treacherous climb-out phase of flight on a hot summer day. John’s solution includes a 7-inch by 24-inch by 3-row brass radiator. John mounted his radiator (seen in a photo to the right) to the belly of the airplane just aft of the firewall, in a fiberglass fairing. At first he found that the system worked well enough on the ground but did not provide adequate cooling in full power mode. Mathematically the volume of the coolant and the dimensions of the radiator were correct, so the problem was thought to be the same as what World War II engineers discovered when they dealt with similar problems in the water-cooled fighter aircraft of the era; it seems the air entering the radiator must be slowed.

So against conventional thinking, the entry orifice was restricted in progressive steps until the amount of cooling air was just enough to provide sufficient volume while allowing the air in the plenum to expand (slowing it down), which gives the air molecules sufficient time to extract the heat while passing through the radiator fins.

Induction System
The intake system is a modified Subaru aluminum manifold with an Ellison throttle-body fuel injector. This guillotine slide style carburetor has found wide acceptance in a large and diverse group of engines used in experimental aircraft. The smallest unit, the EFS-2, intended for engines up to 85 hp, is no longer marketed but is still actively supported by Ellison Fluid Systems, the manufacturer.

The EFS-3A is designed for engines from 85 hp to 140 hp. The model used on our subject engine is an EFS-2 and does not have carb heat in the traditional, manually applied hot air fashion, but rather the intake manifold has hot water pumped past the base flange of the throttle body. Once normal water temperatures are reached there seems to be little chance of ice forming with this system. John Thompson’s experiences to date, which include hundreds of hours flying this component combination, have certainly proven to be trouble-free in that respect. However he does believe that his engine would benefit from the next larger size Ellison.

One effect felt from the Subaru installation is that the standard vertical tail area is insufficient to cope with the additional torque supplied by the PSRU-equipped engine. While these devices do not improve horsepower output (at a given rpm), they are torque multipliers and the tail of this aircraft was never intended to deal with the amount now being applied. These planes were originally designed for a Rotax 582 of only 65 hp. A trip to Nampa, Idaho, offered John the opportunity to discuss the inadequate longitudinal stability with the factory engineering staff. He discovered that staff members were completely aware of this deficiency and had increased the tail height by 7 inches to compensate. Unfortunately, this modification began with serial No. 1717 and John’s airframe is No. 1714. Just missed! The choices then ranged from adding vertical tips at the ends of the horizontal stabilizer to adding height to the factory tail and rudder assembly. Eventually an engineer suggested that just adding some volume to the vertical surface by extending the leading edge several inches might do the trick. This was attempted and found to be just the ticket, and it had the advantage of being by far the simplest modification of those considered.

As a trial-and-error method, John bent up a 5-inch leading edge for the vertical stabilizer and attached it with aluminum tape. This approximated the additional tail area offered by the factory’s modification and proved to be perfect. A permanent corrective part was constructed and rigidly attached. Only a close inspection would catch your attention, since this alteration looks completely natural on the Thompson Kitfox.

While John had the attention of the staff engineers at Kitfox Aircraft, an inquiry was made concerning the possibility of reducing the wingspan. John had always felt that the wing loading was too low for his intended mission of daily flights at relatively low altitude in central California’s San Joaquin Valley. This area is well-known for its bumpy air in warm weather. The farmland kicks up major thermals, and light aircraft tend to get knocked around rather vigorously when flown below 5,000 feet. He was told that he could shorten each wing by one bay, or about 18 inches per side. The resultant 3 feet of reduced wingspan has provided just the right amount of additional wing loading and has transformed this Kitfox into a more comfortable machine.

Electrical System and Instruments
The electrical wiring is all routed to a service bus inside the cabin, between the firewall and the panel. It’s a uniquely tidy installation and quite compact. The instrument panel itself is fairly typical of most Kitfox brand aircraft, with all the basic flight instruments for cross-country flight plus all the necessary instruments to monitor the condition of an experimental water-cooled engine.

Landing Gear
The other major alteration to the airframe is the change from the narrow Cub-style welded steel tube and bungee cord spring main gear. After enough experience with the original gear, the decision was made to install the 2024 T-6 aluminum spring gear made to order by Grove Aircraft Landing Gear Systems, of El Cajon, California. These units are “gun drilled” for the installation of internal brake lines. Anyone who has seen a Grove landing gear (or any product from Grove for that matter) will comment on the craftsmanship apparent in every item it makes. This standard finish* aluminum design bolts right up to a Kitfox airframe with the supplied hardware, between the original forward-gear mount and the float-gear mount that all Kitfox airframes from the former SkyStar Aircraft Corporation have in common.

The standard wheels, tires, and Matco hydraulic disc brakes are still used with the new gear, but with the 6-inch-wider track and the softer suspension qualities of the Grove gear, it’s an entirely different aircraft during landings. Ground handling is also improved with a slightly better view over the cowl due to the lower stance of the plane on its new-style gear.

The Cowl
John has had all of his custom engine installation housed in the standard round cowl with the fake cylinder head bumps, indicative of a radial engine installation, which, of course, there are few if any to be found on Kitfox aircraft. It’s been a cute feature that has always made this brand easy to identify at a glance, but it isn’t the most aerodynamic style by a long shot. Many builders have chosen to go with a less “retro” look and install the sleek and modern Series 7 style cowl. John is one of this group, and he was at one time looking at upgrading his airplane to include this fiberglass engine enclosure and integrate his cooling ductwork more fully than the original factory cowl allowed. The Series 7 cowl has the thrust-line correctly located for an engine with a PSRU, unlike the round cowls that always seemed just a tad “off center” since the props were spinning much higher than the midline of their circular openings. John anticipated an improvement in cruise speed as the primary benefit, besides the obvious beauty factor.

John’s experience in building and flying this modified Kitfox has been a satisfying one. The adventure has proven successful in every regard. Unfortunately, an unscheduled but uneventful off-field highway landing resulted in an overzealous tow-truck driver destroying this beautiful example of John’s craftsmanship. But not to be discouraged, John got right back to it. His current project is another Subaru-powered experimental; this time he chose the all-metal Zenith CH 601 XL and is literally days away from taxi testing. Look forward to reading more about this aircraft in future issues.

Note: *Optional finishes ranging from polished, anodized, and Alodine through painting and powder coating are available from the manufacturer.

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