It isn’t often I can do “scouting party” duty on engines. Powerplants aren’t normally my focus. They are a necessity for flight, but they aren’t my main attraction. A great many pilots I’ve met appear to be just the reverse: They are extremely interested in engines. I enjoyed the chance to fly behind the first European and first American installations of the new 60-hp HKS 700E 4-stroke from Japan’s HKS Company Ltd. With that in mind, this pilot’s report is rather different. We’ll look at this new engine on two different aircraft. As all Ultralight Flying! readers know, Rotax has enjoyed great success for more than a decade as an ultralight engine supplier. After dominating the 2-stroke market, Rotax introduced the 81-hp Rotax 912 4-stroke, which has become a popular powerplant choice for sport aviation aircraft. Is a 4-Stroke Better? All 4-stroke engine manufacturers trumpet their advantages over 2-strokes: lower fuel consumption, quieter operation, longer time between overhaul, and reliability.
The First Two Ultralights with the New HKS Engine
|Gross weight||103 pounds|
|Exhaust system weight||7 pounds|
|Oil tank/cooler weight||6 pounds|
|Total Installed weight||116 pounds|
|Displacement||680cc (42 cubic inches)|
|Gearbox ratio||2.58-to-1 or 3.52-to-1|
|Fuel||Autogas, unleaded premium|
|Time Between Overhaul||500 hours (Initial inspection)|
|Power||60 hp at 6,200 rpm|
|Torque||58 foot pounds|
|Fuel Consumption||2.6 gph at 75% power|
Cosmetic appearance, structural integrity, achievement of design goals, effectiveness of aerodynamics, ergonomics.
Pros - All-new 4-stroke, twin-cylinder horizontally opposed powerplant from Japan. Complete package with technical support during installation. Technically advanced engine product which includes a prop-speed reduction unit. Fitting is in progress for several designs.
Cons - Only available in 60-horse configuration at this time, though a "half-HKS" has been pondered, as has a 4-cylinder model with 120 horsepower. Four-stroke engines have more parts to maintain and this pushes up the purchase price.
Subsystems available to pilot such as: Flaps; Fuel sources; Electric start; In-air restart; Brakes; Engine controls; Navigations; Radio; (items covered may be optional).
Pros - The 700E comes standard with prop-speed reduction unit, electric start and external oil tank and cooler. It features dual carburetors and ignition is dual CDI. Exhaust is stainless steel. Air-cooling suffices for cylinders, while heads are oil-cooled.
Cons - More systems means more pilot work, although the Rotax 582 it competes with requires attention to a liquid-cooling system and radiators.
Instrumentation; Ergonomics of controls; Creature comforts; (items covered may be optional).
Pros - A large diameter, integral flywheel dampens torsional vibration. Though more complex, the 700E installed on either a Flightstar or AX2000 in tractor configuration did not obscure vision more than a 503 would have.
Cons - The engine itself makes a compact installation, but its peripheral items such as muffler and oil cooler make the engine appear busier than a Rotax 582 or certainly a 503.
Taxi visibility; Steering; Turn radius; Shock absorption; Stance/Stability; Braking.
Pros - Start it up, taxi out - with minimal waiting required, you can be flying quickly with the air-cooled HKS engine. The 700E lacks the temperature sensitivity of some engines. Nickel-coated cylinders are said to help this.
Cons - The 700E lacks the quick spin up of a 2-stroke engine, so you must anticipate power needs sooner (though the difference is very minor on light aircraft).
Qualities; Efficiency; Ease; Comparative values.
Pros - While the 700E may yield a slightly longer spool-up than a 2-stroke engine, the torque will make itself felt. Climb didn't seem to give the HKS any extra workout in either the Flightstar II SL or AX2000.
Cons - Quick additions of throttle may not deliver power as quickly as some 2-stroke engines; you must plan ahead a little bit longer (the same as when flying heavier aircraft in general).
Quality and quantity for: Coordination; Authority; Pressures; Response; and Coupling.
Pros - No need to worry about oil mixtures or water temperatures. No additional pilot controls needed to operate the 700E.
Cons - No negatives.
Climb; Glide; Sink; Cruise/stall/max speeds; Endurance; Range; Maneuverability.
Pros - HKS 700E compares well to the Rotax 582 (which has 5 more horsepower) and is substantially more powerful than either a single- or dual-carb Rotax 503 it might also replace. Some top speed increase likely compared to the 503.
Cons - Adds 6 pounds compared to a 582-equipped airplane, or nearly 30 to a 503 aircraft, so climb increase may not be the same as expected when comparing horsepower figures.
Stall recovery and characteristics; Dampening; Spiral stability; Adverse yaw qualities.
Pros - Four-stroke engines may reward their buyers the most when flying cross-country across unfamiliar territory (you should always have a safe place to land within gliding distance). Greater fuel economy can also aid safe flying.
Cons - Installations of different engines require operators to perform new weight and balance calculations. Overlooking this could bring grave consequences; always redo weight and balance after any equipment addition.
Addresses the questions: "Will a buyer get what he/she expects to buy, and did the designer/builder achieve the chosen goal?"
Pros - Marketing in North America by HPower Ltd., whose staff (same as Flightstar Sportplanes) enjoys an excellent service reputation. Buying is easy and service is great. HPower is working with many airframe builders to fit the HKS engine; several are ready and flying now.
Cons - Some aircraft aren't served by the 700E (yet); Part 103 ultralights don't need this power, weight, cost or complexity. Not all aircraft that are appropriate have been fitted yet. New installations take time.