Ultralights have long been defined by their two-stroke engines. Rotax and other brands refined the two-stroke for aircraft use, and today’s two-stroke powerplants deserve their popularity. In the important power-to-weight ratio, two-strokes are hard to beat. Their cost is low, their overhauls cheap; these humble powerplants have lifted many a pilot into the air for many hours. But despite the improvements, despite plentiful businesses to help you maintain your two-stroke engine, and despite a good performance record, two-strokes are still seen as inferior to four-strokes by many aviators. Add to these impressions the regulatory changes many see as inevitable. The blue-smoking two-stroke (even when optimized) presents environmental problems for snowmobiles, watercraft and gas-powered lawn tools. Two-stroke days are numbered—not because of efficiencies, but because of ecological concerns and political decisions. Hail the Four-Stroke General aviation people say, “Well, of course four stroke is better.” But that’s the only kind of engine most of them know, and their aircraft weigh enough to accommodate heavier powerplants. Cessna and Piper pilots often don’t understand ultralight designs and why light two-stroke engines are so necessary. Nonetheless, four-stroke engine enthusiasts win points for reliability, noise, fuel consumption, longevity before overhauls, and intangibles such as the way people feel when they hear a four-stroke rumble compared to a two-stroke whine. Two-stroke powerplants have many strong benefits, but it’s easy to argue in favor of the four-stroke’s appeal. With these factors at work, it is hardly surprising that four-strokes have invaded ultralight aircraft. The invasion will advance when certain ultralights become tomorrow’s light-sport aircraft. Though they seem new to many ultralight pilots, four-stroke engines—at least in the form of the half VW—have been around a long time. The Quiet Invasion Fiberglass-enclosed light-sport aircraft may adopt the Rotax 912 as their near-standard engine, but not all light-sport aircraft will be so heavy or enclosed. Trike ultralights are as ready as any ultralight type to qualify for the industry consensus certification standards now being devised. Many have already passed rigorous certifications in other countries, and the design platform has been proven for decades. In recent years, the Rotax 912 has made many sales to trike pilots. In Europe, the BMW 1100 is gaining popularity. Powered parachutes have created two-stroke installations as clean and functional as any four-stroke. They may use smaller four-strokes like the HKS, but they hardly need the power of the Rotax 912 or Jabiru 2200, nor their weight and cost. Single-place light-sport aircraft—yes, some will be offered—are additional candidates that may not go to four-stroke power. But many ultralight three-axis designs have already jumped to the Rotax 912 HKS or Jabiru, and others are experimenting with choices like the Smart (automobile) engine from Mercedes. Many Choices The photos accompanying this article represent just a few of the four-stroke engines you might choose. I only selected models with lower power and lighter weight to keep such powerplants in range of ultralights and light-sport aircraft. Even with this limitation, the World Directory of Leisure Aviation lists 26 companies that produce 30 engines offering 100 hp or less and generally weigh less than 150 pounds. Power produced by this wide variety of engines runs from 27-100 hp, and the engine weights run from 85-150 pounds. You can see that two-strokes ranging from 50-100 pounds offer the lightest airframe designs a clear advantage, even if they output only 25-70 hp (though Hirth offers two 100-hp models that weigh 93 pounds). Among those larger aircraft which will qualify as light-sport aircraft, four-stroke engines are bound to reign supreme. Our lightest aircraft may continue to use two-strokes until the environmental side brings about a ban on such powerplants. In the meantime, newly designed lightweight four-strokes will probably find growing market share. The companies I’ve identified offer varying degrees of support. Rotax is noteworthy for having a solid distributor network, a series of service centers, and superior technical support. Jabiru’s web site shows many installations, plus nose bowls and cowls of several descriptions available. HKS has built a reputation for responsiveness in the U.S. Hummel is a made-in-the-U.S.A. company and is reported to offer good service. The BMW and Mercedes engines have worldwide reputations behind them, though neither has established any U.S.-based presence. Painless Purchase As the far-and-away leader in this aviation segment, Rotax sets the price pace with the 80-hp 912 selling for roughly $10,000. The 100-hp 912S costs about $11,000. The 60-hp HKS 700E costs less than $7000, while the 80-hp Jabiru 2200 is about $8500. Real bargains for light kit builders come from the 28- to 37-hp Hummel engines at around $3000, though this is for a kit and the engines obviously won’t lift heavier aircraft. The BMW 1100S engine puts out 80- 100-hp depending on the developer. It sells for around $8000 in Europe, but its ability to fit other designs is not known. Unlike most car companies, Mercedes actually supports the use of its Smart engine in aircraft. Despite a world-class brand name, the 70-hp Ecofly M160 with its 1500-hour TBO has only made a small impact on the aviation community. As you choose your next light aircraft project or purchase, a four-stroke is much more likely to be in your future than in past decades. The purring sound reassures and offers greater service life while better satisfying the environmental lobby, making these four cycle powerplants a key to light aviation’s future.

Just when FAA officials believe they have it figured out, individual actions can introduce new questions. I'm referring to the light-sport aircraft category of new rulemaking proposed by the aviation agency. Consider the turbine engine. Why, you may ask, is turbine power the subject for a column that focuses on the lightest and slowest aircraft? The answer is that turbine engines are actually being used on the these aircraft, and they may be the right power for the use intended. Turbine Tug For two decades, hang gliders have been towed aloft by specially built ultralights. Aircraft like Bobby Bailey's Dragonfly have succeeded at launching hang glider pilots who live in flat areas like Florida. They don't need mountains. Just give them a Rotax engine and they'll take to the skies in swarms. Witness the spring ritual of dual hang gliding competitions in central Florida. Each year as the Sun 'n Fun fly-in ends, pilots converge from all over the world. The best distance racing aviators come to prove who's the best. Every one of them was launched by one of a fleet of no less than 17 Dragonfly ultralights and trike tugs. This year they all witnessed something new. The Rotax hegemony was challenged by no less than a gas turbine on the slowest-flying ultralight ever made. (A Dragonfly can stall as slowly as 18 mph and routinely tows hang gliders at speeds in the 20s.) My goal isn't to provide the details of the engine here; it will probably never enter production. Besides, a lesson about turbines is a subject for another writer. But look what this machine can do! Record Flight in Florida Developer Russell Brown of Quest Air (one the two main Florida aerotow locations) has been working on his turbine tug for many months. Before that, he installed numerous engines of increasing power seeking to perfect the powerplant for what is essentially a tractor. Tug pilots climb as fast as possible over a 5-minute window; then they descend as fast as possible to launch another hang glider. It's tough duty for pilot and plane. Brown calls his creation UltraTug. Others have tagged it Tugzilla. The turbine-powered Dragonfly tried to set a record last summer, towing a hang glider to 17,999 feet. Under F.A.R. Part 103, ultralight vehicles may not exceed 18,000 feet msl. On a second attempt, the UltraTug carried its pilot and a hang glider and pilot to the maximum altitude allowed. On the back of the line was instrument seller Steve Kroop. He writes, "Quest Air (where the flight started) is at 118 feet msl, so our AGL (Above Ground Level) is almost the same as our MSL (Mean Sea Level). Out west, thermals regularly take pilots to 18,000 feet, [but] I don't know [of any other location] where they can get that high above the ground. "The view was incredible at nearly 18,000 feet (3.4 miles up). It is a perspective that we don't often get." According to a charted barograph report-from one of Kroop's Flytec instruments, of course-the flight saw an average climb of 430 fpm to the ceiling. They started at 750 fpm and were still doing 300 fpm at 18,000 feet, proving the altitude viability of the turbine engine. The team checked winds at altitudes to find a modest 14-17 knots at Flight Level 180. A cloud deck at 24,000 feet allowed towing clear of clouds as required. Though temperatures were in the mid-90s at sea level, both pilots were dressed for frigid air aloft. Kroop flew a Shadow 148 intermediate-performance glider supplied by Just Fly, the importer of Aeros hang glider wings. A full-body-covering M2 Acer harness kept the hang glider pilot warm, and Flytec's 4020XL electronic digital instrument provided altitude info and produced the barograph. Industry chronicler Davis Straub confirmed the high flight. "Steve may be the hang glider pilot who has gone higher above the ground than any other hang glider pilot in the U.S.," he said. "[But British pilot] Judy Leyden went much higher (40,000 feet) under a balloon in Jordan." The Uncertain Future In the FAA's new SportPlanes| document, the authors specifically excluded light aircraft powered by turbine engines. If the rule passes as proposed, the turbine engine's future in light-sport aircraft is threatened. U.S. Hang Gliding Association leaders agonized over the turbine when they submitted the association's official response to the SportPlanes| proposal. These unpowered flyers were loathe to let turbines be dismissed without due consideration. The turbine Dragonfly is not a product and may never be more than a grand experiment. Its fuel use is disproportionately high compared with Rotax power, and finding parts to make more engines could prove challenging. Nonetheless, Brown is to be congratulated for pushing the envelope. This kind of activity may give FAA regulators reasons to reconsider, and that might keep the doors open to new ideas that can improve aviation. FOR MORE INFORMATION, contact Russell Brown at Flytec Instruments, 6548 Groveland Airport Road, Groveland, FL 34736; call 352/429-8600; e-mail info@flytec.com. To review all "Light Stuff" columns that have appeared in KITPLANES®,  visit www.ByDanJohnson.com, which links to the KITPLANES® with articles of interest.