Aerospace America article about rotaries, June 1986
This article was in the "Aerospace America"
magazine, June 1986, Page 48.
After an extended adolescence the rotary aircraft power plant is ready to go to work. Childhood fame as cover engine on mechanical magazines faded, and the ever promising lab wonder sank into obscurity. Now, following a decade and a half of low-key but persistent development, some engineers look to the mature spark-ignition variant to rev up a low-end general aviation market stuck on idle.
Combined with composite airframe technology, a light, high-power rotary that is relatively inexpensive to buy and operate and that could burn other than increasingly scarce high-octane avgas would make the general aviation market "go like hell," says Edward Willis, assistant branch chief for small engine propulsion at NASA-Lewis. But new technology will have to be coupled with legislative wisdom. About one-quarter of the price tag of every general aviation aircraft goes to pay for the builder's liability insurance for claims against previously built air- planes. At the bottom end of the market that is an intolerable burden. Customers yearning for a small plane look for used aircraft. Builders' biggest competition is their own old airplanes. Congressional action on revising liability statutes could release the brakes on new designs.
NASA-Lewis is doing its part to advance the technology by developing computer codes for analysing cycle performance, internal airflow, and structural strength, doing studies and contracting technology developments that reveal the attractiveness of the rotary engine. Lewis verifies the flow codes using a transparent, sapphire-sided rotary test rig to make holograms of the internal flow and measure it by laser velocimetry.
One or more rotors, each with three curved combustion faces, turn eccentrically about a central shaft within a figure-eight-shaped combustion chamber called a trochoid. Gas seals on the rotor sides are made of piston ring iron. Gas blowing by them passes through ports in the rotor to the engine intake. There are no valves. Trochoids are flame sprayed with tungsten carbide in a cobalt matrix to reduce wear from the surface-following rotor apex seals that isolate the combustion faces from each other. One-piece carbon seals used in early Toyo Kogyo Mazda automobile engines leaked, raising fuel consumption and engine emissions. Curtiss-Wright, which acquired rights to the engine in 1958 and developed a stratified charge version in the '70s, came up with a two-piece sintered metallic seal that was more effective and longer lived. It is spring loaded to feed out from its slot as it wears. It is so good that the Mazda engine generally runs 100,000 miles without internal maintenance and needs about a tenth as much warranty work as the company's piston gasoline and diesel engines.
One rotary advantage is low vibration. Rotary rather than reciprocating motion and smoother torque pulses hold down rivet-loosening shakes. Minus valve trains, cams, and connecting rods, the engine is simpler. Three power pulses for every turn of each rotor give high power to weight. All this adds up to a light, low- volume power plant well suited for aircraft. Rotors can be stacked for more power up to a practical limit of six to eight. More rotors would make the engine too long. Pushing for more power means going to larger rotors. Present engines can extract about 2 hp/cu in. To raise output to 4-5, last September NASA-Lewis signed a three-year contract with John Deere, which took over Curtiss- Wright's rotary engines in 1984. Such an advance will require operating at higher rotor speeds and chamber pressures. Deere will research the high-pressure, electronically controlled fuel injectors needed to give faster stratified-charge combustion-chamber injection to allow such a power increase. Conventional automobile injectors discharge into the intake manifold. The new injection hardware should be ready for testing this month on Deere's 40-cu in single-rotor test engine. If NASA were to exercise the option clauses in the contract, Deere would also look at advanced lubrication and friction reduction, coatings, lightweight rotors, and high-temperature ceramics. And since stresses are lower on shafts for rotary engines than for piston engines, shafts are designed not to a maximum stress but to an allowable deflection. Deere would determine the effects of large deflections on bearings. Federal budget balancing may prevent NASA from pursuing these options.
Deere has teamed with Avco-Lycoming to develop stratified charge rotary aircraft engines. They could be beaten to the light plane marketplace with a rotary engine by Teledyne's Continental Aircraft Products Div., which has licensed development of the British Norton Motors motorcycle engine. Teledyne's 110-lb dry weight GR-36 runs on readily available automotive gasoline and puts out 85 hp from 35.8 cu in displacement, hence the name. Specific fuel consumption is comparable to piston engines, on the order of 0.45 lb/hp-hr at cruise power. The addition of stratified charge and direct injection could enable the GR-36 to burn many other fuels, such as available jet grades. A total loss oil system holds down weight by eliminating sump and filter and boosts fuel economy by reducing friction. Oil injected at the aft main bearing at the rate of 1/4 pint per hour lubricates the shaft, bearings, rotor, and walls as it works its way toward the chamber where it is consumed in combustion.
The GR-36 would suit the two-seat bare-bones trainer market, which, in the three years since the start of engine development, has become one of the most depressed portions of general aviation. William Hill, Continental's director of government marketing, says the GR-36 has completed test-stand work but, because of certification costs and the lack of an aircraft commitment, is not yet "off the shelf and available. Applications engineering would still have to be done if a customer asked for the engine in a specific airplane. This includes mounting the engine, placing the radiator, determining prop rpm, which fixes gearbox ratio and size, and choosing between carburettor and intake injection with turbocharger." In Hill's eyes, the smooth running rotary offers "all the good things of both a two stroke engine [few parts, light weight and compactness, high power to weight] and a four stroke [fuel economy, low maintenance, long, reliable life]."
This summer an even smaller single rotor version of the engine, the R-18, will fly in a military remotely piloted vehicle. An engine in this class should arouse interest among builders of ultra light aircraft. But because these airplanes and their pilots are unlicensed and company liability insurance is so high, Continental is not encouraging such use.
Moving in the opposite direction to- ward more power, Deere and Avco are developing a near-term technology rotary engine having a 400-hp takeoff rating. This output would serve twin-engine business aircraft, high-performance single-engine personal airplanes, and small commuter airliners. The turbocharged and intercooled engine will be able to maintain take off power to 20,000 ft and its 300-hp max cruise power to 25,000 ft. Time between overhauls is targeted for 2,000 hr and fuel consumption at 0.41 lb/hp-hr for takeoff power and 0.39 lb/hp-hr for 300 hp cruise. Avco says comparable piston engines burn 0.6 and 0.43, respectively, and turbo props, 0.65 and 0.55, respectively.
Bench testing scheduled to begin late this year will take the next step toward January 1990 production deliveries. Once the shared development is completed, Avco will build engines for aircraft and pay Deere royalties. Deere will handle non aviation applications.
Designated SCORE 170 for stratified Charge, Omnivorous Rotary Engine, the power plant has two rotors of 105 cu in. (1.7 1) each and is targeted to weigh 506 lb. In size it falls between the two-rotor, 350 cu in. (5.8 1) per rotor SCORE 580 ground engine and the single-rotor, 40-cu in. (0.7 1) "NASA" SCORE 70. Robert Mount, Deere's project manager, says although the SCORE 170 is a new size engine, "performance, reliability, and fuel consumption have been demonstrated on the larger and smaller engines. No breakthroughs are required. It's all near-term technology." Avco hopes to price it just above reciprocating engines at about third the tab for a turbine. Thus it will cost slightly more than a piston engine but run on cheaper, more available jet fuel.
It can burn a fuel unsuited to a spark ignition engine because of stratified charge combustion. A small fuel-rich zone around the plug is ignited first and serves as a pilot light for a mixture in the rest of the combustion chamber too lean to be ignited by spark. In Deere's engine, a mechanically controlled injector meters a small amount of fuel into the recess in the trochoid wall housing the plug. Another injector feeds a larger amount of fuel into the main chamber where combustion is aided and controlled by a pocket scooped in the rotor face and the pocket's movement past the injector. Since only a small, low-compression, rich mixture has to be ignited, the stratified charge engine starts easily in the extreme cold, says Mount. "No aids such as preheating air or oil are needed. Starts at -32 F have been demonstrated."
Rotary engines and composite technology could halve the price of entry-level trainers to $20,000, believes Continental's Hill. But general aviation manufacturers will also have to show some imagination, he adds, perhaps getting ideas from home builders of airplanes. Good technology will be marketable, he feels, and cites the continued strong sales, in an overall depressed market, of the Piper Malibu with its wide, comfortable cabin and high-lift wing and the latest models of the Mooney M20 series and the intense interest shown in the Beech Starship.
Lewis' Willis feels the general aviation market will eventually turn around as the pool of used aircraft dwindles but could soar on the right combination of technology and insurance relief. He expects propulsion improvements to be matched by advances in low-drag aerodynamics and the use of high strength-to-weight composites.
What, then, might the light aircraft of the future look like? Just putting the new developments into conventional configurations may not take full advantage of technology interplay, says Willis. Normally ultraconservative general aviation airframers must learn to innovate to wring out the most benefits. For example, a switch to the smooth-running, quiet rotary may leave the propeller and airflow as the major sources of cabin noise. Contouring the prop may reduce its noise, but converting to a pusher configuration may do more. Moving engine and prop to the aft end of a low-drag, teardrop-shaped composite fuselage would move their clatter and roar and the high-speed, noise-generating prop-wash downstream of the cabin. Lending the fuselage smooth contours with fewer joints would further hush the wind, and constructing cabin walls of materials that conduct sound poorly would help keep out noise. Lowering drag would shrink the engine, again reducing noise and cutting fuel consumption.
Should the rotary engine indeed pull the chocks on a technology race, general aviation companies that are now thinking only of survival will have to throttle up their design staffs to keep from being left in the propwash.
This article discusses several of the non-Mazda rotaries that were in existence in the mid 1980s. Some of these have gone on to further development whereas others have disappeared.
Further reading and acknowledgements:
Other relevant reading at Craig's Rotary Page (Please go via the INDEX
Other relevant sites on the Internet (Please go via the LINKS
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