FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1982
1982 - 2434.PDF
achieve the performance goals in the same time, but consume less fuel. Both engines are designed specifically for light aircraft, with accessories clustered at the rear, and the turbocharger placed further aft. They each produce 320 b.h.p. take-off power. The main difference be tween the two is that the RC2-32 oper ates at higher pressures and temper atures, and spins at 7,000 r.p.m. instead of 6,000 r.pjm. The most striking improvements that come from the change to rotary engines are considerably improved fuel burn, along with multi-fuel ability, small size, and low specific weight. Nacelles are longer, reflecting the for ward position of the engine, necessary for balance, while in a twin-engined air craft they are also much thinner. In the single, the nose keeps its width but the extra space behind the engine firewall can accommodate baggage, says Curtiss- Wright. Another airframe benefit is in wing size. With its light, fuel-efficient rotary, wing area can be reduced some 19-20 per cent. The good news feeds through to the rest of the airframe too, and enables Curtiss-Wright and Cessna to come up with an overall fuel saving of 31-35 per cent over the baseline single. Other performance predictions are equally im pressive (see table). Vibration levels are extremely low with rotaries, as any Mazda car owner will testify. Indeed, Nasa calls it the poor man's turbine, and adds that the smooth running characteristics, plus the exhaust noise absorption inherent in turbocharging, makes rotary engines very quiet. Flyover noise is expected to be cut from 86-2dB(A) in the piston- powered baseline aircraft to 80-7dB(A) in the Advanced RC2-47 rotary-powered single. The secret of success lies in the rotary engine's unique taste for stratified-charge fuel injection, coupled with turbocharg ing. Stratifying (or layering) the in coming charge is a way of burning the leanest mixture of fuel and air, while reaping the maximum thermodynamic performance from the meagre amount of fuel delivered. It means having two fuel injectors, one on either side of a high- energy spark plug (see diagram). The smaller pilot-injector sprays fuel over the plug, establishing an ignition "torch" which continuously ignites the main fuel charge as it is injected. By tailoring the main fuel-flow to the amount of air pushed past the injector by the rotor, a stationary flame front is established. This is energetic enough to ignite any fuel that can be pumped through the diesel-type injection system, as long as its high and low-temperature characteristics suit the outside environ ment. The result is an engine that is able to burn a wide variety of fuels, ranging from diesel, through aviation kerosene, to the avgas necessary for pistons. The SC rotary is also insensitive to timing if fuels are changed. Stratification is much better for rotar ies than piston engines because of the way the rotor sweeps the fuel past the plug. Stratified-charge piston engines have to be designed very carefully to secure the benefits, which makes them more expensive. Stratified charge rotar ies, however, are hardly more expensive than their normally aspirated counter parts. If, on top of stratification, the incom ing mixture is pressurised by turbo- charging, the rotary engine's power-to- weight ratio is improved dramatically, as is its fuel economy. But while turbo- charging is accepted as an ideal running- mate for charge stratification, Curtiss- Wright admits that it has not yet proved that both technologies harmonise if the engine is run for long periods. Turbo- charging puts huge stresses on bearings, seals, rotors and housings, and one of Nasa's main concerns is to see further funding for more work in these areas. Lewis Research Centre rotary-engines chief Ed Willis tells Flight that four-five years of development is needed to "get the technology in hand". It wants money to prove that a test engine is capable of sustained running at the high speeds and tr.rbocharging pressures set out in the Curtiss-Wright/Cessna study. Light weight, high-speed rotors, for example, will need an advanced casting process, and a durable thermal-insulation for PISTON/ROTARY ENGINE PERFORMANCE COMPARED Length Width (in) Engine weight (16) Gross Aircraft weight (lb) Specific fuel consumption Flyover noise (dB(A)) Time to climb reledyne Conti nental TS1-550 59-3/33-4 686 6,625 0-44e 91 2 to 25,000ft (min) 19 4 Rate of climb (ft/ min) Cruising speed (kt) Take-off distance to 50ft (ft) Engine cost 880 230 2,525 (1981 dollars) 356,000 Aircraft opera ting cost (1981 dollars) ($/hr a SOOhr per year utilisa tion) 226 (Note: all aircraft data is Ad vanced RC2-47* 52-0/16-5 467 5,650 0 371 79 1 13-5 1,820 257 1,750 Highly Ad vanced RC2-32* 48-6/16 373 5,375 0-355 78-9 12 8 1,990 260 1,640 344,000 355,000 210 205 for twin-engined ver- sion. * Rotary engine figures are estimated). rotors and casings is still four-five years away. Curtiss-Wright and Nasa are also thinking about advanced rotaries in the 800-2,400 s.h.p. class for commuter air craft. The prospects here are as exciting as they are for general-aviation aircraft, but to beat the advanced turboprops that are in the pipeline some form of waste- heat recovery is needed, says Nasa. While this is commonplace on ground- based engines, it has never been intro duced on aircraft because of its high weight. But when combined with a light, efficient rotary engine, heat recovery turns out to be very effective in the larger commuter aircraft. In fact Nasa reckons that an advanced rotary using turbocompounding (as one method of heat recovery is called), plus improve ments in ceramics and lubrication tech nology, will compete with advanced turboprops in size and performance— and use considerably less fuel. And in an era when fuel is already expensive and true scarcity looms on the horizon, that fact cannot be ignored. Q Right Stratifying the incoming mix ture by having pilot and main fuel injectors con fers the multi fuel capability. In a rotary, the rotor sweeps the mixture past the injectors and sparking plug in the direction shown—ideal for stratification. Left slimline nacelles will be the most obvious sign of rotary power PILOT INJECTOR MAIN INJECTOR ROTATION 1224 FLIGHT International, 23 October 1982
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
