Aircraft Wankel Power Rotary Engines

wankel powered aircraft curtis wright story


Exciting news for homebuilders! Your dream-come-true engine is on the drawing board and in test stages. Read on for more info on applications for and experiments with rotary engines in aircraft.

Catastrophe! Busted engine! Next to a crash that completely wipes out your airplane, a demolished engine is about the worst thing that can happen to an aircraft owner.

Unusable engines can be caused by several things, the most common of which are: internal structural failure; failure to pass annual or 100-hour inspections, or, in commercial usage, arriving at the FAA established mandatory Time Between Overhauls (TBO) limit – but what about the rotary engines.

In today’s aviation environment, your airplane can be returned to airworthy condition only by replacing the offending engine with a new or overhauled power plant of the same kind, or one of a different type or size that is approved for your airplane.

rotary powered test plane

But wait a minute! Before choosing one of the above alternatives, spend a minute or two dreaming about what kind of power plant you’d really like for your airplane if you had unlimited choices. We all know what engines are on the market and what their advantages and disadvantages are.

Any pilot or plane owner could think of numerous improvements that could be — and need to be — made to the engines currently available. If we had the ability and the opportunity, we’d reduce their size, weight and fuel consumption.

We’d also make them smoother, quieter, lighter and reduce their frontal areas. A big improvement in durability would be a primary aim, as would be lower initial cost and ability to operate on the cheapest, most cost-efficient fuel.

Now, isn’t that a dream for airplane owners to drool over?

Impossible, you say? Yes, it is impossible today to buy such an engine; however, engines meeting all of our desires are on the drawing board and on test stands in advanced states of maturity. Just to whet your appetite, two comparisons with today’s horizontally opposed, air-cooled engines will be presented here.

Current engines have an average weight of about two pounds per horsepower; the new engine about one pound per horsepower. Assuming 150 cruising hp, current engines burn about 12.5 gph; the new engine about 9 gph.

Right about now, I’m sure your questions are: What is it? and – Where is it?

first aircraft rotary engine

The first rotary-combustion engine designed exclusively for aircraft use: Curtiss-Wright’s RC 2-90 air-cooled, two-rotor engine of 300 hp.

The new wonder engine is the latest version of the Wankel-type rotary-combustion aircraft engine. Research models of advanced rotary-combustion engines are now running in Curtiss-Wright test cells.

The Wankel engine has had a rather unspectacular history in this country. Prior to 1973, all of the major U.S. auto companies were involved in testing Wankel rotary engines to determine their future adaptability as automobile power plants.

However, the severe 1973 gasoline crisis caught the Wankel with a poor reputation for fuel economy; about the same time, durability problems began turning up. The final blow was General Motor’s announcement that further development of the Wankel would be discontinued.

This General Motors snub gave the general public the impression that rotoray-combustion engines are hopeless failures and all-around losers. As a result, rotary-combustion engines disappeared from the U S. auto companies and were forgotten.

No, not quite forgotten. Curtiss-Wright held the license from NSU (Volkswagen) to develop and sell rotary-combustion engines in the United States, and they still had faith in the future success of the engine. Another Wankel licensee, Mazda in Japan, also had enough faith to develop and produce an extremely successful automotive rotary-combustion engine.

In fact, as early as 1977, Mazda had sold its one-millionth automobile powered by an R/C engine. To date, several million more rotary engines have been sold, and Mazda continues to sell the RX-7 sports coupe to extremely enthusiastic purchasers.

Mazda has made notable contributions to the R/C engine’s power, economy, durability and public acceptance. The company’s latest rotary engines now powering the most recent version of the RX-7 is a two-rotor engine, which produces 120 hp at 7000 rpm.

Fuel consumption, durability and emissions are equal to or better than automotive reciprocating gasoline engines of similar horsepower. As a bonus, size and weight are far less than the equivalent, conventional engine.

Since all discussion of the Wankel thus far has revolved around automotive and industrial use, it would be easy to get the impression that Wankel engines have never been designed for aircraft use or employed to power aircraft in flight. In the course of its research. Curtiss-Wright has installed rotary-combustion engines in several aircraft, including one helicopter.

Curtiss-Wright’s first rotary engine specifically intended for aircraft use was the RC 2-90. two-indicating two-rotor and 90-indicating 90 for each rotor making a engine (designed to run on diesel or jet fuel) The engine produced 310 hp from a total weight of 300 pounds and fit into a two-foot cube.

The RC 2-90 was designed to power a drone helicopter, but when the military project was abandoned, engine development also was terminated. Incidentally, cooling was by forced air over carefully designed and positioned cooling fins.

An axial blower, which was part of the package, provided the forced air. This rotary engines combined all the research gains in the air cooling and proved the feasibility of air-cooling rotary engines in the smaller sizes.

The only rotary engines that has actually powered a flying airplane is the RC 2-60 U5. This is a automotive-type, water-cooled engine with an aircraft carburetor and other minor modifications.

cozy rotary engine conversion

The Lockheed “Q” Star ultraquiet airframe was constructed from a Switzer 2-32 sailplane modified with a conventional landing gear. RC 2-60 engine mounting and an aluminum Corvette radiator on the nose. Since this was an ultraquiet experiment, the engine exhaust was heavily muffled to practically eliminate engine noise.

To reduce propeller noise, a belt-reduction drive of 5.34:1 reduced the engine’s normal 6000 rpm to 500 rpm at the propeller. This experiment was completely successful both in the achievement of the quiet flight levels sought and in the smooth, reliable operation of the rotary engine


To further investigate operation of rotary engines in aircraft, the bulky, belt-reduction drive engine was installed in a Cessna Cardinal airframe, using the extreme propeller gearing and slow-turning prop.

Later an RC 2-60 with internally installed reduction gears allowed use of a normal Cardinal propeller turning at conventional speed. Both tests were classified as complete successes.

The heavy, drag-producing belt-reduction system and automotive nature of the engine did not allow all the natural rotary engines size and weight advantages to be fully exploited; nevertheless, the trials were so promising that a considerable amount of enthusiasm was generated.

The helicopter trial involved a Hughes H-55, which also was powered with the RC 2-60, and came through with excellent marks. All of the knowledge gained through flight testing and research on rotary engines adapted for aircraft use was combined in an aircraft-engine prototype. the RC 2-75 Y1.

unknown aircraft wankel engine

This engine, designed from the beginning as an aircraft power plant, was liquid-cooled and produced 285 hp from a dry weight of 280 pounds and dimensions of 21.5 inches x 23.7 inches x 31.4 inches. Ready to fly, with a complete cooling system and coolant, weight was 358 pounds.

An aircraft carburetor and other aircraft grade accessories were used, and a standard propeller drive with internal .356:1 spur gear reduction allowed use of production-type propellers turning at normal rpms. The reduction drive and general engine configuration were reviewed with Piper, Beech, Cessna, the FAA and accessory suppliers.

This engine has accumulated over 1500 hours on test, including over 100 hours at wide-open throttle, and sustained runs at 7000 rpm. Most likely, this engine could pass the 150-hour wide-open throttle qualification test for certification at this point, which thus far has not been attempted.

During experimentation with rotary-combustion engine prototypes. Curtiss-Wright has discovered several fundamental facts, even though the Wankel-type engine is still at the initial stages of its development cycle.

One of the most fundamental discoveries is that the smallest and lightest engine for a given power output will always be the one with the largest number of rotary power units, although the smallest and lightest will never be the most efficient.

As an example, a two-rotor, 275-hp engine will be smaller and lighter than a single larger-rotor 275-hp engine. The single-rotor unit will be larger, heavier, less expensive and slightly more fuel efficient.

Economic advantages, as well as existing fuel-injection technology, favor the large single-rotor design, though a twin-rotor engine compromises some cost disadvantages for size and weight advantages. Liquid cooling was retained on the RC 2-75 Y1 rotary engines for fuel economy and growth considerations.

Liquid-cooled engines can be operated in flight at the same fuel consumption figures achieved on the test stand. Air-cooled engines generally require a richer mixture to keep head temperatures down to acceptable levels during takeoff and climb, and during extreme warm-weather operation.

Another factor concerns the high-power use of the rotary engine — in other words, its ability to produce high power from a small, light engine. The high-power density makes air cooling difficult and expensive. At some power level, the rotating-combustion, air-cooled engine will become cooling limited.

In response to the favorable results from rotary research, National Aeronautics and Space Administration (NASA) announced a five-year, $15 to $20 million engine-research program to examine conventional, turbine, diesel and rotating-combustion engines with a view toward extensive improvements in durability, power and economy.

Curtiss-Wright is included in the funding to develop an advanced rotary-combustion engine based on the RC 2-75 prototype engine. One of the NASA requirements is the ability of each engine to develop 250 cruising hp at 25.000 feet. To meet this requirement, the RC 2-75 two-rotor configuration was originally studied with turbo-supercharging added to the basic engine.

Six objectives of the NASA program have been identified:

  1. Engine efficiency and performance improved with a specific fuel consumption goal of .38 pounds per horsepower per hour. (Current engines primarily are in the .5 Ib./hp/hr. range.)

  2. Efficient operation on 100/130 aviation fuel and one or more alternative fuels — jet. diesel. unleaded auto gas or low distillate.

  3. Low exhaust emissions.

  4. Manufacturing costs comparable to or less than present aircraft engines.

  5. Overall life-cycle costs and maintenance lower than current aircraft engines.

  6. Altitude capability equal to current engines.

Two engines have been run on test stands, the RC 2-75 and the RC 1-75; both ran normally aspirated with stratified charge fuel injection.

This type of fuel injection requires two injection nozzles — a pilot nozzle and a main nozzle. The pilot nozzle is needed for initiating combustion and for lower-power operation; the main nozzle supplements the pilot nozzle for power.

The pilot nozzle injects an ideal fuel mixture close to the spark plug, which is easily fired by the spark. A flame front is started that lights off the extremely lean mixture injected by the main nozzle, which is too lean to be fired by a spark but which can be ignited by a flame.

Lean mixtures of as much as 28 parts air to one part fuel can be used in turbocharged engines. A perfect mixture (stoichiometric mixture), as injected by the pilot nozzle and as normally supplied by a carburetor, is approximately 16 parts air to one part fuel.

This pilot nozzle, main nozzle stratified charge method allows the engine to operate on a wide range of fuels and provides superior fuel economy due to an extremely lean main-nozzle mixture.

Stratified charge rotary engines have shown essentially the same combustion performance on gasoline, JP4 and JP5 jet fuel, diesel fuel and methyl alcohol without change in the engine.

Present test-stand engines are now operating with a compression ratio of 8.5:1, however, the engines built on the NASA contract will probably have increased compression rations in the range of 9.5 or 10 to 1.

The direct-injected, stratified charge engine offers the great advantage of safer diesel or jet fuel. This engine will operate unthrottled. that is. no butterfly valve will restrict airflow through the carburetor to control engine power as on conventional engines.

In the unthrottled operation, air is pumped into the engine without limitation. Power is produced in direct proportion to the amount of fuel injected into the cylinders. Fuel injection quantity is controlled by the throttle.

rotary powered airplane

A Cessna Cardinal with a Wankel rotary-combustion engine modified to contain a designed-in, gear-type propeller reduction system. The propeller was geared to turn at normal rpms (around 2400), and a normal-sized Cardinal propeller was used. Note the conventional appearance of the aircraft. Note also the cooling radiator at the lower edge of the cowling.

Stratified charge, direct injection, unthrottled engines have demonstrated fuel economy equal to or better than automotive diesels, and further improvement is expected in both low-end and high-end performance. Hydrocarbon emission levels equivalent to contemporary automotive engines have been achieved.

With the light weight and small size of the rotary-combustion engine, even though water-cooled, gasoline engine performance can be surpassed with better than diesel-fuel economy. The already light weight of the engine will be enhanced by the smaller fuel supply for any given mission.

Cessna’s integration study of the advanced rotary-combustion engine into an airframe designed specially for it proved that, except for the shortest mission lengths, the rotary-advanced airframe combination, plus fuel weight, produced a smaller, lighter, cheaper, more-economical-to-operate aircraft than any other combination studied.

The lighter weight of the engine and less required fuel per mission allows a smaller wing and tail, which results in a lighter, cheaper basic structure. Also, remote location of the relatively small coolers allows packaging advantages and thrust recovery at the heat-exchanger air outlet.

At the present time, the most probable configuration for the advanced aircraft rotary engine will be a two-rotor arrangement with 47 per rotor (RC 2-47), turbocharged, stratified charge, direct injection and unthrottled.

RC2 32 rotary brake horsepower

Estimated performance of the highly advanced Curtiss-Wright RC 2-32 rotary engine of 300 hp. Note especially the extremely low Brake Specific Fuel Consumption (BSFC) of about .350 at 55 percent cruise power. Present horizontally opposed piston engines run at about .450 to .500 BSFC. This represents a 23 to 34 percent decrease in fuel consumption. And remember, the engine will run well on the lowest-possible-cost engine fuel, distillate.

The RC 2-75 engine, which has been running on test stands for years, continually improves in power and efficiency. At the beginning of engine development, meeting the NASA contract requirements of 300 hp for takeoff and 250 cruising hp at 25,000 feet required a turbocharged RC 2-75 engine.

Based upon present and expected near-future technology and research improvements, the NASA power requirements can be met with a smaller, lighter, turbocharged RC 2-47 engine. The RC 2-47 engine is known as the advanced rotary engine.

The more distant future promises continuous improvement in Wankel technology, resulting in the ability to meet NASA’s power requirements with an even smaller and lighter RC 2-32 engine. Multifuel capability will be featured on all of these engines and almost certainly, a low-grade distillate will be among the suitable fuels.

Low-grade distillate will produce the most gallons of engine fuel per barrel of crude oil. The advanced rotary engines, unlike our present-day general-aviation engines, will be turbocharged from the ground up to keep size and the power-to-weight ratio far superior than current engines.

NASA has been extremely interested in the technology of turbocharging the Wankel engine and the percentage of power increase that can be expected. For a base engine, a Mazda RX-7 two-rotor automobile engine was chosen, and two different turbo-superchargers were adapted.

To date, these engines both have been run in test cells with excellent results. From a normally aspirated 118 hp at 7000 rpm, supercharging has succeeded in extracting 191 hp at 7000 rpm, an increase of 61 percent.

The NASA contract that covers an engine of 250 cruising hp will be much too powerful, complex and expensive for sport and homebuilt aircraft, however, Curtiss Wright studies prove that all of the basic rotary engines advantages will scale down almost in proportion to size decrease. A smaller, unsupercharged single-rotor engine of 100 to 125 hp is an easy transition from the RC 2-47.

NASA recognizes the potential of the rotary engine for training and sport aircraft, as well as for business aviation for which the supercharged, stratified charge, unthrottled, 300-hp RC 2-47 is primarily intended. Rotary automobile engines have proven to be efficient and reliable enough in the 110-hp range to deserve consideration as aircraft power plants.

Starting with the two-rotor Mazda RX-7 engine, NASA has run many test configurations (both supercharged and normally aspirated) with encouraging results. Power ranges between 100 to 200 hp, exactly the range that is most useful for training and sport airplanes.

rotary horsepower graph

Power chart of the Mazda RX-7 engine both in normally aspirated and turbocharged form. Turbocharging raises the normal power of less than 120 hp to over 190 hp.

These rotary engines do not use stratified charge; they use conventional carburetors rather than unthrottled fuel injection. Not absolutely state-of-the-art designs, they are nevertheless small, light, dependable and surpass the horizontally opposed, four-cylinder engines in every important respect.

Best of all, they are based upon an automobile engine already in fairly high production, which can keep engine cost relatively low. In order to meet present and future noise regulations, high-ratio propeller gearing will be required.

Since the rotary engine will require gearing in any case, the low propeller rpm can be attained with little additional expense. There is no doubt that the rotating-combustion aircraft engine is well within the limits of current technology.

In fact, it is the most likely candidate to replace the obsolescent, horizontally opposed, aircooled, piston engine in sport, training, homebuilt and low-end business aircraft. It offers advantages in size, weight, simplicity, fuel consumption and multifuel capability.

Research to make this engine type available to the general-aviation market continues, and it is urgently needed. Unfortunately, even with NASA development funding, the date for a final engine design is 1986 or 1987.


There is still no certified, production, rotary-combustion aircraft engine available on the market. Nevertheless, changes are taking place and progress is being made.

Most interesting of the new developments is transfer of rights, patents, research, parts and contracts from Curtiss-Wright to the John Deere Company Hold on! I know what you’re going to say,John Deere is a farm-equipment company and airplane power plants are probably the last priority on their development timetable.

Not so according to John Deere spokesmen, who profess a great interest in the potential of a Wankel aircraft engine. Their hope is to cooperate with established engine manufacturers to produce a complete engine.

John Deere most likely would produce a basic rotary-combustion power-plant package to which engine companies would add dual ignition, propeller governors, vacuum pumps, propeller reduction gearing and other miscellaneous pads and drives necessary for aircraft use.

John Deeres basic package probably will come in two sizes, an RC 1-40 and an RC 1-350. These units can be multipled to produce RC 2-80s, RC 3-120s, etc.. for an almost unlimited number of horsepower ratings from 125 hp up.

Rotary-combustion engine technology in the automobile field has also taken a leap forward with the new Mazda-type 13B engine and a turbocharged Mazda racing engine. Those who build their own airplanes have a better chance of powering them with a rotary-combustion engine than we who buy our airplanes finished from a dealer.

Turbocharged Mazda rx 7 rotary combustion engine

NASA is test running Mazda RX-7 rotary-combustion engines on test stands under various conditions. This turbocharged version produces 191 hp at 7000 rpm. The automobile transmission is used only as a convenient connection between the engine and the dynamometer.

Duncan Aviation Engines of Comanche, Oklahoma is offering rotary power plants in sizes ranging from a single-rotor, 40-hp, aircooled, to a twin-rotor, turbocharged, geared 350-hp liquid-cooled engine.

These rotary engines all feature the smooth, powerful, lightweight performance characteristic of the Wankel and are advertised to have a service life before overhaul (TBO) of 3000 hours. NASA is well into application of testing of a rotary-combustion engine in a flying aircraft. A Cessna Super Skymaster (Army O 2-A) has been obtained surplus from the Army.

The front engine has been removed and replaced by a two-rotor, turbocharged, 210-hp Mazda RX-4 engine with a belt-reduction gear system and a constant-speed propeller. The aircraft has been comprehensively instrumented to measure almost any engine parameter worth measuring.

NASA is on the brink of significantly advancing the knowledge of rotary-engine performance in the actual flight environment. Unfortunately testing of the Skymaster/RX-4 project is in jeopardy due to shifting of funds among in-house projects. Let us all hope that the rotary-combustion engine flight testing is allowed to proceed.

We need it badly.

Dawn Star SP 440 38hp rotary engine


Aircraft Wankel Power Rotary Engines
Article Name
Aircraft Wankel Power Rotary Engines
Rotary engines have been around for a lot longer than you may realize. Here we investigate the early days of research and development looking for the ultimate aircraft engine, smooth, reliable and powerful.

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