We need this lightweight engine NOW!

ARTICLE DATE: September 1976

Aviation magazines and, sometimes, even the Sunday newspaper supplements print articles about “the” great new aviation advance that’s going to revolutionize private aviation and put airplanes into every garage.

Anyone old enough to hold a private pilot (license has read enough of that drivel to acquire a permanently distorted attitude toward any aviation developments reported in the news media. I Let’s face the fact that over the last thirty years, there haven’t been any developments. Everything we are currently using is old. Materials, structures, aerodynamic shapes, and engines.

True, small improvements slowly filter their way down to small aircraft. Such things as Fowler type flaps, fuel injection and turbo supercharging. But at what a fantastic slowness of pace and what a fantastic increase in price. We’re not even using the best ideas and technology we have available to us today.

Undoubtedly, the cost of tooling for and certificating new developments to advance the status of General Aviation is fantastic. But what other choice is available? Present airframes and engines have been refined almost to the absolute limit.

Future improvement of the same old technology is going to come in smaller steps and at greater expense. The old rule of diminishing returns! Some day we’re going to have to start at the beginning with something new that has current possibilities and great growth potential.

Teledyne Continental spent a fortune developing and tooling up for the “Tiara” line of engines. The Tiaras are magnificent engines but they are developed almost to the last gasp of reciprocating engine technology.

How much better for us all if Continental had spent as much on a new powerplant concept which is already good and which has development possibilities reaching far into the future. There is such a starting point, you know, and it is called the Wankel rotating combustion engine.

The rotating combustion concept has terrific potential for aircraft use since already it is smaller, lighter, smoother, more powerful, and quieter than any existing certified aircraft engine of equal power.

You ask, “If it’s so good why aren’t we using it?” To which I can only reply “Yes! Why aren’t we?” In his laboratory in Neckarsulm, Germany in the late 1950s Prof. Felix Wankel may have opened a whole new chapter in the book of General Aviation airplanes.

The primary purpose of his experiments was certainly not consciously directed toward production of a new power source for light aircraft. And just as certainly, the first product of his research had little or no utility. The first runable model of a rotating combustion engine was kind of an inside-out product.

Similar to old World War I rotary engines, the outer envelope of the engine revolved, while the internal rotor and eccentric stood fast. Torque and inertia of the heavy rotating mass made the whole concept rather unappealing to say the least.

Dr. Walter Froede of the NSU automobile firm in Germany, produced what was called a “kinematic inversion” on the original which resulted in a single three sided rotor revolving in a stationary housing. The first prototype of this more practical design was run on a test stand in 1956.

Licenses to produce and develop that engine were purchased by several companies, among them Curtiss-Wright of the U.S.A., and Toyo Kogyo of Japan, builders of Mazda cars. Millions of partially or completely successful rotary combustion powered automobiles are now operating along the highways and byways of the world.

Among them are the NSU Sport Prinz and R0-80, several Mazda models, and other low production prototypes from Mercedes Benz, General Motors, and Curtiss-Wright. Even in its earliest forms, RC engines displayed several very attractive advantages. Chief among these were potential for compactness and light weight for any given power output.

Mechanical simplicity of the basic engine is fantastic. Only two moving parts are involved; the three sided rotor and the eccentric (output) shaft. No valves or complicated valve train are required.

Induction of fresh fuel charges and exhaust of spent gases are through ports uncovered and covered by the rotor itself as it rotates with in its outer casing. The entire power cycle is on the four stroke principle with the same familiar cycle of intake, compression, ignition, expansion, and exhaust.

The basic rotating three sided rotor can be assembled in multiples; as a single rotor, double rotor, three, four or more. Probably the most common configurations in the future will be single or double rotor designs.

More than two rotors require complicated and expensive machining arrangements for assembling crankshafts. A single rotor engine has one power impulse per revolution, a two rotor has two impulses per revolution.

However, since the output shaft rotates three times for each revolution of the rotors, a single rotor RC engine has the smoothness of a piston six cylinder and a two rotor the smoothness of a twelve cylinder.

Tell any airplane owner that a new engine is small, light, powerful, and smooth; immediately his eyes will reflect envy and anticipation. Slowly the gleam will fade as the realization sinks in that cost is also involved.

The pure turbine and turbo prop are small, light, and powerful. They are, also, prohibitively expensive and thirsty. Rotating combustion aircraft engines promise to be an exception. Exotic high temperature-resistant metals are not required as in turbines.

Casting and machining for major assemblies of the engine are straightforward and conventional. The major remaining technical problem is sealing combustion chambers from the remainder of the engine. This is not a simple problem, but solutions are at hand.

The rotating combustion engine is considerably lighter for a given output than conventional engines. Part of the greater power to weight ratio comes from higher rpm, the remainder from compactness and simplicity. For equivalent power, RC engines require only about half the size and weight.

A two rotor RC engine is equivalent in power output to a four-cycle engine of about twice the displacement. Another endearing feature in these energy crisis days is modest octane requirement in comparison to compression ratio, enabling high compression for efficiency without need for high octane fuel.

I’m sure you are all familiar with the Wankel story as it applies to automobiles. Many producers have tried RC type engines but today only one is left: Mazda. General Motors’ experimentation with RC power is well known but now the concept is shelved to emerge who-knows-when, if ever.

The fuel crisis set back the Wankel and it may never recover. There is a great temptation to dismiss the RC concept for aircraft use because it isn’t feasible for automobile use. Nothing could be further from the truth.

Use of the RC in automobiles runs into two conflicting requirements;

1. High torque and lugging power at low rpms to accelerate a heavy automobile fast enough to please American motorists.

2. High power output and good fuel economy at cruise speeds. These two demands are mutually antagonistic. Inability to resolve these problems has kept the RC engine out of the mass automobile market.

In aircraft use, however, most operation is at 50%-100% power which allows the aircraft RC engine designer to concentrate on the high power, low fuel consumption problem. It appears that RC engines are basically much better suited to aircraft than automobiles. How do we know the RC power will ever be suitable for aircraft?

We are not talking about an untried concept that may or may not prove out. Several engines have been developed exclusively for aviation use by Curtiss-Wright, some air-cooled, others watercooled.

The RC2-75Y1 was designed as a liquid-cooled general aviation prototype. RC2-75Y1 meaning Rotary Combustion, two rotors, with each chamber displacing 75 cubic inches. Design horsepower is 285 with a 15% growth early after introduction and more later.

Dry weight is 280 pounds. Wet, ready to fly with coolant and radiator, 358 pounds. All this comes in a package 21.5″ x 23.7″ x 31.4″. A representative air-cooled engine is the RC 2-90 Y2 with an axial-flow air blower as an integral part of the engine. Output of the RC 2-90 was 310 and weight about 300 pounds.

Another air-cooled military version was the YRC 180-2 with 310 horsepower and 278 pounds. These engines are still in the experimental stage and no attempt has been made to certificate them as production airworthy engines.

Lockheed Aircraft Corp. offered the rotating combustion engine its first chance to fly. Under a Navy contract, Lockheed was experimenting with ultra-quiet aircraft for undetected low altitude reconnaissance. Several airframe configurations were developed culminating in the QT-3.

Basically the QT-3 (QT for quiet thruster) consisted of a highly modified Schweizer 2-32 sailplane equipped with art amidship mounted Continental 100 horsepower engine turning a large slow-turning propeller through a reduction drive and long overhead propeller shaft.

The QT-3 yielded airframe and propeller noise so low that the most noticeable remaining sound- was valve action in the engine. Endeavoring to eliminate valve noise, Lockheed’s engineers seized upon the RC engine since it has no valves, only ports. Replacing the air-cooled Continental with an RC 2-60 U5 liquid cooled engine required extensive re-engineering.

A Corvette aluminum radiator was grafted to the nose and redesigned reduction gearing was required. A 5.34:1, two-stage, “V” belt reduction system reduced 6,000 rpm at the engine down to 500 propeller rpm. Only 185 horsepower was used in the Q Star due to carburetor limitations.

Nevertheless, power was increased by 85 % with only a 6% increase in airframe weight. A three blade 90-100″ constant speed propeller converted power to thrust. Laminated birch was used for blade material but at least one propeller had a balsa wood core covered with glass fiber.

Incidentally, throughout the QT project, Lockheed tested five 4-blade, two 6-blade, and two 3-blade props. Add of the propellers were fabricated by old-time propeller builder Ole Fahlin. Lockheed engineers cite Ole as the reason they have been able to accomplish so much propeller research for so little cost.

Flight testing revealed hitherto unattainable levels of quiet flight. Compound muffling culminated in a discharge pipe pointing straight up. Residual noise was thereby directed away from the ground.

As a test a Cessna 182 and the Q Star, both loaded to 2,600 pounds gross weight, were flown over the airport at 800 feet. The 182 was easily detectable by engine and propeller noise; Q Star was almost impossible to detect. Even at 400 feet the Q Star sounded only like leaves rustling in a light wind.

In the cockpit, engine noise is similar to the hum of an electric motor and even then, most noise in the cockpit seemed to be aerodynamically originated. In a follow-on Navy project, a Cessna Cardinal was equipped with an RC 2-60 engine, propeller gearing, and wide chord 100″ three blade propeller.

Compound muffling was also employed on the Cardinal. Need for “V” belt gearing and propeller clearance required a large cowling bulge but few other external changes. Test flights were very successful and silence was impressive.

As a follow-up to the quiet Cardinal, an RC 2-60 engine was installed in a second Cardinal which turned a standard sized propeller at conventional prop rpm. External lines of the Wankel Cardinal were identical to the production version and performance was much improved.

Exact performance figures have never been released by either Curtiss-Wright or Cessna. The RC 2-60 was also installed and tested in a Hughes H-55 helicopter with very satisfactory results. Most any type of engine can be adapted to airplanes and made to work in one way-or another.

Automobile, motorcycle, outboard marine, and even stationary industrial engines have been used with varying success in aircraft but everyone involved in those ventures realized the penalties incurred due to weight, unreliability, and low power. How about the Wankel? Will it have to contend with the same problems?

To find out the potential application for the RC concept, the National Aviation and Space Administration awarded Lockheed-Georgia a contract to assess the impact of advanced technology applicable to general aviation for the 1985 general time frame.

Among other things, the study group concluded that general aviation has the potential of performing an increasingly important role in the national transportation system. However, to reach its potential, advanced technology relating to aerodynamics, materials and propulsion must be introduced.

Four categories of aircraft were studied:

Category 1: Four Place, 145 K cruise, 1000′ field length.

Category 2: Four Place, 200 K cruise, 500′ field length.

Category 3: Six Place, 250 K cruise, 1500′ field length.

Category 4: Four Place, 150 K cruise, vertical takeoff and landing.

Investigation of propulsion aspects occupied a significant part of the contract. One major constraint was a noise level limit of 75 PNdb at 500 feet. Propulsion investigation included engine types, propulsarms, propeller technology, pollutants and propulsion noise. Today, reciprocating powerplants are most efficient in Category 1 and turboprops on the other three.

Propellers were found to be most efficient in the 130-250 knot speed range, howeever, propeller speed for a 75 PNdb noise level must be low, requiring geared propellers. When we look into the future; the true potential of the RC engine comes to light.

Examining the same performance parameters in categories 1-4 in light of new aerodynamic, structural, and powerplant developments be- comes very interesting. Due to development, weight and size for the same payload will be considerably reduced, requiring less power for each category as follows:

Category	Present Power Required	1985 Power Required	Percent Reduction
1	175	152	17
2	545	452	17
3	1064	1000	6
4	641	500	25

The crucial fact appearing throughout the entire study is that among the powerplants studied — reciprocating, turbo-prop, turbo-fan, and RC, the rotating combustion would be the clear choice for these classes of aircraft in the 1985 period.

In each category the RC engine plus fuel usually proved lighter than an equally powerful turbo-prop gas turbine plus fuel on all but very short missions. The more one studies the present state and future possibilities of RC engines, the more difficult it is to suppress enthusiasm and anticipation.

The Wankel engine with its latest refinements such as fuel injection and stratified charge for economy and low emissions fits right into an emerging pattern of developments promising to revolutionize general aviation aircraft.

Visualize the combined effects of an engine half the size and weight of current products, turning a new technology propeller utilizing the supercritical airfoil; the airframe incorporating airfoil technology growing from NASA’s research into the new GAW airfoil series, spoilers instead of ailerons allowing full span flaps.

Maybe even some of Princeton’s research on radical pilot control concepts will be included in the package. Good as the RC engine now is, future prospects are even better. Wankel type powerplants are just entering their initial phase of technological growth while the conventional four cycle is near the end of its development.

Great potential exists for increasing power to weight ratios by upgrading rpm capacity from the present 6000 rpm range to 10,000 rpm or more. Even liquid cooled, the RC 2-60 surpasses power to weight ratios of reciprocating engines, with bonuses of quiet, smoothness, lack of vibration and low pollution level.

If we try to get the needed power by continuing to squeeze existing designs for one last drop, no one will be able to afford the product. The future is open. In its infancy the RC concept affords a superior General Aviation powerplant. It is not untried. On the contrary, it has proved itself in many applications.

Future possibilities and growth potential are attested to by a great aircraft engineering company. But somehow we need to get started. General Aviation is in dire need of a lighter, smaller, quieter, smoother, and I6wer polluting power source.

Where are our Wankels?