How the Stirling will Benefit the Aircraft.

A key feature of the Stirling is the regenerator, which stores and re-uses heat energy. The Stirling cycle comes the closest to the Carnot limit of efficiency. Contemporary Otto cycle powerplants, on the other hand, actually use fuel as expendable coolant during takeoff and climb, whenever power is set above 75%. This is exceedingly poor use of a depleting natural resource, and results in a high level of unburned hydrocarbon emissions, as well as being a waste of payload and an increase in operating cost.

Vibration is another area where the Stirling excels. Shaft torque on a 4 cylinder spark ignition engine varies from a negative 100% to a positive 350% of mean torque twice each revolution, while a Stirling with the same number of cylinders may vary only 5%! Besides the obvious increase in comfort to the occupants, the aircraft itself also benefits. With less vibration, airframe fatigue is greatly reduced and the weight of engine mount and vibration isolators can be largely eliminated. An even greater factor is the advantage of smooth, non-reversing torque to the propeller. At present, variable pitch propeller designs are hampered by the extreme torque pulses. As long as the prop is also the flywheel it must be heavy and robust. When the propeller can be treated as an aerodynamic surface first, rather than a flywheel first, designers can concentrate on silence and efficiency.

[Shaft torque as a function of crank rotation]

Powerplant reliability stands to materially improve with the Stirling. The most trouble-prone part of the piston engine is the ignition system. Magneto failures are common, as are problems with spark plugs and ignition harnesses. In the Stirling, no ignition is needed once the fire is started. The second most common failure is in the valve train. Stirlings have no valves. Other failure mechanisms are eliminated, too. While the increase in reliability will not be achieved overnight, the possibilities are tremendous.

Altitude performance is an immense potential of the Stirling aircraft powerplant. The piston engines of today are rated for continuous operation at 75% of maximum power. As the aircraft climbs and air density lessens, it is necessary to open the throttle further to maintain 75%. Full throttle 75% power is reached in non-supercharged designs at about 8000 ft. MSL. Above that altitude, today's engine cannot achieve 75% power. The airframe is experiencing less drag due to thinner air and wants to go faster, but the engine won't allow it. As the climb continues, airspeed decreases and finally the air is so thin that the volumetric engine cannot develop power to climb any higher. Service ceiling is often between 12 and 15 thousand feet.

On the other hand, Stirling engines are sealed systems with no reference to ambient air density, so they are not directly affected by altitude as Otto engines are. Since the Stirling operates on the difference between combustion and ambient temperatures, it actually benefits at altitude. As the outside temperature declines, engine power increases. This compounds the natural ability of the aircraft to fly faster as air density decreases. If a plane could hold constant power, it would fly twice as fast at 40,000 ft. as it did at sea level due to drag reduction alone! And the Stirling will substantially increase power at high altitudes, allowing even faster speed. This will lead the way to reasonable coast-to-coast nonstop operation of single and light twin engine aircraft.

[Altitude performance]

Safety in the event of mishap or accident is a prime concern in aircraft design. The present requirement for large quantities of high octane gasoline is detrimental to safety. Of necessity, aircraft are lightweight structures and cannot provide a level of physical protection between fuel and occupants equal to the protection found in road vehicles. Plus, a single engine plane often carries 70 or more gallons of fuel. Compared to the automobile, this represents several times the fuel quantity at a higher speed in a lighter structure. It is inevitable that fuel containment will sometimes be breached in accidents. Simple incidents such as botched landings or forced off-airport landings occur where the occupants are not injured. The extremely volatile gasoline sometimes ignites and kills people who would otherwise have walked away. For safety alone, we need a less hazardous and explosive fuel.

The Stirling can burn anything. Turbine fuel is the obvious choice since it's widely available at airports. This may not be the ideal fuel from a safety standpoint, but it's a big improvement over high octane gasoline. Long-term availability of aviation gasoline is in question. This is a specialty product with a tiny market, faced with legislated deadlines to eliminate lead content. Fuel suppliers have expressed concerns as to their capability or willingness to continue to supply avgas to the general aviation market.

Many aircraft accidents are weather related. As outlined above, the Stirling will allow the plane to cruise above the weather rather than through it. The safety implications are tremendous, as are the improvements in utilization of the aircraft investment. Planes will always have to climb and descend through the weather, but the ability to cruise in clear air and avoid enroute weather is a powerful inducement to develop this powerplant. In addition, when the pilot has a wider choice of operating altitudes, he can better optimize his use of winds. That is, he can climb into high altitude winds if they are blowing his way, but he can also cruise below them if they aren't. Turbine operators usually don't have this luxury, operating efficiency often dictates that they climb regardless of the wind penalty.

Another safety consideration is noise. The exposure to high levels of noise and vibration hour after hour is fatiguing to the pilot. Headsets are typically worn and this helps. But we don't need intercoms in our cars, why should we need them in our planes? Many pilots suffer from permanent hearing loss, which has public health ramifications. The Stirling will go a long way toward reducing noise fatigue, a benefit to both pleasure and safety.

Still another benefit of the Stirling involves ease of operation. No carburetor heat is needed. No concerns for shock cooling exist. No mixture control, no worries of overboost, the list goes on and on. Each of these gives the pilot less to worry about, more time to fly the plane.

In addition to engine noise, it is recognized that much sound originates from the propeller. As noted above, propeller design is linked closely to the characteristics of the internal combustion piston engine. The Stirling tends to produce peak power at somewhat slower RPMs, this will lead to lower propeller tip velocity, further from the noise-producing transonic region. Once the engine is a silent producer of smooth torque, we can go to work on the propeller.

Exhaust emissions are another environmental plus for the Stirling. Present aviation gasoline is termed 100LL, the "LL" standing for Low Lead. But low is only relative to the formerly higher lead content. Unlike auto fuel, aviation gasoline still contains lead. For environmental reasons we need to get the lead out! Much research in MTBE and other octane boosting compounds has been accomplished, but major compatibility problems remain. In California, EPA has proposed regulations beginning in the year 2001 that will severely restrict aviation because of exhaust emissions. Aviation needs to begin work on a new powerplant now, not waste time on more bandaids for the same cycle flown by the Wright brothers in 1903.

Besides concerns for lead and other pollutants, gross fuel efficiency of the Stirling cycle has positive benefits to society and the environment as well. Whatever we burn, we need to burn less of it. The Stirling offers the greatest fuel efficiency of all the thermal engine cycles.