by
Darryl Phillips
Once each year, major federal agencies solicit proposals from small businesses on a wide variety of topics. In the 1994 cycle, NASA listed 108 topics. While I'm not a great believer in socialized research and development, I found one topic that was too good to pass up:
"General Aviation Propulsion Systems... Proposals are invited for innovative concepts or integration of technologies in aircraft propulsion that are appropriate for use in general aviation aircraft. Objectives are to improve performance, safety and reliability, simplify operation, reduce maintenance and costs, and improve environmental compatibility (e.g. reduce community noise from aircraft operations). Areas of interest include the following:
Anticipated performance and/or cost benefits of introducing or integrating proposed innovative propulsion technologies into general aviation aircraft shall be quantitatively defined in the proposal using appropriate theoretical or experimental data. Proposals must be hardware-oriented for near-term problem solutions and applications; proposals for analyses or system studies are not acceptable."
Readers who have followed my love of the Stirling engine will understand why I jumped at the opportunity to make a proposal. Reduced interior and exterior noise YEAH! Reduced vibrations SURE THING! Improved propeller performance ABSOLUTELY! Alternative fuel THEY'RE TALKING ABOUT MY ENGINE! They may not know it, but they've just described the Stirling aircraft powerplant.
Here's a sampling from the proposal that AirSport submitted to NASA:
Fuel efficiency is a prime element in the design of any aircraft. Every pound of fuel is a lost pound of payload. For a given payload, every extra pound of fuel spirals into extra airframe weight, thus drag, thus increased engine power, which in turn requires even more fuel. This spiral runs both ways, however. A design that requires less fuel needs less of everything else, and thus even less fuel.
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.
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.
Supercharging, usually turbocharging, permits higher altitudes to be reached, but at some point the same limit applies. And turbocharging is a costly and high maintenance game. Even with boost an aircraft cannot fly above all the weather. And the majority of planes, those not supercharged, can only fly through the weather, not over it.
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.
This is NOT to suggest that a Cessna 172 or a Bonanza should be flown at forty thousand feet. And that is the point. Totally new aircraft designs will be needed to realize the altitude and speed potential of the Stirling powerplant. These new designs will shape the renaissance of general aviation. As always, every advance in aircraft design is preceded by an advance in powerplant design.
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.
Airplanes make a lot of noise, and they do it over people's homes. Society objects to this intrusion, and rightly so. This incompatibility between aviation and the rest of society has led to the closing of many airports, the land has been given to other uses and will never be recovered. To keep peace with our non-flying neighbors, it is incumbent on us in aviation to quiet our machines. Plus, there is a perceived link between noise and danger. This is the source of much of the societal bias against general aviation, resulting in more and more regulation. If personal flying is to survive in this regulatory society, we must fly safely, AND WE MUST SOUND LIKE WE'RE FLYING SAFELY. We need the silent Stirling powerplant, if for this reason alone.
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.
That's only a small part of the proposal. It went on to describe an engine suitable to power an ultralight. We didn't want to bite off too much, especially since there was a $70,000 contract limit, so an ultralight seemed like a good compromise between a model engine and a larger aircraft powerplant. With the right pilot and airframe, we figured a 15 horse engine would be adequate to demonstrate the concept. Personally, I wanted to include actually flying the ultralight in the contract, but wiser heads convinced me not to propose too much. Credibility and all that. It was agreed around here that we could sneak to the airport and fly the engine, contract or not!
After agonizing over every word, every table, every illustration, every cost analysis, the proposal was mailed. All eight required copies! Then the waiting began. First the short wait for the certified mail receipt, then the wait for confirmation that the proposal had met NASA's criteria and would be considered. Then months of excruciating waiting. NASA's acceptance date got pushed back (twice), and in October we went to the AOPA Palm Springs convention not knowing our fate. It was a long trip.
Finally the announcement came. We didn't get the contract. NASA made a total of 412 awards, including three in this classification. One was titled "Quasi-constant speed fixed-pitch composite propellers". The second was for "Regenerated engines for general aviation propulsion". And the last was "Single lever power control for general aviation and unmanned aircraft".
I'm still trying to figure out who would operate the single lever in an unmanned aircraft.