by
Darryl Phillips
In the year 1816 a Scottish minister, Robert Stirling, invented an engine. It runs without noise or vibration and burns any fuel from 100 LL to kerosene to whale blubber. It competed with steam engines of that time, and was even sold by Sears Roebuck to pump household water in the 1920s. Stirling engines are used today in much of the "undeveloped" world, yet most Americans have scarcely heard of it.
This is a fictional account of what it might be like to fly a Stirling-powered aircraft. It hasn't happened yet, but work is progressing. Read on, and share the dream....
It was early, he wasn't supposed to be here for another hour, but I wanted to be sure to witness the arrival. I'd heard a little about the Stirling powerplant, mostly how they were used a century ago in mines and so on, how they weighed 800 pounds and only put out a few horses. So of course I was curious to see the Stirling principle applied in a modern design, all the more so since it was powering an old Cessna 150.
Turned out he was early too. In fact I just barely got to witness the landing at all. He taxied in, and it was obvious the aircraft itself had seen better days. He explained that he had purchased it from an EAAer who wanted the Continental O-200 for his homebuilt. Good deal for both parties.
I was anxious to get a better look at the Stirling, but he began with a familiarization walk-around of the plane. It was mostly plain-vanilla C-150, no major surprises. The prop was stock, as was the cowl, though the cooling-air intakes had been closed over. Sort of strange, seeing these intakes missing. He explained that the engine was liquid cooled, and we examined the ducted radiator on the belly between and behind the main gear. He mentioned that it was somewhat larger that would be necessary with an internal combustion engine, only later did I learn why.
We checked fuel quantity and climbed aboard. I thought we'd missed checking the oil, but he said no, the oil is like the lubricant in a refrigerator. It is sealed inside a pressurized system and if the pressure is there, the oil is too!
Controls and instruments didn't look much different. There was the familiar look of the throttle, although it was labeled "Bypass" and had a vernier knob. The mixture control had been replaced with another vernier, labeled "Temperature". There was the normal looking EGT, but it was marked "Heater Temp", and another similar gage read "Cooler Temp". The other unusual gage was marked "Engine Pressure", and it's 300 PSI reading let us know we still had our lubricant. The familiar oil pressure and oil temp gages were missing altogether.
Oh, yes, there was a horn button!
Starting the Stirling engine is a bit different from anything I had experienced before. He explained each step, indicating that he expected soon to have the whole process automatic, just turn the key. But for now it is a manual operation. The first step is to turn on the electric combustion air blower, then the electric fuel pump and ignition spark. Once the fire is started the spark isn't needed anymore. We watch heater temp and after a few seconds it passes 1000 degrees F. He eases the Bypass control forward.
It was then that I realized the prop had begun to spin. Somehow it didn't seem so much like an aircraft prop as a lightweight toy windmill that had caught a breeze. It was just sitting out there spinning, without any engine noise, without any starter grind, without much of anything. It just began to turn.
Checking the gages, he shut down the electric blower and pump, with the comment that they weren't needed while the mechanicals were being driven by the engine. Temperature was still climbing, and he kept the engine at about 400 RPM by occasionally retracting the Bypass control a little more.
It wasn't silent, but it sure was close.
Checking for other aircraft, we taxied out. He left the temperature control set to produce about 1400 degrees, and controlled engine power with the Bypass.
With no mags or carb heat to check, run up consisted mostly of just running it up and looking at temperatures, and the checklist was more concerned with the usual takeoff items. We set the temperature to maximum, 1700 degrees, and noted the engine pressure, now up a little from the cold reading. He was particularly interested in the Cooler temp reading, which has a bigger effect on engine output than does the heater. Scanning for traffic, he taxied into position, pushed the Bypass full forward, and released the brakes.
I don't know exactly what I had expected, but somehow had thought there would be a trumpet fanfare or breathtaking acceleration or something equally dramatic. But what we had was a rather normal Cessna 150 takeoff. Except for the noise, it was different. The sound of wheels and tires, perhaps a brake dragging a little. The creak and groan of an old airframe that had seen too many students. Definitely noise from the prop. And the sound of air flowing over, under, and around all the non-aerodynamic parts of the plane.
But I didn't hear much from the engine. And perhaps more surprisingly, I didn't FEEL much from the engine. The power impulses were missing. We have come to accept two of them each time the prop goes around on a four cylinder engine, but they were gone. He explained that the engine mount had no rubber isolators, everything just bolts together.
We climbed out at the same rate I'd have expected with the O-200, and leveled out at 2500 AGL. During the takeoff and climb, both the Temp and Bypass controls had been full forward. Setting cruise power was just a matter of pulling back the temperature to select the desired RPM. Since it took a few seconds for the temp to stabilize, the response was somewhat slower than we've come to expect from a throttle, but quicker than the present practice of setting cruise RPM and then carefully leaning the mixture.
I remembered being surprised at the noise level the first time I'd taken a ride in a sailplane. Quieter than a powered craft to be sure, but not silent. This was the same feeling. But the lack of engine vibration was wonderful. And we were able to converse easily without an intercom.
He pointed out that we were consuming about 0.44 pounds of fuel per horsepower per hour. Engineers call this SFC, and it compares with something like 0.59 for the O-200. Better than that, though, is the fact that we were burning turbine fuel. He said it really didn't matter, he would have burned heating oil if it had been available, but most anything including diesel fuel would be fine.
There are several reasons for the superior fuel economy. First, the Stirling is a much more efficient powerplant. An internal combustion engine takes in new air and fuel for each stroke, saving nothing from the previous one. But the Stirling re-uses the same heat energy on successive strokes, the fuel is only needed to make up the losses. The second reason is that the fuel is always burned full lean, at the best air/fuel ratio, while normal aircraft engines actually use gasoline as a coolant. Expensive coolant. The Stirling also uses the exhaust from the burner to preheat the incoming combustion air. Since the Stirling exhaust is cool, it is obvious that less energy is being thrown away.
We flew it around for awhile, as he demonstrated the operation of the engine controls. There are basically four ways to control Stirling power output, he said. These are to change the temperature, or the pressure, or the phase angle, or to waste power with a bypass valve. Each has it's good and bad points.
Temperature control is very fuel efficient, and simple to accomplish by just turning the burner up or down. The problem is that it is slow.
Pressure control can also be fuel efficient, and response time is quite rapid. This is the method used in road vehicles NASA and General Motors and Ford have developed. The problem here is complexity, as the several hundred PSI must be introduced into and removed from the engine quickly in accord with throttle position. This takes sophisticated compressors, reservoirs, servo loops, and so on. Too costly and trouble prone.
Phase angle control can be rapid, and fairly simple mechanically, but not very fuel efficient.
Wasting power sounds like the worst possible choice, and for cars and trucks it is, but it turns out to be OK for aircraft.
Here's why. The "mission profile" only requires that a small part of the flight needs rapid power control. During taxi, takeoff, and landing the pilot needs to be ready to instantly add power. Most of the trip, however, is made at a steady climb or cruise power setting, and it is during this time that fuel efficiency is of paramount importance.
So, the aircraft Stirling powerplant uses temperature control to set the power. Takeoff and landing is made at maximum temperature. When landing, the unneeded power is wasted with the bypass. During this time operation is fuel inefficient, but it gives the pilot the ability to apply power instantly if needed, and doesn't last long anyway.
Then he demonstrated how it worked. We entered the pattern, no other traffic, and turning base to final he pulled the Bypass all the way back. I saw that the temperature remained at maximum, but no power was being produced. He initiated a go-around just before we touched, and by advancing the Bypass control, full power was instantly available. The next time around we made a normal landing, and taxied in at modest temperature, controlling power again with the Bypass. Shutdown consisted of turning off the fuel.
Since the Stirling is so well insulated, there isn't much to see under the cowl. It looks perhaps a little more like a small turbine from the outside because it has a cylindrical shape, but the similarity ends there. The combustion air intake is at the bottom, the coolant hoses exit the engine toward the belly-mounted radiator, and the alternator and vacuum pump are clearly visible. Other than that, not much familiar can be seen.
He explained about the oversized radiator. An internal combustion engine takes in fuel, makes heat, and divides it roughly into thirds. One third becomes shaft horsepower, another third exits as heat in the exhaust stream, and the final third is disposed of by the cooling system. This is equally true in air-cooled and water-cooled engines. The Stirling, on the other hand, puts over 40% into shaft horsepower, very little out the exhaust, and half into the cooling, thus the bigger radiator requirement.
Furthermore, since the Stirling works on the ratio of hot and cold temperatures, it needs a somewhat bigger radiator to keep the coolant as near ambient as can be. No thermostat is ever used. On cold days the engine has greater output, and at high altitudes where the temperature gets very cold this engine puts out even more for the same fuel burned.
Since a given aircraft naturally wants to go faster at altitude where the air is thinner and drag is less, and since the Stirling puts out more at the high altitudes, the possibility for major improvement in light aircraft speed is evident.
He made a quick walk around, climbed aboard, started the Stirling, and as he taxied silently out, gave me a couple of toots of the horn!
I was impressed. We had consumed less fuel than in internal combustion engine would have needed, and it was safer, less explosive fuel. As 100LL production declines the price is bound to rise and availability will become a problem. We need an engine that can burn jet fuel. Plus the benefits of less operator fatigue due to lower noise and vibration levels, and less airframe fatigue too.
But perhaps a bigger benefit from the Stirling is less tangible. It involves the public image of general aviation. We make a lot of noise, and do it over people's homes. People equate noise with danger. This may be a false impression, but it is a fact that most non-fliers connect a sailplane or balloon with peace and tranquility, and connect poorly-muffled screaming engines with danger. The motorcycle manufacturers learned this lesson, and now it is acceptable to park you Yamaha next to your Oldsmobile, without being considered a member of Hell's Angels.
Quieting the motorcycle didn't make it safer, but it certainly changed the public perception. Perception drives legislation, and aviation cannot survive much more of that.
It's not the 1920s anymore, friends. Barnstorming is gone, Waldo Pepper has been replaced by EPA and Friends of the Owl. And those people have some valid points. We cannot keep making noise and be accepted in modern society. Do we want to give up noise, or just give up?