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Editorial: Rotary Reborn?

By Tracy Crook. Originally published in Rotary Aviation News Issue 1, 1997

When I was choosing an alternative engine for my airplane, I was looking for the best available mass produced engine that fit my requirements. This turned out to be the Mazda 13B. From that point on, all my research and efforts were directed toward this series of engines. The section on ‘history of the rotary’ in the conversion manual covers only this engine and the earlier Mazda models that led to it (10A & 12A).

Several readers have correctly pointed out that my rotary history is very incomplete and does not give credit to the earlier work done by NSU, Curtiss Wright, John Deer and of course Dr. Wankle himself.

I confess to being totally pragmatic about this subject when I started my engine project.. I didn’t care diddly squat who did what and when. All I knew was that here was this fantastic engine that had everything I wanted and I could buy it for less than the price of one cylinder assembly for a Lycoming.

Several readers have generously sent me copies of books long out of print which cover the subject in great detail. The story behind rotary engine development is truly fascinating to read. You may have asked yourself why the rotary engine has not been more of a success. There are several reasons but the main one is that there was over 75 years of development behind the piston engine before anyone began working seriously on the rotary design. There is a great resistance to change when there is that much experience and infrastructure devoted to an existing technology. A designer of a new piston engine can choose pistons, rings, etc. from a long list of manufacturers. If you wanted to build a rotary, where do you go for parts? It is similar to the situation in computers. The first IBM PC was based on one of the worst microprocessor architectures available at the time (Intel 8088) but because it got established as a standard upon which 99% of all development took place, it has become the unquestionable leader.

I sometimes wonder what it would have been like if the rotary had been developed first. Once you understand and are comfortable with how the rotary works you would have laughed at anyone who proposed something as clumsy and complicated as the piston engine. The fact that the rotary succeeded at all in the face of the recip’s 75 year head start is evidence that it has some significant advantages.

Unfortunately, the economic engine which drives internal combustion development is the automobile. In this application, there really was no compelling reason to abandon the piston engine. In automotive use, fuel economy of the rotary is not quite as good as the recip and the introduction of the rotary occurred at about the same time as the Arab oil embargo. Talk about bad timing….

Anyway, with the odds against it, the rotary engine would never have seen the light of day if it had not been for a few good men that saw the inner beauty of this design. While there was some very good work done by NSU who introduced the first rotary powered car, my personal hero in the story is a man named Kenichi Yamamoto. He is the Mazda engineer who understood what it took to make this engine work satisfactorily in the automotive environment and get it into production. Ironically, he also understood that the automobile was the worst possible use for the rotary. He never came right out and said this in his book “Rotary Engine” but buried in all the charts, graphs and equations was the unmistakable message that this was an engine born to run hard and it was not really happy idling down the street on the way home from the grocery store.

This is illustrated by a couple of fuel consumption numbers that you will rarely read in the typical aircraft performance chart. During warm-up and taxi at high idle, my engine is burning 2 gph. This is pretty rotten fuel efficiency when you consider that it only takes 3 gph to FLY the same plane at 95 mph. Fuel injection would improve this a bit but the engine just doesn’t like to loaf which is what it is doing most of the time in a car.

The good news is that there is very little fuel efficiency penalty for the rotary when operated at mid to high throttle settings and in some cases the rotary actually does better than a recip.

Any Future For it?

So, what are the future prospects for our favorite engine? Most of you are aware that the RX-7 is no longer marketed in the US (after 1996) and the latest word from Mazda (under Ford control now) is that the RX-7 is being dropped altogether. You have also probably seen the new concept cars Mazda has been showing using a new generation of rotary engines using a side port exhaust configuration. Still based on the 13B, these engines make 217 HP when normally aspirated and show very good efficiency and emission characteristics. As nice as all this sounds, I don’t think it will ever make it to production. Those guys at Ford only look at quarterly profits and ‘ain’t got no romance in their souls.’

There really wouldn’t be any problem finding engines and spare parts for our airplanes during our lifetimes if no one ever built another 13B but the real future hope for the rotary comes from a completely different direction.

About 2 years ago, Rotary Power Marine (RPM) began producing a line of jet pump drive boats powered by Mazda 13B engines. Mazda has agreed to furnish them with new 13B’s so they will not be relying on the rebuilt market for engines or parts. Not only does this keep the production line going, it also puts the rotary into an application where it can clearly show its advantages over the recip engine.

Most interesting to me was the performance testing RPM did on identical boats powered by different engines. The test boat was an Ebbtide RXR-19 equipped with a Dominator 12 TD jet drive. Engines tested were the Ford 302 and Chevy 350 V8 in addition to the Mazda 13B. Top speed for the Mazda was 45 mph burning 12.5 gph. The Ford managed only 40 mph and burned 20 gph. The Chevy equaled the top speed of the Mazda but burned twice the fuel while doing it (25 gph).

At first glance you might conclude that the rotary makes as much power as the Chevy 350 and has twice the fuel efficiency. Neither would be true. The normally aspirated rotary engine used in the tests produced 175 HP @ 6000 rpm which is considerably less than the 350 Chevy. (RPM also offers a supercharged rotary rated at 240 HP) One of the misconceptions about engines is that Dynamometer figures tell the whole story. Not so. Engine performance has to be evaluated in the environment it is used in.

A boat (using a planeing hull design) is similar to an airplane in its power requirements in that they are both sensitive to weight. In fact, a boat is even more sensitive than a plane. The more weight the hull has to carry, the lower in the water it planes giving it higher ‘wetted area’ and drag. The physical size of the engine affects things as well. A larger engine takes up more space and requires a larger boat (more weight) for a given passenger load. (The RXR-19 had room for two less people with the V8 engine installed). The bigger engine burns more fuel so more must be carried adding still more weight and taking up more space. Bottom line; traveling small and light is a huge benefit to both planes and boats.

RPM’s parent company Rotary Power International is also designing the rotary into some other unusual applications. When equipped with silicon nitride apex seals and run on natural gas, Mazda believes the rotary will have a 20,000 hour TBO. I never would have thought of this application but they plan on turning refrigeration compressors at large retail food stores. The fuel savings are said to pay for the engines in only six months (compared to electric motor driven compressors).

Airplane Engines Too?

Another company (Wankle Rotary Gmbh, Germany) has introduced an aircraft engine they call the Wankle Twinpack. It consists of two small 70 HP rotary engines driving a common prop shaft through belt reduction drives. Sprag clutches are used on each so that if one engine should fail, the other will continue to power the prop.

While I am glad to see more work being done on the rotary engine, I think they missed the boat on this design. One of the big advantages of the rotary is its simplicity. The added complexity of coupling two engines negates much of this advantage and ends up weighing more than the aircraft engines they hope to replace. With cooling systems it will weigh as much as the 180 - 200 HP Lycoming 0 - 360; not good enough for an engine rated at only 140 HP. The side by side configuration of the two rotary engines is designed to fit in the same space as the horizontally opposed aircraft engine but you still need to find a place to put radiators and oil coolers.

The advantage of redundancy is questionable as well. Unless a constant speed prop is used, the best power you could expect in the event of a failure of one engine is about 30%. On most planes this would only give you more options in choosing your emergency landing site.

No price information was given but you can be sure it isn’t cheap. As much as I would like to be wrong, I think this one is a no-brainer.

Sorry about taking up so much space with this rotary rambling. Hope you find it as interesting as I do.