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Hummmmming or Boing, Boing, Boing....

A Mazda Rotary powered RV-6A won the first Sun & Fun 2001 "Best Auto Engine" award at Lakeland, FL. In addition to Ed Anderson's award winning RV-6A rotary installation, there were three other rotary powered RVs on row 19 at Sun & Fun 2001. There are 10 known RV rotary powered installations flying at this time as well as Long Ezs, Cozys, Lancairs, Coots, GlasStars and other airframes. Also, a large number of projects are underway with the rotary engine as the planned power plant. So why a rotary engine as an alternative power plant? This article will attempt to address that question as well as show photos of a number of successful installations.

There is no question that the alternative engine movement is rapid growing as the cost of purchase and upkeep of certified aircraft engines continues to climb steeply. However, it would be a mistake to assume that cost was the driving motivation behind this growth. While it is certainly a contributing factor, there is a sense that small reciprocating aircraft power plants have not evolved much since the 1930s and certainly has not shown much technological innovation - although there is some evidence that may be beginning to change.

With low (comparatively speaking) cost of automotive power plants which commonly last 100,000 miles and beyond these days and the advanced engine management systems provided just simply high lights the contrast between certified engines and auto engines. There are certainly drawbacks to auto installations and their use must be carefully analyzed and thought out to minimize risk. But, experimentation is what experimental aircraft are about. So the challenge of rolling your own power plant does attract a certain subset of this community. It is certainly not an endeavor that is suited for everyone. An auto engine project is a miniature Research and Development project and like all such projects, it is certain to take longer and cost more money than planned. However, there is likely no greater satisfaction than powering into the air with your homebuilt powered by your own engine installation. So why a rotary engine?

The rotary engine that is found in the Mazda RX-7 automobile is rapidly gaining converts despite its somewhat scary reputation as a strange, exotic engine. It is different than a reciprocating engine, but is still an Otto cycle, four phase (intake, compression, combustion, exhaust) engine. It is not a 2-cycle engine as many think - although it does have some characteristics that make it understandable why it might be considered one. However, this article does not intend to delve into the technical workings of the rotary but instead to focus on those characteristics that make it well suited for aircraft use (as well as some of the challenges).

Characteristics of the Rotary.

Reliability, power, weight and cost are probably the four most critical areas to consider when selecting an engine for an aircraft.

Reliability is perhaps the most important area, for if not reliable - then regardless of power, weight or cost, it is unsuitable for aircraft use. Reliability is reliant on several factors, but reliability engineers will point out that the number of parts and the stress on these parts are the inherently dominating factors in reliability. This of course assumes good maintenance in all cases.
The rotary engine is inherently more reliable than a reciprocating engine simply because it has fewer parts and these parts are less stressed than in a reciprocating engine. The rotary does not have a camshaft, valves, valve springs, valve springs keepers, rocker arms, or connecting rods and therefore can not suffer failure from these parts. Additionally, the rotors are under much less inertia stress than is a piston that must reverse direction any number of times per second and the enormous loads that imposes on piston engine parts. There is no translation of linear motion to rotary motion. Furthermore, the 2 rotor Mazda 13B has two firing pulses for each revolution of the eccentric shaft (crankshaft) there is no region of negative torque between firings as there is with piston engine, which again reduces loads, on components.

Even thought the eccentric shaft has less loads that a crankshaft, its implementation in the 13B is massive. A forged billet chunk of steel 1 3/4" dia at its smallest part, 12" long and with only two journals for the rotors. Even when turbocharged in drag racers and producing over 800 HP it is extremely rare for one to fail. The engine is redlined at 7500 rpm in automobile use, but with a few modifications - mainly balancing of components and porting will enable to the engine rev to 9000 rpm with few problems. Most aircraft application use the 5000-6500 rpm region with a drive reduction unit to reduce the rpm to that appropriate for a propeller. The most common reduction unit uses the Ford C-6 planetary gear drive set to achieve a reduction ratios of 2.17:1 or 2.85:1 with 2.17:1 being the most common. Other reduction systems such as belted systems have been used, but most rotary engine fliers favor the planetary gear approach. The engine will run hour after hour at 6000 rpm with no undue stress on the engine. To date, 2500 hours on a rotary powering a gyrocopter is the longest flying rotary followed by Tracy Crook in his rotary powered RV-4 with over a 1000 hours. A university study indicated that a 13B rotary engine in aircraft use could potential have a 4000 TBO, again an indication of the robust reliability of this engine.

Power. The power produced is of course dependent on a number of factors. The most basic is whether the engine is turbocharged or naturally aspired (NA) The most common installation is the naturally aspired (even when using turbocharger engine block) due to the reduce complexity and weight. In this state, the engine will reliability produce power in the 150HP to the 185HP range depending on induction and exhaust system, state of tune, and whether the engine has been ported. Most installations use Electronic Fuel Injection with a few relying on carburation. PowerSport has extensively modified the basic 13B design to produce 215HP using a modification known as a peripheral port. If willing to tackle the complexity of a turbocharger then 220-250 HP can be expected for a well designed system. So this puts the power band right in the center of that needed for a number of popular hombuilts. Power is of course dependent also on fuel consumption. The early rotary had a reputation for high fuel consumption. That problem has been solved with a well set-up rotary getting slightly better fuel consumption that an air-cooled reciprocating engine of the same power. The NA rotary engine can not be damaged by overleaning (there are no valves to burn) and will run at extremely lean mixture (of course power is reduced as well), which, if willing to take longer to get there, can give the aircraft increased range. There is a three rotor engine (Model 20B) that was never imported in this country, but which can be acquired, which will produce 250-300 HP without turbocharger and a few are going into larger homebuilts.

Weight. The short block of the 13B two-rotor engine most commonly used weighs 190 lbs. However, by the time the auxiliary systems (including coolant and radiators) are added in - the total FWF weight comes in at between 290 lbs and 340 lbs depending on installation specifics. This places the 13B in the ballpark of the 0-320 and 0-360 in terms of overall weight. The rotary engine itself comes in a surprisingly small package given its power potential. The engine block is 20" long and approx. 13" wide so is quite small. But, once the radiators and gear reduction drives and other auxiliary system s are installed the normal installation will leave a little more room under the cowl that an aircraft engine, but not a large amount.

Cost. Cost depends on more factors than are worth mentioning here as they are for most auto conversions. However, roll-you-own FWF rotary installations have been done for between $5000-$8000. A zero time rebuilt 13B will run from $3600 to over $6000 depending on what is done to the engine and who does it. I personally rebuilt a 91 turbo II 13B for a total of $3300 including the $1200 cost of the core. You may find a suitable engine in a wrecked RX-7 for several hundred dollars. The engine and the PSRU are the two most expensive components. The PSRU can run from $2500 to over $6000 again depending on who you purchase it from and its design features. PowerSport who continued the development after Everett Hatches demise produces the only FWF rotary package currently available. This is a super package with plenty of power, availability of a controllable pitch prop, Glass cockpit type engine management system, etc. As you might imagine, the cost of this set up is greater than if you rolled-you-own. But, still very reasonable compared to what a certified aircraft engine of similar set-up can cost. So there is the potential to save a few dollars over an aircraft engine, but as mentioned earlier that is not the principle reason folks are choosing the rotary engine.

The major reason is that the rotary has shown it can be adapted to a wide variety of popular airframes and give reliable, satisfactory results. It is a very easy engine to maintain and has a robust ability to continue to run under adverse conditions. It is often said that once the engine is running it will get you back to the airport provided it has fuel and ignition. Two separate individuals lost engine-cooling miles from an airport, which saw the rotary engine quickly exceed all temperature redlines. However, due to an inherent characteristic of the 13B rotary the engines never seized and delivered their pilots, pounding hearts and all safely on the ground. The reason is that the rotors are cast iron alloy revolving in aluminum housing. With any overheating, the aluminum housing expands faster than the rotor precluding seizing due to overheating. Yes, the engines did incur damage and all the rubber seals were fried. However, small price to pay for saving airframe and possibly life and limb.

The Downside of the Rotary
Ok, what is the downside of the rotary compared to a reciprocating engine? First, the biggest disadvantage the rotary has is that it is simply not well know by many people. We, who cut our teeth on the old flat head V-8s, hemis, rat motors or other engines of our youth find the rotary to be completely different, exotic and somewhat strange. In fact, it turns out the rotary is a simpler engine in most ways and only takes a little effort to understand it. Second, the rotary carries a lot of baggage from its early introduction by Mazda when there were problems with seals and the fuel consumption was high. Mazda had the worst luck to bring their rotary powered automobiles to this country during the 70's fuel crisis. Lack of understanding of the particulars of the engine including by auto mechanics did little to enhance its image. Sales lagged and before long the only rotary powered automobile imported to the US was the Rx-7 sports car, which the rotary engine and the performance it provided practically created an Rx-7 cult.

More to the technical side, the rotary's small physical size and high power density makes it critical to remove heat from the engine. The rotary is not as tolerant of high temperatures for this reason, no - it is unlikely to seize due to overheating, but it can incur damage. The rotors themselves are cooled by oil, which removes approximately 1/3 of the engines internal waste heat. This requires a large, efficient oil cooler with plenty of cooling air. Cooling has undoubtedly been the biggest challenge most has encountered. One reason is that reciprocating engine with which most are familiar seldom require much in the way of oil coolers even in race cars. But, once acknowledged the cooling problem goes away with a few improvements in the oil cooling subsystem.

Some may consider the need for a gear reduction drive a downside as well. It is certainly an added weight and expense, however, almost all auto conversion require some type of rpm reduction. A few folks have apparently been able to use direct drive with large displacement auto engines with some success, but the vast majority use gear reduction. So at least for the alternative engine this requirement is not solely a rotary requirement.

Another detraction that is fortunately starting to be overcome, is that there are not a large number of vendors with such things as motor mounts, radiators, fuel management system and other components and subsystems from which to choose compared to the wide variety of parts, and performance enhancing components one can get for say - a Vortec V-6, or Volkswagen air-cooled four. But, as mentioned this is changing. Lou Ross of Ross Areo got many started with his Propeller Speed Reduction Drives. Tracy Crook provided the first customized engine electronics for the flying rotary and now also produces an excellent PSRU as well as other products including a book and video (video produced by rotary engine rebuilder Bruce Turrentine of Raligh, NC) on converting and overhauling the rotary engine. Dave Atkins also provides engines and subsystems for the rotary and of course PowerSport can provide a FWF package. Other people are starting to address niche needs for the engine and installation. This is likely to grow as does the rotary movement.

When the rotary movement first started, few knew of each other and there was little exchange of information. We now have two rotary aviation web pages and e-mail list focused on the challenges of adapting the rotary to aircraft use. These now have hundreds of members many with in-depth technical knowledge and experience in the fields of thermodynamics, electronics, engines, etc. which is shared. This is certainly making it easier to roll-you-own without making the same mistakes as others have and to benefit from their success and without out beating your own head against that same wall.

To further promote the rotary movement, Paul Lamar (host of the rotary engine e mail list and AREEP Web page) has offered a cash award for the first rotary powered aircraft to average 200 MPH in one of the EAA sanctioned air races. Paul has also been a tireless contributor and promoter of the use of rotary engines in aircraft. He is currently working on a KIS Cruiser with a Turbocharged 13B as the power plant. Tracy Crook continues to provide electronics and PSRUs for the rotary movement as well as informative books and videos.

One thing you can count on - there will be many more rotary powered aircraft in the future.