The Conversion of the Rotary Engine

Mazda Rotary Engine Conversion for the “Coot-A” Amphibian

Rotary Engine on test stand

In 1975, it was a problem to fine a suitable bed mounted engine, from 150 to 200 horsepower for use as pusher for an amphibian, as the engines available were limited. The Franklin engine was not manufactured or sold new in the United States anymore, only used or run-out engines were available. The Continental 10-360, 210 horsepower cost approximately $4000 for a run-out engine, and up to $9000 for a new one. The Lycoming engine was a very good engine, but not really recommended by the designer for use as a pusher prop engine in the “Coot Amphibian”. It is not a bed mounted engine and there were other problems confronting a pusher installation. The price range of the Lycoming engine ran from approximately $3500 for a run-out to $8000 for a new one.

I purchased my “coot” plans in December, 1974 and immediately started looking into the possibility of converting an automobile engine for the plane. In the mid 1950′s, when I first read about the Wankel engine, I felt it had great possibilities. I began looking at the Mazda Rotary engine. I liked it and started my project of conversion for use on my “Coot A” aircraft.

In the Spring of 1975, I purchased a Mazda rotary engine from a wrecked car. The engine had only 22,000 miles on it. It took almost two years to make the conversion, but when I put the engine on the test stand two years later, it would not start. It would fire but not continue to run. I found the problem was caused from suck seals due to setting up and not running for so long. Even though during the two years I had the engine, I turned it over by hand quite often, but it had set up a year before I bought it. I was able to purchase a completely re-manufactured Mazda engine which I converted. After about three years of jumping back and forth between building the “coot”, working on the engine and taking care of other problems, I had the engine about completed and running on the test stand. Except for running it on the test stand at times, to keep everything lubricated, I did not continue additional work on the engine until I was ready to install it on the “coot”.

My engine was a 1973 RX-3 rotary with two distributors. It had manual transmission so the engine bell was just what I wanted and the starter mounting pad was at 7:00 o’clock. Earlier models had the starter on top at 12:00 o’clock. The 1974 and later, Mazda rotary engines have only one distributor but still have dual ignition. The 1976 and later models are larger rotaries with more power.

In my conversion of the Mazda, I designed and manufactured the following parts: the planetary reduction gear system, drive coupling, gear box adapter plate, motor mounts and supports, exhaust manifold, carburetor heat chamber, engine drive pulley for governor drive, prop governor drive, radiator and oil cooler support propeller coupling and modified the oil pan for eight quarts of oil. The flywheel was machined for aligning the drive coupling and to obtain the correct dimensions. All mating surfaces were machined with alignment bosses and all dimensions and surfaces were held within .001 inches.

The following materials were used: the planetary gear was made from an automatic transmission drive coupling of mild steel that holds the drive splint gear box and adapter plate that are made of 2024-t3.

The exhaust manifold is 2″ mild steel pipe with a .125 wall thickness. The carburetor chamber, drive pulley and governor drive housing and radiator support are aluminum. The propeller coupling is 4340 heat treated and propeller shaft is 4130 heat treated (both of these materials were pre-heat treated before machining). The oil pan was modified with a sump from a Plymouth oil pan. A jig was made to hold the oil pan for welding to eliminate warping. The engine mount supports are 4130 condition n.

The total weight of the engine with eight quarts of oil and nine quarts of coolant was 400 pounds. The engine is 40 inches long from radiator to propeller coupling, 26 inches wide and 28 inches high from bottom of oil pan to top of air filter. The motor mounts are 19-3/4 inches from front to rear and 12-1/2 inches apart. The planetary reduction gear has a 2.4 to 1 ratio with lubrication that is force fed that drains back to the, engine oil pan.

The propeller governor receives oil from the oil filter at 70 lbs. of pressure and is pushed to the propeller through the gear box. The governor is set for 2700 rpm maximum and its drive ratio is the same as the reduction gear. With the propeller turning 2700 rpm static the engine is turning 6480 rpm and the constant speed propeller is at approximately 12 degree pitch at 30″ station and is producing between 180 to 190 horse power in thrust. With this power, I can drop the static rpm to about 2600 and cut down on the noise level some.

The reduction gear housing is designed to take thrust in either direction. The drive shaft is a floating shaft and rides in a spline and pilot bushing at the flywheel and a bushing in the propeller shaft at the opposite end. A small radial thrust bearing takes the helical gear thrust. The engine handles its own thrust. I plan to replace the present carburetor with a 1974 or later model as the newer models have an altitude compensator on them. In using the carburetor some vacuum lines have to be plugged and the intake manifold has to have certain openings blocked off. The air injection nozzles have to be removed and replaced with plugs to eliminate hot gases from backing up into the engine.

The oil pan must be removed before the air injection nozzles can be removed. The retard points in the leading distributor should be removed and all other unnecessary items removed. Just one set of points and a condenser is all that is needed in each distributor. A ballast resistor should be used for each coil. I set each distributor at 3 degrees btc but the trailing distributor still has About 200 rpm less than the leading distributor. I believe this is due to a time lag in the ignition of the fuel because of such a small hole which is about 3/16“ in the trailing spark plug.

The air pump can be used as a vacuum pump and with a vacuum regulating valve it works fine. The exhaust manifold gets very hot and air will have to be forced through the hot air chamber to help cool the manifold. This can be done by the exhaust air from the vacuum pump or duct air from the radiator. The high speed fan that Molt Taylor used at the propeller coupling is a must in order to cool the engine when it is idling and taxing.

The compactness and lightness of the reduction gear assembly which weighs only 53 lbs., brings this engine into the same weight to horsepower ratio as any other aircraft engine.

It is my opinion that the formulas used in figuring reciprocating horse power cannot be used to figure rotary. The rotary is a four cycle engine firing as a two cycle engine that is  firing every 60 degrees of the rotor with no piston inertia to overcome.

The simplicity of the engine with only four moving parts: Rotor, shaft, oil pump and distribution drive, and the absence of valves make for a low maintenance and lower chance of engine problems.

The engine does have some drawbacks such as; proper cooling must be maintained so that the engine does not overheat. If the engine overheats from lack of coolant, the assemblies will warp, water jacket seals will leak and the engine will have to be rebuilt; this is also true of almost any engine. The engine temperature is also critical due to the aluminum and cast iron difference in expansion. The engine would normally operate about 220 to 230 degrees Fahrenheit, and 4 to 7 lbs. pressure in the radiator but by being liquid cooled, the engine will run at a more consistent temperature thereby giving the engine a possible longer life. The engine cannot be allowed to set up for long periods of time as the seals can stick. If the seals stick, too bad, the engine has to be disassembled to correct the problem. The seals may not stick as bad in an airplane as it would be in a car due to the higher speed the engine will run eliminating carbon build-up in the engine.

The engine can run on any gas from 70 octane up and the engine manufacturer recommended below 90 octane.

I feel if we will look up over the ridges and the ruts that aviation and time has made, we will see the possibilities that the Wankel rotary engine offers us and with some experimenting can come up with a dependable engine with reasonable power and cost for our home-builts.

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