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Mercury Outboard Conversion for an Aircraft, Part 2

Bits & Pieces Newsletter - July 2014

By Ted Kiebke, EAA Chapter 1498, EAA 837375

  • Mercury Outboard
    An outboard engine on an Acro Sport!

July 9, 2014 - The saga of the Mercury outboard engine conversion to an aircraft engine continues. Last month, I ended with three major problems to confront: the engine cooling system, the propeller rpm, and engine mounting to an Acro Sport aircraft.

Let’s start with the cooling system. An outboard engine draws cold water for cooling from the water it is running in, and the water can vary from 32°F on up. The engine has its own water pump that is located in the lower unit and is driven off the propeller shaft. The bottom of the power head mounts to the lower unit and provides the necessary passages for both the inlet and outlet of the water. As the boat moves through the water, it pulls water into the water pump, then up the lower unit into the power head passages, cooling the power head, and then back down the lower unit and out of the exhaust port.

Converting the outboard to an aircraft engine will then require a closed-water system. Two other systems which also go between the power head and lower unit are the crankshaft and the exhaust. The mounting surface between the power unit and the lower unit is completely flat, so isolating one system from the other, i.e. water inlet and outlet, crankshaft and exhaust, was very simple.

I started with a piece of cardboard. I placed the cardboard on the bottom of the power head, transferring the image to the cardboard. Then I cut out the pattern and the bolt holes, and I had a workable adapter plate made of cardboard. The crankshaft and exhaust were still open, and an inlet port was cut into the adapter to allow water to flow into the engine inlet passages.

The water outlet was very simple because the engine has a pressure relief valve that was removed and an outlet adapter was fabricated to bolt directly to the same port. With the engine water system closed and the crankshaft exposed, I figured the next project would be adapting some kind of reduction to the engine. With the engine developing 230 SHP at 5,500 rpm, I wanted a gearbox that would turn at 2-to-1 to bring the prop rpm down to about 2,600 rpm.

After a quick search of Barnstormers, I found an Eggenfellner gearbox with a ratio of 2.1-to-1 that could do the trick. However, there seemed to be a few issues with the gearbox, one being no shock absorption between the gear box and the Subaru engine it was designed for. The Mercury engine crankshaft has a 6-tooth internal spline, and the Eggenfellner has a 32-tooth internal spline. A coupler had to be designed to accommodate both units with a shock-absorbing device in between.

A lot of cars use a dampener on their drive shafts to smooth out any roughness through the universal joints. I found a Toyota dampener that might work. With both drive shafts, a dampener, and a computer, the coupler was designed and machined in no time.

Using the gearbox mounting flange as a pattern, I made another cardboard adapter plate. After aligning the engine crankshaft with the gearbox input shaft, I found that the engine adapter plate had to be modified to mount the gearbox.

Once I had it in my head that my prototype adapters would work, it was time to make the real adapters. I chose 3/8-inch steel plates instead of aluminium because of a weight and balance problem that was lurking in the future. All measurements from the engine block and gearbox were transferred into the computer, then to a computer numerically controlled (CNC) mill, and the adapters were complete. Spacers were machined to separate the two plates to give enough room for the coupler to run without interference. The units were bolted together for a perfect fit the very first time.

As for mounting the unit in the Acro Sport, I had a total of 32 inches from the firewall to the propeller shaft opening in the nose bowl. The engine is 34 inches long. I figure a special-order prop spinner will cover everything very nicely. I chose the tube-style engine mounting which requires a lot of tube cutting and welding.

After finishing the engine mount and installing the engine, I realized it was time to start looking for the “real” engine that would be built for the aircraft. Now was the time to see what this beast was going to cost.

“Two hundred and thirty SHP costs how much? No, that can’t be right! Did I add correctly? Better check those figures with a calculator. Hey, Bert, would you check these figures? That includes the prop, too? That can’t be right. Hey, Burt, let’s build a twin-engine experimental. Wow!

In my next article in this series I will talk a little more about how much my firewall-forward cost me. I am pretty good at getting things done and knowing the right people, and I’m a pretty good scrounger. I think you will be surprised.

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