Acro Sport with Mercury Engine — She Flew, and Then!

By Ted Kiebke, EAA 837375

I had purchased an Acro Sport 2 that I used for a test bed, mainly because the airplane normally used a Lycoming for power. That gave me lots of room under the hood for engine adjustments. I planned to install an inexpensive Mercury outboard engine.

The engine was very easy to mount with its small size and numerous bolts and flanges that could be used as attachment points. The only concern I had was getting the propeller shaft in the center of the nose bowl and making sure it stuck out far enough for prop clearance. Using an aircraft for a test bed is a big mistake. I should have mounted it to an engine stand.

Left-side view of the engine.
Right-side view of the engine.

With the engine mounted to the airplane I had two things to worry about. If a mod was required I had to make it on the aircraft. The other thing was when it came time for flight testing, finding an airplane that was very stable and had a little weight to it for in-flight stability. Don’t get me wrong, the Acro Sport was fun and I got the info I needed but a test stand is the place to be. The biggest problem during all the testing of the engine was cooling. At high power with the cowling closed it doesn’t take long before the temperatures are in the red.

Water pump.
Mercury engine installed —front view with air intake.
Reduction drive —top view.
Spinner post on reduction drive.

The engine power comes on so fast and hard that any coupler I put on would come apart during the initial high power tests. I did make a couple of flights with the gearbox, but I was very gentle with the power. That put me back to the drawing board and the design of a belt drive system. There are many different belt drives out there and all use the same concept so I chose one, modified it to fit the Mercury and it sure runs well. Of course it didn’t start out perfect, but it is now.

Another issue that popped up is the exhaust. The exhaust system is very simple, a modified six into one, and a two-stroke is very loud without a muffler. I chose to run without a muffler during testing and used a straight pipe that allowed the engine to run great but sound bad. It’s funny how much time is spent doing research and modifying the system to make it work.

For the flight testing, again the Acro Sport is not a good platform to use. It’s really squirrely on the ground, but once airborne, talk about fun. The engine develops power so fast, idle RPM to full power in about a second, and so smooth. Each flight I was on I had to make a cautionary landing, mainly for overheating, and it all boiled down to the size of the radiator.

What’s in the future for this project? I am changing gears and have chosen another Mercury engine block for its 150-, 200-, and 300-hp engines. The only difference is the porting. The engine is more compact and more user friendly for its accessories. The most important thing is the RPM it develops. The maximum is 5,500 RPM at full throttle for each of the three engines. That makes it very easy for the propeller and re-drive. More to come on all that later.

Some of the most interesting flying I have ever done is during the flight testing of this engine. Again, it was 220-hp, the propeller was a 74-inch three-blade Magnum made by IVO. The combination of the two was perfect. In flight it would advance the power from idle to full in about three seconds. I was at 100 mph, advanced the throttle and after a few seconds I was at 170 mph. On my very last flight in the Acro Sport I was on a low final approach to the grass runway, when the aircraft began to settle a bit. I applied power and the RPM did not move. The aircraft continued to settle and I knew I couldn’t make the runway. A railroad, four-lane highway and a small hill were in front of me. I slowed the aircraft and picked my landing spot. My stall speed was 50 mph and when I got to around 60 I began the falling leaf routine. The altitude came off very nicely and at about 100 feet AGL I pushed the nose over for some airspeed and immediately flared. It was perfect, right into a 30 degree uphill slope. When the aircraft hit the little hill it stopped very fast. To make matters worse, all this took place in a corn field. As the aircraft came to a stop it slowly flipped over onto its back. There was no fire and egress was routine. I was out of the aircraft in seconds. Minor aircraft damage, and minor injuries to me.

What happened? A switch box failed that powers three plugs on the engine. The remaining three plugs were running and were putting out about 50 percent power. In the boating world they call this limp mode, enough power to get home. All I got was a little altitude.

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