Subaru Powered Scorpion Helicopter Modification

Notes from Ron Froburg: The Subaru EA81 engine was modified with a custom built flywheel made from 6061 T6 aluminium, the ring gear was from an automatic flywheel. The drive pulley was machined to fit the all aluminium flywheel. The drive pulley was also 6061 T6 aluminium with a one inch stub shaft out of the top for the upper support bearing.

The oil system was modified by building an oil pan with a lower sump and building the oil pump pick up tube to go to the bottom of the new sump. For the oil to drain to the new sump, two holes were drilled next to the front pulley on the engine (now at the bottom) also valve cover drains were welded in each cover to the oil pan.

The rotation on the Subaru engine was backwards compared to the original two stroke boat engine installation so the rotor system had to be reversed with new rotor blade straps being made for the change. The tail rotor was reversed by undoing the pitch links and rotating the blades 180° (degrees).

To sum it up, the helicopter did fly but power wasn't so good. The engine was advertised as putting out 80hp but was probably closer to a more realistic 60hp. We ran it at 4500rpm then tried it at 5000rpm with little improvement. I then ported the engine and raised the compression and finally got it to fly with only marginal power reserves.

Scorpion 1 Experimental Kit Helicopter - rear view
Rear 3/4 view of the completed helicopter. The engine installation is neat and clean and eliminates several belts and pulleys that are used in the stock Evinrude engine installation.

Editors note: In the early 90's the EA81's power potential hadn't yet been realised. Through further experimentation over the next few years it was discovered that significant horsepower gains could be made through improved understanding of the engines ability.

There were a few auto engine aero conversion companies that were able to extract around 115 to 120 hp reliably from the EA81 in normally aspirated configurations with some pricey custom modifications. Had these been incorporated to this particular helicopter it could have become a popular conversion option re-birthing the old Scorpion helicopter to a whole new generation.

Those engine options were obtained without even considering turbo charging which Doug Schwochert later discovered as a solution to the lack of power issue. Unfortunately the stock turbo engine wasn't matched well to the engines output potential. Custom EA81 turbo charged engines have been able to deliver around 140 hp built correctly. Now though, the EA81 is over 40 years old and far and few between in the junk yards.

Rotorway Scorpion 1 Helicopter With Modified Subaru EA81 Engine
Subaru powered Scorpion 1 helicopter hover testing.

Update: (1992) - Ron said that after replacing the Hughes rotor blades with the stock Rotorway rotor blades the stability problems went away and he appeared to have more power (hp). The Huges blades have a built-in twist which were apparently flexing and causing the instability problems and draining the engine of power.

Froburg has flown the ship through transitional into forward flight up to a cruise of about 40 mph. At the time of his latest report no major problems had been encountered with testing progressing positively.

Apparently Ron sold the Subaru powered Scorpion helicopter to allow time to take on a new Rotorway Exec project recently. The new owner had reported that for some unknown reason, the engine had blown while hovering and inquired if he had a possible solution?

It could just be a "fluke" and Ron recommend that he install another engine and give it a go. Unfortunately persisting with research and development is the only way to get these answers! Only time will tell if the little EA81 will be a reliable powerplant for light homebuilt helicopters.

Subaru EA 81 Powered Single Seat Scorpion Helicopter
Subaru EA81 powered Rotorway Scorpion 1 helicopter taxiing.

Editors note: It is unknown where this helicopter is located today and no further information has surfaced. It is interesting to note though that many helicopters now use later versions of the Subaru engine both as experimental helicopters and kit helicopters. There have even been quite a few Scorpion Two helicopters re-powered using Subaru auto conversions.

Please note that Ron Froburg no longer has this helicopter or offers any support for the modifications made to the Scorpion One Kit Helicopter. All information is © copyright protected.