Diesel Power for Helicopters
By David Hyatt

AS MANAGER of Alpha Squadron 101 Air Show Team, I was once faced with a large and immediate problem. The quarter-scale Piper Cub that we used to tow banners for our show had been the victim of an accident. Jack Walton, its proud and creative owner, simply didn't have the time to put it back into operation to meet the deadlines of coming shows.

I'm the helicopter flier on the team and, although the banners are rather heavy, it dawned upon me that perhaps the 60-size GM P Pro Competitor helicopter I was flying might be able to pull them. I made up a releasable towing device on the rear skid arm to work off the fifth (retract) channel, went to our flying site with great trepidation, and hooked the whole thing together. The Competitor housed an OS. 61 FSRABC for power and flew the chopper extremely well for acrobatic helicopter flight, but would it lift and pull a heavy banner?

I shouldn't have worried so much. The Competitor/OS. combo lifted that heavy banner off the ground with authority and forward flight proved to be more than adequate to stretch out the banner fully and display the message. Flying over the field in larger and smaller circles in both directions and at various speeds did not uncover any control problems with the chopper. I knew we had a winner. The chopper could indeed fill the towing vacancy.

Since I discovered that an R/C helicopter is a very adequate "banner towing craft with several advantages over fixed wing, I felt the need for that 'reserve power" we all seem to want. The chopper was adequate as it was, but for air-show duties more engine power would give me that built-in confidence when performing under less than optimum conditions. Also, the added dimension of being team manager pushed me to find answers to any question about reliability and safety and the feasibility of such a "public" project before the questions could arise.

Enter the diesel-powered R/C helicopter! Six years ago, I had used a Davis Diesel Development conversion head in a Trainer 40 on an O.S. 40 FSR with great success I remembered the ease of starting, the brute power that could swing a much larger prop (see chart), the utterly reliable idle, the instant throttle response and, of course, the fuel economy. One flying buddy had timed one of my flights at 321/4 minutes on a 10-ounce tank.

Things finally began to click together. Why not try a Davis Diesel conversion head on that O.S. 61 in my Competitor? I did and the results were spectacular. The awesome power that is available under that left thumb of throttle/collective control is truly impressive. It's a very reassuring experience to ask that much of an engine and helicopter and have them answer so reliably over and over again.

We have flown the Davis Diesel Development conversion in the GMP Competitor on an O.S. 61 FSR-ABC in seven air shows to date, towing banners, and doing acrobatic flight and a candy drop. The candy and box weigh about 4 pounds.

The engine and chopper seem to play with the tasks we assign them. There has never been an engine failure, an aborted flight, an engine quitting at idle or high rpm, or a failure to start quickly every time. The diesel is extremely tolerant of needle valve settings. Cooling is not critical as diesels love heat. The rpm can go right up there with the glow engine. The only thing that suffers at high rpm is fuel economy. Bob Davis of Davis Diesel reminded me that the diesel can do its best and most economical work at lower rpm. If we re-geared our choppers for diesel power so the engine could run at lower rpm, we chopper pilots would all take a giant step forward in our quest for a much better power-to-weight ratio and greatly increase fuel economy at the same time. The reliability of the Davis dieselized engine and the power available allow the pilot to fly with great confidence.

We have dieselized other choppers and found the same Impressive results. We have also put an OS. 25 FSR-ABC in a GMP Cricket, and an OS. 40 FSRABC in the new GMP Cobra and the results arc as dramatic with those machines as they were with the larger, heavier Competitor.
These are the advantages of diesel power in R/C helicopters and other fixed-wing applications as I have experienced them:

1. Diesels have a much greater powerto-weight ratio than standard glow engines of the same size.
2. Diesels have an extremely reliable throttle response through the whole range, with a "won't-quit" idle when properly adjusted.
3. Diesels are quieter. They tend to purr rather than bark.
4. Diesels give a much longer engine run per ounce of fuel-up to twice as much.
5. Diesels offer very low maintenance. There is no glow plug to maintain or change, no glow plug support equipment needed, such as a 1.5-volt battery, glow plug clip, plug wrench, etc.
6. Diesels love heat but run cooler than a glow engine. There are no cooling problems with a diesel.

On the negative side, the only irritants I've found are getting used to the different exhaust fumes and raw fuel smell (which contains ether) and the black residue of oil around the engine and chopper from the burned hydrocarbons.

My 28-car love affair with radio control has been given a giant boost with the exciting application of diesel power to helicopters. I can envision all sorts of benefits, such as more adequately powering the larger, heavier, scale helicopter, with no glow plug worries, including the sometimes half-day task of changing them in some helicopter applications! Running them at lower rpm will result in a quieter environment and more mpd (miles per dollar) of flying enjoyment.

Since diesels like heat, tightly-cowled engines in scale, fixed-wing aircraft are a natural for diesel power. Scale props become a distinct possibility for true scale flying and not just static display.

After all of the positive experience I've had with diesel power and the Davis Diesel Development conversions in particular, I am beginning to wonder: Why is R/C diesel power (like radio control itself) the best kept secret in the country?

I believe that secret is now getting out.