Scratch Building a 160 Gnome Rotary at 1/3 Scale Able to Fly a Vintage RC Airplane


Two years ago I built the 1/4-scale 4-cylinder inline Gypsy DeHavilland engine per Merritt Zimmer's design. This was flown in my scratch-built Gypsy Moth--what a blast to fly especially with a homemade engine. My hero for internal combustion engines is Les Chenery in England--he started my interest in rotaries. He has designed many and is world famous.

Each year I visit the Old Rhinebeck Aerodrome for the model meet. The Camel with the 160 Gnome rotary stands out. What a whacking roar when it powers up! So there was my challenge. Tom Polpink said he had a dismantled engine in the hanger so I borrowed Ken's Vernier calipers and started blueprinting. Not having all the parts I needed, Tom gave me Fred Murrin's number.

Fred runs Murrin Antique Aero and I met him in Daytona at the biplane meet. He had a full scale Fokker Triplane with a real Lehrone engine. What a piece of work! I set up a meeting as he had a 160 Gnome apart. This I blueprinted. Fred really knows his engines and was very patient with me in sorting out the details. Using my CAD I was able to draw the outside to scale. The inside was a different matter as I wanted to maintain the mechanics and be as antifriction as possible. One third scale is small enough to replicate with all the original mechanicals. Most 1/4- scale model engines are never dynoed for power, run only once, and are mantle pieces. To create a strong setup I had to use some needle bearings. The crutch rod carrier is the only bronze bearing and is lubricated with castor oil from a piston pump as per original.

The fuel is standard two-stroke 32/1 gasoline mix. The castor oil does not mix with the gas maintaining lube. C&H made the sparkplug wires and ignition. My electronics friend Tim Spoon designed a flip-flop circuit to duplicate the mag interrupter that the 160 engine had. This gives all, one half, one quarter, or none of the spark firings. As the engine is four stroke and nine cylinders, the high speed is 4.5 firings per rev, second speed is 2.25, and third is 1.125 per rev. In the video listen for the burping (OFF) and the bark of the two lower speeds. I wanted that low frequency sound so was looking to run about 2500 rpm. With a scale prop it runs 2600 rpm and dynos at 42 lb thrust and 18 ft/lb torque. Stroke (2") and bore (36 mm) is scale. The original dyno gave a thrust of 58 lb--75 ft/lb torque at 3300 rpm with a Zinger 36/22 prop. This torque scared me for takeoff and right turning. The true scale prop detuned enough to give more than enough power for flying. It will even fly at half speed--14 lb thrust.

It took 6 months to build the engine and it fired right up--what a surprise owing to the 20 degree January. Six more months were required to get reliable power. The engine uses 25 oz of gas per 15 minutes and 2 oz of castor oil. Finished weight was 13.5 lb and is about 13" in diameter. The cowl is 15.5" in diameter.


Tom also said everybody builds the Camel but not the AVRO. Thanks Tom as that was a good choice as there were many unknowns in applying a rotary to a model. I bought the WINDSOCK data file 28 and the cover by Brian Knight is my color scheme. Next I took hundreds of digital pictures of engines and planes. Kermit Weeks' Fantasy of Flight in Florida has a 504K with the 160 Gnome. As it had been damaged in a hurricane, the wings and tail feathers were uncovered. These were built precise and as beautiful as fine furniture. I used some of the building techniques as the aluminum wing tips and tail feathers. Thanks Paul Ftecewycz and Ken Kellet for the special behind the scenes escort.

The Canadian National Air Museum in Ottawa has an Avro with a Bentley. Gerry Nadon really went out of his way lending me a stepladder and escorting me through the archive library. Gerry's a modeler also and president of Stetson Flyers Model Club in Ontario. They even unbound Avro maintenance and operating manuals so I could copy them. What a great bunch of guys--even got to meet the reconstruction crew in the workshop. It's a class "A" job they do putting back together history. My Canadian flying buddy, Lou MacDonald, made the Ottawa connection.

I usually scratch build, but Arizona Models had a 1/3 Bob Holman plan and would put together a kit. All of my early warbirds are built with proven construction design and techniques by Jon Tanzer. The airfoil is semi-symetrical modified Clark Y using Wingmaster software. I needed a thicker airfoil to fit a 1-1/4" wing tube at 2-1/2 deg dihedral. With John's stamp of approval, I had Arizona laser-cut the ribs. I used only the rear formers in the kit as I again used John's way of locking the firewall, cabane, lower wing, and landing gear. Per John, I used 0 deg wing and 1-1/2 pos stab incidence.This seems to stop the vintage model from power porpoising. The engine has 2 deg down and right thrust. The model flew without any trim and handles well with power surges. Arizona furnished a 15-1/2" aluminum cowl. I contacted World War I Aeroplanes, Inc. for the Danish Gov't Archives Set--that had all the drawings of every bracket and detailed the wood construction. After enlarging at Quick Copy to scale, I redrew on my CAD the details needed. Arizona then had them water-jet cut in stainless steel. They are black chromed not painted. I even die folded the sheet development for the control horns and pulley guards to scale. Making the scale gas tank was a challenge as I had to learn how to spin the end cones in brass. The prop is scale 35/32 from Tennessee Propellers--custom made. They do ultralite props. 10" wheels are from Reid's Model Products. Spruce Aircraft Specialties supplied me with some beautiful 6-ft long 1/4 x 1/2" spruce for the spars and 1/8 x 1/4" stringers. The leading edge is a sheeted "D" section. When fitting the wings, I noticed they were strong enough to fly even without wires.

The 12-ft wingspan is assembled with 4 each thumbnuts on the 1/4 x 13 L threaded rods. This makes the wings pull on each other rather than glued wood. All wires are prestrung with 6 passes of 30 lb Kevlar using 23 homemade turnbuckles with 6-40 thread and 20 with 4-40 thread. These, as per original, have phosphor bronze centers and heat-treated chrome molly eyes and forks with nickel plating. All forks have a 17-4 PH stainless clevis pin, No.2 washer and 1/32 cotter pin. This is how the real ones are rigged. At the field, all that has to be done is to nut the wings, attach the two front fly wires that go to the cowl, hook up the ailerons and fly.

The plane balanced perfectly--guess that 13.5 lb engine helped! Total weight is 52 lb and at 5500, it is 22 oz/ft sq. The pilot is 1/3 vintage by Diane Chevalier--note the scarf flowing in the wind in the video.


The first flights on 12/4/01 were only bits and pieces since everybody wanted to watch rather than look through a camera. The first two were dead sticks as I was too lean. The third flight was 5 minutes. My friend Ron Fancett volunteered to film the fourth flight on 12/12/01. With the upstate New York winter creeping in, I only had this gloomy, windy (15-20 mph), gusty day. Temperature was 40 degrees with wind chill and prop wash near 0. I should have had my head examined as I wouldn't have flown my sport planes in those winds. The turns were rough owing to the weathervaning. At half way through I had to reverse aileron and rudder to stop the whip. In the first flights we had 5 mph winds and made beautiful flat rudder only turns. The rough weather flight shows some aileron stall typical of a bipe without differential and lots of sideboard. It is going to take some getting used to the throttle. The landings are like a Cub--if you only have Cub weather! It appeared the engine needed some cam timing change but seemed to power up on takeoff okay and maybe too much for the high speed scale flyby. As I get used to the throttle, the other two speeds will get some use. Thanks to my ground crew, Dick Owens and Bob Pickney. Notice Bob was prepared for the wind chill with his snowmobile suit-up!


The photos show individual pieces of the engine after machining but prior to assembly. Click on the link below to view the photos. Use the link at the bottom of the photo page to return to my web page.

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Once again I want to thank everyone who helped me in creating and testing the Gnome 160 and the Avro 504.