Flying for Fun
D.B. Mathews 909 North Maize Road, Townhouse 734, Wichita KS 67212
In the Beginning
In the late 1970s, a few RC (Radio Control) modelers began to build and fly much larger aircraft than previously seen. Several factors were involved in the development of these seven- to 12-foot-span model aircraft that weighed 15 pounds and more, including:
- my airplane is bigger than yours
- smoother flying characteristics (particularly in windy weather)
- improved visibility
- the use of less-expensive building materials and engines
Recalling those early days, the Bud Nosen series of large-scale models was originally designed around .60 cubic inch (cu. in.) two-stroke engines. Why? Because they were the largest model aircraft engines available. Several manufacturers, most notably Du-Bro, produced belt-driven reduction-gear propeller-drive units to enable those .60s to turn larger propeller sizes. With them, it was possible to use 16- to 20-inch-diameter propellers on .60s in order to clear the cowls of these new large models.
Several pioneers in large models began to convert gasoline-powered chain saw and grass trimmer engines into propeller-swingers — the development that really set off the big-airplane explosion.
The Canadian-built 35 cubic centimeter (cc) Quadra chain saw engine was really the first to be converted to model-airplane use, and others, such as Kawasaki, Kioritz (now Zenoah), McCulloch, and Roper were also modified. Almost all conversions were done by individual users, but several ready-to-go engines became available shortly thereafter.
There is a bewildering selection of gas-burners, ranging from 20cc (1.20 cu. in.) to gigantic 240cc (14.4 cu. in.) units, capable of pulling huge models straight up. Actually, the latter may be better suited to ultralights than model aircraft, but that's another subject.
Not only are these "industrial" power plants much larger and more powerful than traditional glow model aircraft engines, they run on much-less-expensive gasoline-and-oil mixes and are much heavier (a distinct advantage for nose weight on such large models). Most importantly, their construction is much more robust than model aircraft engines.
The big engines usually have bearings on both ends of the rod and crankshaft. They are engineered to survive considerable punishment from their intended misuse: being used to cut trees for hours on end, then tossed into the back of a pickup without so much as a wipedown, and getting an occasional dose of old, dirty fuel-oil mix. It's little wonder, barring crash damage, that chain saw/grass trimmer engines last hundreds of hours in modeling use.
Dennis Reichenberger, a flying buddy, has a Quadra 35 that he converted to model use almost 20 years ago, and he flies it regularly. It's gone through a few carburetor kits and some coil changes, but it runs well after all of that use.
Chain saw and grass trimmer engines have pump-type two-jet carburetors that can be set and forgotten, greatly simplifying tank placement in a model. The final virtue of these conversions, in my mind at least, is that their power curve favors lower rpm, justifying the use of large propellers.
In spite of the wonderful user-friendly qualities of these converted power plants, there are several distinct problems.
Foremost was the requirement for cranking them at relatively high speeds to obtain a spark from their magneto to fire the plug. Recoil spring starters were developed for this shortcoming. Also, the magneto, coil, and flywheel are usually bulky and inconveniently located, making them difficult to cowl neatly on the nose of a model airplane.
Bill Carpenter's C.H. Electronics solved the ignition problem many years ago with his Capacitive Discharge Ignition (CDI) system. It has become something of an industry standard on many of these engines as sold by converters. With the Carpenter units, the gas-burners start easily, have a solid spark throughout their speed range, and produce a minimum of radio interference.
There is only one major remaining complaint about the big bangers: noise. If you are in denial about the amount of sound coming out of these large engines, you've probably never noticed how flying-field conversations stop when they make big noise. Although several innovators have developed excellent mufflers that greatly reduce exhaust noise, they have never really caught on; in almost every case mufflers and exhaust deflectors work best. Perhaps the nonmodeling public's feelings about noise can best be illustrated by an ordinance passed in Los Angeles County, California, outlawing the use of grass trimmers, leaf blowers, and chain saws. The ordinance has subsequently been amended to allow use during certain hours of the day. This piece of news seems pertinent when you consider how many public flying sites across the country have been lost because of noise, public drunkenness, and obnoxiousness.
The Start of Something New
There are flying fields in the US and Europe limited to electric power or four-stroke engines.
A four-stroke is quieter than a two-stroke because the fuel-air mixture explodes half as many times per revolution. What earthly good does that do for those who prefer big airplanes? Bill Carpenter has given a glimpse into what may be the forced future of large model engines.
Honda Motors of Japan has developed, and is now marketing, a four-stroke power plant for grass trimmers and blowers to begin to solve not only some of the noise problems, but also the pollution concerns associated with lawn care.
Honda contacted Carpenter to inquire if its engine might have modeling applications, and he accepted the challenge. After many months of development, the model conversion is being demonstrated and is available from C.H. Electronics, Box 1732, Riverton WY 82501; Tel.: (307) 857-6897.
I saw the Honda 35cc four-stroke conversions run in June 1998, and the noise levels are remarkably low. One can talk in a normal tone immediately behind the running engine and propeller and be heard. It consistently starts on the first flip and is exceptionally well made.
The Honda is as big as a house and ugly as sin. Nonetheless, it's hopefully the first example of what will be a series of developments in large-but-quiet four-stroke power plants suitable for modeling use.
The Numbers
With all of the extraneous grass-trimmer parts removed and equipped with Bill Carpenter's CDI auto-advance system, the Honda four-stroke weighs four pounds. Its footprint for mounting directly on a firewall is 4 inches × 4 inches.
The oil (10W-40) is circulated through the engine by a pressure system, not mixed with the gasoline. Honda recommends changing oil after 50 hours of running and adjusting the valves after 200 hours. That is a bunch of 10-minute flights! Imagine your flying buddy saying, "I'll be a little late to the field; I've got to change the oil in my model." That's different!
Fuel consumption is about one gallon of regular unleaded gasoline per hour.
Bill and Terry have developed several modifications to the stock engine — most notably increasing the lift on the valves by changing the cams on the single camshaft, and using a larger carburetor.
They are measuring 5,800 rpm on a 20 × 8 propeller and 7,200 rpm on an 18 × 8. To make those numbers mean something, one of the Hondas is flying a Sig 1/3-scale Spacewalker with enough oomph to do loops from level flight. It won't do torque rolls, but then neither will the full-scale prototype. The Honda is comparable to an old-style, non-Surpass O.S. 120FS.
The most important number of all, and the rationale behind this write-up, is 80 decibels at nine feet. Not exactly electric-power sound numbers, but the fuel is certainly much lighter.
The Honda's current price, completely assembled and ready to run with all machining done, is $485. A conversion kit for do-it-yourselfers is also available.
Editor's note
A reminder here that for AMA liability insurance to apply, models must have a takeoff weight of 55 pounds or less.
Looking Ahead
Bill has been advised that Honda will market a 50cc single and a 100cc twin later this year. He and Terry will pursue development on these and will keep us informed. While the conversion in the photos is rather large and bulky relative to its power, the future may well bring strong, yet quiet, four-stroke big-bore engines.
Out of the Pulpit
I've been preachy this month, so I'll inject a little levity into the proceedings with a granddaughter story.
I took nine-year-old Melanie out to a Radio Control flying site to have some fun with her rubber-power models. She noticed the frequency board with all its numbers and asked what it did. Mel quickly grasped the "shut-down" concept, but was a bit baffled by so many numbers.
"Grandpa, do those numbers mean the flier's age?" she asked.
"Well, honey, I'm 64," I replied, smiling.
Quick as a flash she responded, "Well, that sure can't be it."
Transcribed from original scans by AI. Minor OCR errors may remain.
