Magic of Radial Engines, The

Model Airplane News, Jul 2003 by Yarrish, Gerry

=These Beauties are Beasts! =

A GUIDE TO RADIAL POWER AND OPERATION

An old aviation adage says: "The only real airplanes have two wings and round engines!" It's that second part-the round engines-that makes a model airplane really stand out. There's something magical about radial engines; whether they have three, five, seven, or nine cylinders, round engines capture everyone's attention. The sound is unmistakable, and the look? As I said, they're in a class all by themselves.

This guide highlights some of the most popular radial engines available today. Whether you love classic civilian biplanes, early turn-of-the-century WW I dogfighters, or round-nose WW II warbirds, there's a radial engine to hang on each plane's nose. Let's check out some of this round-engine magic!

DESIGN BASICS

Though this article is specific to the radial engine, there are two types of round engine. The earliest was the rotary engine, and in this configuration, the propeller was attached to the engine case and the rear-facing crankshaft was attached to the airframe. The engine and prop revolved in unison around the fixed crankshaft. This was the most popular engine type during WW I and was used by almost all countries to power many of their flying machines (see "Replica Engines" sidebar).

Developed soon after WW I, the radial engine had its case mounted solidly to the airplane, and the propeller was attached to the crankshaft. Powered by multiple pistons moving up and down in their cylinders, the crankshaft faced forward. Both engine types were air-cooled. Basically, a radial engine is made up of multiple 4-stroke cylinders attached to a common engine case. Each cylinder has its own intake and exhaust valves, and commonly, a single carburetor feeds all the cylinders. For exhaust systems, radials have either individual exhaust stacks or they are attached to a single collector ring. Either way, the sound of a running radial is unforgettable!

RCS 215

Distributor: RC Showcase

Number of cylinders: 5

Displacement: 13.lci (215cc)

Bore: 1.535 in. (39mm)

Stroke: 1.417 in. (36mm)

Power output: 13.5hp (turns a 30x12 prop at 5,600rpm)

Weight: 11 lb.

Major diameter: 12.25 in. (311.15mm)

Price: $2,375

Comments: this big radial runs on gasoline and comes with a 4.8 to 6V microprocessor-controlled, auto-advance onboard ignition system. It has a 6-bolt prop hub and uses miniature NGK CM-6 spark plugs. A standard choke-equipped Walbro carb is positioned center aft on the engine case and inside the circular cast-aluminum engine mount.

RC Showcase (301) 374-2493; rcshowcase.com.

ROBART R780

Distributor: Robart Mfg.

Number of cylinders: 7

Displacement: 7.8ci. (128cc)

Bore: 1.125 in. (28.575mm)

Stroke: 1.125 in. (28.575mm)

Power output: 10hp (turns a 26x12 prop at 5,500rpm)

Weight: 7.65 lb.

Major diameter: 10 in. (254mm)

Price: $4,000

Comments: the Robart R780 is a 22.5-percent-scale miniature of a Jacobs 7-cylinder radial engine. The output shaft has a 6-bolt prop adapter that is slipped over a tapered brass collet and is attached to the main output shaft with a single nut in the center. Pistons feature twin rings, and the hemi-head design uses large valves with steel seats and bronze guides. Roller lifters and ball bearings are used throughout, and the engine is equipped with an oil pump. A boost fan aids air intake. A steel tube mount with rubber dampers is included. Collector-exhaust ring and 7-cylinder onboard glow driver, battery and charger are available separately.

Robart Mfg. (630) 584-7616; robart.com.

O.S. SIRIUS FR5-300

Distributor: Great Planes Model Distributors

Number of cylinders: 5

Displacement: 3.035ci (49.763cc)

Bore: 0.945 in. (24mm)

Stroke: 0.866 in. (22mm)

Power output: 4hp at 8,500rpm

Weight: 94.3 oz.

Major diameter: 9.25 in. (234.95mm)

Price: $1,349.99

Comments: the Sirius FR5-300 features pushrod-driven overhead valves, a gear-driven camshaft, a ball-bearing-supported crankshaft and ringed pistons. It comes with a motor mount with mounting hardware, a single locking prop-nut assembly, a display stand, a gauge-tool set, a glow-plug extension-wire harness and five O.S. "F" glow plugs.

O.S. Engines; distributed by Great Planes Model Distributors (800) 637-7660; osengines.com.

SAITO FA-325R5D

Distributor: Horizon Hobby Inc.

Number of cylinders: 5

Displacement: 3.234ci (53cc)

Bore: 0.976 in (24.8mm)

Stroke: 0.866 in. (22mm)

Power output: 3.8hp (turns 20x8 to 20x10 props)

Weight: 84.86 oz. with muffler

Major diameter: 9.125 in. (231.775mm)

Price: $1,449.95

Comments: this powerful 5-cylinder, 4-stroke ABC engine is equipped with two glow plugs per cylinder (10 total). To start the engine, only the rear five plugs need to be lit; the front plugs come online alter the engine reaches operating temperature. The engine comes with a strong, formed-metal engine mount with mounting hardware, engine primer lines and fittings, a fuel primer injector, five short exhaust stacks and five glow-driver attachment wires with plug clips attached. It also includes basic tools and a high-efficiency fuel filter.