Third Line Theme

This article is reproduced from Kirt Blattenberger’s website http://www.airplanesandrockets.com/resources/third-line-theme-aug-1957-am.htm.

It was in turn reproduced from 1957 American Modeler magazine. It is “reproduced” here rather than “linked to”, as hot-linking is a form of bandwidth theft. No infringement of copyright to either Kirt Blattenbergeror American Modeller is intended.

I’m not sure how long the 3rd-line for throttle control has been around, but this article from the August 1957 edition of American Modeler seems to suggest that it was introduced formally around time of the 1957 model hobby industry trade show in Chicago – maybe a few years before. There in an exhibitors’ booth was a special bellcrank featuring a three-wire control line system offered by the J. Roberts Model Manufacturing Company, of Baker, Oregon.

The scene is the big annual trade show for the model-hobby industry in Chicago. Three large halls are jammed with exhibit booths of new plane, train, and boat kits, engines (glow) and motors (d.c.), accessory and supply materials.

Sauntering about are fat-cat distributors and their coveys of salesmen, rival manufacturers, solemn-looking designers checking the other guy’s brainstorm -and almost lost in the vast crowd, a few build-it-yourself control line modellers.

Although outnumbered a hundred to one, it’s easy to spot the yo-yo fans. All you have to do is plant yourself in front of the small display by a small concern from a small town in Oregon. This booth features “Flight Control;” a three-wire control line system offered by the J. Roberts Model Mfg. Co., of Baker, Ore.

Here come some CL’ers; quickly come their comments. “Hey, look at that landing gear retract!” “Now that’s what I call a real sharp motor control . . .” etc., etc., etc. You get the idea.

What is this third line deal that’s the theme of J. Robert “Bob” Smurthwaite’s pitch? Bob describes it as “a mechanical triple-control connection in which three wires can be selectively manipulated while an equalizing pull or tension is exerted on all three.”

Translated from the Patent Office lingo it means you move the Roberts Flight Control handle (photo E) like any standard Ukie handle for “Up” and “Down”; at the same time you can trigger a third line back and forth. Different, claims JRS, is the fact that the centrifugal pull of the airplane in flight is divided among the 3 control lines which share equally the pull load.

System consists of three lines (which you provide), the handle and the “Plane-Unit” (photo A).

Lines hook up and detach in conventional manner for storing on any standard reel. Most modellers are surprised to find no springs in the set-up. Reason; a doubled leverage mechanism within the handle compensates for every action of the plane-unit regardless of the manoeuvre or control operation.

Consequently, as Smurthwaite points out, it requires very little line tension to maintain full control. As a result solo flight operations are possible. In fact, the first motor-speed control flight with the experimental unit was a I-man deal from hooking up the lines, starting the engine, walking to the center of the circle, then taking off the model. A spring held the motor speed restrictor at low when the lines were slack.

This original test plane was a scale model of the Danish KZ-3 light plane. The 5-pound, 54″ wingspan model used a Madewell .49 on ignition. An exhaust rotary restrictor had been installed in a stack extension. After 8 years of flying Bob’s first all-aluminium plane-unit is still in good condition. Its bulkiness has been cut down in the latest version. Final tests are near completion on a small one for use on planes with motors up to .15 cu. in. displacement. Regardless of the size of the plane-unit or model, the one size handle works with all.

Ron Moulton, noted English designer, reported to Bob that “the great revelation was that the control can be set and left alone. We had been under the impression that it would be essential to maintain finger tension on the 3rd line to retain engine setting, but in fact, this is only required when … (one) blips the motor … ”

Shown in sketch D is the new style full-length “slide-restrictor” Smurthwaite now recommends. This type which fits almost every size and shape exhaust stack works more smoothly and wears longer with its lengthened, edge surfaces. You make this by drilling starting holes in the saw blade “slide” material and filing out the open “port.” It is best to grind and fit the blank slide to the necessary incisions and tracks that you make in the exhaust stack before drilling and porting the slide. Your motor, naturally, starts each time with restrictor slide back in open, full-speed position.

The Oregon manufacturer sees the Navy Carrier, the new Air Force Rocketry event (AM, pg 42, 2/57) and rat racing as logical places for his units. Beyond that he thinks motor speed control and wheel brake possibilities will stimulate greater activity in all phases of control line flying be it sport or competition.

“In recent years,” declares JRS, “precision aerobatic events have been won by flyers who enter large models with stunt flaps in the wing. A smaller precision-stunt model utilizing Flight Control will wrap up more points than ever before possible with the following advantages: The necessity of stunt-flaps to perform those square comers can be eliminated by the combination of a full power surge and up/down elevator control at the same moment to snap the plane around sharply. Smaller models can be slowed to a smooth cruising speed between each manoeuvre to allow the judges more time to record manoeuvres.

“By smaller model, I refer to a model without stunt-flaps with a span between 38 and 46 inches and a wing area from 320 to 450 square inches.”

Production-line control units were first used at the Dallas National Meet last July. Donald Storner (photo B), 15, of Belleville, Ill., won first in Junior Navy Carrier using Flight Control to operate a Bramco throttle on a Fox 35 motor. His friend from Belleville, 16-year-old John Corrough, won second in Senior Carrier event with the same combination – both flew J. Roberts “Sabre” models equipped with their own version carrier-hooks. John’s model is shown on ground in photo B.

Glen Magree of Cleveland took first in Senior Carrier using Flight Control to operate a Roto-valve on the Fox 59 in his scale model “Bearcat.”

Besides the items mentioned JRS says his system can be utilized to operate landing flaps, carrier-arresting hooks, bomb-bay doors and racks and related mechanism to drop any desired number of bombs at any time from any position including dive-bombing, also multi-engine control, landing lights, pilot ejection seat, even change of propeller pitch.

Bob is an old-time modeller having been a fan of this magazine since its Bill Barnes days. He recalls building from our early plans Gordon Light’s 1935 Wakefield winner and Al Judge’s ’36 championship plane as well as many of Alan Booton’s scale jobs. As owner of one of the first Ohlsson .23 power plants, he scaled down Ben Shereshaw’s Cavalier to take that engine.

Bob’s forthcoming kit models includes a scale-line prop-driven Crusader (photo F) for Navy Carrier and other racing events. Span is 28″, length is 30″; it takes .29 to .35 motor. Original has carrier-hook; complete horizontal tail surface moves like the full scale job. All-balsa moulded construction to be featured; kit ready by mid-summer. Later he promises the scale Corsair for Fox 59 with motor-speed (photo C). Model has retractable landing gear, including tail-wheel, controllable rudder offset, droppable carrier-hook-all operating through one handle.