R/C Triplex: Three Controls on One Channel, November 1956 Popular Electronics

This "R/C Triplex: Three Channels on One Channel" article that appeared in a 1956 issue of Popular Electronics magazine was written by a fellow who was well known in the aeromodeling world at the time - Claude McCullough. Claude won many titles in precision scale for both control line (C/L) and radio control (R/C). As was the case with many R/C modelers of the era, he did a lot of experimentation with transmitters, receivers, and electromechanical devices used to move control surfaces. Rubber-band-powered escapements dominated the field, but some servomechanisms were being developed to provide a means for proportional control and/or a more powerful means of multiposition control. As can be seen in the video I produced showing how a typical escapement worked, the output drove the airplane's rudder to either neutral, full left, or full right deflections, with no position in-between. To actuate the control, the R/C pilot pushed a button on the transmitter the number of times required to affect the desired control movement. That made for somewhat jerky flights, but it was a very popular setup. At some point people started adding more positions via more pulses, which allowed the number of controls to be expanded to rudder, elevator, and engine throttle, as reported here in this article. It was still very crude by today's standards with precision digital proportional R/C systems, but it did enable more complete control over models and allowed scale types like Mr. McCullough flew in competition to demonstrate behavior closer to the full-size aircraft.

To the R/C fan who has been flying single-channel models, the challenge of multiple controls is an enticing one. The "Triplex" circuit shown here was developed over a period of years and flown in a number of the author's models. "Triplex" provides three controls using any ordinary single-channel R/C receiver. Most "rudder-only" models may be easily modified to carry it.

The basic principle of this method is the use of a pulse system of proportional control. Addition of extra control features is accomplished in an uncomplicated manner. They are simple to maintain and also achieve a degree of "fail-safe" operation.

The R/C transmitter is keyed in the usual pulse method by a mechanical or electronic pulser, in which control stick movements vary the pulse length from no signal at one end of the range to a steady signal at the other. For the author's purposes, the movement of the stick is limited, so it doesn't quite reach the extreme positions. Two push-button switches have been added as shown in Fig. 1. One is a normally closed switch which gives "full off" when depressed; the other is a normally open switch which gives "full on" when depressed.

Fig. 1 - Control switch hookup for proportional pulser S5 and S6 (shown on top of unit at right, which was built by the author) are normally closed for motor control and normally open for elevator control.

The receiving unit is mounted as an extension of the chassis of the McNabb 465-mc. receiver.

Fig. 2 - Schematic diagram of "Triplex" control system. Component values are given in parts list above. See text for details of motor control and elevator escapements, actuator and pulser. Note that all switches are shown open in this diagram; they would be closed for normal operation.

When the pulsed signal is received by receiver relay RL1 in Fig. 2, the twin-coil actuator displaces the rudder in proportion to the pulse rate. At the same time, RL2 is keyed, which in turn keys RL3 and RL4. Relays RL3 and RL4 are delayed for about one-half second by the 50-μfd. capacitors C1 and C2. A pulse rate above 150 pulses per minute will cause relays RL3 and RL4 to remain closed.

If a full-off signal of about 3/4-second duration is sent via S5 (Fig. 1), RL3 opens after a 1/2-second delay period and keys the motor control escapement. If the motor control is an escapement-operated air-bleed valve, then holding the full-off signal for several seconds will cut the motor. And since the continuity of the actuator is broken, the rudder will be left in neutral position. This prevents the model from a failure in flight due to the interruption of the radio signal in some manner (such as transmitter failure or flying out of range).

Similar action occurs in RL4 when "full-on" is sent via S6 in Fig. 1 to operate the elevator escapement. A four-position escapement is used in this case to give half positions. The rudder control will give maneuverability while the elevator is in any position.

When a full-on signal is sent, the rudder again falls to a neutral position after the first 1/2-second. If you have interfering radio signals present, simply switching off the elevator with S4 will allow you to fly with only rudder and motor control. The single effect of an interfering signal will then be to return the rudder to neutral.

Since the "Triplex" is to be added to your present equipment, the method of mounting and installation is up to you. The photo shows a McNabb Citizenship 465-mc. receiver, with the extra components mounted on a Micarta extension to the chassis.

Any sensitive relay with coils of 5000-10,000 ohms can be used for RL3 and RL4 with the delay circuits shown; I used the Neomatic 7250-ohm subminiature type in this original unit. It may be necessary to adjust the value of capacitors C1 and C2 and the spring tension of the relays to give the required delay. The keying relay in this circuit is a Neomatic with a 300-ohm coil, employed in conjunction with an actuator using 6 volts through 20-ohm coils. Be certain that the keying relay will close at the lowest voltage used to operate the actuator.

The escapements will determine battery voltage for the elevator and motor controls. Do not employ the same battery for both escapements or tap into the actuator battery. Use a 15-volt Eveready #411 battery or a one-ounce equivalent to power the delay relays. I recommend a twin-coil activator and a fully variable pulser. Since an elevator is a much larger load than a rudder, use a heavy-weight escapement and try balancing the elevator.

All of the components may be purchased from firms which specialize in R/C equipment.

B1 - 15 volt hearing-aid battery (Eveready #411 or one-ounce equivalent) - see text for other battery voltages

R/C Triplex: Three Controls on One ChannelNovember 1956 Popular Electronics

R/C Triplex: Three Controls on One Channel
November 1956 Popular Electronics