Often when I see photos of some of the early radio control gear
for model airplanes, I have a simultaneous reaction of aghastness
and marvel at the crudity and ingenuousness, respectively, of the
electromechanical devices - the same kind of reaction I have to
stories about early surgical procedures and equipment. In 1940,
when this article appeared in the ARRL's QST magazine,
successful takeoffs and landings were considered notable events
not so much because of pilot ability (or inability), but because
of the low reliability of available electronic and mechanical gear.
Vacuum tubes with attendant heavy, high voltage power supplies,
and heavy metal gears and shafts required large airframes to support
all the weight and bulk. A less than greased-in landing usually
resulted in a lot of repair to airframe and radio control equipment.
A modeler had to build (and often design) his own radio and electromechanical
actuators since commercial availability was sparse. Modern-day low-cost,
readily available R/C models incorporate, depending on your requirements,
autopilot, total prefabrication of airframe, propulsion, and guidance
components, all of which places the burden of success entirely on
the operator and none on the airplane.
August 1940 QST
These articles are scanned and OCRed from old editions
of the ARRL's
QST magazine. Here is a list of the
QST articles I have already posted. All copyrights are
New Radio Control Gear for Model Airplanes
Three-Way Control Installation for Gas-Powered Models
By C. E. Bohnenblust, W9PEP
The control of machines by radio has been a fascinating field
of endeavor for a long time, but not until the last couple of years
has there been really fool-proof radio control of planes by amateurs.
This story is the result of a mighty interesting demonstration staged
at the Midwest Division Convention held at Wichita in April. Since
then successful flights and landings have been executed, and as
we take off to press Siegfried and Bohnenblust are vying with the
best of them at the National Meet in Chicago.
The gnome, "A chain is no stronger than its weakest
link," still holds water when it comes to radio-controlling a ship.
The author is shown giving each component the once over. Note the
belt-driven generator run from the Dodge driveshaft.
About eighteen months ago Mr. C. H. Siegfried approached me with
a request to assist him with the design and construction of radio
control gear for his model gas-powered planes. Siegfried is a model
builder of great ability and his planes fly - something quite vital
before radio control can be applied to a plane. I became interested
in the job and this article describes the gear we are using, its
adjustment and operation.
The receiver, with minor changes, is identical to that shown
in a previous issue of QST.1 The use of the RK62 tube
in a conventional "super-regen" circuit provides a consistent receiver
which will operate the Sigma relay at distances of slightly over
a mile on level ground. The circuit is given in Fig. 1.
The receiver, less the relay, is assembled in a 2 3/4-inch diameter
aluminum shield can. A hole through the can allows the tube to protrude
an inch or so. The base is fitted with a plug so that the entire
receiver can be easily removed.
The antenna is coupled to the receiver by means of a three turn
coil. We found this to be somewhat better than capacity coupling.
An air tuning condenser of about 15 μμfd. capacity connected
from the center of the coil to the plate end, with the rotor connected
to the center of the coil, makes tuning easier since hand capacity
is largely eliminated. A hole cut through the base of the can provides
access to the tuning condenser for adjustment. The antenna is vertical
and about three feet long. No potentiometer is used for plate-voltage
control, nor is an r.f. choke used.
The large-size "D" flashlight cell is used for the filament supply
because constant voltage for the filament is very essential for
continuous, stable receiver operation. A medium-size cell might
be used with satisfactory results. However, a "penlite" cell is
not satisfactory. Note that but one cell (1.5-volt) is used for
the filament. We have found this to be as satisfactory as higher
A five-ounce 45-volt "B" battery is used for the plate supply.
This size will last for some time.
Adjustment of this receiver is simplicity itself. Starting with
no signal and with loose antenna coupling, the plate current will
be well below 1.5 milliamperes. Coupling is increased until the
plate current is about 1.5 ma. Then, with the carrier on, the signal
is tuned in. This results in a lower value of plate current. Some
readjusting of antenna coupling may now be necessary to hold the
plate current at 1.5 ma. with carrier off.
Try other values of grid leak until the value is found which
gives greatest plate current change. To do this it is probably best
to separate the transmitter and receiver some distance from each
Fig. 1 - Circuit of the single-tube receiver.
The transmitter is of conventional design, using an 807 doubler
in the final for 5-meter output. Using a 500-volt power supply,
about 20 watts output is obtained. An 89 with a 20-meter crystal
drives the 807.
The transmitting antenna is a "J" type. This is equivalent to
a vertical which gives coverage in all horizontal directions.
The Control Unit
The control unit presented by far the most difficult problem
of the entire job. There are of course many methods of applying
the controls, and final choice of the method used was influenced
by (1) weight, (2) speed of application, (3) ease of construction,
(4) consistency of operation, and finally (5) controllability.
Our first was a simple rudder control unit which operated in
sequence each time the carrier was turned on. The sequence was (1)
neutral, (2) right rudder, (3) neutral, (4) left rudder, (5) neutral,
and so on. Such an arrangement has been used by several experimenters
with good results, and since constructional data have appeared previously
no details will be given here.
The complete control set-up installed in the
model plane. The third motor (throttle control) is mounted to the
left of the control mechanism. At the right are the relay and the
receiver, the latter completely enclosed, except for the tip of
the tube, in a shield can.
With the simple rudder control we obtained several nice flights;
however, we soon concluded that we needed more controls to do the
job up "brown." After much discussion we decided upon the following
controls: (1) rudder right or left, (2) elevators up or down, (3)
motor speed high or low, and finally (4) shut off motor.
The use of but one receiver makes a pulsing or "dialing" arrangement
necessary in order to select and operate anyone of several spring
assemblies. Pulses for this unit must be made uniformly in order
to avoid interference with the restoring feature. This will be explained
later. We use an ordinary telephone dial to pulse the carrier. A
switch turns on the carrier, then by dialing a number the carrier
is pulsed. This rate of pulsing is about 9 per second.
Rather than using a stepping switch, actuated by an electro magnet,
a dialing wheel driven by a rubber motor is used. Its rotation is
controlled by an escapement. This device is lighter and the battery
required to operate the escapement is smaller. Thus an overall saving
in weight is effected.
Reversible electric motors were decided upon to apply the controls.
Model DC-7 electric motors, manufactured by the Pittman Electrical
Developments Company, Philadelphia, Pennsylvania, are used in this
unit. Three separate motors were used. These motors weigh three
ounces each and come equipped with reducing gear assemblies. The
speed is reduced to one revolution in 18 to 20 seconds. By using
but 1/20 revolution to apply the control the time required is about
Motor shut-off is obtained by opening the ignition circuit sufficiently
long to cause the motor to die. Then other controls may be selected.
Now for the hard part of the job. A dialing wheel about three
inches in diameter carries a cam which, when the wheel is rotated,
successively operates the spring assemblies. The operation of the
spring assemblies in turn closes electrical circuits to the motors
which apply the controls. Fig. 2 shows the circuit arrangement for
motors Nos. 1 and 3. The circuit arrangement for No. 2 motor is
identical to No.1 and is not shown.
A side view of the plane with the control unit
in place. This shows the opposite side of the unit to that given
in the other photograph. The commutators and wipers on the motors
are visible through the left window, while at the right are the
10:1 gears, the escapement assembly, and the magnet which operates
the escapement. The throttle on the engine is controlled by a cord
driven by the electric motor mounted at the front of the plane.
Reversible motor No. 1 has two wipers, A and B, attached. These
wipers engage commutators C, D and E respectively. It will be noted
that C and D are spaced apart just far enough so that wiper A can
rotate to the center gap where it clears, electrically, segments
C and D. Wiper A, segments C and D and the normally-closed contacts
at positions 1 and 2 constitute the automatic restoring feature
for motor No. 1. By tracing the circuit it will be seen that, with
1 and 2 at normal, if wiper A is in contact with either C or D,
the circuit is closed to the motor so that it rotates to the gap
between C and D, which is normal. The rudder is adjusted to normal
with the motor in this position.
Rotation of the dialing wheel to bring the actuating earn to
position 1 operates the spring assembly at that position. Operation
of this spring assembly opens the restoring circuit and closes a
circuit which causes the motor to rotate to the right. It will be
noted that this circuit is in series with commutator segment E and
wiper A which open the circuit when the motor rotates a given amount.
This position is of course determined by the length of segment E
and determines the amount of right rudder applied.
The control remains operated so long as spring assembly No.1
remains operated, and when the spring assembly is released will
rotate back to normal as described above.
Operation of spring assembly at position 2 results in rotating
the motor to the left for left rudder control in a manner identical
to that described above for right rudder control.
Motor No.2 is an exact duplicate of No.1 and is wired to the
spring assemblies at positions 3 and 4 exactly as shown for motor
No. 1. This motor controls the elevators.
Fig. 2 - The electrical circuit of the control
unit. Motor No.2 is not shown because its connections are similar
to those of No. 1.
Motor No.3 rotates right or left as spring assemblies at 5 and
6 are operated, respectively. This motor is connected to the throttle
and has no automatic restoring feature, since after setting the
throttle at a given point it would not be desirable to have it rotate
back to "neutral" automatically. This arrangement makes possible
the setting of the throttle at any given point.
The spring assembly shown at position 7 is used to open the ignition
circuit of the motor, which of course causes it to die.
The Mechanical System
Fig. 3 shows the gear layout and the escapement. Gear A is on
the same shaft as the dialing wheel shown in Fig. 1. Gear A drives
gear B in the ratio of 10:1. Gear H, driven by the rubber motor,
drives gear I, which is also on the same shaft as the dialing wheel
and gear A. This ratio is 4:1. Therefore 4 turns of rubber are used
to turn the dialing wheel one revolution.
The dialing wheel normally cannot rotate because the "flipper"
D strikes the edge of the escapement. When the magnet is energized
the" flipper" clears and is caught by E of the escapement. This
permits the dialing wheel to turn nearly 1/20 revolution and the
cam is now located at position 0 shown in Fig. 2. One pulse of the
magnet similarly rotates the dialing wheel to position 1. Likewise
by pulsing the magnet the dialing wheel cam can be rotated to any
position from 1 to 9. The pulsing of the magnet obviously is accomplished
by the relay in the receiver circuit.
The control unit, showing batteries and two of
the reversible motors. The 4:1 bevel gears are mounted on the front
plate; the hook for the rubber motor is on the right-hand end of
the shaft of the driving gear. Spring contact assemblies are attached
to the rear dural plate. At the top, one of the assemblies can be
seen riding on the rim of the dialing wheel.
Position 0 is left dead because in operation the carrier is first
turned on by use of a non-locking switch and then the dial is operated
to pulse the carrier. Because of the interval of time involved before
the dial is operated, a spring assembly at position 0 would be operated
an appreciable length of time.
It will be noted that the cam on the dialing wheel is at position
0-9 inclusive only when the carrier is on and the magnet operated,
holding the "flipper" at E. Therefore the control selected is on
so long as the carrier is on. When the carrier is released the"
flipper" rotates to normal. However the cam C on the same shaft
with the" flipper" closes spring assembly F through normally closed
assembly G. This causes the magnet to operate and thus the escapement
is automatically operated until the cam on the dialing wheel reaches
neutral, whereupon assembly G is opened. This stops the dialing
wheel at neutral. Note that spring assembly G shown in Fig. 3 is
the same assembly as shown in Fig. 2.
Fig. 3 - The mechanical gear system and control
The automatic restoring of the dialing wheel to neutral as described
above does not interfere with dialing so long as "carrier off" time
interval of the pulsing is not too great. Pulsing the carrier means
simply cutting the carrier off for a short time (about 1/18 sec.)
and then restoring the carrier. The carrier is left on about 1/18
sec. and this cycle repeated as desired for the various controls.
During the "carrier off" intervals of pulsing, the magnet is
released and the "flipper" rotates to normal. This closes spring
assembly F which operates the magnet even though by this time the
carrier may not be on again. However in order to avoid interference
the carrier must be on again before assembly F opens. If interference
is experienced, shorten the "carrier off" time of the pulsing or
add some weight to some of the moving parts to build up inertia
which of course will slow down the speed of rotation of the dialing
One turn of the dialing wheel (four turns of the rubber power
plant) is required for each operation. It will be noted that all
the spring assemblies are operated when selecting a control; however,
the time closed is so short that the controls are not applied. If
a control would be slightly applied in this manner, the restoring
device .would immediately bring it back to neutral.
Reviewing the operation we have this sequence:
1. Carrier is turned on. Cam on dialing wheel rotates to position
2. Carrier is pulsed; cam is rotated to positions 1-9 corresponding
to the number of pulses.
3. With carrier left on cam is held at that position which applies
4. Carrier is turned off. Cam wheel rotates back to neutral,
and simultaneously the control applied restores to neutral.
The operation of the spring assembly at position 7 when the dialing
wheel is restoring to neutral is but momentary and not of sufficient
time to kill the motor.
The weight of the unit is two pounds. The receiver and batteries
bring the total weight up to 3 1/2 pounds. The ship has a wing spread
of 12 feet and weighs 13 1/2 pounds complete with the radio and
This solution of the control-unit problem, I feel, is all that
the experimenter could expect from a performance viewpoint. However,
I do believe that the same performance could be duplicated in a
strictly mechanical unit without the use of the electric motors.
Such a solution would be lighter and less costly. I hope to have
something along this line in the near future.
1 DeSoto, "Radio Control of Powered Models," QST, October, 1938.
Posted February 25, 2016