Radio Communication: Remote control of a target plane (1937-1940)


Remote control of a Fokker C.VII W/C.VIII W target seaplane (1937 – 1940)

On December 21, 1937, the Office of the Chief of the Material Department of the Navy Artillery asked the Meetgebouw to develop a method to remotely control from a ship an aircraft to act as a target for shooting practices. According to the report, a Fokker C.VII W would be used as the target aircraft. At that time, the Dutch Fleet Air  Service (MLD) possessed some 30 Fokker C.VII W seaplanes.

Our archives show that this plan is older: in a letter from 1936 the Ministry of the Interior states that the frequency band 175-250 MHz (1,714-1,200 m*) is reserved for ‘UKG communication, for automatic control and the reconnaissance signal (Navy)’.  [* wavelengths in metres] 

Fokker C.VII W (bron: Wikipedia)
Fokker C.VII-W (source: Wikipedia)

The plan is to equip a still-to-be-chosen Netherlands Naval Aviation Service (MLD) aircraft with an Alkan autopilot. Robert Alkan developed the autopilot in 1936 (US patent 2,221,748). Around April 1938, the Alkan autopilot will be delivered. In parallel, the Measurement Building could develop equipment to ‘remotely command’ the gas lever annexe contact button.
A problem is that the MLD Fokker C.VII aircraft in the Netherlands are from different series. Technical drawings show that the aircraft deviate from each other. This could present problems with the dimensions of the components to be designed for remote control. The Navy will consider whether a specific plane can be designated for the conversion and installation of the equipment.

The report of the meeting shows that it was not certain that the Armstrong-Siddeley Lynx engine would not electronically interfere with the autopilot and the remote control to be developed. A letter is sent to the Siddeley factory for ‘information about the shielding’.

The plan is that as soon as the remote control is ready, test flights will take place. The test pilot has to intervene in the event of problems with the remote-controlled autopilot. Therefore, he must be able to communicate with the ‘remote pilot’ on the ground. Reference is made here to a short-wave radiotelephonic device from the Commission of Physical Armament. It is not clear whether the meeting attendees refer to the UHF telephone (NSF type DR-42).
There was also talk about the necessity to install a radiographically registering altimeter, which can be derived from the existing meteorograph.

It is said that placing a directional antenna on a ship is very possible; the iron masses will not influence the radio transmission and receipt too much.

1938 developments

Our archives show that the Meetgebouw was allocated a budget for the project of NLG 3,000 at the end of 1937. The amount was charged to the 1938 budget. Work starts on the development of a transmitter on the ground and for mechanical parts in the aircraft needed for remote control.

On March 3, 1938, a Royal Dutch Navy C. VIII-Wis, an 11.5 metres long and 18 metres wide seaplane, is designated as the test plane. On March 14, 1938, a report on “measurements on the control parts of the aircraft” was sent by the Meetgebouw to the Navy.

Fokker C.VIII-W (source: Beeldbank NIMH)
Fokker C.VIII-W (source: Beeldbank NIMH)


1939 developments

In early January, the Measurement Building reported that at the end of 1938, a budget of NLG 522,80 for this project was left untouched. On January 18, a budget of NLG 1830,92 was released for further development of the remote control.

The museum archives contain a drawing of the radio transmitter for the control of the aircraft on the ground. The transmitter was designed to transmit information about the throttle, the verticle angle (pitch), and the course angle (yaw).
In addition, the museum has six highly detailed drawings of the mechanical adjustments to the throttle and direction information. These drawings date from April 1939.

On 24 October 1939, an invoice by Alkan (Valenton, France) was sent to the Commission for Physical Armament for the travel and subsistence costs of a chief engineer from France to “Vliegkamp de Mok” for the “assembly of the automatic pilot Alkan “. The invoice erroneously ended up in a part of our archives which was not destroyed at the outbreak of the war. The assembly work took place between 11 and 23 June 1939 -more than a year later than the original plan of April 1938.
The costs: 13 days of Dfl 30, -/day for labour: Frs. 7,800), 2 times return 1st class The Hague-Den Helder (Frs. 106) and 1st class return by train Paris-The Hague (Frs. 684).

On December 29, the Meetgebouw was commissioned to produce an electronic receiver for the remote control of the aircraft.

Status in 1940

On April 10, 1940, the remote control system was at an advanced development stage. The director of Department II. A of the Department of Defence, however, temporarily stopped this development. He considers this project less urgent -given the German threat- than other projects [NL-HaNA, Cie. Physische Strijdmiddelen, 2.13.94, 2].

At the outbreak of the war in May, the completed parts were destroyed as well as most records on paper. What remains is an electronic drawing of the radio transmitter, a detailed design proposal of the throttle steering mechanism, and six very detailed mechanical drawings. The latter drawings are dated April 6, 1939.

The design of the remote controller/transmitter

Transmitter to remote control a Fokker C.VIII-W
Transmitter to remote control a Fokker C.VIII-W (click to enlarge)

From the figure above, we can determine that 2 x TB04/8 “apple valves” are used as a power oscillator. These tubes were a favourite with Von Weiler, he also used them for the “electric listening device” and the ultra-high frequency field telephone. One circuit can be switched on with the filament switch, which is then able to transmit at full power within approximately 0.5 seconds. There are two circuits for reliability. If the mA meter shows movements, the relevant oscillator/transmitter circuit works. To reduce the hot filament voltage of the transmitter from battery voltage from 4 V to 2.8 V, the resistance of the existing wiring is probably used. We, therefore, conclude that only one transmitter circuit could be used at a time.

Apple valves
Apple valves TB04/8

To use amplitude modulation (AM) for the transmission signal, the ELL1 valve-based circuit “modulator” causes the 400 V transmitter supply voltage to fluctuate between 50 and 750 V. The valve contains two pentodes and forms a balanced circuit (note: when using a single pentode, the core of the output transformer can go into saturation due to static magnetisation of the core, something which is undesirable. The remedy would be to take a heavier output transformer, preferably with an air gap).

Double pentode ELL1 (photo courtesy Studieverzameling EWI)
Double pentode ELL1 (photo courtesy Studieverzameling EWI)

The 6 x EBC3 tone generators (Colpitts) are all active. The outputs are bridged in three pairs with switches. The capacitors (0.05 uF) will differ per oscillator to form, for example, a tone series rising from left to right in the diagram. The outputs jointly form a circuit that goes to the input transformer of the aforementioned modulator of the transmitter. If we look at the left pair of generators, it is almost certain that the switch had three positions. For the reliability of the signal, the switches had a double operation. In the middle position, all outputs are short-circuited with the 700 Ohm resistors ensuring that the tone generator oscillators continue to run.

EBC3 (photo courtesy Studievcerzameling EWI)
EBC3 (photo courtesy Studievcerzameling EWI)

The operation works as follows: throttle lever to the left: oscillator 1 transmits a signal (gas reduction) as soon as the switch(es) make contact; the throttle lever in the middle position: no changes; the throttle lever to the right: oscillator 2 transmits its signal (gas increase). The same for the verticle angle and heading controls with oscillators 3-4 and 5-6.

It must have been possible to transmit three tones at the same time, whereby the amplitude per tone would not be influenced by the transmission of the tones from the other generators. For this purpose, the input impedance of the input transformer of the ELL1 circuit will be high and therefore form a small load. This design also prevents the oscillators from “seeing” each other.
The voltage as a result of the addition of the three tone-signals will be almost three times as high as that of one tone, so one has to be careful not to let the transmitter “clip”. However, the design does not contain a volume control. The most simple solution would be an adjustable cathode resistance of the ELL1 (600 Ohm).

The power supply design is more or less self-explanatory: a heavy 400 V power supply, presumably 30 to 50 W, for the transmitter; a lighter of 280 V (approximately 20 W) for the modulator and oscillators. Switch S1 switches the 2.4 V hot filament voltage of the transmitter. Switch S2 turns on the high-voltage inverters. Given the battery capacity of lead batteries, we conclude that only one transmitter could be used at a time.


Mechanical design

Design of the remote control throttle
Mechanical design of the remote-controlled throttle


Mechanical design of the remote-control throttle
Mechanical design of the remote-controlled throttle