Running up the 4 Merlin engines.

Simulations - August 2003

The latest simulation to be incorporated into the Lancaster generates the sounds of the four Merlin Engines, controlled from the throttle levers and producing signals to drive the four RPM gauges. The simulation is arranged such that the Merlins go through the normal starting procedure, the Magneto switches have to be switched on and the start switches pressed on each engine in turn until that engine is running at idle speed with the throttle just cracked open. The throttles can be opened up to full power by advancing the throttle levers. The RPM meters are also functional.

The sound is generated electronically and drives two hi-fi speakers hidden behind the pilots panel, one for the two port engines and other for the starboard engines. The sound is impressive and with all four engines running at full power the sound output is sufficient to vibrate the fuselage.

The Lancaster has a complex engine and propeller pitch control system. Once the engines are at their operating level, each throttle lever provide an input to a servo system that holds the boost pressure of that engine at a constant level demanded by its lever and indicated on its boost gauge. The boost is independent of altitude (up to a certain height) and held at the demand level by a servo system controlling the carburettor. At higher altitudes the supercharger can be switched to a higher speed (FS) allowing boost control to continue up to even higher altitudes.

The prop RPM is held constant depending on the settings of the four RPM levers mounted below the throttle levers by separate control systems. As the boost pressure and hence the engine power is increased the pitch of the props are automatically increased, absorbing the extra power, thus increasing the thrust produced by the engines whilst maintaining the RPM constant. At take off, the RPM is set to maximum which will result in a fine pitch setting, similar to starting a car in high revs, low gear.

This simulation however is not that complex, it simply increases the RPM from idle (approx. 600 RPM) to a maximum of 2800 RPM as the throttles are opened but the overall effect is quite realistic especially with the working RPM gauges.

The simulator must have sensors fitted to the levers, some electronics to generate the sound signals and a stereo power amplifier to provide the audio output. Fitting sensors to the levers proved difficult. The levers are spaced close together and movement of one must not affect the other. Variable resistors or potentiometers are the most convenient but proved impossible to fit in the space available, in the end variable capacitors were used. These were home made and consisted of discs of single sided PCB material with an insulator between them, one disc fixed to a lever and one to the frame. The copper on the PCB was cut into 4 insulated sectors, opposite ones being joined, the other pair ignored. The two discs when placed face to face with the insulator between then gave a maximum change of capacitance over a rotational angle of 90 degrees. Discrete digital electronic circuits converted the capacitance change into a variable voltage change for driving the sound signal circuits. Generating the sound produced by an aircraft engine is not easy. Using recorded sounds was considered but abandoned as complicated and not versatile enough. Computer generated sounds were considered but lack of space prevented this approach. In the end a combination of analogue and digital integrated circuits were used which proved both versatile , inexpensive and extremely compact. A conventional stereo amplifier, speakers and power supplies were salvaged from a defunct hi-fi system to route the Port and Starboard engine sounds to the left and right hand speakers respectively.


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