CAC BOOMERANG CARB/OUTSIDE AIR SELECTOR SWITCH

The Carburettor/Outside Air Temp Switch was a critical component of the CAC Boomerang's Carburettor Air Heat Control system.

It’s primary aim was to prevent or remove ice formation in the Boomerang's carburettor which could occur when the engine was run at mid throttle under conditions of high humidity or at high altitude and in adverse weather conditions. Uncorrected, the ice formation could quickly result in engine failure or significant reduction in engine efficiency.

It did this by enabling the pilot to direct hot air from the exhaust manifold shroud directly into the air mixture chamber below the aircraft’s carburettor.

Unfortunately, the Boomerang’s Pratt & Whitney R-1830 twin Wasp 1200 hp engine required high manifold pressure for take-off and landing which in turn, meant high cylinder head temperatures and increased the risk of engine overheating. 

Engine overheating could also occur during prolonged climbing and full throttle operations and pilots were given detailed instructions to ensure they maintained cylinder head temperatures within acceptable limits.

As the cylinder head temperature rose, Cold air - not Hot was required to be directed into the carburettor's air mixture chamber.

To enable this ability to ‘alternate' the flow of Cold or Hot air into the mixing chamber, the pilot toggled the Boomerang's Carburettor/Outside Air Temp Switch as conditions required.

The 2 Position Carb/Air Temp Switch linked an air temperature thermometer to two thermocouple units - one fitted in the air mixing chamber below the carburettor and the other, enclosed in a chamber of the wing section’s leading edge which provided fresh air ventilation to the pilot’s cockpit.

A small Exhaust Gas Analyser was also installed which monitored the proportions of the air/fuel mixture entering the engine by electrically analysing the exhaust gasses. 

When at high altitude or in cold adverse weather conditions, the pilot toggled the Carb/Air Temp Switch to 'Position 1’ , which directed Hot air from the exhaust manifold shroud directly into the mixing chamber.

For takeoff and landing, the Carb/Air Temp Switch was toggled to ‘Position 2’ so that Cold Air from the wing’s leading edge could be directed into the chamber. This was also true for prolonged climbs, diving and full throttle operations.

As the Carburettor Air Heat Control System was operated by the aircraft's 12v battery, it was always important for the Switch to be toggled to the ‘OFF’ position after engine shutdown.

Still bearing its ‘service inspection’ stamp on the side of its bakelite casing, and with its large rotary knob still operating and its rear electrical connectors still connected, this is a rare example of early Australian aviation innovation.

What a special gift for the aviation enthusiast in your life!  - An original, WWII CAC Boomerang Carb/Outside Air Selector Switch mounted in its 100yr old Mango Wood Display Stand with engraved Plaque, printed and laminated Fact Sheet plus a highly detailed, custom-built Scale Model of this iconic Australian designed and built fighter perched atop its magnetic display arm.

This CAC Boomerang Instrument comes complete with detailed 1/72 or larger and more detailed 1/48 Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit

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