The CAC Wirraway G6A/3317 Turn & Slip Indicator is a rare and unique piece of WWII Australian aviation history.
Built in Australia under licence from the British Air Ministry at the (CAC) Commonwealth Aircraft Corporation in Victoria, it features the RAAF stamp and G6A/3317 serial numbers just below the British Crown and was initially installed in Australian designed and built WWII aircraft such as the CAC Boomerang single-engine fighter and the rugged two seater trainer/fighter/bomber, the CAC Wirraway. Later it was to also find its way into Australian built versions of the Bristol Beaufighter and Bristol Beaufort - all built by the CAC under licence.
An aircraft's Turn & Slip Indicator is one of the pilot's primary flight instruments and is usually mounted near the centre of the main flying panel. Sometimes called the Turn & Bank Indicator, the unit was effectively two flight instruments in one.
Into the turn...
One component indicated the Rate of Turn or the ‘rate of change' in the aircraft’s heading and is based around a small gyro built on a gimbal. As the aircraft yaws or swivels on its central axis, the gimbal and gyro rotate against a calibrated spring. Usually measured in degrees per second or minutes per turn, the unit alerts the pilot to the speed of his turn.
In later units and in particular, those installed in all USAAF aircraft like this P-38 Lightning shown opposite, the Rate of Turn featured ‘Hash' marks spaced on either side of a central mark. Known as ‘Dog Houses’ for their shape, when the needle was aligned with the central mark, the aircraft was said to be performing a ‘Standard Rate Turn’ which is roughly equivalent to three degrees per second or equivalent to two minutes for the completion of a 360 degree turn.
Being a much earlier model, the RAAF unit featured here does not have the ‘Dog House’ hash marks of later units and instead the Rate of Turn is simply measured in minutes of turn from 1 to 4 minutes across the bottom of the face. A 4 minute turn would have a relatively slow and wide turning circle whereas a 1 minute turn would indicate a fast and tight turn.
For modern civil aviation aircraft beginning their runway approach, their pilots would be implementing a slow and comfortable turn to ensure passenger comfort and safety rather than risk plastering passengers up against the cabin walls.
The other component is the Inclinometer located at the top of the instrument face.
Again, in later and all USAAF models, this usually consists of a small ball inside a sealed curved glass tube filled with liquid for damping.
In this early RAAF unit, the instrument uses a simple pendulum needle and spring dampener rather than the ‘ball in a glass’.
Operating much the same as a spirit level, the ‘ball’ or in this case, the ‘pendulum needle’ gives an indication of whether the aircraft is slipping, skidding or in coordinated flight.
The ball or needle’s movement is caused by the force of gravity and the aircraft's acceleration around its central axis. When they are centred in the middle of the dial, the aircraft is said to be in coordinated flight. If they are on the inside (wing side down) of a turn, the aircraft is said to be ‘slipping’. If on the outside (wing side up) of the turn, the aircraft is ’sliding’.
When an aircraft is in a balanced turn, pilot and passengers experience a slight increase in gravity directly inline with their seat and in a well balanced turn, might not even realise the aircraft is turning unless they are looking out the window.
Slip sliding away...
Slipping or Sliding within a turn are sometimes referred to as a sloppy turn, due to the apparent discomfort it can cause to the pilot and passengers but the manoeuvre does have some practical implications.
Intentionally putting an aircraft into a forward or side slip by applying opposite inputs of the aileron and rudder controls is often used by the pilot to either, allowing the aircraft to drop altitude without gaining unnecessary speed (a forward slip) or utilising the aircraft’s sideslip to counteract a crosswind landing (a side slip).
What a great gift for the aviation enthusiast in your life! - An original, WWII CAC Wirraway G6A/3317 Turn & Slip Indicator 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.
All CAC Wirraway Instruments come complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit.
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