HELICOPTER INSTRUMENTS by Donald Jensen Kollsman Instruments Too often the importance of aircraft instruments and man's genius which they represent, are obscured in the overall appraisal of an airplane, in spite of the fact that every significant step forward in the history of aviation has depended to a great extent on the ability of the aircraft instrument industry to keep pace with new developments. The advent of heavier-than-air ships, multi-engine aircraft, high altitude flying, long range navigation, jet propulsion and helicopters typify aeronautical advances that have resulted in a demand for specific types of instrumentation. Here we are concerned with two special helicopter devices, a tachometer and an air speed indicator, details of which have been disclosed by the Square D Company's Kollsman Instrument Division. Both insrruments, developed in the Kollsman laboratories during the war and produced for the helicopter units of the Army Air Forces, are now in production for commercial applications. The helicopter tachometers are dual electric tachometer indicators on which the engine speed is indicated by one pointer moving over an outer dial and the rotor speed by a second pointer moving over an inner dial. When the rotor and engine are operating together without slippage, the scales are so arranged that the pointers move coincidentally and are in direct alignment. This is accomplished by providing a rotor speed dial that has its graduations spaced to suit the engine-rotor ratio of the helicopter. Two types of indicators are available, with or without synchroscope. The former has a small subdial which remains practically motionless when the engine and the rotor are operating in the correct ratio. However, should slippage or any other cause of slowdown of the rotor in relation to the engine occur, the subdial rotates in a counter-clockwise direction. The speed of rotation is proportional to the difference in speed of the engine and the rotor. The synchroscope will also operate in the opposite direction if the engine is throttled down and the rotor continues to operate at advanced speed. The Helicopter Tachometers are available with a range of up to 4000 rpm in engine speed and can be made for any ratio of engine speed-rotor between 5: 1 and 15: 1. To make use of the standard tachometer generators, and for synchroscope indication, both outlets must operate at 1/2 engine speed. With an unusually low indicating range desired for helicopter operation, the Helicopter Air Speed Indicators function in much the same manner as standard air speed indicators. Pitot (dynamic) and- static (atmospheric) pressures are introduced into the indicator from connections to a (Continued on Page 46) Kollsman Helicopter Tachometer records the engine speed on the outer dial from 0-4000 rpm. Rotor speed is shown on the inner dial. Small subdial shown is the Synchroscope which indicates slippage between the engine and rotor. Kollsman Helicopter Air Speed Indicator makes one complete revolution for first 80 mph. One and three-quarter revolutions for full range. Standard mounting.