The Aerosystems 91-029 is a miniature, electric, two-pole, AC Tachometer Generator for use in generating electrical signals for transmission to aircraft indicating instruments.

The model 91-029 is a lightweight, permanent magnet AC Tachometer Generator for aircraft designed to the requirements of MIL-G-26611.

It consists of one stator with 3 phases 2 poles windings and a permanent magnet rotor. By turning the rotor, a current is induced in the windings of the stator with a frequency of one cycle per revolution of the generator shaft. It is designed for use with percent RPM indicators in turbo-jet engines. It can be employed up to 5000 RPM in either rotation direction when mounted in any position.

The Aerosystems Tachometer Generator 91-029 has been completely qualified for helicopters and airplanes applications.


  • Conform to: MIL-PRF-26611 (ex MIL-G-26611)
  • Mounting Pad: AND20005
  • Volts: 21 +/- 0,5 VAC
  • Hz: 70
  • RPM: 1000-4200
  • Weight: 480 gr
  • Temperature Range: -65° +177°C

Aerosystems Tachometer Generators are housed in aluminium case Black Hard Anodized in accordance with MIl-A-8625. Alnico permanent magnets are used.  Shafts are stainless steel, and rotate on shielded stainless steel ball bearings.  The entire armature is impregnated.

Tachometer generators are special direct current generators that produce a very linear voltage per rotation speed.  The linearity accuracy may deviate at ultra-low speed or at speeds beyond the tach-generators maximum recommended speed.  This allows a remote voltage meter to be calibrated to the tachometer generator and indicate tach-generator speed (RPM) or speed of any device/apparatus coupled to the tach-generator.

Use and Applications

Tachometer generator use has numerous advantages. For example, when a corresponding shaft reverses direction, a permanent magnet DC tachometer generator’s voltage polarity will also reverse. Tachometer generators are therefore ideal in control or measurement applications which require directional indication.

Tachometer generators are frequently used to measure engine and motor speed, as well as the corresponding speed of powered equipment such as conveyors, mixers, fans, and machine tools. They are therefore employed as feedback components in speed control circuits.

Our company designs, manufactures and homologates its first aviation product, a small Tachometer Generators MIL-G-26611, MIL-PRF-26611 (still used on some aircrafts) which were boarded on Bell and Agusta helicopters until today. The pictures show the first Tachometer Generator and a vintage drawing.