GP-21

Constant-speed drive function inside the GP-21

The constant-speed drive is one of the key parts of the GP-21. Its task is to keep the generator rotor at a stable operating speed while the aircraft engine runs through different regimes, from low power to takeoff power. Without this drive, the generator frequency would follow engine speed and the aircraft AC network would become unstable. In the GP-21, the drive maintains the required generator speed so that the output frequency remains close to the standard aircraft value. This is why the unit is used in systems that require stable three-phase 400 Hz power.

GP-21 generator output and onboard network supply

In generator mode, the GP-21 supplies three-phase AC power to the aircraft electrical system. The output is used for onboard consumers that require a stable AC source with fixed voltage and frequency. The generator section may be associated with built-in generator types such as GT30NZh412 or GT30NZhCh12, depending on the exact version and installation. Correct output depends not only on the generator windings, but also on the condition of the drive, oil system, voltage regulation unit, current transformers, protection circuits, and aircraft switching equipment.

GP-21 on D-36, D-436, AL-31F and RD-33 engines

The GP-21 is installed on several aircraft engine families, including D-36, D-436, AL-31F and RD-33 applications. It is used on passenger, transport, amphibious and military aircraft where a compact engine-driven AC power source is required. Aircraft applications include Yak-42, An-72, An-74, An-148, An-158 and Be-200, as well as fighter platforms such as Su-27 and MiG-29, depending on engine and aircraft configuration. Before supply or replacement, the exact aircraft type, engine version and generator-drive installation must be checked against the technical documentation.

Control and protection equipment used with GP-21

The GP-21 works together with external regulation, protection and current-measuring equipment. Voltage regulation units keep the generator voltage within the required limits when load changes. Protection and control units monitor the quality of generated power and can disconnect the generator from the aircraft network if unsafe conditions occur. Current transformer blocks provide current signals for overload and differential protection circuits. Depending on the aircraft system, associated equipment may include BRN-120T5A, BRN-208M7A, BRZU115VO-2s, BZUSP376T, BZU-376SB, BZU-376SP, BTT-30VT, BTT30BTA and APP-1M-5.

Oil system and service points of the GP-21

The drive section of the GP-21 uses aviation oil as both working fluid and lubrication medium. The oil condition matters because the constant-speed drive is a hydromechanical unit, not just a mechanical gearbox. Incorrect oil type, contamination, overheating or poor storage condition can affect speed regulation and service reliability. During inspection, attention is normally paid to the oil system, drive condition, generator output, connector condition, mounting points, signs of leakage, overheating, mechanical damage and documentation. A serviceable GP-21 must be checked as both an electrical and hydromechanical assembly.

Identification and supply of GP-21

When ordering or repairing the GP-21, the full designation should be stated together with the required version, condition and documentation status. It is important to clarify whether the unit is required as serviceable, overhauled, repaired, from storage or for repair stock. Since the GP-21 is used across different aircraft and engine installations, interchangeability should not be assumed by designation alone. The unit should be matched with the engine documentation, aircraft parts catalog, generator control system and applicable test requirements before installation or shipment.