Do Helicopters Have Auxiliary Power Units (APUs)?

The question of whether helicopters are equipped with auxiliary power units (APUs) is an interesting one, especially for those who love aviation or are involved in the industry. This article delves into the intricacies of APUs, exploring their presence, functionality, and importance in various helicopter models.

Introduction to Auxiliary Power Units (APUs)

Essentially, an APU is a small engine used to provide energy for services other than propulsion, such as starting the main engines, powering the electrical systems, and supplying air conditioning or cabin heat while on the ground. This article will discuss the presence of APUs in helicopters and highlight some notable examples.

General Presence and Notable Exceptions

Not all helicopters have an APU, and the presence of an APU is often influenced by the size of the helicopter and its intended use. Some larger helicopters or those designed for specific roles such as transport or search and rescue typically have APUs due to the additional power and systems required. However, many smaller helicopters might rely on the main engines for power during ground operations.

Real-Life Examples

Take, for instance, the CH-47 Chinook. This helicopter is known to have an APU, located at the back inside the aft pylon between the gearbox and the squared-off back end above the ramp. The APU has its own drive shaft into the main aft gearbox:

Leslie Smith got it in a nutshell. Here's a little amplification on the "why." The biggest power demand on most helicopters is associated with starting their turbine engines. For bigger helicopters like the CH-47, with their larger engines, it's far safer to have an auxiliary power unit or an "APP" to provide the necessary power without the risk of a hot start. This is crucial for safety and performance.

Why APUs Are Crucial

Let's dive deeper into why APUs are so crucial, especially in larger helicopters:

Starting the Engine

One of the primary reasons for having an APU is to assist with starting the main engine or engines, particularly in larger helicopters with bigger engines. Starting engines, especially large ones, requires a significant amount of power. An APU can provide the necessary power to start the engine safely and efficiently, significantly reducing the risk of a hot start. For example, in twin-engine helicopters like the Huey (UH-1N), a battery-only start is often only used in emergencies. In my old Sikorsky H-3, a battery-only start was a strictly emergency procedure.

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This service demand is even higher when the helicopter's rotor brakes are off, as in the case of the H-3, where the transmission had to be spun up alongside the starting of the first engine. This extra demand on the battery can make a battery-only start tricky. The APU helps to mitigate this, providing the necessary torque to get the engine running smoothly.

Hydraulic Power and Flight Controls

Another reason for the presence of an APU is its capability to provide hydraulic power, which is essential for ensuring the flight controls are working correctly. In larger helicopters, the APU turns the "accessory section" at the back of the transmission, ensuring the main rotor blades are brought up to speed before the transmission takes over. Without hydraulic power for the flight controls, it can be risky to start the engine, and this is why the rotor brake is usually kept on during a battery-only start.

Conclusion

The presence of an APU in helicopters varies based on size and intended use. While many smaller helicopters rely on the main engines for ground power, larger helicopters benefit greatly from the additional safety and performance provided by an APU. Whether it's the CH-47 with its onboard APU or the CH-53 with its powerful auxiliary unit, the role of an APU in modern helicopters cannot be overstated.