By Jim Davis    2021-03-12   (Last Updated October 26, 2023)

Helicopter Engines

Most modern helicopters use turboshaft engines with electronic controllers. Some older or smaller helicopters use piston engines. Light, unmanned helicopters and new models may also use electric motors. In this article, we discuss the following aspects of helicopter engines.

• History
• Engine Types
• Engine Operation
• Performance
• Transmission and Drive System
• Engine Manufacturers and Models

History

Some of the earliest experiments with helicopters utilized short-term power to the rotor, from a spring, rubber band, or pulling a string (toys). In the late 1800s, Gustave d’Amecourt, Enrico Forlanini and others attempted to power helicopter-like machines with steam engines. Some of these models were able to climb off the ground and hover briefly, but no useful designs emerged.

In the early 1900s, internal combustion engines were adopted by helicopter pioneers. There were several attempts to use radial engines, but useful, sustained flight was not achieved. Some of these designs would use separate engines to power the main and tail rotors.

Piston engines soon became the norm. In 1939 Igor Sikorsky used such an engine with 75 HP to power both rotors of the VS-300. Many consider this to be the first successful helicopter. A piston engine powered the Bell 47—the first helicopter certified for civilian use. Piston engines were the primary powerplant for helicopters until turboshaft engines matured.

Turbomeca made the first turboshaft engine for a helicopter in 1948. This 100 SHP model 782 was used in many helicopters in the 1950s. However, the first turboshaft to fly a helicopter was the Boeing T50 which powered a Kaman K-225 in December 1951. Turbomeca produced the Artouste with 280 SHP soon after. Larger, modern helicopter may use two or even three turboshaft engines.

More recent history has been filled with attempts to use electric power. For example, Robinson has adapted their R44 helicopter to make an all-electric e-R44. Bell has experimented with electric tail rotors on its 429 model.

Engine Types

Turboshaft Engine

Turboshaft engines are used in most modern helicopters. While piston engines are cheaper and have better fuel efficiency, turboshafts provide much more power per unit weight. Turboshafts are like turbojet engines, except the exhaust is used to spin an output shaft rather than directly provide thrust. This output shaft feeds a gearbox which ultimately spins the helicopter’s rotors.

A turboshaft engine performs three key functions: compression, combustion, and extraction. The compressor pulls in outside air and compresses it. The compressed air is mixed with fuel in the combustion chamber and ignited. The explosion of air flowing out of the combustion chamber turns one or more turbines which supply power to the helicopter (and to the compressor).

Compressor

A compressor is like a bunch of fans lined up, each with many rotating blades pushing air into the engine. Unlike fans, this air is “squeezed” into a small space increasing its pressure and temperature. Helicopter compressors typically spin around 2 to 50 thousand revolutions per minute. The compressor is powered by engine exhaust. It’s circular: the compressor provides some of the energy that creates the exhaust, and the exhaust provides all the energy for the compressor.

Combustion

A portion of the compressed air flows into the combustion chamber, where it mixes with fuel and is ignited. Unlike a piston engine, this chamber remains lit (on fire) throughout operation—it’s not periodically lit and extinguished (unless there’s a “flame-out” malfunction).

Extraction

The explosion exhaust from the combustion chamber blows through a series of turbines, akin to how wind powers a wind turbine. The first turbine powers the compressor, and the last turbine typically turns the output power shaft. The power shaft is ultimately geared to the rotors and turns them at a proportional speed. As discussed in the “engine operation” section below, the engine controller adjusts fuel flow to keep this shaft (and hence, the rotors) turning at a constant speed.

Piston Engine

Piston engines, also called reciprocating engines, powered the earliest successful helicopters. They are still used today in the smallest/cheapest helicopters. They are less expensive and more fuel efficient, but generally less powerful than turboshaft engines.

Piston engines contain a number of pistons that move up and down, each driven by periodic “explosions” when a spark plug ignites a compressed fuel/air mixture. The explosions put pressure on the piston, pushing it in a direction that torques the crankshaft, which ultimately powers the helicopter.

Helicopter piston engines are typically four-stroke engines. The pistons in these engines have four-strokes of movement per engine cycle, two in each direction. The strokes are described and shown in a diagram below.

1. First, the piston moves down with the intake valve open and the exhaust valve closed. This brings a mixture of fuel and air into the cylinder, above the piston.
2. Next, with both valves closed, the piston moves up and compresses the fuel / air mixture.
3. Near maximum compression (with the piston up) the spark plug fires, ignites the fuel, and creates the explosion. This creates a very high pressure that pushes the piston down and torques the crankshaft.
4. After the piston moves all the way down, the exhaust valve is opened and the piston moves upward, pushing out the exhaust.
5. At this point the exhaust valve is closed, the intake valve is reopened, and all these steps are repeated.

A good, high-level discussion of piston engines, with animations, is provided by Energy Education. You may also be interested in Wikipedia's article on piston engines.

Engine Operation

Throttles

Although it’s not considered a primary flight control like the collective, cyclic and pedal, helicopters often have a throttle control for each engine. Each throttle is typically a twist grip on the collective stick. In most cases, throttles will not be moved in flight. Instead, they are fixed in a specific "fly" position, where engine controllers modulate fuel flow to govern rotor speed and optimize efficiency. A pilot can turn or "roll the throttle" to override the engine controller. This may be done if an engine controller malfunctions, or if the pilot wants to practice autorotation.

When manually controlling the engine via throttle, a pilot must maintain proper rotor speed without damaging the engine. For a dual engine helicopter, the pilot would only take control of the failed engine—the other engine controller would continue to maintain speed for the other engine (at least within the engine's limitations). Normally, a pilot would control the failed engine’s throttle to provide roughly the same power as the controlled engine, mimicking the action of the electronic controller.

Not all helicopters provide this level of throttle control. For example, the Bell 505 has a knob that can be turned to off, idle, or fly. There may not be a way to manually control fuel flow.

Starting an Engine

Here, we discuss the engine start process. This process can vary significantly from one helicopter to the next. Always consult information specific to a helicopter model for precise details.

Most larger helicopters have an auxiliary power unit (APU). The APU is a smaller, gas turbine engine used to start the main engine(s) and possibly pressurize hydraulic systems, among other things. The APU accelerates the main engine’s gas generator (the compressor fans) until it reaches a self-sustaining speed. At this point, APU power is not needed, and the engine controller will command fuel flow and ignite it for power. The gas generator speed (NG) will normally ramp up on a smooth schedule until one of the governors kicks in. There are typically governors for NG, NP (power turbine speed), torque, fuel flow and/or mean gas temperature (MGT).

Normally, the first "steady" condition after starting the engine is idle. The pilot may select this by rolling the throttle from "closed" (off) to a set idle position or, in some newer helicopters, using a discrete switch. In idle, the NG governor will typically be active, maintaining a specific NG around 65%. Once the engine is in fly mode, either by further opening the throttle or a discrete switch setting, the NG governor’s max NG will increase so that the NP/NR governor becomes the limiting factor, as discussed in the next section—flight mode.

Flying with an Engine

Once the engine has reached flight mode, the NP/NR governor keeps the rotors at a set speed. For example, the main rotor may run at 300 RPM. If the pilot pulls collective, or otherwise maneuvers in a way that causes NR to drop, the engine control unit (ECU) will increase fuel flow (and therefore power) to push NR up to the target speed. Likewise, a reduction in collective, or other maneuver that increases NR, will be countered by a reduction in fuel flow. ECUs and engines typically react quick enough to keep the rotors within 1% of the target speed in normal flight.

Some helicopters are equipped with an "anticipator." As the name suggests, this feature anticipates power changes, adjusting fuel flow before rotor speed change is detected. This can be done, for example, by monitoring the collective control. When the pilot pulls collective, the ECU can increase fuel flow before NR reduction is seen.

Many helicopters can operate at multiple rotor speeds. For example, a Bell 412 pilot can set the operating NR to 100 or 103. In other aircraft, the flight control system may command a target NR as a function of airspeed or other parameter(s). The variation will generally not exceed 4%. Some ECUs must therefore be capable of using a variable NR target.

On rare occasions, other factors (NG, MGT, …) could cause the ECU to reduce fuel flow in flight. For example, if a torque limit is reached (perhaps a danger to a gearbox), the ECU may reduce fuel flow even if it results in a loss of rotor speed.

Very rarely, an ECU may malfunction. Perhaps a sensor failure causes an inappropriate governor to kick in, driving the rotor to an unsafe speed. For this reason, helicopters often have an override allowing the pilot to directly control fuel flow by turning the throttle. For example, in the UH-1Y and AH-1Z, pilots can turn the throttle past the full open position to manually control fuel flow.

Performance

The performance of turboshaft engines is dependent on properties of the outside air sucked in by the compressor. Generally speaking, the denser the air, the more powerful and efficient the engine. It is typically more useful to examine the relationship with outside air temperature and pressure altitude instead of density, since those values govern the density and are more readily available to the pilot or engineer.

Engines are more powerful/efficient using denser air. Since warmer air is less dense, performance decreases with temperature. (We will see below that there may be exceptions where extreme cold air actually reduces performance.) Since higher pressure altitude (lower air pressure) air is less dense, performance decreases with pressure altitude. While humidity also impacts engine behavior, it’s generally considered negligible (well under 1% power impact). For more detail on humidity effects, checkout GE Gas Turbine Performance Characteristics.

The chart above shows the amount of torque available at various combinations of outside air temperature and pressure altitude, at fixed engine/rotor speed. Values are shown from -40 to +40 C, as shown on the horizontal axis. Values are given for each of 3 pressure altitudes 0 ft, 5k ft and 10k ft as shown by the 3 curves within the plot. To find the torque available at 0 C and 5k ft follow the center (red) line to where it’s over 0 C and it should be at a vertical value around 108. This means 108% of the “nominal” torque is available.

Here are a few things to notice about the chart.

1. Differences in performance are substantial. For example, torque drops from 108% to 85% going from 0 C and 5k ft to 6 C, 10k ft. That’s more than a 20% reduction!
2. As discussed above, available torque mostly decreases with temperature. This is evidenced by the downward slope of all three curves on the right side of the plot. However, at extreme cold temperatures, performance actually improves with temperature.
3. As pressure altitude increases, available torque decreases. For example, going from the red curve (5k ft) to the yellow curve (10k ft), much less torque is available at every temperature.

Transmission and Drive System

The engine power shaft is geared to the main and tail rotors and keeps them turning at a constant, but different, speed. A typical main rotor gear ratio is around \(\frac{1}{20}\), meaning 20 turns of the power shaft result in one turn of the main rotor. The tail rotor typically spins about 5 times faster than the main rotor, with a gear ratio of about \(\frac{1}{4}\) to the engine. The tail rotor also rotates slower than the power shaft.

The power shaft cannot slow the rotors, it can only maintain or increase rotor speed. If the power shaft slows, e.g. during an engine failure, it unclutches from the rotors. The rotors can maintain speed due to inertia and/or aerodynamic forces. This is what happens in autorotation—the helicopter descends at high speed, so the upward flow of air powers the main rotor. Think of it like a wind turbine. Since the power shaft unclutches, it and the engine do not "drag down" the rotor speed. However, the main rotor shaft always remains clutched to the tail rotor shaft. In autorotation, the main rotor effectively powers the tail rotor to provide yaw control.

Engine Manufacturers and Models

The main players in the helicopter turboshaft market are listed below.

• GE Aviation. GE has built a wide variety of helicopter engines. The T700 and derivatives power the Apache, UH-1Y and AH-1Z helicopters.
• Rolls-Royce. Rolls built the T406 specifically for the V-22 with 6150 SHP, but also built the popular M250 series of engines.
• Safran/Turbomeca. Safran has focused on small/medium engines providing 500 to 3000 SHP, with the Arriel being the most popular model.
• Pratt & Whitney Canada. Pratt focuses on smaller turboshafts. Its engines are used in the Bell 212 and 412.
• Lycoming. Lycoming was the first to put a turboshaft engine in a helicopter: the UH-1. Lycoming’s T55 engines power the large Chinook helicopter.

Below are some popular engine models.

GE-T700-401C

This engine, made by General Electric, powers the latest generation AH-1Z and UH-1Y military helicopters. The T700 series of engines may be the most successful turboshaft engines ever, proven in battle, extreme environments, and in commercial service. The 401 variant (without the “C”) was originally designed to power the SH-60 Seahawk.

These engines have 6 compressor stages and can output up to 1890 shp.

Safran Arrius 2R

This engine, certified in December 2015, powers the Bell 505 helicopter. It delivers up to 500 shp and, according to Safran, is the only engine in this class with a dual channel full authority digital engine control (FADEC).

PT6C

The PT6C is the latest family of engines from the PT6 series of engines produced by Pratt & Whitney (P&W). P&W claim these engines have the highest power-to-weight ratio in their class. PT6Cs have dual channel FADECs and can produce up to 1600 to 2000 shp, depending on the variant.