If you have ever stood close to a helicopter and heard it start up you may have noticed that some sound like an old truck firing up while others sound like a fighter jet. This is all down to the engine hiding deep inside the belly of the beast.
Helicopters use turboshaft gas turbine engines once helicopters begin to have over 4 seats. The lightweight, small, gas turbine engines produce more power for their size compared to piston engines and therefore helicopters are able to lift more. This equates to more passengers, cargo & fuel.
Jet engines were a game-changer when they were first fitted to a helicopter, but they are not like a jet engine you see on a plane when you head off on vacation. Lets find out why…
Why Do Only Some Helicopters Have a Jet Engine?
Piston engines, like the one you have in your car, are a great engine for an aircraft. They are cheap to manufacture, service, and run. The fuel consumption is very low and they can produce lots of power, but they are heavy! The more power they need to produce, the bigger the engine has to be, which means size and weight.
Once a helicopter reaches 5 seats it needs to have a very powerful engine to make the aircraft useful enough that people will buy it. Having a helicopter that is cheap to run is great, but if it can only fly for an hour before it needs refueling, then the buyers will soon get sick of stopping to get gas. This makes marketing a helicopter very tough!
To get around this and make a helicopter as useful as possible, helicopter manufacturers select gas turbine engines to match the required power for that particular helicopter design. This will then help make the helicopter appeal to more market buyers.
Gas turbine engines produce way more power for their size and weight compared to a piston engine. This allows helicopter designers to fit gas turbine engines into smaller spaces, which allows for more passengers/baggage/cargo and fuel to be taken, and because they are lighter, the helicopter can lift more passengers/baggage/cargo and fuel.
The one downside to a gas turbine engine is that they are expensive to buy, maintain and run!
Helicopter Jet Engine Type
A helicopter runs a type of jet engine known as a Turboshaft Gas Turbine engine. A jet engine you see on the wing of a plane pulls air in the front, compresses it, mixes it with fuel, ignites it and fires it out of the back.
This creates thrust which pushes the plane down the runway and into the air. Helicopters need a different kind of propulsion.
The Turboshaft is very similar except before the air fires out the back, it passes through a ‘Power Turbine’. This airflow turns the turbine, which connects to a gearbox mounted on the engine.
This gearbox then connects to the main transmission system of the helicopter which turns the main and tail rotors at the required speed for them to generate lift.
A Helicopter Turboshaft Gas Turbine Engine
The ‘Jet Engine’ used on a helicopter is an incredible piece of engineering and it is made up of several sections, all with a specific role to play to allow the engine to produce its power smoothly, consistently, and reliably.
This is the first part of the engine. It draws in air, compresses it, increases its velocity and smooths out the flow before sending it into the Centrifugal Compressor
This stage further compresses the air and raises its temperature ready to enter the combustion section.
The air is mixed with with vaporized jet fuel and fed into a self sustaining fireball once the engine has started. This rapidly expands the gas volume. The rapidly expanding gas now heads towards the exhaust but first passes through the Gas Generator
This 2 stage turbine controls the speed of the two compressors, the more fuel added, the more power required, so the faster the gas generator turbines spin and drives the compressors faster, to suck in more air. This keeps the air/ fuel mixture ratio constant no matter the power setting.
The gas exiting the engine under high velocity and pressure drive this single stage power turbine. It is only connected to the reduction gearbox. This is what powers the helicopter.
The power turbine spins up to 46,000 RPM which is far too fast for the helicopters gearboxes. This reduction gearbox reduces the rpm and drives the main driveshaft at 6,000 rpm.
This is used to drive the compressor from the starter motor during engine start. The fuel and oil pumps also connect here so they pump as soon as the engine begins rotating.
By powering a main driveshaft this is where the engine gets its ‘Turboshaft’ name from. These engines are also used on fixed-wings to power propellers and those are known as ‘TurboProps’. Propellers cannot be mounted directly to the power turbine because of the immense speed they rotate at, so just like a helicopter, they need the rpm from the engine reducing.
Do Helicopters Have One Engine Or Two?
Depending on the size of helicopter it could have one engine, two engine or even three engines.
As helicopters get larger or are required to be safer, then a second engine is added to the airframe. Adding a second or third piston engine to a helicopter would make it far too heavy to lift it off the ground.
Planes are able to have two piston engines mounted on each wing because they are able to have bigger wings to generate more lift, but a helicopter needs immense power to lift it its entire weight into a hover a keep it there.
Creating a helicopter rotorhead and blades to overcome the weight of two piston engines makes it uneconomical to build because it would have to be such a large helicopter. Using gas turbine engines solve this problem – To a certain extent!
Having a second engine gas turbine engine allows for larger loads to be lifted as the power has been increased, but the main benefit is that it makes the helicopter safer.
Having two engines does have a downside though. It will add more weight to the helicopter because of the second engine and the transmission which now has two input shafts and the fuel and maintenance costs go up because of the second engine.
The Bell 205 and Bell 212 are prime examples of this. Practically the same airframe but the 205 has one engine and the 212 has two engines. The empty weight of the twin-engine aircraft increases, but so does its useful load.
- Engine: Lycoming T53-17B
- Maximum Takeoff Weight: 10,200 lbs Internal
- Empty Weight: 5,725 lbs
- Useful Load: 4,475 lbs
- Fuel Burn: 350 litres/hour
- Engines: 2x Pratt & Whitney PT6 3B
- Maximum Takeoff Weight: 11,200 lbs Internal
- Empty Weight: 6,450 lbs
- Useful Load: 4,750 lbs
- Fuel Burn: 410 litres/hour
The Bell 212 may not be able to carry that much more but what it does do is increase the safety factor because if one engine were to quit, the other will be able to get the helicopter to a safe landing area. This is one of the reasons why the Bell 212 is such a popular helicopter for wealthy Heli-Skier’s!
Helicopters that use a ‘Jet Engine’ are designed to lift more people and their cargo. Getting the same power from a piston engine would make the engine far too big and heavy.
Although the piston engines are far cheaper and cost less to run, there comes a time when they use is not perfectly matched for the airframe. This usually occurs once helicopters have 5 seats or more.
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