Today, one of the most common helicopter engines is the turboshaft engine which is a type of turbine engine. Turboshaft engines are essentially jet engines that run on kerosene-based fuel. There are many turboshaft engine variations, but all follow the same basic concepts and principles. To better understand how helicopter engines operate, this blog will outline their working principle and useful tips.
The working cycle of turbine engines is similar to that of piston engines wherein there is an induction, compression, combustion, and exhaust phase. The major difference between the two lies in the fact that turbine engine cycles are continuous. Unlike aircraft jet engines where exhaust gasses provide thrust, the exhaust gasses in helicopter turboshaft engines are intercepted by a turbine that transfers the energy from these gasses to a gearbox that provides power for the helicopter.
For this reason, it is crucial that helicopter owners do not over-torque or over-temp the engine, because this can have disastrous consequences for pilots weeks or months later. In fact, misusing the engine will not cause a problem until much later, catching many owners and operators off guard. As such, we are going to outline some proper handling procedures.
To begin, a helicopter engine imposes greater stress to its components during start up than in any other part of operation due to thermal shock and wear. With this in mind, the start cycles are counted and recorded in a flight log after each flight. Since helicopters usually fly for short periods compared to airplanes, the likelihood that the engine will reach its start count limit before it reaches its useful life hour limit is high.
Start-ups are costly; thus, it is better to keep the engine running for ten minutes when waiting for a passenger rather than shutting the engine down and starting it up again. Keep in mind that the engine relies entirely on the battery or Auxiliary Power Unit (APU) for start up. Moreover, the engine has to turn fast enough to keep ample cold air flowing through the turbine; otherwise, it will become too hot and damage the internal components. A weak battery may run out of power before the combustion phase is self-sustaining, and this can be disastrous.
Hung Start v. Wet Start
A “Hung Start” occurs when the engine fails to accelerate and the RPM remains constantly low. This type of start uses up battery power, and if the battery runs out of power, the engine slows down, less and less cold air is drawn through the engine, and the temperature in the combustion chamber gets really hot in just a few seconds, causing a lot of damage.
A “Wet Start” is used to describe a flooded engine and an igniter that has failed to light the fuel. It is recommended to wait for a minimum of 5 minutes, and then vent the engine. Because the battery has already been partially drained by the failed start and venting, you should obtain an external start utilizing an APU to ensure power does not run out on the next start attempt.
What To Keep In Mind
If all the appropriate steps are followed, then you should have no problem starting your helicopter engine. It is worth noting that hot starts usually take place when starting the engine with the throttle already partially open. Additionally, turbine engines necessitate time to “spool up” or “spool down” during power changes. That being said, you should operate the collective or throttle smoothly to prevent engine “surging.” Generally, surging happens when airflow over the compressor blades is disturbed, causing them to stall. This leads to loud banging noises and excess vibration from the engine. If this occurs, shut down the engine immediately.
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