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The Helicopter Engine (Turboshaft)

Time:2012-04-05 15:02Turbochargers information Click:

helicopter helicopter engines

In order to meet the design requirements, the engine must have a relatively high power output, good fuel consumption and fast acceleration characteristics. For these reasons it is beneficial to operate as closely as possible to the stall angle of the compressor blades. Operating close to the stall angle has the following benefits:

If too much fuel is supplied to the burner, there will be more than enough air to allow proper combustion. However as extra air is used during this combustion, there will be less air available for cooling and therefore the temperature inside the combustion chamber will rise. As the temperature rises, there will be more gases to be exhausted. It is possible that the volume of gases to be exhausted may exceed the capacity of the turbine and the turbine will “choke”.

During startup there is a bleed valve that opens and allows some of the air to escape from the compressor. This makes it easier to get the engine up to speed and takes less power from the battery. When the engine reaches a sustainable speed the valve closes automatically.

A “Wet Start” is the equivalent of a flooded engine and the igniter has failed to light the fuel. Wait for at least 5 minutes and then vent the engine. Because the battery has already been partially drained by the failed start and the venting, it is probably a good idea to get an external start using an APU to make sure you do not run out of power on your next start attempt.

When more power is required, the compressor speed (N1) increases to supply more air. At the same time more fuel enters the combustion chamber and therefore N2 is maintained at a constant speed.

Compressor stalls may or may not have an audible sound but there will often be a vibration. If the stall is severe a flame may emanate from the exhaust or a very loud backfire may be heard. Smoke may also be seen.

If the pressure in the combustion chamber exceeds the pressure of the compressor discharge air, then not only will the compressor stall but also the hot gases will flow from the combustion chamber into the compressor section.

Both of these conditions will result in a loss of air into the combustion chamber. The flame will not have enough oxygen and will die, resulting in a rapid drop in temperature. As the temperature drops, the expansion is stopped (or greatly reduced). The turbine is no longer choked and the combustion chamber pressure drops to a very low value.

So what is compressor stall?


Comment by Richard G on August 29, 2011 @

In the axial compressor, each stage is separated by stator vanes to make sure the air hits the following blades at the correct angle. As the air flows past these blades it becomes compressed more and more. The pressure rises and its velocity decreases.

If the pressure in the combustion chamber is equal to the pressure of the compressor discharge air, then the compressor will stall.

Comments

The Air Inlet

The air inlet is designed to stabilize the air before it enters the compressor. A particle separator may be fitted at this point to remove any foreign matter such as dust or sand which could cause erosion of the compressor blades.

Helicopter Engines

Turboshaft Engine

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