Friday, December 13, 2013

Thrust Reversers

There will be cases when the aircraft is required to land on runways that are shorter than those for which the aircraft can normally land.  It is also beneficial to use less runway than that available in order to have a safety margin in the event of a failure.  Although the brakes on the aircraft are designed to be adequate under all normal operating conditions, the engine can assist in shortening the landing distance by using the thrust reversal systems.In this system the engine thurst is vectored to the front to slow down the aircraft.

On a propeller engine this is very simply done by reversing the pitch of the propeller blades so that they now thrust air forward instead of backwards. Therefore the prop aircraft can change the direction of thrust by 180 degrees. But, This is impossible to achieve with a jet engine, The result of this is that the reversed thrust from a jet engine can never be as fully effective as that from a propeller.  The thrust level obtained from the engine can be as high in reverse as it is in forward thrust but, because of the angle involved, the effect can never be the same as the forward thrust.

types of reversers

As far as jet engines are concerned there are three basic types of thrust reverser: 

  • Clamshell Door
  • Retractable Ejector
  • Cold Stream Cascade Reverser


Friday, December 6, 2013

Gas Turbine Combustion Systems

The combustion system is designed to burn the fuel as efficiently as possible over the whole range of engine operating condition.  All the energy released by the fuel is converted into heat and velocity energy. Very high temperatures exist in the combustion system, the burning temperature of the fuel being in the region of 2,000°C.  To protect the material from which the system is manufactured, about 60% of the total air flow is used for cooling and the rest is used for fuel burning.

 The combustion chamber is quite short therefore it must be efficient in completing the combustion. To achieve this fuel air mixture is made with it best ratios.

The combustion process requires 15 unit of air to 1 unit of fuel for a successful combustion.  This is known as an air/fuel ratio of 15:1 by weight

The air flow leaving the compressor is first split into two, approximately 20% - 40% being used for combustion, the other 60% - 80% is further divided for combustion support and the greater proportion for gas cooling.

These three flows are known as:-

  • Primary air flow, for mixing with the fuel and to support combustion.(20% )
  • Secondary air flow to shape the flame and complete combustion. ( 20%)
  • Tertiary air flow, to cut off the flame and reduce gas temperature to a figure acceptable to the turbine. ( Cooling 40% & Dilution air 20%)