Flat rated

[Olendirk]

Hey!

Could somebody explain what the advantages of a flat rated engine are and how it works?

Thanks

OD

[Keith.Williams.]

The thrust produced by a jet engine is equal to the mass flow rate of the air passing through it multiplied by the acceleration given to that air. Anything that reduces the density of the air will reduce the mass flow passsing through the engine, and hence reduce the thrust. Such factors inlcude high pressure altitude (which means low ambient pressure), high ambient temperatures and high humidity.

In order to recover this lost thrust it is necessary to open the throttle to increase the fuel flow and make the engine turn faster. But the extra fuel also increases the turbine temperatures and a limit is eventually reached where any further throttle opening will destroy the turbines.

If an aircraft has just enough thrust to take-off from a low altitude, high pressure, cold, dry airport, any changes in these conditions will require take-off mass to be reduced. This is clearly unacceptable in terms of disruption to passengers, so a better method is required.

The term flat rating means that an engine is able to produce at least a certain advertised thrust within a wide range of ambient conditions. In effect the engine is more powerfull than it needs to be in hgh density atmospheric conditions. So in conditions of high pressure, low temperature and low humidity, the required thrust is available using a reduced RPM and reduced turbine temperatures. This increases the life of the engines.

When atmosheric pressures reduce, or temperature or humidity increases the engine control system automatically increases fuel flow to give the standard thrust. So provided the atmospheric conditions are within the flat rating limits, the standard thrust is available and there is no need to off-load passengerd or cargo.