Found another one
'THE EFFECTS OF INCREASED DENSITY ALTITUDE'
On engine performance (power available to the rotor)
Turbine performance will be adversely affected by increased density altitude.
1. As DA increases, air density is less. For a constant speed compressor in a fixed spool engine the mass of air entering the combustion chamber decreases. For a given throttle opening the combustion chamber temperature increases as there is less air to cool the flame.
The throttle can be opened to give more power until the T4 (TOT) limit is reached. As DA increases this limit is reached at smaller throttle openings and the power available is less. (Note: See previous posts as to the internal workings of the turbine)
2. In a free power engine the compressor speeds up as DA increases, to maintain the same mass of air entering the combustion chamber. The throttle can be open to give more power, and the compressor speed (Ng) increases the mass of air, until a limit is set by the forces on the compressor blades. As DA increases this limit is reached at smaller throttle openings and the power available is less. Thus the power available from the gas turbine engine reduces with increasing DA (constant pressure altitude with increasing temperature for example). However at low DA when the power available is greatest there will be a limit set by the maximum transmission torques permitted'
There are your limits and the reasons why DA effects them so greatly. How it physically happens in the engine is described in a previous post but I do have a power point presentation detailing the internal workings of a gas turbine, written by me for tuition purposes, if you wish a copy Nick then PM me.
I find it interesting as I have been teaching this stuff for years now and I am surprised that there is so much opposition to the theory. Look at the common pressure versus highly differing temperature example to see how your performance will drop purely by the increase in temperature.