Question:

How do I calculate power output of a turboshaft (or turboprop) engine?

(I see PWshaft unchanged in the output table although I change off-design conditions. Also if I change ambient conditions I see SCF remain unchanged. )

Answer:

This problem is due to the settings in the power turbine and (if present) load control components.

Also: make sure you do not use the design point calculation when you actually want to predict off-design performance. Note that if you change ambient conditions for example and re-calculate design point, you actually calculate a design point of a different engine.

Load specification and calculation for a turboshaft engine with free power turbine is a little more complicated than specifying jet engine power setting due to various options to power level for maximum flexibility:

a)Without the load control component, for a power turbine running at a fixed user specified speed, GSP calculates the Power surplus (PWsurplus) as a difference between power taken from the shaft by the External Loads specified in the Turbine component and the mechanical power delivered by the turbine. In the design point PWsurplus=0 by definition and the total design power load specified is used to calculate the turbine design point performance. The PWshaft output field in this case only represents what is specified as loads: not necessarily equal to power delivered.
Of course, one can derive actual power delivered as the difference between PWshaft en PWsurplus, but this is not as practical as using option b ). Also note that the PWshaft_x (with x for component nr. or string) represents gross turbine power output. PWshaft_x times mechanical efficiency equals net total turbine power output.

b)With the load control, a design load can be specified in the Load control (and none in the turbine), while the off-design loads are all left to 0. Now GSP uses the load control to correctly calculate actual power output from the power loads specified and the surplus power calculated: the PWshaft will always show the correct power output provided the turbine and load control components are correctly configured (see below).

Power levels and torques can be combined (to be calculated as a total load) in the turbine and load controls. It is usually best to use a Loadctrl component (option b) ) to efficiently use GSP for turboshaft/turboprop performance calculations.

There are basically two alternative approaches to calculate off-design performance:

1)specify fuel flow and calculate power output:
You then normally have selected the Specify fuel flow and Fuel flow input User specified options in the Manual fuel control (or PLA in a detailed fuel control model). In the power turbine component, set Model options to User specified rotor speed and check the Free Power Turbine check box. Leave all design and off-design loads in the turbine component at value 0, unless a fixed auxiliary power is continuously extracted. Use a Load control component to specify turbine load. Set the right shaft nr./suffix (equal to the power turbine component shaft). Leave the off-design loads (in the General tab sheet) at value 0 and only add a design point load. It is usually better to specify a torque load instead of a power value because with torque, the power will vary with any user specified rotor speed changes accordingly. Torque as input is also more practical if analysing engine transient responses to load variations (torque steps). Make sure you clear the Auto reset input to design option in the Model General options (Model Options|General menu) to avoid the initial setting of off-design load to design load at design point calculation, otherwise you must reset off-design load to 0 after every design point calculation.

2)specify power output and calculate fuel flow (or power setting PLA) required:
Activate the Auto reset input to design option in the Model General options (Model Options|General menu) to initially set all off-design input to design at design point calculation. In the Manual fuel control component (or control system model component PLA) change the Fuel flow input option to Free state (this adds a 'State variable' to the GSP equation system). In the Power turbine component change the Model Options to Power balance at rotor speed (this adds a corresponding 'Error variable' to the GSP equation system). After design point calculation the off-design load in the Load control now is initialized to design values. With subsequent off-design (stead-state or transient) calculations at varying load levels in the Load control General (or transient) tab sheet, now GSP calculates the corresponding fuel flow required.

As an alternative, Load control input can be specified as a state instead of fuel flow, which in effect turns 2) back into option 1). Then the load corresponding to a user specified fuel flow is solved form an extra equation. This option however is not recommended because of potential iteration stability problems with the extra equation in the system.

These guidelines equally apply to single spool turboshaft engines (fixed power turbine). The only difference is that a part of the turbine power delivered is used for driving the compressor.