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How to Correct Engine Data to SLS Conditions

1. There are essentially two steps to correcting data to standard day conditions, the first is to correct atmospheric conditions for aircraft forward motion and altitude so as to determine engine inlet temperature and pressure (the conditions the engine ‘sees’), and the second is to correct engine operating parameters to standard day.


2. The latter process has some added complication due to aircraft weight and bleed air configuration that cannot be directly corrected (without OEM input) until some reasonable amount of data has been collected at which time an empirical correction can be derived. However, if you want to go to this level of sophistication, you will need additional input data such as Ramp weight and fuel used.


3. To calculate pressure and temperature at the engine inlet the following calculations can be used, Note: the aircraft Pressure altitude should be set to 29.92 in Hg before reading altitude a) Total pressure (Pt) at the engine inlet:



In this case P is external static pressure and can be derived from the aircraft pressure altitude using the ICAO standard atmosphere calculation available from many sources.. Ɣ is the ratio of specific heats of air and a good approximation is 1.4 at all altitudes. M is the aircraft Mach number. b) Total temperature (Tt) at the engine inlet:


Ts is the outside Static Air Temperature and is available as SAT in the aircraft. Note: It is important to note that in all cases the values for Temperature must be expressed in absolute terms for use in these equations i.e. aircraft SAT expressed in Degrees Celsius must be converted to Kelvin prior to use in the above equation, this done by adding 273.15 to the C value.


4. Having derived the inlet conditions conversion to standard day conditions can be performed using the following equations: a) Corrected Shaft Speed, N corrected =



Ɵ is ratio of the Tt calculated 3b) above to the standard day temperature of 288.15 Kelvin i.e



x is typically quoted as 0.5, for simple turbojet engines however for turbofan engines this will approximate 0.5 but will typically be higher, in the range of 0.525 – 0.55. The engine OEM should provide these value for you and they will be different for N1 and N2. b) Corrected Temperatures (such as EGT), T corrected =



y is engine and parameter dependent and will be approximately 1.0, you should contact the engine OEM for this value stating the precise temperature you wish to correct (likely ITT/T5 or EGT). c) Corrected Fuel Flow (WF), WF corrected


z is engine dependent and will be approximately 0.5, you should contact the engine OEM for this value. ϭ is the ratio of Pt derived in 3a) above to the standard day pressure of 29.92 ins Hg or 14.67 psi, i.e.

d) EPR is a non-dimensional parameter and therefore requires no corrections and can be used as a baseline for trending the other parameters.

5. Once corrected data is obtained curves of these parameters versus a primary parameter such as EPR or N1 for Turbofan engines, or Torque for turboprop engines can be developed and then a best fit curve fit equation can be derived. These curves will normally be linear, or second order polynomial fits, use of higher polynomial order fits is not advised. Deviations from the predicted parameter value at any given EPR/N1Torque can be used to trend the engine.


6. There are three significant items that can affect the accuracy of these curves: a) The aircraft bleed system configuration will have a measurable effect on the N2 or NG vs EPR/N1/Torque, WF vs EPR/N1/Torque and EGT vs EPR/N1/Torque relationships. Part of the data recording process should record the status of Anti Ice on or off and these two conditions may need to be plotted separately. b) With engines that have a variable bleed valve and Compressor Variable Guide vanes and there will be a lower limit of operation where the data will not be repeatable. As a rule of thumb 90% HP rpm is probably the low end of the reliably repeatable data. c) Large variations in aircraft weight may cause variability in corrected data that may need to be corrected out using an empirical formula derived once sufficient data has been generated.

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