Effective Date: 15 June 98
The method used to compute the in-flight thrust of the generic high bypass engine is a conventional nozzle thrust calculation procedure. As such it requires the use of nozzle flow coefficients for airflow determination, and nozzle velocity coefficients for thrust determination. The definitions of these coefficients are conventional; i.e.,
CT = Cv x CD where:
CD = W act W ideal
CV = W act V ideal C-D
The values of these coefficients have been derived from full scale engine tests with engines having the production engine nozzle system. To obtain the necessary data, engines have been run on both sea level and altitude engine test stands.
The sea level beds generally used are those located at the engine manufacture facilities. The altitude facility generally used is a government developed test facility. The coefficients being employed for thrust determination were developed from data obtained on a test engine which may be from an early batch of standard of high bypass engines having an exhaust system representative of those on production engines.
As noted in Section xxx4, the thrust calculation uses an internal total to free stream static pressure ratio to compute the ideal airflow. A CD is applied to the ideal value to obtain the actual airflow which is then used to obtain in-flight gross thrust as shown below.
Note: This equation combines equations -13, -14, -15 and -16.
The value of # Fg, used in the above equation, is obtained from the NGTE force accounting equation shown below:
The velocity coefficient of the fan nozzle is determined by taking the #Fg term from equation -2 and substituting it into equation -l. The latter equation is then solved for CVB using the following inputs:
o WE calculated from the choked turbine nozzle guide vane area and known pressure and temperature.
o WB calculated from measured W2 and the solution of equations 72-8, 72-10, 72-11 and 72-12.
o TTB and TT8 as measured.
o FgB and FgE as determined by equation 72-13.
-Wa # TTB i WE ./TT8 i
o CVE set at .998 which is the value recommended by Engine manufacture.
All the CVB data thus computed is then fit by root-sum-square methods to a single valued curve of CVB versus PTB/PAM.
The flow coefficients of the fan and primary nozzles are used to obtain the temperature-corrected flow terms WB # TTB and ###TT8 equation -2. When divided respectively by the terms PTB AB and PT8 AE appropriate to the data point, the "actual" flow parameter required to determine the flow coefficients CAB and CAE are obtained. The thrust calculation uses the conventional expansion ratio, PT/PAM, to define the "ideal" flow parameter, and thereby, the flow coefficient. Since PAM exists only in the expanded exhaust stream and not uniquely at the nozzle plane, external influences will affect nozzle plane pressure and, therefore, the flow coefficient. As an example, fan nozzle flow coefficient is affected by engine position, aircraft angle-of- attack, and flight Mach number.
The core engine nozzle flow coefficient is influenced by these same variables plus the fan efflux. The NGTE full-scale engine data in this case are only used to define the flow coefficients in a quiescent environment (M = 0). Scale model test results used to define the correction to the airflow reflect external effects. This correction is defined as a flow coefficient ratio; i.e.,
CD (in A/C flow) CDR = CD (in quiescent air)
Figures -l and -2 present the values of CDR to use in calculating nozzle airflows. The values have been derived from test results obtained on two 1/20 scale models; one a semi-span aircraft model equipped with a Tech Development powered nacelle and the other an empennage model with blown nozzles. The CDR was derived by taking the ratio of measured airflow to calibrated airflow in the quiescent envirorment. Where direct airflow measurements were not available in the aircraft environment, e.g., fan flow on the powered nacelle, an airflow calibration using internal nozzle static pressures was used to obtain CDR. A flow diagram showing how the various flow coefficient data interface with the thrust validation procedure is presented in Figure -3.