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Dynamic Analysis of XB-70-1 Inlet Pressure Fluctuations During Takeoff and Prior to a Compressor Stall at Mach 2.5

The capability of predicting the combined performance of an inlet and engine under
distorted flow conditions has increased in recent years through the development of
various steady—flow distortion parameters based on total—pressure measurements at
the inlet—engine interface plane. In addition, the necessity of understanding unsteady
phenomena occurring in the air inlets of high—speed aircraft has arisen. These
unsteady interactions can lead to severely reduced aircraft performance because of
abrupt compressor stalls and Violent inlet unstarts and, as such, pose a major problem
for both civilian and military aircraft inlet and engine designers. Accordingly, con—
siderable emphasis is being placed on inlet dynamics by government and private
industry.

Turbulence—induced compressor stalls may have been first recognized in wind—
tunnel engine—inlet compatibility studies of a one—third—flow-scale XB—7O inlet modified
to fair into a single J93 engine (ref. 1). More than 20 stalls at Mach numbers of 2. 24
to 3. 00 were induced by operating the inlet at highly supercritical conditions, that is,
with the normal shock relatively far downstream of the inlet aerodynamic throat.

Turbulence and dynamic distortion parameters may be defined by dynamic—pressure
measurements. Values of a turbulence parameter are local measurements of normal—
ized fluctuating pressure amplitude which, of course, may vary depending on the
location and the local flow conditions. Values of a dynamic distortion parameter,
however, are the calculated values of instantaneous distortion, implying a multiplicity
of high—reSponse pressure measurements. This report deals only with turbulence
aspects of the dynamic measurements. Because of the random nature of the transient
pressure signals, statistical methods can be applied in their analysis.

To provide inlet dynamics data from full—scale aircraft under actual flight con—
ditions, the NASA Flight Research Center conducted flight studies with the XB—70—1
airplane. The airplane was instrumented with high—response static— and total—pressure
sensors to detect the pressure fluctuations caused by any type of flow—disturbing
phenomena, but particularly shock—induced boundary—layer separation and flow—turning
separation. To minimize tubing transmission problems inherent in measuring dynamic
pressures at remote locations, close-coupled probes and sensors were used in the
left—hand inlet of the XB—70—1 airplane.