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Photograph of subsonic, supersonic and forebody/inlet performance models.

Aerodynamicists use wind tunnels to test models of proposed aircraft and aircraft components. The model is placed in the test section of the tunnel and air is made to flow past the model. In some wind tunnel tests, the aerodynamic forces on the model are measured. In some wind tunnel tests, the model is instrumented to provide diagnostic information about the flow of air around the model. Model instrumentation can include static pressure taps, tufts on the surface, or total pressure rakes. Some wind tunnel models are designed to determine the performance of a particular component of the aircraft.

The aircraft inlet and nozzle bring airflow into and out of the gas turbine propulsion system. The performance of the inlet and nozzle depend on both the flight conditions of the aircraft and the operation of the turbine engine. Special wind tunnel models are used to determine the performance of the inlet and the nozzle.

On this page we show several examples of inlet performance wind tunnel models. The model at the lower left is a forebody/inlet model mounted in the NASA Glenn 8x6 foot transonic tunnel. The model at the upper center is a subsonic inlet model mounted in the NASA Glenn 9x15 foot subsonic tunnel. The model at the lower right is a supersonic inlet model mounted in the NASA Glenn 10x10 foot supersonic tunnel.

There are several unique features to inlet wind tunnel models and testing. Inlet performance is characterized by three factors:

  1. Total pressure recovery is the ratio of the average total pressure at the exit of the inlet to the free stream total pressure. A higher pressure recovery indicates a better performing inlet. The maximum possible value of recovery is 1.0.
  2. Compressor face distortion characterizes the variation of pressure across the exit of the inlet. There are many different distortion factors used in the aerospace industry. The simplest factor takes the maximum measured total pressure minus the minimum measured total pressure divided by the average total pressure. More sophisticated models use mass or area weighted averages. Inlet distortion is an important factor because highly distorted flows cause compressor stalls that can damage the engine or disrupt flow through the engine.
  3. Inlet spillage drag is a drag that occurs when the engine cannot handle all of the flow that approaches the inlet. The airflow through the engine is set by choked conditions in the nozzle. Any excess flow that approaches the inlet is spilled around the inlet generating additional drag on the airframe.

The airflow through an inlet model is varied during a test to determine the effects of engine airflow on both spillage drag and recovery. Engine airflow is varied by using a calibrated plug at the exit of the inlet. The flow going into the inlet is influenced by any object upstream of the inlet. Therefore, an inlet model often include a model of the forebody of the aircraft. The flow is also affected by the angle of attack and angle of yaw of the aircraft. Inlet models must often be pivoted during a test to determine the effects of maneuvers on inlet operation. For supersonic inlets, shock waves are generated by the airframe and inlet surfaces. Boundary layers generated on the surfaces can interact with the shock waves to create separated regions in the inlet that decrease recovery and increase distortion. Flow control devices used in the inlet, such as boundary layer bleed, must be properly modeled in the wind tunnel test.

Inlet testing usually contains a combination of flow visualization, diagnostic instrumentation, and performance instrumentation. The chief performance instrumentation for an inlet test is the 40 probe rake. The rake is used to both determine the inlet pressure recovery and the inlet distortion parameter values.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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