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AEDC adds large-scale mass flow assembly calibration to test capability offering

  • Published
  • By Jill Pickett

The Arnold Engineering Development Complex team stood up a new capability in the 16-foot supersonic wind tunnel at Arnold Air Force Base, Tenn. – hardware calibration for engine inlet testing.

An Aerodynamics Test Branch customer needed to calibrate their mass flow assembly, or MFA, for an upcoming high priority propulsion integration test. The MFA consists of an aerodynamic interface plane (AIP), a mass flow plug and ejector. The MFA system is used to simulate the throttle lever angle’s (TLA) mass flow through the test article. A facility capable of providing the needed uncertainty levels could not be found due to the large size of the MFA hardware.

The Aerodynamics Test Branch stepped up to meet this customer’s need, and as a result additional customers quickly requested to use the new capability for their future MFA calibration needs.

“MFA calibrations like this are unique in nature and are specific to wind tunnel testing,” said Nathan Payne, propulsion subject matter expert for the Aerodynamics Test Branch. “In the past, our test customers have had their own facilities to do this work. However, as those facilities wear out, or are sold off, our customers need a new solution.”

An MFA is attached to an aircraft model for testing in an aerodynamic wind tunnel. The MFA represents the engine mass flow rate into the inlet in order to characterize how the airflow into the engine is disrupted by the aircraft inlets and inlet ducting during maneuvers made by the aircraft throughout the flight envelope. Changes in the airflow can effect engine operations, specifically stall margin and pressure recovery.

The data gathered during the inlet testing are used to make distortion screens to replicate the altered airflows to the engine to increase real-world conditions during full-scale engine testing in engine test cells at Arnold AFB or at other original equipment manufacturers’ (OEM) engine test facilities. In an engine test cell, the engine is in a fixed position, therefore, it does not have airframe effects and is not representative of a full-mission envelope of conditions. The distortion screens allow test engineers to more closely simulate actual flight conditions.

  The MFA is calibrated in clean, dry, smooth and choked flow air so all losses between the AIP to the flow plug are captured. When the MFA is attached to an aircraft model, the only effects to data are the distortions caused by the model design and model attitude. This allows for comparisons to different configurations at the same TLA, or corrected mass flow, setting.

Each engine that is being simulated by an MFA has to have a mass flow assembly calibration or, at minimum, a calibration verification. There are multiple facilities around the nation that have this capability. However, the large size of the system under test drove the need to perform the work at Arnold AFB. A large scale model provides high fidelity data which leads to a lower margin of error when scaled up for engine testing.

The 16-foot supersonic wind tunnel, or 16S, will provide a unique flow measurement capability that utilizes a choked venturi for more accurate mass-flow measurements. This measurement technique is one that most OEMS do not have in their wind tunnels. A choked venturi allows test engineers to reduce the number of measurements that need to be taken, which reduces instrumentation, preserves flow quality and reduces overall measurement uncertainty.

“Our customers in the past have not had the ability to calibrate MFAs at different atmospheric pressures, which ensures no Reynolds numbers effect in the data,” Payne said. “They also have not been able to provide dry air to the MFA resulting in humidity errors, or the need for a humidity correction. Last, in some cases customers could not keep the venturi choked for all flow rates needed.”

The calibration is conducted by mounting the MFA to the scavenging scoop in 16S. The scoop was designed for the purpose of removing exhaust from the airflow when testing air-breathing engines and rockets ensuring non-vitiated flow because 16S is a continuous flow tunnel. A set of venturis was mounted inside the scoop and then the MFA was mounted to the front of the scoop via an adaptor plate. When not needed for MFA calibration, the scoop can be returned to the standard configuration.

Standing up this new capability was accomplished through a collaboration of expertise from across AEDC. Payne drafted the plan and did the initial calculations. Computational fluid dynamics engineers then verified that the theoretical should be possible. From there, design engineers took over to design the hardware to ensure it could withstand the test conditions. Payne approached the Propulsion Test Branch to secure the venturis needed. Then, AEDC craftsmen fabricated the parts needed to modify the scoop and installed the hardware.

“AEDC was able to utilize the different skill sets here on base to retrofit the suction scoop and added a new metered airflow capability for our customers to an existing world-class facility,” Payne said.

 The wind tunnel, known as Tunnel 16S, has been the focus of a major multi-year effort to return the facility to service to provide additional testing capacity for large-scale models, and to expand the test envelope available for testing those models.

This capability does not require the use of the main drive compressors, but did contribute to the return-to-service effort through checkouts of the data systems, which are the same systems that will be used once the tunnel is fully operational. Other systems checked out and utilized during the calibration were the controls system, the real-time data displays and the analysis stations. The tunnel was also pumped down using the plenum evacuation system (PES), providing an opportunity to “exercise” the tunnel, including valves that must be opened and closed for operations.

The high demand for the 16-foot transonic wind tunnel increases the importance of returning 16S to service and of this new capability. If the large-scale MFAs had to be calibrated in 16T, there would be a significant loss of test time.

“The original design I had was to utilize the 16T scoop, however, as more customers showed interest, the 16S systems started to come online, and with a successful leak check of the 16S circuit, the 16S scoop presented itself as a viable test option,” Payne said. “A walk-down of the flow path by the TOE (test operations engineer), myself and plant engineers; favorable CFD runs; and a fairly open test schedule, we knew we had found the right place for this capability.”