SOO Atch 3 - External Panel Balance.pdf

Distribution A. Cleared for Public Release. AEDC IRA-26-1262.

Technical Requirements: External Panel Balance

1. Introduction

This document outlines the requirements for the procurement of a flush-mounted external panel balance for the AFRL-operated supersonic wind tunnel. This wall-mounted device is intended to allow loads measurements on wall-mounted approximations of vehicle acreage panels.

This document specifies the required degrees of freedom, minimum/maximum loads, acquisition rate, and integration requirements. The values in this document are estimates; the performer shall work with the Government to revise and confirm these values prior to final fabrication.

2. Scope

This document covers the functional and performance requirements for the flush-mounted external panel balance, including:

• Degrees of freedom

• Expected loads

• Operating environment and conditions

• Physical dimensions and mounting interfaces

• Data acquisition and output requirements

• Calibration requirements

Where possible, requirements will be specified as Thresholds and Objectives, denoted by (T) and

(O), respectively.

3. Requirements: External Panel Balance

3.1 Functional Requirements

The flush-mounted external panel balance shall measure up to six components of aerodynamic forces and moments (drag/lift/side-force + rolling/pitching/yawing moments). The Government encourages early discussions to understand how changes to these requirements may impact the final product.

3.2 Performance Requirements

• Degrees of Freedom: Single-component drag (T); 6 forces/moments (O)

• Expected Drag Load: The balance will be used to measure loads on acreage panels, including pristine/flat as-designed panels and damaged/deformed surfaces. For the conditions in the tunnel, at the lowest Reynolds number conditions at Mach 5, wall shear stress τw ≈ 5 Pa. When multiplied over the wetted area of the panel (5” x 9”), the resulting drag force is approximately 0.145 N. On the upper end of drag estimates, a half-diamond shape spanning the entire 9” panel length (with amplitude of 20mm) would generate a drag force of ~ 300 N at Mach 1.5. The vendor is encouraged to provide feedback to the

Government as to what range of drag loads can be conceivably measured in a single configuration; while 0.145N and 300N lie at the extreme ends of expected measurements, typical values are expected to fall within 5-100N.

• Maximum Lift Load: The largest lift load (measured normal to the panel surface) is likely to occur during the transient tunnel shutdown process, and is a function of the instantaneous pressure differential across the panel test article. In prior work by the Government, thin-walled acreage panels were rigidly mounted in the tunnel floor (see Section 3.3) above a pressure-matched cavity. While the venting was sufficient to produce a differential pressure ∆p ≈ 0 while wind-on, the time lag through the vent tubing produced an instantaneous ∆p = 0.15psid during shutdown after a 90-second duration run. For a full-duration run, this value is expected to be closer to ∆p = 0.45psid. Integrated over the panel wetted area, this yields instantaneous lift loads of 6.75lbf (T) and 20.25lbf (O), corresponding to ∆p = 0.15psid and ∆p = 0.45psid, respectively.

• Operating Temperature Range: The typical total temperature within the facility is less than 200 F, with adiabatic wall temperatures close to ambient (i.e. 70 F).

• Operating Pressure Range: Operating static pressure in the test section during operation at high Mach numbers can be assumed to be at vacuum (<0.1 psia). The balance must be designed to operate under these conditions.

• Operating Duty Cycle / Conditions Exposure Duration: The tunnel is a blowdown wind tunnel facility. Test runs are typically less than 5 minutes each at operating conditions, occurring typically fewer than 20 times per shift. Pre- and post-operational procedures may require the test section to be at stagnant vacuum conditions for longer (typ. 30 minutes), during which time balance tares and/or checkouts may be performed.

3.3 Physical Requirements

• Dimensions: The balance shall have the necessary bolt pattern to mount Government-supplied 9” x 5” test articles (Fig 1); CAD and drawings for this bolt pattern can be made available.

• Mounting Interface: The balance must sit flush with the tunnel floor, mounted within a

Government-supplied adapter hereafter called the Multi-bay Frame; Figure 2 shows possible mounting locations within the tunnel. CAD and drawings for the wind tunnel and

Multi-bay Frame can be made available. The Multi-bay Frame is currently used for mounting thin (0.010”) compliant panels, which are first mounted into carriers (Fig. 3) before installation into the Multi-bay Frame. The balance supplied by the vendor may replace the carrier in Fig. 3, as long as the legacy interface to the test articles (Fig. 1) and

Multi-bay Frame are maintained.

• Alignment and Tolerances: Any gaps / steps on wetted surfaces must be less than 0.005”.

3.5 Data Acquisition

• Acquisition Rate: The balance will be used for measuring steady-state loads; anticipated acquisition rates are < 200 Hz.

• Wire Routing: If the balance will incorporate a pressure-matched cavity (similar to the current implementation shown in Fig. 3), then any cabling or wires must not compromise the seal of this cavity.

3.5 Calibration Requirements

• Calibration Process: Recommended calibration procedures and intervals should be supplied by the performer, along with as-built data sheets for converting signals to engineering units.

Figure 1 Required bolt pattern to interface with Government-supplied test articles

Figure 2 Installation region for the panel balance, showing the Multi-bay Frame and possible mounting locations for the balance (A - C). Flow is left to right.

Figure 3 Build-up of the Multi-bay Frame and associated carriers.