SOO Atch 2 - External Balance for Fin Loads.pdf

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

Technical Requirements: External Balance for Fin Loads

1. Introduction

This document outlines the requirements for the procurement of an externally mounted balance for measuring fin loads in AFRL-operated supersonic wind tunnels, referred to in this document as tunnel 1 and tunnel 2.

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 externally mounted fin balance, including:

• Degrees of freedom

• Expected loads

• Operating environment and conditions

• Physical dimensions and mounting interfaces

• Data acquisition and output requirements

• Calibration requirements

• Articulation Range and Control

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

(O), respectively.

3. Requirements: External Balance for Fin Loads

3.1 Functional Requirements

The externally mounted fin balance shall measure at least three components of aerodynamic forces and moments (lift, torque, and bending moment about the shaft). The Government encourages early discussions to understand how changes to these requirements may impact the final product.

3.2 Performance Requirements

• Maximum Lift Load: Maximum loading experienced by a test article and its support structure in tunnel 1 occurs during the unsteady, chaotic, asymmetric flow breakdown at the end of every tunnel run.

As an upper bound, the differential pressure experienced by a test article is theoretically largest when flow is “unstarted” (subsonic, with high static pressures behind a normal shock at test Mach number) and fully started (supersonic, with low static pressures).

Practically, the maximum loading experienced by test articles during breakdown are shown via experiment to be less than this and can be predicted empirically from Eq. 4 of Maydew

(“Compilation and Correlation of Model Starting Loads from Several Supersonic Wind

Tunnels”, 1962) as N = CN*S*P0, where

N = maximum unstart / blockage / breakdown load (lbf), CN = empirical force coefficient (unitless), S = cross-sectional area (in2), and

P0 = upstream pressure at breakdown (psia).

For tunnel 1, the maximum breakdown loading occurs at the end of a maximum-duration run at a Mach number of 2.6, where CNc = 0.262 and P0 = 66 psia. Thus

Nthreshold = (17.3 lbf/in2) * S, Given an estimated area S = 21.7 in2, Nthreshold = 375 lbf.

An objective of running the same configuration in tunnel 2 would require a higher maximum loading. For that case, where the Mach number is 6 and P0 = 2000 psia, Nobjective = (51.6 lbf/in2) * S, Given the same estimated area of S = 21.7 in2, Nobjective = 1120 lbf.

It is well-understood that maximum design load is a driving factor for balance and mechanism design, and that high loads required for survivability are in opposition to a desire to optimize balance sensitivity and uncertainty. The maximum design load for the combined test article and support system may require tailoring, in coordination with the government, to provide an optimal compromise of balance survivability, balance sensitivity, maximum model size (S) and tunnel runtime (i.e., P0 at unstart). This requirement should be clearly defined and agreed upon early in the effort such that any operational and/or experimental-design limitations may be fully understood. Note any side-loads generated by the model positioning system must also be considered.

• Maximum Bending Moment: Maximum bending moment is configuration dependent but minimally is assumed to be defined by the application of the above maximum lifting load applied at the center of pressure of the fin, a minimum of 3 inches from the balance-tunnel interface. Thus, approximately, 1164 lbf-in (T) and 3468 lbf-in (O), corresponding to the conditions in tunnel 1 and tunnel 2, respectively. The Government encourages early discussions to understand how occasional configuration changes in increasing the distance between the balance-tunnel interface and fin by about 1-2 inches may impact the bending moment.

• Operating Temperature Range: 294 K (T); 611 K (O)

• 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: Tunnel 1 and Tunnel 2 are blowdown facilities. 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 will be integrated with Government-supplied control surfaces for identifying stability & control characteristics. Due to packaging constraints, the desired maximum diameter of the balance is 3.0” (T) or 1.5” (O). The diameter of the hollow portion of the torque tube the fin balance will need to connect to is 0.300” (T) and 0.375”

(O). The overall diameter of the torque tube needs to be 0.625” (T) and 1.000” (O).

• Mounting Interface: The balance will be mounted to the test article, with the torque tube passing through the article into the control surface. Based upon the size of the test article, this may place the balance either partially within the test section in the test article (the more desired configuration to decrease bending moment) or mounted externally through the facility sidewall. Regardless, the balance should be operable at vacuum conditions (> 0.02 psia) in order to minimize sealing constraints.

3.4 Data Acquisition

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

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.

4. Requirements: Model Positioning System

A model positioning system integrated with the fin balance to rotate the torque tube such that the fin is controlled to move to set angles of attack within the wind tunnel test section during facility operation is desired as an accessory. The term “accessory” is used due to the fact that the fin balance system may have the versatility for the model positioning system to be removed from it in the case the fin deflection angle needs to be set manually. When in use, the model positioning system shall seamlessly integrate with the performer-supplied fin balance. This includes mechanical mounting, wiring, and data handling.

4.2 Performance Requirements

• Angle Accuracy: < 0.1°

• Actuation Rate: > 0.5° / second

• Control Interface: Vendor shall supply positioning-system control software or a documented interface coordinated to be compatible with existing facility data and control systems (NI LabVIEW-based). Control shall allow for scheduling of programmed sweeps

(continuous or ‘yaw-pause’) and positioning shall be closed-loop. Pertinent time-stamped position feedback and status indications shall be available as logged output or available to the facility data systems.

• Data Handling: The actuator control system shall allow for synchronization/alignment of control and position feedback with data acquisition from fin balance and other facility data systems. Data from all vendor systems should be time-synced (NTP, PTP, and IRIG-B are available) and output in a clearly documented format.

• Wiring: All wiring for the balances shall be routed through the actuated mechanism and terminated at a designated connection point outside the wind tunnel test section.

Figure 1. Swept fin geometry, previously studied, in flat insert with the fin geometry in flow and model components under insert outside of test section. The torque tube diameter here is 1" with hollow part of torque tube being 0.375”.

Figure 2 Diagram showing preferred integration of fin balance system