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Water Tunnel Force/Movement Measurements and Dynamic Experiments

5-Component Balance

RHRC has developed a 5 component internally mounted balance (no axial force) for model testing in Water Tunnel facilities. The coupling of simultaneous flow visualization and force measurement provides powerful insights. The balance is 0.75 inches in diameter and 5.1 inches in length. The balance uses high-sensitivity semi-conductor strain gages that are waterproofed for submersion. High sensitivity is required because the model loads encountered in Water Tunnels are quite small. Balance calibration hardware and the required weights accompany the balance. RHRC provides a complete calibration for the balance upon delivery, including an accurate accounting of gage interactions and instructions on how the customer can perform calibrations, if desired. Information on the balance calibration equipment and the general procedures used for calibration are provided, along with examples of previous calibrations.


1-Component Balance

RHRC introduces a new 1-component balance for Model 0710 University Desktop Water Tunnel. The new 1-component balance uses an extremely sensitive, semi-conductor strain gauged section to measure either normal force and pitching moment, or side force and yawing moment, on a model. The balance allows students to observe the correlation between what they see in the flow field and the resulting forces and moments on an aircraft. The 1-component balance comes with a 12-bit analog to digital converter card for a PC using the PCI bus. Temperature compensation circuitry, an excitation voltage source and a zero trim pot are also provided in a convenient package.

In addition, RHRC's software is provided which teaches the student the basics of experimental measurements. The software contains balance calibration routines, weight tares, and up/down zero measurement functions. The output from the software is provided in ASCII format, which can easily be loaded into your favorite plotting software for analysis.  For additional information, please download the 1-Component Balance Description in Adobe Acrobat format.


Comparison Between Wind and Water Tunnel Force/Moment Measurements

Results from several experiments performed in the Water Tunnel using the balance show good agreement with wind tunnel tests performed on similar configurations such as delta wings and fighter aircraft. The example shows the normal force coefficient for an F/A-18 during an angle attack sweep at zero sideslip. The Water Tunnel results agree well with several wind tunnel data sets at angles of attack up to the point where the separate flow takes on a wake-like structure. With wake blockage corrections applied to the Water Tunnel data, the agreement is greatly improved.

Data Acquisition/Reduction Software

RHRC has also developed a complete data acquisition and reduction software package for balance and operational testing using LabVIEW, a graphical programming language based on virtual instruments.

Water Tunnel Dynamic Tests

In addition to static tests, the Water Tunnel balance, coupled with RHRC's computer controlled model support, allows performance of dynamic experiments, including oscillatory motion, pitch-up/down and hold maneuvers, rotary balance tests, etc. The advantage of the Water Tunnel for dynamic experiments is that, because of the slow test section velocity (1 ft/sec), the model rotational motion is also slow, typically less than 10/sec. By providing simultaneous measurements of the forces and moments and visualization of the flow field, there is a direct cause and effect link established, without the need for speculation on how the flow field is affecting the model.

RHRC can also provide the hardware and LabVIEW-based software to perform different types of motion, including pitch, roll and yaw, and to acquire and reduce dynamic data. Understanding the relationship between dynamic forces and moments, and the dynamic flow field that produces them is essential for future aircraft design, in particular for agile aircraft whose role depends on high performance with robust controllability. Simple and low-cost dynamic experiments that combine flow visualization and force/moment measurements provide a unique means for teaching the aerodynamicists of the future about the complex flow fields they must deal with, whether experimentally or computationally.


Model Support System

Model Support System



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