Design of a six component internal strain gage balance system
Creators: name: Ford, W. W.
Publisher: name: California Institute of Technology
Year: 1949
Subjects: Aeronautics

description: A. Statement of the Thesis Problem:  When the aerodynamic forces acting on a model are transmitted through struts, stings, etc. to the wind tunnel rigging for determination there are several corrections to be made to compensate for the various other forces which have unavoidably entered into the rigging. Among these are the tare corrections for the deflection of the model supports. These corrections were satisfactorily made in the case of slow speed aircraft in which the tare drag is small in comparison with the drag of the airplane. However, the performance of the new jet airplanes and missiles with extremely low drag and high speed, approaching the speed of sound and higher, cannot be satisfactorily evaluated using the same balance system. A balance system to give satisfactory data on the aerodynamic forces acting on these low drag high speed airplanes should measure only the forces acting on the model with every high degree of accuracy. This can best be achieved through use of an internal balance system.

Among the many advantages which can be realized through use of a balance system which can be placed inside a model are: (a) The above mentioned corrections can be eliminated, (b) The balance system can be designed for the loads acting on the model alone, thereby increasing the sensitivity and accuracy, (c) The complexity of the wind tunnel rigging can be reduced and, (d) The electrical sensing of all the measured components simultaneously permits their direct automatic recording, thereby saving time and reducing personnel requirements.

It is the purpose of this thesis to describe a small internal balance system which has been designed to measure the six aerodynamic forces and moments which act on a model being tested and thereby realizing the above mentioned advantages.

B. Survey of the Field:  Internal balance systems for measuring three or four of the aerodynamic forces and moments have been used for several years. These balances, in general, have been large in size thus restricting their use to large size models. Furthermore, galling of bearing surfaces and internal friction has in many cases reduced their load capacity and accuracy.

During the past year the author has been able to visit the following activities for the purpose of becoming acquainted with the details of the balance systems being used: (a) The Cooperative Wind Tunnel at Pasadena, California, (b) Ames Laboratory of NACA at Moffett Field, California, (c) North American Aviation Inc., at Inglewood, California. The Cooperative Wind Tunnel has three internal strain gage balance systems of the beam type. Two of these balances measure lift, drag and pitching moment while the third also measures rolling moment. These balances are large in size. A fourth balance system in use at the Cooperative Wind Tunnel, while not of the internal type, is of particular interest. This is the transonic bump balance to which the half model is secured for testing. This balance is also of the beam type and measures all six aerodynamic forces and moments.

Ames Laboratory 1 x 3 foot supersonic wind tunnel uses a four component beam type strain gage balance system in which all the beams are located in the sting and shielded from the airstream by a two inch diameter shield. The features of this balance are: (a) Use of ball bearings in races for transmitting the load to the beams, (b) Drag and rolling moment beams are readily interchangeable and, (c) The electrical system is set up to record all readings simultaneously by punching a single button.

North American Aviation, Inc. has developed a small six component internal strain gage balance system for use in their new supersonic wind tunnel. This gage is being used satisfactorily. It is fully described in ref. (a).

Due to the rapid lapse of time and after a conference with Mr. Bell and Mr. Gayman of the Cooperative Wind Tunnel it was decided that an internal strain gage balance system based on the North American design promised the quickest and most satisfactory solution of the design problem at hand. This met with the approval of Drs. Sechler and Klein.