AEROLAB has designed and built over 100 strain gauge balances for subsonic, transonic and supersonic wind tunnels. Usually complete systems are supplied including the model positioning linkage, power supply and digital readout. Either manual or motorized operation is offered for angle of attack and angle of yaw adjustment.

Internal One-Piece Balances

Internal balances are, almost universally, used for measurements in supersonic and transonic tunnels. There are also popular in subsonic tunnels. Recently AEROLAB has developed an internal balanced cut from a single piece of precipitation-hardened stainless steel. No screws, pins or welds are used. This one piece design is superior to earlier designs in strength, accuracy and in providing lower interactions between components. These advantages are particularly significant in Axial Force because of its sensitivity compared to the other components. The accompanying photograph shows the geometry for insulating Axial Force from the other components.

Standard balances from 3/8-inch to 1-inch Closeup of sting balance

Referring to the close-up, the balance is separated into halves by the diagonal cut with the two halves joined by sixteen flexures (8 shown in the picture and 8 on the back side). These flexures offset normal and side loads and permit limited axial movements. The axial movement is resisted by the cantilever beam in the center which is pulled by the thin, flexible strip at the top. Strain gauges are installed near the bottom of the beam to measure the strain as the beam is deflected. The axial force measurements beam is thereby shielded from the loads produced by normal and side forces.

Design Details

The balances are normally designed with cylindrical ends, one of which is clamped into the model positioning system and the other fits into a reamed hole in the model, secured with a set screw. If desired, the balance may be fitted with a tapered end to support the model. Two problems are commonly encountered in the use of the internal strain gauge balances. The foil strain gauges and the fine wires are vulnerable to rough handling; and the sting must be enclosed in the model to protect against airflow over the gauges (which causes erroneous readings).

Accessory Equipment

AEROLAB eliminates the problems by enclosing the sting in a metal sheath. All balances are supplied with calibration barrels and roll bars (for six-component models) which are individually grooved to apply loads at known locations. Reamers are supplied to machine models for proper fit on the balance.


Supersonic Model Positioning System

In supersonic tunnels models are generally adjusted in angle of attack with a half quadrant, as shown in the illustration. The model is rotated about a point near the end of the sing so it stays substantially in the center of the test section as the angle is varied. The quadrant is driven though a gear train by an adjusting knob which also drives a mechanical counter indicating the angle of attack to the nearest tenth of a degree. The quadrant is well supported by adjustable eccentric ball bearing guides and aluminum plates on the sides.

Supersonic tunnels are almost invariably of the blow-down type wherein a run of short duration (typically 15 to 30 seconds) is followed by a relatively long pump-up time (typically 3 minutes). It is therefore advisable on the larger tunnels (4 by 4 inch and larger) to motorize the angle of attack system and to employ a computerized Data Acquisition System. AEROLAB drives the angle of attack quadrant with a variable speed motor through a Geneva Drive unit which converts the steady rotation at input to rotation for ¼ of a cycle and stopped rotation for ¾ of a cycle, a so-called pitch-pause drive.

The system is gear driven so that consecutive pause positions are exactly one degree apart to ensure the accuracy of angle of attack settings.

The smallest AEROLAB one-piece balance measures 3/8-inch in diameter, suitable for a 4 by 4 inch supersonic tunnel. A special balance with a 3/16-inch diameter sting is offered for the 2 by 2 inch tunnel with motorized drive option.

The AEROLAB 1 by 1 inch supersonic employs and angel of attack quadrant but space is insufficient for a practical balance.


Subsonic Model Positioning System

Subsonic balances are usually associated with larger dimensions and lower loads (than supersonic) which make parallelogram model positioning system more appropriate. The parallelogram linkage is versatile: the balance may be removed and the test section made “clean” by removing the two vertical links; the front vertical link and the balance may be removed and the rear vertical link may be used for traversing pressure or hot wire instruments in a vertical direction (with small stream-wise motion); and finally additional holes can be drilled in the vertical links for testing automobiles or models in ground effect near the floor.

Model positioning systems designed for yaw (as well as pitch) require an opening in the floor of the test section to permit yawing of the vertical links. For these yaw systems AEROLAB provides an aluminum turntable. The turntable is supported by the frame of the balance and rotates integrally with the balance in yaw. It is only necessary for the customer to cut a circular hole in the floor of the wind tunnel slightly larger than the turntable diameter.

The magnitudes of angle of attack and angle of yaw are indicated on digital counters to the nearest tenth of a degree. As an option, the angle of attack mechanism can be driven by an adjustable speed motor (replacing the hand adjustment) and the angle of attack read on a digital meter with both the controls and the meter housed in a control box. As a second option, motorized control and remote readout of both pitch and yaw is offered.

Model positioning system with motorized angle of attack and motorized angle of yaw. Manual angle of yaw with mechanical counter.