Motorized 6-component sting balance with roll, pitch and yaw

A complete design package for measuring the aerodynamic forces and moments on 3D-printed aircraft models in a university low-speed wind tunnel. The model sits on a strain-gauge balance at the tip of a sting; stepper motors pitch, roll and yaw it from a Raspberry Pi control station while the balance measures all six load components.

70-100 mph

test speed, sea level

6 components

lift, drag, side force + 3 moments

±90° / -6..+20° / ±15°

roll / pitch / yaw

~0.3 % FS

target accuracy (student-built)

The whole system as it sits in the tunnel (CAD, near wall and ceiling cut away): model on the balance and sting, roll spindle, motorized pitch sector and yaw carriage in the downstream bay. Pitch +8°, yaw +10°. The bay floor and the floor pocket under the test-section exit are assumed.

Roll spindle at the sting root (CAD render; housing transparent): bearings, 50:1 worm gearbox, NEMA 23 motor and encoder.

The one-piece 7075 aluminum 6-component balance: two tall-narrow measuring necks and a drag flexure in the middle.

What the system is

The model attaches to a balance - a precisely machined aluminum bar whose thin sections strain under load. 24 strain gauges in 6 Wheatstone bridges turn those strains into voltages; a calibration converts the voltages into lift, drag, side force and pitching, yawing and rolling moments. The balance sits inside the model's pod and is held by a steel sting that runs downstream into a three-axis positioning mechanism outside the test section.

Goals

Teaching-lab quality aerodynamic data (lift curves, drag polars, stability derivatives) on 3D-printed models.
Remote, repeatable attitude control: roll ±90°, pitch -6° to +20°, yaw ±15°, all about the model's own centre.
Buildable by a university shop and students; purchased parts where sensible (gearboxes, bearings, electronics).
Documented, calibrated and safe: drawings with GD&T, a calibration procedure and software, interlocks.

Status at a glance

Item	Status	Where
Requirements, loads, envelope analysis	Done	Requirements
6-component balance design + CAD + drawing	Done	Balance
Model pod with pressure taps, model interface	Done	Models & pod
Roll spindle (±90°) CAD + 5 drawings	Done	Motion: roll
Motorized pitch sector CAD + 6 drawings	Done	Motion: pitch
Yaw carriage CAD (option B) + 6 drawings	Done; drawings preliminary (bay dims to confirm)	Motion: yaw
Calibration stand + fitting software	Done (6-component hardware CAD + fit software)	Calibration
Electronics design (wiring diagrams, pinouts, cables, parts list)	Done	Electronics & control
Control software (rig service, station GUI, data reduction)	To write	Software services
Project costs (JLCCNC + JLC3DP quotes, electronics, total)	Done	Project costs
GD&T drawings (36 sheets: 5 assemblies + balance, pod, roll, pitch, yaw, calibration parts)	REV A, unchecked	Drawings

How to read this package

New to the project

Start with Requirements, then Balance and Motion system.

Building it

Go to the Implementation plan, the Drawings and the CAD files under References & files.

Why it looks like this

The Design history explains the options that were tried and dropped, and what the design reviews caught.

Glossary

Term	Meaning
BMC	Balance moment centre - the reference point (x = 0) for loads and rotations; the model's wing quarter-chord goes here.
N, A, m	Normal force (lift direction), axial force (drag direction), pitching moment - in balance (body) axes.
Y, n, l	Side force, yawing moment, rolling moment.
FS	Full scale - the design load of each component (e.g. N = 25 lbf).
q	Dynamic pressure, ½ρV² (12.5 psf at 70 mph, 25.6 psf at 100 mph).
mV/V	Bridge output per volt of excitation; ~1 mV/V at full scale is the NASA design target.
Drag count	ΔC_D = 0.0001. Drag accuracy on this site is quoted as ΔC_D (e.g. 0.002 = 20 counts).
GD&T	Geometric dimensioning and tolerancing (ASME Y14.5) used on the drawings.