Design history and decisions
How the design got here, including the options that were evaluated and dropped, and the errors the reviews caught.
| Step | What happened | Outcome |
|---|---|---|
| 1 | First concept: internal 3-component sting balance for 70 mph | Sizing method established (two-moment necks + drag flexure) |
| 2 | Literature review: 6 papers (NASA LaRC design concepts, calibration regression, gauge procedures, sting interference; 2 others) read page-by-page | Design rules adopted: one piece, ~1 mV/V, calibration method; flaws found in the first sizing |
| 3 | Re-scoped for an undergrad lab, 3D-printed models, 70-100 mph | Full scale set at 100 mph; manual arc-sector AoA mechanism; calibration stand and fit script |
| 4 | 3D-printed balance studied: polycarbonate, then PPS-CF (CORE One + HT hotend) as a hybrid with metal end fittings; creep-test coupons designed | Abandoned polymer creep/hysteresis and moisture make drag unreliable; aluminum chosen |
| 5 | Requirement added: motorized roll ±90°, pitch and yaw from a Raspberry Pi station | Balance upgraded to 6 components; yaw option B (carriage, wings level) chosen over the pitch-roll combination |
| 6 | Motion envelope analysis | Found the model-span limit for rolling in an 11.25 in tall test section |
| 7 | 6-component balance CAD; roll spindle CAD; GD&T drawings | Current design |
| 8 | Motorized pitch sector: cam followers on the roll housing, braced cheeks, internal curved rack on the -y side | First sweep found the roll gearbox and motor hitting the base at +20°; base notched - clear over the full range |
| 9 | Yaw carriage: curved V-edge rails, captive V-wheel trucks, floor rack drive | Carriage notch widened for the gearbox flange at +20° pitch; a stray cylinder in the roll-gearbox envelope found and removed |
| 10 | 6-component calibration on the roll spindle: square sleeve, roll arm, 45° stirrup, head plates; 6-output fit script and 233-point schedule | Synthetic test passes (check points ≤ 0.23 % FS); bench mode needs roll -90..+180° |
| 11 | Whole system rendered in the tunnel; GD&T drawings extended to 27 sheets (calibration 3xx, pitch 4xx, yaw 5xx preliminary) | Yaw carriage found to reach 2.3 in under the test-section exit (new open item) |
| 12 | Electronics & control detailed: wiring diagrams, power/E-stop chain, pinouts, cable schedule, parts list | Pitch motor speed limited to 2°/s (rack ratio left out of the drive sizing); custom readout PCB deferred |
| 13 | Machining cost estimated; quote package (36 STEP files, manifest) sent to JLCCNC | Real quote $1,618.80 + $274.63 shipping for 34 of 36 lines (47 of 49 pieces) as plain 6061; precision parts still to be re-specified |
| 14 | Full consistency review of this package | Added assembly sheets WT-6C-100 to 500 (render + parts list) and stand sheets 308-310 (pulleys now on R5 bearings); STEP files for the wheel spacers and pinion shaft; drive-sizing script corrected (R22 rack, 1/8 step, 2°/s); 35 sheets in all |
| 15 | Drawing for the 3D-printed reference pod (WT-6C-004) | Interface and inspection sheet: puck-fit bore, screw holes, tap layout, keep-outs, print orientation, blow-through test; 36 sheets in all |
Problems caught by checks and reviews
| Found in | Problem | Fix |
|---|---|---|
| AoA sector CAD | index holes at the same radius as the lock slot (slot would cut them) | lock at r 18.1 in, index at r 18.9 in |
| Calibration sleeve | load pins would have passed through the balance | load holes in the side walls, stub pins + yoke |
| Clamp block CAD | the D-D bore for the sting flats was never cut (wrong x) | fixed; sting-vs-block interference check added |
| Stand README | yoke CG offset added to the moment | a pinned yoke loads at the pin - corrected |
| Roll caps | rear cap lip had ID > OD (empty) | lip sized to clamp the outer ring and clear the KM7 locknut |
| Drawing review (housing) | cam-follower stud holes broke into the clearance bore and the rear bearing bore | studs at x 17.3 / 19.4 in, 0.45 in deep, 0.150 in wall left |
| Drawing review (encoder) | bracket foot only touched the gearbox edge-on | flat plate on the gearbox face, slots + M3 standoffs |
| 6-comp calibration physics | the yoke must hang in the lab frame, and at 45° roll no sleeve wall is horizontal | yoke kept lab-fixed; corner stirrup added for 45° / 135° / 315° |
| Drawing review (stirrup) | nothing located the stirrup at a station, and its V-seat was only 0.05 in deep | 0.30 in deep V-seat with a knife-edge rib that drops into the sleeve's corner groove |
| Drawing review (roll arm) | clamp screws started inside the sleeve window (could not be fitted); notch load height taken at the top edge | nylon-tip set screws through the top rim; fit uses the notch apex (z 1.155 in) |
| Drawing review (pitch drive) | bearing bore 0.56 in for 22 mm OD 608 bearings; two M6 holes ran through the bearing block | 22 H7 bore in a larger block; M6 holes moved to z ±0.90 clear of the block |
| Drawing review (yaw) | eccentric preload spacer could not pilot into the wheel hole | outer wheel hole of each truck opened to 0.500 in |
| Whole-system render | yaw carriage plate and inner rail reach under the test-section exit at ±15° yaw | open item: confirm the space is open, or trim the carriage |
| Quote package review | manifest had 1 each of the stand uprights and axle | corrected to 2 each (49 pieces) |
Why aluminum and not a printed balance
Printed PPS-CF can reach the stiffness needed if printed on its side with thicker sections, and a hybrid with metal end fittings was fully designed. But polymer creep (several % of the reading within minutes in published data and estimates), moisture and temperature sensitivity are of the same size as the cruise drag signal, and no filament maker publishes creep data for printed parts. The aluminum balance avoids all of that; printed parts are used for models, fairings and fixtures instead.