Requirements and design basis

The tunnel, the operating conditions, the models and the loads the balance must measure.

Tunnel Flow conditions Design loads Accuracy Motion requirements

The wind tunnel

Closed-return tunnel with a 30 in long test section, 26.25 in wide x 11.25 in high. The model's balance moment centre (BMC) sits 17.0 in aft of the test-section entrance (13.0 in ahead of the exit). The positioning mechanism stands in the wider bay downstream of the test section (assumed 36 in wide x 22.7 in high from the tunnel CAD - to be confirmed).

Flow conditions

Dynamic pressure vs airspeed

Speed	q (psf)	q (inH₂O)	Re (3.5 in chord)	Lift at C_L 1.3	Min drag (C_D 0.012)	ΔC_D = 0.001
70 mph	12.5	2.41	1.9 x 10⁵	7.1 lbf	0.066 lbf	0.0055 lbf
85 mph	18.5	3.55	2.3 x 10⁵	10.5 lbf	0.097 lbf	0.0081 lbf
100 mph	25.6	4.91	2.7 x 10⁵	14.5 lbf	0.134 lbf	0.0112 lbf

Reference model: 18 in span x 3.5 in chord wing (0.44 ft²) on a 2.25 in pod. Low Reynolds number: trip strips or low-Re airfoils recommended.

Design (full-scale) loads

Component	Full scale	Basis
Normal force N (lift)	25 lbf	C_L 1.3 at 100 mph = 14.5 lbf, x1.7 for buffet, model weight and handling
Axial force A (drag)	4 lbf	post-stall wing ~3.4 lbf; bluff printed bodies ~1.1 lbf
Pitching moment m	15 in-lbf	C_m ~0.1 + 0.5 in quarter-chord placement error
Side force Y	8 lbf	sideslip up to ~15-25° (assumed; to confirm)
Yawing moment n	10 in-lbf	assumed; to confirm
Rolling moment l	30 in-lbf	one wing panel stalled (~7 lbf at 4.5 in)

Accuracy expectations

NASA Langley's own balances reach 0.06-0.17 % of full scale (2σ). A student-built balance should plan on ~0.3 % FS: lift ±0.08 lbf, pitching moment ±0.045 in-lbf, drag ±0.012 lbf - about ΔC_D 0.002 (22 counts) at 70 mph and 0.001 (11 counts) at 100 mph for the reference wing. That is fine for lift curves, stall, polars and model comparisons; minimum-drag values at 70 mph carry roughly ±15-20 %, so drag-critical runs should be made at 100 mph.

Motion requirements

Axis	Range	Mechanism	Speed
Roll	±90°	Spindle at the sting root, NEMA 23 + 50:1 worm	±90° in ~10 s
Pitch	-6° to +20°	Arc sector centred on the BMC, NEMA 17 + 40:1 worm + curved rack	up to 2°/s
Yaw	±15°	Carriage on curved floor rails centred under the BMC (option B: wings-level sideslip)	up to 3°/s

Model size limit for rolling. At 90° roll the wing span stands vertical in an 11.25 in tall test section. Full ±90° roll is possible only for spans of about 9 in or less; larger models get a software roll limit (see envelope).