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Motor / prop stand · Planned build files · Rev. Oct 22, 2026

One motor, five sensors, and the curve the drone numbers rest on.

A bench fixture that spins one 2207 drone motor through scripted throttle steps while logging thrust, current, voltage, RPM and temperature. Its first job is to check the speed quad's propulsion predictions. After that, any motor and prop can be compared on the same rig.

Companion: Hammerhead speed quad build files ↗.

Peak thrust13.4 N predicted, 6S, 5.1″ prop

Peak current~31 A predicted, static

Thrust rate80 samples per second

Parts cost$94.40 bought, plus ~$20–30 local

A / Bill of materials

A test rig for about $100

Prices are what I paid on Oct 11, 2026, in USD before tax and shipping. The motor, props and batteries come from the speed quad build, so the stand tests the exact hardware that flies.

Qty	Part	Why	Source	Each	Line

B / Wiring

Battery current through the sensor, everything else at 5 V

Battery current runs pack → current sensor → ESC → motor. The Arduino reads four sensors and drives the ESC's throttle. The only pack voltage it ever sees comes through the divider.

Schematic, not to scale.Fig. B1

Common ground, one rule

The ESC's signal ground has to connect to the Arduino's ground, or the throttle signal floats. That ties the laptop's USB ground to the pack's negative. It's fine on a laptop running on its own battery. On a laptop plugged into the wall, use a USB isolator or unplug the charger during runs.

Connection schedule

10 runs

#	Run	Wire	Notes

Log format

One row per 100 ms

t_ms,state,throttle_us,thrust_N,current_A,voltage_V,rpm,temp_C

The Arduino streams this over USB serial. A Python script on the laptop saves it, sends the heartbeat, and plots the sweep.

C / Fixture & method

Measure the motor, not the rig

The load cell stands upright with the motor mounted on top, so thrust bends the cell in its sensing direction. The prop blows away from everything, and the operator stays out of the prop plane.

Side view, roughly to scale. A plywood barrier panel stands on the operator's side, covering the prop plane.Fig. C1

Measurement chain

Per channel

Quantity	Sensor	Resolution	Rate
Thrust	Bar load cell + HX711, gain 128	set by calibration	80 Hz
Current	ACS758-100B on the Uno's 14-bit ADC	0.015 A per count	≈1 kHz, averaged
Voltage	10k / 2k divider, 14-bit ADC	1.8 mV per count	≈1 kHz, averaged
RPM	TCRT5000, one mark, interrupt timing	< 0.1% at hover	every revolution
Temperature	MAX6675 + K thermocouple	0.25 °C	4 Hz
Throttle	Servo PWM to the ESC	1 µs	50 Hz

Derived results

Feeds the speed quad

Result	Formula	Used for
Thrust coefficient	C_T = T / (ρ n² D⁴)	Quad hover and top-speed calcs
Efficiency	g/W = thrust (g) / (V × I)	Prop and motor comparisons
Full-throttle current	measured	Quad power budget, ESC margin
Thrust curve	T = a · rpm²	Checks the quadratic assumption

Air density ρ comes from the room's temperature and pressure on test day. Calibration uncertainty follows the regression and prediction-interval method in the speed quad hand calculations.

Firmware safety logic

Runs on the Uno

States: SAFE → ARMED → SWEEP, and ABORT from anywhere. Power-up always lands in SAFE.
Arming needs the arm switch on, the throttle at 1,000 µs, the thrust tared, and an arm command from the laptop.
Abort on: the button, an x on serial, no laptop heartbeat for 0.5 s, current above 45 A, motor above 80 °C, or the pack below 21.0 V (3.5 V per cell).
Abort action: throttle straight to 1,000 µs and hold. Re-arming needs the switch cycled.
Sweep: 0 → 100% in 10% steps, extra 5% steps from 15–35% around hover. 3 s per step, mean and spread of the last 2 s, throttle ramps no faster than 20% per second.
Between sweeps: 60 s cool-down, and a fresh pack for every full-throttle run.

Predictions to check

From the quad hand calcs

Quantity	Predicted	Assumption behind it
C_T	0.12	Typical for 5″ tri-blades
Thrust at 12,100 rpm	1.645 N	Quad hover: 670.8 g ÷ 4
Full-throttle rpm	34,440	2,050 KV × 21 V × 0.8 under load
Full-throttle thrust	13.4 N	C_T at that rpm
Full-throttle current	~31 A	Momentum theory, figure of merit 0.5, motor + ESC 80%
Efficiency at hover	5.7 g/W	Same assumptions

The full-throttle current is new here: the quad calculations bounded thrust but never predicted current. If it comes in well above 31 A, the figure of merit or efficiency assumption is too optimistic, and the quad's hover endurance estimate drops with it.

D / Test log

Each prediction, and how it gets tested

Calibration comes first: no sweep result counts until the sensors pass T01–T05. Predictions and pass bands are written before testing.

Planned build document — not measured test results. Every test entry is a prediction or proposed pass band; no measured results are reported here.

ID	Quantity	Predicted	How it's measured	Pass band	When	Measured	Status

Limits of this rig

Static thrust only. A prop in forward flight makes less thrust at the same rpm, so the quad's top-speed check still needs GPS speed runs. The upright cell also feels a small share of the motor's torque, and the static calibration can't fully separate that out.