Lab 3 – DC Motor / Propeller Characterization Unified Engineering 27 Feb 06 Learning Objectives • Measure thrust, torque, electrical power with prop test rig and wind tunnel • Create motor/prop p erfo r mance curves Secondary Objectives, for Flight Competition Project • Get familiar with propulsion terminology and propeller performance para meters Experimental Rig • Test Article: Propeller/motor unit in 1′ × 1′ tunnel in Bldg 33 hangar • Instrumentation: – Tunnel’s pitot-static probe for airspeed measurement – Prop test rig with thrust and torque gauges – Power supply with voltage and current meters – Strobe for measuring RPM Test Conditions • Fixed motor terminal voltages: v = 3 Volts , 6 Volts • Nominal wind tunnel speeds: V = 0 . . . Vo (static to zero-load condition) Suggested tunnel speed increments: ΔV ≃ 1 m/s for v = 3 Volts , 2 m/s for v = 6 Volts Wind-off Sensor Tare data, prop mounted, wind off. • Ttare , Qtare from load sensor. Enter these in Tread , Qread columns for convenience. Take tares before and after each tunnel-speed sweep to check for drift. Use average va lues. Note: Do not bump the rig during a run. This is likely to disturb the tare values. Motor-mount Aero Tare data. The motor mount has a small negative thrust (i.e. drag) T0, and a very small aerodynamic torque Q0, both of which must be corrected for. To reduce your test time, we provide this aero tare data in the form of aero-tare thrust area and torque volume: qCTA0 ≡ T0 = −1.6 × 10−3 m 2 ∞ qCQA0 ≡ Q0 = +1.1 × 10−6 m 3 ∞ Raw Prop Data Acquired for each each tunnel speed sweep • Wind-off tares Ttare , Qtare from load cells, at beginning and end of tunnel-speed sweep. • v from voltmeter (in Volts). Adjust power supply to hold v fixed for all tunnel speeds. • q∞ from tunnel’s pitot-static pro be in Torr. Convert to Pascals. • Tread , Qread from load cells. • i from ammeter (in Amps). • RP M using strobe. Convert to Ω (in r ad/s). To avoid aliasing, start with strobe in a too-high setting, say 150 00 RPM, and then reduce setting until you see the prop stop. 1Data processing • The data recording sheet can be used for hand data reduction, but a spreadsheet will be much faster and less error-prone. • Load cell tar e correction, calibration conversions, and aero-tare correction. T [ N ] = 5.00 × 10−3 × (Tread − Ttare) − q∞ CTA0 Q [ Nm ] = 1.47 × 10−4 × (Qread − Qtare) − q∞ CQA0 • Thrust and power coefficient calculations. These are analogous to CL and CD for a wing. (note extra R factor in CP denominator!). TQCT = 1 ρ(ΩR)2πR2 , CP = 1 ρ(ΩR)2πR3 2 2 • Advance ratio λ (analogous to α for a wing), efficiency calculations. V T V CT QΩ T V λ = , ηp = = λ , ηm = , η = = ηp ηmΩR QΩ CP iv i v Lab Report Contents • Name of author, and members of the lab group • Abstract • Sketch of experimental setup • State the pro peller being tested: APC7x4. Motor: Speed-400-7V. • One plot of dimensional T (V ), two curves, one for each voltage (see Fig 2 in Lab notes). • One plot of rotation rate Ω(V ), two curves, one for each voltage (see Fig 2 in Lab notes) • One plot of efficiency η(V ), two curves, one for each voltage (see Fig 2 in Lab notes) Comment on how the best-efficiency speed V appears to vary with voltage. • One plot of motor current i(V ), two curves, one for each voltage (see Fig 2 in Lab notes) • Two plots of component and total efficiency ηp(V ), ηm(V ), η(V ) curves, one plot for each voltage (see Fig 7 in Lab notes). Comment on whether this prop and motor ar e well matched. • Determine Vo and aerodynamic pitch Paero for each voltage. Compare your Paero values with manufacturer’s stated pitch P • Three dimensionless coefficient plots. Two curves on each plot, one for each voltage: – CT (λ) – CP (λ) – ηp(λ) Comment on the usefulness of these curves fo r determining the propeller’s dimensional t hrust and to r que for any V , Ω combination. • Estimates of uncertainty and errors in results. 2Propeller: Diameter: Pitch: R = D/2 = [m] Motor: v1 = 3 V q∞ Tread Qread i RPM q∞[Pa] T [N] Q [Nm] V [m/s] Ω[rad/s] λ CT CP ηp Tares: — — — — — — — — — Tares: — — — — — — — — — v2 = 6 V q∞ Tread Qread i RPM q∞[Pa] T [N] Q [Nm] V [m/s] Ω[rad/s] λ CT CP ηp Tares: — — — — — — — — — Tares: — — — — — — — — —
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