Tools › Aerodynamics › Rotors and Propellers › Fan Noise Estimate › Example
NACA TN 2968 Table I: rotational noise of a 3-blade, 8 ft propeller at M_t = 0.5 and 1,000 hp, 300 ft away at 105°
| Tool | Fan Noise Estimate |
| Source | Hubbard, H. H. (1953). Propeller-Noise Charts for Transport Airplanes. NACA Technical Note 2968, National Advisory Committee for Aeronautics, Washington, D.C. Table I and Appendix C. |
| Level | Professional |
Problem
A three-blade propeller of 8 ft diameter runs at a tip Mach number of 0.5 (c = 1126 ft/s, 1,344 rev/min) absorbing 1,000 hp at sea level; its thrust follows the TN 2968 Appendix C relation C_T = 0.75 C_P^(2/3). Find the sound pressure levels of the first four harmonics of the blade-passing frequency 300 ft away at 105° from the thrust axis, including the 6 dB ground reflection.
Inputs to Enter
- T = 2688 lb (from C_P = 0.628, C_T = 0.550 at ρ = 0.002378 slug/ft³), P = 1000 hp
- n = 1344 rev/min, D = 96 in, N_b = 3
- Altitude 0, distance 300 ft, β = 105°, ground reflection (+6 dB) checked
- Blade area as desired (it enters only the vortex-noise term)
Answers
- Harmonic 1: tool 97.5 dB, Table I 96 dB
- Harmonic 2: 82.9 dB (83)
- Harmonic 3: 67.0 dB (68); harmonic 4: 50.5 dB (50)
Notes
Gutin's formula in the engineering form of TN 2968 eq. (1) with Bessel functions J_mB; the table was read with a ±2.5 dB tolerance in test-scripts/test_ducted_fan.php. The tool's default inputs are the optimizer's 48 in fan, not this case.