Tools › Aerodynamics › Rotors and Propellers › Fan Noise Estimate › Example
NACA TN 2968 Table I: rotational noise of a 4-blade, 8 ft propeller at sonic tip speed (M_t = 1.0) and 1,000 hp
| 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 four-blade propeller of 8 ft diameter runs at a tip Mach number of 1.0 (2,688 rev/min) absorbing 1,000 hp at sea level, with thrust from C_T = 0.75 C_P^(2/3). Find the first four harmonic sound pressure levels 300 ft away at 105° from the thrust axis with the 6 dB ground reflection.
Inputs to Enter
- T = 2688 lb (C_P = 0.0785, C_T = 0.1375), P = 1000 hp
- n = 2688 rev/min, D = 96 in, N_b = 4
- Altitude 0, distance 300 ft, β = 105°, ground reflection checked
Answers
- Harmonic 1: tool 111.6 dB, Table I 111 dB
- Harmonic 2: 110.9 dB (110); harmonic 3: 108.8 dB (108); harmonic 4: 106.2 dB (106)
- The other Table I cases checked: 4 blades, M_t 0.6, 2,000 hp → 103.5 / 88.7 / 72.5 / 55.8 (102 / 89 / 73 / 56); 8 blades, M_t 0.9, 10,000 hp → 123.4 / 113.6 / 102.5 / 90.9 (123 / 114 / 103 / 91)
Notes
At sonic tip speed the harmonics fall off slowly, as the table shows. Values from test-scripts/test_ducted_fan.php (±2.5 dB).