The aerodynamic and acoustic charaeteristics of two scale model propellers are compared experimentally.A three blades model(Model A)is a scale model of a production type.A four blades model(Model B)is a scale model of a new designed propeller.Their rotational speeds vary between 1500 to 3000rpm.Experimental results indicate that under identical operational conditions the four blades propeller has higher thrust coefficient but lower noise level than the three blades propeller and also has lower noise level at the same power coefficient.The four blades propeller

of course

is superior to the three blades one in the efficiency and overall performance.