Noise of counter-rotation propellers

Theory is presented for noise generation of counter-rotat ion (CR) propellers with special emphasis given to the effects of acoustic and aerodynamic interference between the two rotors. New radiation formulas are given for noise caused by unsteady loading. Spinning mode characteristics similar to those of turbofans are explicitly displayed so that reinforcements and cancellations between acoustic fields of two rotors or between acoustic modes of one rotor can be studied. Mode orders and cutoff criteria are compared with well-known results from spinning mode theory of turbofans. A theoretical wake model is also presented so that the roles of viscous and potential flow can be seen. The potential wake, which is ordinarily negligible in turbofans, is more powerful for CR propellers because of the smaller blade numbers and the strong load gradient at the blade tips. Noise predictions for an existing CR propeller airplane are compared with experimental results.