Abstract: Method of reducing the overflying noise of airplanes having a propeller den by a piston engine. The propeller is arranged on the engine shaft in such a way that positive components of the engine sound pressure fall on negative components of the propeller sound pressure. It is preferable to use an engine/propeller combination in which the number of engine ignitions per revolution of the propeller shaft divided by the number of the propeller blades is an integer, preferably being equal to 1.

Abstract: Directly driven propeller whose propeller blades are provided with slats and which is designed for a lower blade tip Mach number in order to reduce the propeller noise. The slats are rigidly connected to the propeller blades and the slat tips lie on a diameter given by the equation ##EQU1## where LTLP=lower transonic limit of the particular blade configuration including slata.sub.o =velocity of sound in airv.sub..infin. =flight speedn=propeller rotational speed.

Abstract: A method of determining in a propeller-driven aircraft a propeller pressure ignal from a combined signal derived from the propeller and from the engine exhaust during flight. Where the propeller driven aircraft is powered by a four-stroke internal combustion engine, pressure signals from the propeller and the engine exhaust coincide where the number of propeller blades corresponds to half the number of cylinders in the engine. In order to determine the clean propeller signal from a propeller/engine exhaust combined signal, a succession of combined signals are detected by means of a microphone arranged on the wing of the aircraft behind the plane of rotation of the propeller. In addition to this, the exhaust signals of the individual engine cylinders are measured at the mouth of the exhaust pipe. Both the combined and the cylinder exhaust signals are recorded synchronously on a multi-track tape recorder.