– Prop Tip Speed & Noise

Tip Speed & Noise

Have you ever had the privilege to hear the beautiful sound of a Merlin-powered aircraft take-off or fly overhead? Or heard the rasping sound of a rowdy radial engine, or the thundering roar of a jet aircraft taking-off? All this noise is sweet music to a pilot’s ears, but not so for local residents living near an airport. To the locals, it is a very disturbing nuisance. To this end, aircraft and propeller manufacturers all attempt to reduce aircraft noise as much as possible. In fact, certification requirements for all new aircraft designs stipulate the maximum allowable noise limits.

The Cause of Noise

Noise is generated by the engine, exhaust system, propeller propwash and the prop itself. The prop noise is dependant on the blade loading, number of blades, prop diameter, and the location of the prop on the aircraft. However, the main cause of noise is the propeller tip speed

Consider a two-blade prop installation: the prop produces an inherent vibration once per revolution that will vibrate through the airframe to be heard as noise. The greater the number of blades, the less is the vibration and noise produced. Single-engine aircraft have their prop wake striking the cockpit windshield adding to the vibration and noise as opposed to multi-engine aircraft with their props further away from the cabin

Tractor props mounted on the aircraft nose or in front of the wings are generally quieter than pusher props, which operate in the disturbed air flow passing over the aircraft creating resonance or noise in the cabin. However, the greatest amount of noise is heard the prop’s plane of rotation. On a twin, or multi-engine aircraft, any occupants seated in line with the props will suffer the most noise. Moving the engine/ prop further out board on the wing will help to reduce the noise heard in the cabin, but this will also increase engine-out asymmetric forces, as mentioned above.

‘Shrouded props’, or ‘Propulsors’ are claimed to be considerably quieter than conventional props due to the shroud around the propeller and also the lower tip speed (they are usually props of smaller diameter). But, the disadvantage here is, the prop noise can be directed more fore and aft by the shroud. Therefore the amount of noise heard to a certain extent is dependant on one’s external position relative to the aircraft, or their position inside the aircraft