Unmanned Aerial Vehicles (UAVs) have become very affordable and small in recent years, and are increasingly used in a wide range of monitoring and surveillance applications. A major problem when operating with swarm of UAVs is the risk of mid-air collisions. Sensor technology to detect other aircrafts in order to prevent collisions currently receives a lot of attention in the research community. Therefore acoustic technologies could play an important role in anti-collision systems for small lightweight UAVs. Since most of aircrafts rely on active propulsion using propellers or jets, they emit noise which reveals their presence and position. It is therefore possible to detect the range and the bearing of other aircraft to avoid collisions without relying on active communication within the swarm. Because acoustic sensing is passive, tracking acoustic sources in air faces number of substantial challenges because of low signal to noise ratio of the surrounding aircrafts noises compared to the own propeller noise. In this context we propose a propeller noise reduction technique for on board microphone array processing.