# Functionality Investigation of the UAV Arranged FMCW Solid-State Marine Radar
## 3 Experimental Setup  
The weight of the radar is 7.4 kg. Taking into account the weight of additional equipment such as a separate power supply, minicomputer, communication equipment, and the AIS receiver, the UAV must be capable to lift a payload weight at least 10 kg. For this experiment, only lifting the radar to high altitudes is required. The altitude is limited to 120 m due to regulations. A rotary-wing UAV is the optimal choice for this task. Since the payload of approximately 10 kg was foreseen, the multi-rotor UAV was specially designed for this task. The maximum takeoff weight (MTOW) of this vehicle is 25 kg (though potentially it could lift off with greater load). The MTOW of 25 kg was chosen to ensure easier certification and flight clearances (in most of the countries 25 kg is the maximum mass for small UAVs). To ensure convenient mounting of the radar and possibilities of modification, the multi-rotor was fully custom built having the size of the radar, its mounting points, endurance,needed electronic systems, transportation possibilities, possible modifications and etc. in mind. To increase the safety in case of failure of one of the engine systems (since it is supposed to carry quite expensive equipment) the 8 motor layout (octocopter) was chosen. It must be mentioned that the increase of number of motors and especially the number of propellers decreases the overall efficiency of the vehicle both due to lower efficiency of multiple electric motors (though not so obvious) and due to increase of induction drag and tip losses on the multiple propellers. It is always more preferable from an efficiency point of view to have as few propellers and motors as possible (and propellers with as little RPM as possible), nonetheless from the safety point of view, increase of the number of motors provides additional safety to the flight in case of failure. In the case of the octocopter's configuration, the failure of a single motor does not lead to a crash and the vehicle in most of cases can safely return to the takeoff point. Even a failure of several motors in most of the cases leads to safe landing of the vehicle. According to preliminary calculations, a combination of Turnigy Multistar 9235 100 KV brushless motors and Multistar carbon fiber 32 \(\times\) 5.5 propellers was chosen. These 2.8 kW maximum power motors are designed to work using a 30-48 V supply (8-12S Li-Po battery) with a maximum current of 57 A, the weight of the motor is 674 g [22]. Mentioned motors, while having a quite low rotation speed (100 RPM/V), provide comparatively high propeller efficiency. The experimental engine/propeller thrust data from the chosen 48 V supply are given in Figure 1.  
As it can be seen from Figure 1, each motor/propeller provides around 100 N (9.81 kg) of static thrust, which is excessive for a 25 kg multi-rotor UAV (normally 400-500 N of total thrust would be sufficient). Nonetheless, this octocopter configuration (8 motors) was chosen in order to increase safety and versatility of the vehicle. Since the multi-rotor was designed by implementing non-aviation grade composite materials, additional experimental laboratory static strength testing was performed. According to the mentioned testing, the maximum static strength of manufactured carbon fiber materials was in range of 255 MPa. In accordance to that data, modeling of full strength properties was performed and presented in Figure 2. According to the modeling data, the frame strength is far in excess of planned maximum loads. Figure 1: Experimental motor/propeller thrust data compared to the manufacturer provided. To ensure convenient transportation (and some possible modifications) the multi-rotor motor arms were manufactured in a detachable manner, i.e., the arms together with the motors and propellers could be detached from the central part, which houses all of the systems of the vehicle. That ensured convenient transportation of the vehicle and simple and quick replacement of the motor arms, which were the most vulnerable parts of the vehicle (Figure 3).