## 1 Introduction  
Currently, the Unmanned Aerial Vehicle (UAV) market is snowballing--thanks to many successful civilian and military applications, including, but not limited to photogrammetry [1], remote inspection [2], parcel delivery [3], disaster recovery [4, 5], etc. Most UAV missions will benefit from extending the drone range as much as possible while maintaining the Vertical-Take-Off-and-Landing (VTOL) capability. We may consider medical supplies [6], Search-And-Rescue (SAR) operations [7, 8], and even non-critical missions such as ordinary remote inspection of distant objects (e.g., wind turbines) or simple parcel delivery. In this paper, we make the following contribution: we investigate a rare type of UAV (Figure 1), designed for a mission where a small-sized, agile, long-range VTOL drone is capable of precise hovering over a distant target. Many aspects of the drone are discussed, including but not limited to test flight results (Figures 2 and 3). We define the size and weight constraints as follows: a ready-to-fly drone should fit into a square of \(1\times 1\) m and weigh less than 3 kg (without payload). It should be able to carry at least 300 g of payload and stay airborne for at least 15 min. This research aims to design, build, and test a UAV with maximized range and hovering time. There are many different types of UAVs, each having unique features. Figure 4 presents the most common configurations:  
1. A flying wing [9, 10]--typically used for long-endurance missions, e.g., photogrammetry or aerial photography. It is not a VTOL drone. 2. A fixed-wing plane [11, 12]--similar applications as for a flying wing. Fixed-plane UAVs are usually bigger and can carry more payload. If the plane has landing gear, it can operate from a runway. Again, it is not a VTOL drone. 3. A helicopter [13, 14]--can fulfil VTOL missions when a heavy payload is required. Complex mechanical design, many moving parts, and inefficient due to the energy required for the tail rotor, which does not contribute to the lifting force. * A hexacopter [15; 16]--similar applications to the helicopter. Especially popular in aerial photography, the film industry, and crop spraying. Less complex design; typically, it has no moving parts except the motors and propellers. It can survive if one motor or propeller is lost. * A quadcopter [12; 17; 18; 19]--the most popular type of non-professional UAV. It is very robust and agile; it can be tiny, as well as bigger and more powerful. Usually, it has a limited range because its electric propulsion always comprises lifting force (static thrust) and manoeuvrability (pitch-speed). * A co-axial helicopter [20; 21]--a rare type of UAV. It basically has the same features as a helicopter, except there is no tail rotor; thus, it is more energy-efficient. However, double main rotors mounted on a long shaft make it prone to wind gusts. * A quadplane [22; 23; 24]--combines the benefits of a VTOL quadcopter with all features of a fixed-wing plane. The drawbacks include higher take-off mass and increased drag due to extra motors and motor holders or additional motor tilting mechanics. The wings and the tailplane make this type of drone less agile in hover and more prone to wind gusts. * A tandem-wing quadplane--combines essential advantages of VTOL and fixed-wing UAVs. There are a few full-scale designs, and prototypes [25; 26] mainly use multiple motors to hover (e.g., 6, 8, 12, or more). Existing tandem-wing drones mostly use a tilt-motor [22; 27; 28] or tilt-wing design [24; 29]. We decided to select this type of UAV because it has no extra motor holder beams (the two wings support the motors), and it can have a smaller wingspan than a quadplane with a comparable wing surface. It is agile in hover because it acts almost like a regular quadcopter. Thanks to the wings and the pusher motor, it can fly fast in a level flight without using significant energy.