## 2 Materials and Methods
### Mechanical Design
#### 2.3.1 Wings  
Typically, drone arms are made of carbon-fibre tubes because they are very stiff and lightweight at the same time. However, such a single tube could have a too big a diameter to fit into the drone's wing. Instead, we decided to use double \(6\times 2\) mm carbon-fibre flat bars as wing spars. Additionally, the space between them forms a convenient tunnel for electric wires. The wings are built of two matching full-balsa wood elements: a bottom and a top half, both CNC 3D milled and glued together. The leading and trailing edges of a wing are usually prone to accidental damage (especially a very thin trailing edge); therefore, both edges are reinforced with carbon-fibre \(4\times 1\) mm flat bars. The carbon-fibre wing spars at the wingtips support the main motor holders (CNC milled from a 3 mm-thick aluminium sheet). The two elements of the holders are screwed together to catch protruding wing spars tightly. Finally, the surface of the wing is covered by Oracover [32] film. The wing construction proves to be light and very durable. We could say it is a perfect balance between stiffness and elasticity. Initially, we chose a wing profile (an airfoil) optimized for high-speed flight: the P-51D tip (BL215) airfoil (see Figure 10). Generally speaking, high-speed airfoils have low drag, but, on the other hand, have a low lift coefficient, which results in a high stall speed, and that means the plane has to maintain high enough speed to stay airborne in a level flight. That should not be an issue if the pusher motor can accelerate the drone to that speed. Due to safety reasons, we decided to modify the original wings--we made them much thicker (see Figure 11). Such a thick airfoil (thickness increased from 12% to 25% of the airfoil chord) gives us a much higher lift coefficient (resulting in a lower stall speed) at the cost of lowering the top speed. Nevertheless, lower stall speed means we could perform the in-flight experiments of switching between quadcopter and plane mode at lower (i.e., safer) speed, and we could do that in a less spacious airfield. Figure 10: The original airfoil for both drone wings; generated by Airfoil Plotter [33]. Figure 9: An overview of the Elka1Q drone—side view. The wing configuration used in the drone is called a \"tandem-wing\" or sometimes a \"lifting-tail plane\". Those names refer to the fact that the aft wing is not just a horizontal stabilizer, like in a classic \"tailplane\" configuration, but it contributes to the total lift force produced by the plane. It is a rare configuration due to possible stability and controllability issues [34; 35]. Sometimes, quite the opposite statements can be found--tandem-wing planes are easier to pilot because of safer stall behaviour [36]. However, there were at least a few successful tandem-wing planes, e.g., Quickie designed by [PERSON] (and later QAC Quickie Q2) [36; 37] and the Proteus [38] built by Scaled Composites ([PERSON]'s company). Another famous tandem-wing plane is the \"Flying Flea\" (French name: \"Pou du Ciel\"), designed by [PERSON] in 1933. A thorough study of many more historical and modern tandem-wing planes and UAVs, as well as their aerodynamic and stability studies, can be found in [34]. A wing that produces lift force also generates a downwash, i.e., the airflow direction behind the trailing edge of the wing is deflected down by the aerodynamic action of the wing. That phenomenon changes the effective Angle of Attack (AoA) of the rear wing in the tandem-wing configuration. Most tandem-wing planes have the front wing mounted lower than the rear wing to minimize the downwash effect of the front wing [34; 35]. Additionally, it is recommended to set a higher AoA of the front wing than the aft wing--such a wing setup affects the stall behaviour of the tandem-wing plane. The front wing with a higher AoA will stall first while the aft wing still produces lift force--that situation will cause the plane to pitch down, increase the speed, and ultimately, end the front wing's stall (bring back its lift force) [36].