All the UAVs we build have conventional rear-mounted tail surfaces. We use both normal rudders and elevators and also combined V or inverted-V elevons, in most cases made of closed-cell foam, supported by small CFRP spars and covered with Mylar or glass-fiber cloth. These can either be directly attached to the rear of an elongated fuselage or are attached to long CFRP tubular booms. Booms generally allow for smaller tail surfaces because of the increased moments that can be generated by shifting the tail further aft, but this can limit rotation angles during takeoff. Figures 4.20 and 4.21 show a selection of the arrangements we have used. In most designs, we use conventional flaps on the tail surfaces, but we have made use
Figure 4.21 Tails attached using CFRP booms, both circular and square in cross-section.
Figure 4.22 All-moving horizontal stabilizer with port/starboard split to augment roll control and pro vide redundancy.
of all-moving horizontal stabilizers and even split horizontal stabilizers that can augment roll control, see Figure 4.22.
When sizing tail surfaces, we follow the idea of vertical and horizontal tail volume coefficients - these coefficients are given by the projected vertical or horizontal areas multiplied by the distance between the quarter chord points of the tail surface and the main wing, all divided by the main wing area and either the main wing span (for vertical coefficient) or the main wing mean chord (for horizontal coefficient). For aircraft with conventional operating characteristics, we generally choose a horizontal coefficient of 0.5 and a vertical coefficient of 0.04 following the advice offered in Raymer . If we wish to have a particularly stable aircraft or one that has significant shifts of the CoG in flight or has to operate well at very low flight speeds, we increase these, sometimes to as much as twice the normal values.
Whether a tail boom is in use or not, suitable attachment points for the tail items must be provided, which are sufficiently rigid to avoid flexibility and flutter issues in the tail. When attaching tail surfaces directly to the fuselage, this is rarely a problem, but equally it is then almost inevitable that one uses V or T configurations of some sort. Booms offer much greater flexibility in tail arrangements, although we now rarely use a T or inverted-T tail with a single tail boom because it is then difficult to generate sufficient torsional rigidity without very large boom diameters. For directly attached surfaces, we generally place the servo actuators inside the fuselage and then use small cantilevered CFRP tubes to provide rigid hinges for the flaps. When using tail booms, one has to provide both a stiff location inside the main fuselage to mount the boom (see again, for example, Figure 4.1) and a small self-contained structure to support the tail and attach it to the end of the boom, see Figure 4.23. Sometimes, these small tail structures are also used to house steerable tail wheels.
Figure 4.23 SLS nylon part to attach tail surfaces to a CFRP tail boom.