The horizontal tail and vertical tail along with wing are referred to as the lifting surfaces. The major difference between wing design and tail design originates from the primary function of the tail which is different from the wing. The primary function of the wing is to generate the maximum or substantial amount of lift, while the tail is designed to operate normally at only a fraction of its ability to generate the required lift potential. During a flight mission, if at any moment the tail reaches its maximum angle of attack which refers to the tail stall angle; is an indication of design process error.The tail in a conventional unmanned aerial vehicle often has two components, horizontal tail and vertical tail, and carries primary two functions:
Trim (longitudinal and directional)
Stability (longitudinal and directional)
Control (longitudinal and directional)
Trim is referred to the generation of a lift force which acts through the tail moment arm about the center of gravity, balancing some other moment produced by the aircraft. For the horizontal tail, trim primarily means the balancing of the moment generated by the wing. Hence, longitudinal trim is the first and primary function of a horizontal tail and is also referred to as the equilibrium or balance. An aft mounted horizontal tail has a negative incidence angle of about 2-3 degrees to balance the wing pitching moment.
The vertical tail is not required to generate longitudinal trim force because the aircraft is usually left-right symmetric i.e. an aircraft is symmetric about the XZ plane. Hence the primary function of vertical tail is directional stability.
The second function of the tail is to provide stability. Aircraft stability is defined as the tendency of an aircraft to return to its original trim conditions if diverted by a disturbance . The major disturbance source being atmospheric conditions, namely gust. The horizontal tail is responsible for longitudinal stability; while the vertical tail is responsible for maintaining directional stability.
The third important function of the tail is control. The elevator which is designed as a part of the horizontal tail is designed to provide longitudinal control, while the rudder which is designed as part of the vertical tail is responsible for providing directional control. Hence, tails must be powerful to control the aircraft, in such a way that the aircraft can switch flight conditions from one trim condition to another. Thus, tail must be sized to provide adequate control at critical conditions. The critical conditions for horizontal tail include nose wheel liftoff, low-speed flight with flaps down, and transonic maneuvering. For the vertical tail, the critical conditions include engine out flight at low speeds, maximum roll rate and spin recovery. It must be noted that the control power depends upon the size and type of the movable surface as well as the overall tail size.
Fig : The tail...