Flight Controls

example, the horizontal tail surfaces may be attached near similar recovery problems are also found with conventional the lower part of the vertical stabilizer, at the midpoint, or tail aircraft with an aft CG. [Figure 6-11] Deep stalls can at the high point, as in the T-tail design. occur on any aircraft but are more likely to occur on aircraft with “T” tails as a high AOA may be more likely to place T-Tail the wings separated airflow into the path of the horizontal In a T-tail configuration, the elevator is above most of the surface of the tail. Additionally, the distance between the effects of downwash from the propeller, as well as airflow wings and the tail, the position of the engines (such as being around the fuselage and/or wings during normal flight mounted on the tail) may increase the susceptibility of deep conditions. Operation of the elevators in this undisturbed air stall events. Therefore a deep stall may be more prevalent allows control movements that are consistent throughout most on transport versus general aviation aircraft. flight regimes. T-tail designs have become popular on many light and large aircraft, especially those with aft fuselage- Since flight at a high AOA with a low airspeed and an aft mounted engines because the T-tail configuration removes CG position can be dangerous, many aircraft have systems to the tail from the exhaust blast of the engines. Seaplanes and compensate for this situation. The systems range from control amphibians often have T-tails in order to keep the horizontal stops to elevator down springs. On transport category jets, stick surfaces as far from the water as possible. An additional pushers are commonly used. An elevator down spring assists in benefit is reduced noise and vibration inside the aircraft. lowering the nose of the aircraft to prevent a stall caused by the aft CG position. The stall occurs because the properly trimmed In comparison with conventional-tail aircraft, the elevator on a airplane is flying with the elevator in a trailing edge down T-tail aircraft must be moved a greater distance to raise the nose position, forcing the tail up and the nose down. In this unstable a given amount when traveling at slow speeds. This is because condition, if the aircraft encounters turbulence and slows down the conventional-tail aircraft has the downwash from the further, the trim tab no longer positions the elevator in the nose- propeller pushing down on the tail to assist in raising the nose. down position. The elevator then streamlines, and the nose of the aircraft pitches upward, possibly resulting in a stall. Aircraft controls are rigged so that an increase in control force is required to increase control travel. The forces required to The elevator down spring produces a mechanical load on the raise the nose of a T-tail aircraft are greater than the forces elevator, causing it to move toward the nose-down position if not required to raise the nose of a conventional-tail aircraft. otherwise balanced. The elevator trim tab balances the elevator Longitudinal stability of a trimmed aircraft is the same for down spring to position the elevator in a trimmed position. both types of configuration, but the pilot must be aware that When the trim tab becomes ineffective, the down spring drives the required control forces are greater at slow speeds during the elevator to a nose-down position. The nose of the aircraft takeoffs, landings, or stalls than for similar size aircraft lowers, speed builds up, and a stall is prevented. [Figure 6-12] equipped with conventional tails. The elevator must also have sufficient authority to hold the T-tail aircraft also require additional design considerations nose of the aircraft up during the roundout for a landing. In to counter the problem of flutter. Since the weight of the this case, a forward CG may cause a problem. During the horizontal surfaces is at the top of the vertical stabilizer, the landing flare, power is usually reduced, which decreases the moment arm created causes high loads on the vertical stabilizer that can result in flutter. Engineers must compensate for this by increasing the design stiffness of the vertical stabilizer, usually resulting in a weight penalty over conventional tail designs. When flying at a very high AOA with a low airspeed and an aft CG, the T-tail aircraft may be more susceptible to a deep stall. In this condition, the wake of the wing impinges CG on the tail surface and renders it almost ineffective. The wing, if fully stalled, allows its airflow to separate right after the leading edge. The wide wake of decelerated, turbulent air blankets the horizontal tail and hence its effectiveness diminished significantly. In these circumstances, elevator or stabilator control is reduced (or perhaps eliminated) making it difficult to recover from the stall. It should be noted that an aft CG is often a contributing factor in these incidents, since Figure 6-11. Aircraft with a T-tail design at a high AOA and an aft CG. 6-6