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A T-tail is an empennage configuration in which the horizontal surfaces (tailplane and elevators) are mounted to the top of the vertical stabilizer. The resulting arrangement looks like the capital letter T when viewed from the front or back, hence the name. This differs from the traditional configuration in which the horizontal control surfaces are mounted to the fuselage at the base of the vertical stabilizer.
The tailplane surfaces are kept well out of the disturbed airflow behind the wing and fuselage, giving smoother and faster (more dynamic pressure) flow, more predictable design characteristics, and better pitch control (with the exception being in aircraft using a tractor propeller, because the T-tail configuration places the horizontal control surfaces above of the high velocity flow from the propeller) This is especially important for planes operating at low speed, where clean airflow is required for control. deHavilland Canada's line of larger STOL aircraft all use this arrangement for this reason. T-tail configuration also allows high performance aerodynamics and excellent glide ratio as the horizontal tail empennage is less affected by wing and fuselage slipstream, has a better effective aspect ratio (better lift slope), less interaction drag than a cruciform tail and a more efficient vertical tail (the horizontal tail plate effect increases the effective aspect ratio so the lift slope of the vertical tail).[further explanation needed] Therefore the T-tail configuration is especially popular on gliders.
The aircraft will be prone to suffering a dangerous deep stall condition, where blanking of the airflow over the tailplane and elevators by a stalled wing at high angles of attack can lead to total loss of pitch control. The F-101 Voodoo suffered from this throughout its service life.
The vertical stabilizer must be made considerably stronger and stiffer to support the forces generated by the tailplane. Unless expensive composite materials are used, this inevitably makes it heavier as well.
The T-tail configuration can cause several maintenance concerns as well. The control runs to the elevators are more complex, and elevator surfaces are much more difficult to casually inspect from the ground. The loss of Alaska Airlines Flight 261 was directly attributed to lax maintenance due to the complexity of the T-tail.
Because of concerns about being able to clear the tail, the first high-speed aircraft with a T-tail, the Lockheed F-104 Starfighter, was originally fitted with a downward-firing ejection seat. This prevented pilots from escaping at low altitudes, causing several deaths. For later models of this aircraft, the ejection seat was improved and changed to fire upwards in order to overcome problems in low-altitude escapes.
Due to a lack of airflow over the elevator from a forward mounted engine (piston or turboprop), low speed control is reduced and low speed operation is more difficult for aircraft not designed for low speed operation.
In order to mitigate some of these drawbacks, a compromise is also possible. The tailplane can be mounted part way up the fin rather than right at the top, known as a cruciform tail. The Sud Aviation Caravelle is an example of an aircraft with this configuration.
- Hoerner and Borst, Fluid Dynamic Lift, Directional Characteristics, T-tail page 13-11
- "Gloster Javelin - History". Thunder & Lightnings.