T-tail
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In aircraft a T-tail is an arrangement of the tail control surfaces with the horizontal surfaces (tailplane and elevators) mounted to the top of the fin, rather than the more common location on the fuselage at the base of the fin. The resulting arrangement looks like a T when viewed from the front, hence the name.
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Missing image Vc10.tail.arp.jpg The T-tail of an RAF VC10 refuelling tanker |
There are pros and cons to this arrangement.
Pros
- The tailplane surfaces are kept well out of the airflow behind the wing, giving smoother flow, more predictable design characteristics, and better pitch control. 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.
- The effective distance between wing and tailplane can be increased without a significant increase in the weight of the aircraft. The distance between the two planes gives the "leverage" by which the tailplane can control the aircraft's pitch attitude - with a greater distance, smaller, lighter tailplanes and elevators can be used.
- The tail surfaces are mounted well out of the way of the rear fuselage, permitting this site to be used for the aircraft's engines. This is why the T-tail arrangement is also commonly found on airliners with rear-mounted engines. The Douglas DC-9, Boeing 727, Vickers VC-10, Hawker-Siddeley Trident and BAC 1-11 all used the T-tail for this reason.
Cons
- The aircraft will tend to be much more prone to a dangerous deep stall condition, where blanking of the airflow over the tailplane and elevators by a stalled wing can lead to total loss of pitch control.
- For similar reasons, T-tailed aircraft can be much more difficult to recover from a fully-developed spin.
- The fin must be made considerably stronger and stiffer to support the forces generated by the tailplane. This inevitably makes it heavier as well.
- The control runs to the elevators are more complex.
- The elevator surfaces are much more difficult to casually inspect from the ground.
- In aircraft fitted with an ejector seat and a T-tail, F-104 Starfighter, there were (mostly unfounded) concerns about being able to clear the tail. The solution was a downward-firing ejection seat, this presented obvious problems in low-altitude escapes.
A compromise is also possible, with the tailplane mounted part way up the fin rather than right at the top. The Sud Aviation Caravelle is an aircraft with this configuration.