Airplanes with a T-tail design have a shorter vertical stabilizer than the conventional design , what is the reason?
- T-tail design is often imposed on designs with twin engines mounted at the aft fuselage. This means they have a small moment arm in the yaw direction, the vertical tail is dimensioned to compensate for engine failure. The further away form centreline the engine is mounted, the higher the yawing moment that the remaining engine exerts upon engine failure.
- The horizontal stabiliser at the top of the vertical stab functions as an aerodynamic barrier, like a winglet does. This prevents a vortex from streaming off of the top of the vertical tail when yawing, and thereby increases the effectivity of the vertical tail surface. This effect was researched in NACA TN 2907, at five different locations and three different spans:
The graph below shows calculated effective increase of aerodynamic aspect ratio, as function of where on the fin the horizontal tail is located. If completely at the top or at the bottom, the aerodynamic aspect ratio is about 50% higher than when at the centre, resulting in increase of the lift curve slope of the fin of roughly 15%.
With a T-tail, the horizontal stabiliser at the top functions as an aerodynamic barrier, while the fuselage at the bottom end provides some barrier function as well. Depending on the local fuselage circumference and shape of course.
Raymer gives the following answer:
The 'T-Tail' is also widely used. A T-tail is inherently heavier than a conventional tail because the vertical tail must be strengthened to support the horizontal tail, but the T -tail provides compensating advantages in many cases.
Due to end-plate effect, the T -tail allows a smaller vertical tail. The T -tail lifts the horizontal tail clear of the wing wake and propwash, which makes it more efficient and hence allows reducing its size. This also reduces buffet on the horizontal tail, which reduces fatigue for both the structure and the pilot." (pg. 69)
- D. P. Raymer, 'Aircraft Design: A Conceptual Approach', AIAA, Washington DC, USA, 1992.