Most aircraft have horizontal tails. What is the advantage of increasing the horizontal tail surface area?

  • $\begingroup$ Better pitch stability, $\endgroup$ – ratchet freak Oct 1 '14 at 20:51
  • $\begingroup$ More explain please @ratchetfreak $\endgroup$ – SAMER BABIKIR Oct 1 '14 at 21:05

Increasing horizontal stabilizer area increases pitch stability and the permissible range for centre of gravity at the cost of some aerodynamic efficiency.

Stability in pitch means that the aircraft tends to maintain constant angle of attack (which roughly corresponds to maintaining constant speed with pitch). An aircraft is stable in pitch if the aft surface flies at lower angle of attack than the front surface (this applies to both conventional tails and canards).

Making the horizontal stabilizer larger does the following:

  • The absolute change in lift force on the horizontal stabilizer when angle of attack changes is larger, so the aircraft returns to the trimmed angle of attack faster (increased stability).
  • As centre of gravity moves aft, the difference in angle of attack between the surfaces decreases to keep the plane balanced, reducing pitch stability. If the horizontal stabilizer is larger, the angle of attack changes more slowly for the same change in lift, so the centre of gravity may be permitted further aft before the aircraft becomes not stable enough.
  • As centre of gravity moves forward, the difference of angle of attack between the surfaces increases until angle of attack of the horizontal stabilizer exceeds its stall angle of attack and control is lost. If the horizontal stabilizer is larger, it can provide more (downward!) lift before it stalls, so the centre of gravity can be permitted further forward before loosing control authority.
  • $\begingroup$ Maybe it is worth mentioning control forces. They should increase with the square root of the increase - less deflection needed, but a bigger area to move. $\endgroup$ – Peter Kämpf Oct 2 '14 at 9:24
  • $\begingroup$ Whereas the question does not ask for drawbacks, you may consider including them in your answer for completeness. $\endgroup$ – Manu H Apr 22 '15 at 8:41

Previous answers highlight two separate issues: effects on stability and effects on control power. Its difficult to predict control power changes in the context of your question - is it an all moving stabiliser or is it an elevator (how is the configuration of the elevator changed?). If we look at the effect of increasing horizontal area when a perturbation occurs without motivator (control surface) movement, thus isolating control power changes if any, and assuming moment of inertia isn't changed in the pitch axis (which it probably will with a larger tail), then the dynamic and static stability of the aircraft will change.

Assuming a statically stable aircraft: Dynamically, the natural frequency of the aircraft should increase. Statically, assuming the motivator generates the same lift force for a given deflection, then more control surface movement will be required to achieve the same alpha. Ultimately, the control power left will be reduced as more power is needed to counter increased stability. Hence as mentioned, the aircraft CoG working range will be reduced.

WRT control forces and what is 'apparent' to the pilot, this will depend on the flight control system employed. For reversible controls, conventionally stronger stick forces and larger displacement may occur (not guaranteed), however non-reversible systems with complex computer arrangements may manifest as something completely different. A moot point given we're in a completely hypothetical question.

So we talk about the 'effects' of changing area as opposed to 'advantages and disadvantages'; the aircraft design is tailored to achieve the intended application.


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