Why do trijets (3 rear engines) usually have a T-tail instead of a normal tail?

Question

I'm thinking of trijets like the Tu-154, which has three jet engines in the rear. Not something like L-1011 Tristar which has only one engine at the rear.

Tu-154:
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L-1011 TriStar:
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Why do these kinds of jets have T-tails? Maybe the side jets get in the way of a conventional tail, but it looks to me like you could just raise those engines a little bit and enough room would appear.

My understanding is that T-tails add complexity. The vertical part of the tail has to be strengthened (more weight) since it undergoes more forces. So a conventional tail seems preferable.

Answer 1

A T-tail is not so bad. Its main advantages are:

• A smaller vertical tail is required, because the horizontal tail acts like an endplate and enhances the efficiency of the vertical tail.
• When the vertical tail is swept, a T-tail will allow to make the horizontal tail smaller as well, because it gains additional lever arm in this configuration.
• By designing the junction with the vertical well, the T-tail has less interference drag. It also helps to reduce wave drag, especially when using a well designed Küchemann body (the round, long, spiky thing on the tail junction of a Tu-154) by stretching the structure lengthwise.

As soon as the cruise Mach number demanded tail sweep, the T-tail became the preferred solution. Placing the engines at the rear fuselage put them higher, so FOD became less of a worry, and the wing could be kept clean for maximum lift resulting in shorter runway requirements. In the days before CFD, it was much easier to get the wing right when no pylons with barrels at their ends were sticking from it.

If an engine is mounted near the place where normally the horizontal tail would be, it is much easier to relocate the horizontal to the top of the vertical than trying to join both together. The mass of the engines will require a relatively rearward wing position, so the lever arm of a conventional tail would be rather small. By shifting it up to the top of the swept vertical tail, its lever arm is much larger, making the T-tail especially attractive for configurations with rear-mounted jet engines.

A second reason is better failure tolerance. Note that the T-tail designs are from the early jet age. Their first flight years were: Caravelle 1955, Jet Star 1957 (both not real T-tails, I know, and having little tail sweep), Trident 1962, VC-10 1962, Ilyushin 62 1963, Boeing 727 1963, Tu-134 1963, C-141 1963, Hansa Jet 1964, Tu-154 1968. The designers were afraid an engine failure would also damage the horizontal tail and would turn a survivable accident into a lethal disaster.

Answer 2

You want clean, undisturbed air flowing over the control surfaces. You could probably squeeze it in someplace above or below the engines, but it would not be easy without negatively influencing the aerodynamics over the elevators. As you can see from the diagram, you would need to get them pretty far out of the way of the engine cowlings.

If you place either engines or elevators low down along the fuselage they suffer from the disturbed air coming of the wing; this is not desirable. As it stands, the side engines (#1, #3) are already angled for the downwash.

As for the more need for a sturdier rudder, this is partially weighed against that you have a longer moment arm.

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Answer 3

Complexity. The worst possible design would be like this: Horizontal and vertical stabilizers are mounted to an engine each...

So you triple the maintenance difficulties you have with a MD-11 or DC-10. And you want an easy access to the engines as possible - which requires enough space.

Answer 4

You can't have a horizontal stabilizer where the engines are mounted. It's either one or the other at this location.

Answer 5

A horizontal tail that is actually mounted to the fuselage wouldn't work well in a design with jet engines mounted to the fuselage, because it would be directly in the jet exhaust, so it comes down to chosing between a cruciform tail (Caravelle style), or a modified version with the horizontal tail closer to the fuselage, but still above the jet exhaust stream, or a T-tail. Clearly the T-tail has won out, in the airliner world. Other answers give some hints as to why.

You might say that if you are going to have the complexity of a horizontal tail mounted to the vertical tail at all, you might as well get the additional benefits of the T-tail.

Yet the T-tail does have some disdvantages over a horizontal tail mounted lower on the vertical tail.

The Falcon family of business jets shows that 3 engines in the rear and a horizontal stabilizer mounted fairly low on the vertical tail can work well- see https://www.dassaultfalcon.com/en/Aircraft/Models/7X/Pages/overview.aspx .

Every aircraft design is influenced by many competing factors and it's hard to set hard-and-fast rules as to what will work best.