How do conventional and T-tails differ?

Question

What design considerations go into the decision between conventional tails and T-tails? Functionally the horizontal stabilizer/stabilator are the same in both cases, providing negative lift, the elevator control and a method for pitch trim. What are the differences though?

As far as I am aware the T-tails I have flown have T-tails for avoiding propwash (PA-44) or aft engine placement (EMB-145). Are there other reasons for having a T-tail? What are the aerodynamic consequences a pilot needs to be aware of with a T-tail (e.g. avoiding hard de-rotation on touchdown, issues at high AOA, etc)?

Answer 1

There is more to a T-tail than that:

Aerodynamics:

1. The placement on top of the vertical gives it more leverage, especially with a swept tail.
2. Depending on wing location, it stays in undisturbed flow in a stall. Note: This is really depending on the details, the HFB-320 had a forward swept wing and a T-tail, which made a deep stall possible (and in one case fatal).
3. 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.
4. It can help to increase the effectiveness of the vertical tail by keeping the air on both sides of it separated. At the other end, the fuselage does this already, so moving the horizontal tail up does not hurt so much there. As a consequence, the tail can be built lower.
5. Rear-mounted engines pretty much force a T-tail, but allow to keep the wings clean. Before CFD, mounting the engines on the wing created lots of problems, prompting the engineers to move to tail-mounted engines in their next design (DC-8 -> DC-9, B707 -> B727)

Structure:

1. The mass of the horizontal tail on a long lever arm (= the vertical tail) means that the torsional eigenfrequency of the fuselage will go down. This will be a problem in case of flutter.
2. As a consequence of the smaller vertical tail, a T-tail can be lighter. Note that the increased leverage means that the horizontal tail can be smaller as well. This reduces friction drag and is the main reason why most modern gliders have T-tails.
3. However, now the fuselage must become stiffer in order to avoid flutter. During flight test of the C-141 it was found that the antimetric wing bending mode would nicely couple with the torsional Eigenmode of the the tail, resulting in some scary footage. Also, a coupling between elevator rotation and horizontal tail torsion required to generously add mass balance to the tail.
4. Rear mounted engines also require more fuselage structure. The Boeing 737 was initially planned with rear-mounted engines, like the Sud-Aviation Caravelle, which it was meant to replace. By selecting the final version with wing-mounted engines in the underslung design, Boeing could reduce the empty weight of the 733-100 by 700 pounds.

Control:

A T-tail produces a strong nose-down pitching moment in sideslip.

If it were not for the flutter and pitch-down, T-tails would be more widespread ...

Answer 2

There's a lot to this, and I'm no aircraft engineer, so if there are any other answers, I'll happily delete this. Anyway, from what I've been told:

The T-tail sticks the elevators out of the disturbed air of the wings, prop, and (usually most of) the fuselage which gives you better elevator authority, and makes a tail stall less likely.

It has some drawbacks though, by putting the elevators directly in the (turbulent) separated flow from the wings during a stall can put you in a (more or less) unrecoverable deep stall.

(Picture from the linked Wikipedia article)

Answer 3

The considerations in the roe's answer are entirely correct but there might be other factors to take into account.

First, it is true that using conventional tail leads to the fact that the airflow over the tail might be disturbed by the main wing and/or the engines and/or the fuselage. However, the downwash induced by the main wing on the flow is taken into account (for the cruise conditions) in the design of the tail in order to reduce some negative aspects of the interaction between the main wing and the tail.

Another major difference between these two configurations concerns the stability. As I already explained in this answer, the tail is used to create some lift that is required to fulfil the trim relations. Regarding the "vertical" force equilibrium equation, there is no real difference between the two configurations but there is a big one for the moment equilibrium.

Assuming that you have the same amount of lift generated by the both configurations (this is relevant due to the "vertical" force equilibrium), a quick sketch will convince you that both the angle and the lever arm are different. The conclusion of this study cannot be drawn without a specific example but I hope it is clear for you that stability is really impacted by the choice of the tail.

From a structural point of view, when flying transonic (or even supersonic) it is not good to have a T-tail configuration because it usually induces flutter on the tail.

Finally, at a lower level but still a difference, using a T-tail increases the wake (compared to a conventional configuration, where the tail is almost in the wake of the main wings and the fuselage) behind your aircraft and thus the drag you need to overcome is larger.

Answer 4

A T-tail has structural and aerodynamic design consequences. The structural considerations are of course the increased weight of the vertical tail due to now having to support the forces and moments on the horizontal tail, including strengthening for flutter. The vertical tail can be shorter due to the end plate effect of the horizontal tail, and the moment arm to the CoG is longer - however for most higher subsonic speed aircraft these effects merely reduce the weight penalty.

The T-tail stays out of ground effect for longer than the main wing. Upon approaching the ground, the increase in wing lift causes an auto-flare: the aircraft lands itself. From the wikipedia page of the Handley Page Victor:

One unusual flight characteristic of the early Victor was its self-landing capability; once lined up with the runway, the aircraft would naturally flare as the wing entered into ground effect while the tail continued to sink, giving a cushioned landing without any command or intervention by the pilot.

The aerodynamic consequences of a T-tail have most to do with stability and control in stall and post-stall behaviour, and can be grave. The Fokker 28 and F100 had stick pushers that acted upon detecting a high angle of attack, making it pretty much impossible to keep the columns at aft position. The reason for this is the reversal of the $C_M$ - $\alpha$ slope of T-tails, as depicted below.

• Graph A is for a tail height of 2 * MAC
• Graph B for 1 * MAC
• Graph C for same height as MAC

The aeroplane is aerodynamically stable when the $C_M$ - $\alpha$ slope is negative, such as in cases B and C. For configuration A, the slope becomes positive after the stall point, meaning that the nose wants to increase upwards after reaching the stall - not a good situation.

The stall speed must be demonstrated during certification, and safe recovery from a stall is a requirement. A stick pusher prevents the aeroplane from entering the deep stall area.