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Looking at several modern airliners, the root of the vertical tail doesn't extend right to the aft-most point of the fuselage. This seems counter-intuitive given extending the tail further aft would increase the moment arm and its contribution to the aircraft's directional stability.

In this photo of a 787, you can see what I mean.

enter image description here

There is still a significant length of fuselage behind the tail.

The same can be said for the A320:

A320 Planform

This is far less prevalent on the 737 Classics and NG

737 Classic

And the BAE-146 follows (to my ignorance) what seems like the most logical tail design, with the root reaching the very aft of the fuselage.

BAE-146

The potential reasons for the tail not extending fully aft, that spring to mind are:

  1. CG limits
  2. APU placement
  3. Torsional loads on the fuselage where it becomes very thin at the back, not being able to support the loads imposed by the tail.

Is this why it is or is there more to it?

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  • $\begingroup$ "CG limits APU placement Torsional loads on the fuselage" So, you have basically answered your own question 😜 $\endgroup$
    – sophit
    Dec 22, 2022 at 12:02
  • $\begingroup$ @sophit ah yes I presumed there was more to it than that but maybe that's it...! $\endgroup$ Dec 22, 2022 at 12:04
  • $\begingroup$ @sophit ok thank you - so to that end, it seems to me to make the most sense to put the tail further aft and have a longer APU exhaust with APU a bit further fwd to allow tail structures to be put in. Then there's a less aft CG and longer tail moment arm, meaning the tail can be smaller (less weight and wetted area)? $\endgroup$ Dec 22, 2022 at 12:20
  • $\begingroup$ @sophit Nobody in their right mind oversizes a tail surface. The A318 vertical is larger than that of an A320 precisely to compensate for the shorter lever arm. $\endgroup$ Dec 22, 2022 at 12:42
  • $\begingroup$ @PeterKämpf: correct, I meant A319 Vs. A321, which all share the same tailplane. $\endgroup$
    – sophit
    Dec 22, 2022 at 12:47

3 Answers 3

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Spin resistance and area ruling

Locating the vertical a bit ahead of the horizontal tail brings it out of the probable wake of the horizontal at high angle of attack, so it still is effective in damping the yawing motion. If this were not the case, the airplane would be much harder to bring out of a spin.

As you correctly note, the T-tail of a BAe 146 allows to put the vertical all the way back. Same for the DC-9 and most other T-tails.

The Boeing 727 and the Tu-154 have a longer fuselage because they have the hot parts of the center engine at the end of their fuselage. Other T-tails (Fokker 28 / 100, Tu-134) can afford to move the vertical all the way back.

The APU is small enough so it will not force a less than ideal tail placement.

EDIT: Phil Sweet's answer adds another good point: Area ruling. Placing horizontal and vertical tail a bit apart helps with area ruling. Which one comes first, however, is determined by high angle of attack aerodynamics.

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    $\begingroup$ If the APU is within the aft fuselage ahead of the last bulkhead, it will be in a stainless steel enclosure box for fire protection and containment. Most however put it after the last bulkhead that has the fin main spar so it is outside the structural hull and enclosed only by an aerodynamic tailcone fairing that is more or less a cowling. $\endgroup$
    – John K
    Dec 23, 2022 at 1:19
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The three main duties of a vertical (as well as horizontal) stabiliser are:

  1. to trim;
  2. to control;
  3. and to provide with stability the airplane.

the root of the vertical tail doesn't extend right to the aft-most point of the fuselage

All of these three characteristics develop in respect to the CG and/or the aerodynamic center AC of the airplane and it is therefore the distance from CG/AC which enters in the design process of the tailplane and not the distance from the fuselage's end. Where the fuselage actually ends has nothing to do with the sizing/location of the tailplane: the tailcone simply aerodynamically "closes" the fuselage, possibly taking into account the area rule like it was done for example for the Concorde; obviously this is not important for general aviation airplanes. Tailcone can be used for example also to hold the APU: this location for the APU is ideal since it is far from any important structure or pipe that could be damaged in case of APU's failure; it is easily accessible; the exhaust is short and straight and doesn't point toward anybody.


This seems counter-intuitive given extending the tail further aft would increase the moment arm

The sizing condition for the vertical stabiliser is normally given by stability against Dutch-roll and, for the rudder, by landing @OEI. Locating it further aft would maybe reduce its size and weight but at the expense of a longer and heavier fuselage's structure. The tailplane can even be designed keeping in mind future stretching/shrinking of the fuselage's length like in the case of the A319, A320 and A321 which all share the same tailplane.


is there more to it?

At least two other points come to mind:

  • as already pointed out in another answer, for spin recovery is indeed important the longitudinal placement of the vertical stabiliser but, again, not in respect to the fuselage's end rather in respect to the horizontal stabiliser: this latter should be placed well forward or well backward in respect to the vertical stabiliser, as can be seen in the following picture (source) together with the rule of thumb for a safe spin recovery (rudder as much a possible outside the tailplane's wake extending 60° and 30° upward): tailplane design for spin recovery Anyway this is less of a concern on modern jetliners where the avionics (try to) keep the aircraft far from this condition. Off the top of my head I don't think that any modern jetliner with conventional tailplane respects this rule of thumb.

  • structural clearance between horizontal and vertical stabiliser: by a structural point of view, the horizontal stabiliser is only one single piece going through the fuselage, just like the wing. That means that no important structure can be placed at the same fuselage's station: the structure of the vertical stabiliser is attached to the fuselage just in front of (or behind) the horizontal stabiliser. This can be seen for example in this B737's cutaway: B737 cutaway The horizontal stabiliser's structure 118 goes through the fuselage and the vertical stabiliser is attached just in front of it 99.

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For fast aircraft, there is a drag penalty. You want the curve of cross-sectional areas to be nice and smooth. If the tail is right at the back, then there is a big lump at the rear of the area curve.

Whitcomb area rule: https://en.wikipedia.org/wiki/Area_rule

The area rule says that two airplanes with the same longitudinal cross-sectional area distribution have the same wave drag, independent of how the area is distributed laterally (i.e. in the fuselage or in the wing). Furthermore, to avoid the formation of strong shock waves the external shape of the aircraft has to be carefully arranged so that the cross-sectional area changes as smoothly as possible going from nose to tail. At the location of the wing, the fuselage is narrowed or "waisted". Fuselage cross-sectional area may need to be reduced by flattening the sides of the fuselage below a bubble canopy and at the tail surfaces to compensate for their presence, both of which were done on the Hawker Siddeley Buccaneer.1

You can't flatten something that isn't there. You have to shift the tail forward to be able to flatten the fuselage and make significant volume adjustments.

Taking this one step further, sometimes you have a tail-like junction of foils without a fuselage, such as a T-foil hydrofoil. In that case, you can actually lower total drag by adding an extended fairing body to lower the junction drag. Coke bottle fairing (Tspeer is an aero guy with a boating side-hustle.)

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  • $\begingroup$ If there's a need to obey the area rule then it's better not to move the whole stabiliser forward, possibly altering its stabilising effect and needing a bigger and heavier one, but it's enough to simply prolong the tailcone backward just like they did for example with the Concorde as explained in the Wikipedia link. $\endgroup$
    – sophit
    Dec 23, 2022 at 23:18
  • $\begingroup$ Sweep takes care of the area rule. No need to extend the fuselage. But placing horizontal and vertical tails a bit apart is indeed helpful in getting the area distribution right. $\endgroup$ Dec 24, 2022 at 8:35
  • $\begingroup$ @PeterKämpf: well, Concorde's engineers did extend the fuselage without touching the stabiliser $\endgroup$
    – sophit
    Dec 24, 2022 at 9:25
  • $\begingroup$ @PeterKämpf, perhaps counterintuitively, sweep doesn't really help much. I actually apply sweep when I'm producing the area rule curve, so it cancels out. But I don't design planes, I am an amateur boat designer. We have essentially the same area rule in the form of a displacement curve. But near the surface, it works better if you apply sweep when producing the curve. We "bustle" the stern in front of the rudder to improve wake drag. The near-surface regime in a boat is mathematically identical to the transonic regime in planes. $\endgroup$
    – Phil Sweet
    Dec 24, 2022 at 12:33
  • $\begingroup$ @PeterKämpf Also, the taper really diminishes the outboard volumes, so the area curve is dominated by the root sections. The spanwise rake of the foil areas is usually less than the aerodynamic sweep angle due to mechanical design constraints. $\endgroup$
    – Phil Sweet
    Dec 24, 2022 at 12:37

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