Tails - What Are They Good For?

Tails are there to keep aircraft pointed in the proper direction, and not much else, right? Well here are some ideas from new patents awarded to Airbus designers for other things tails could do as well.

All graphics: USPTO

First we have noise reduction. Above is an annular tail (US patent application 20120325958), in which the horizontal and vertical stabilizers form closed surfaces aerodynamically optimized around the engines. This tail completely encloses the engines - here open rotors - to maximize noise shielding, and has to hinge upwards to provide access.

Then we have drag and weight reduction. A conventional tail has horizontal and vertical stabilizers attached to a tailcone that tapers to reduce base drag. This prevents the pressurized cabin being extended rearwards, says a new patent (US patent 8,292,225) for a concept Airbus calls the "queue-de-morue" (flat brush) tail. This flattens and widens the rear fuselage (below left), reducing weight and drag by eliminating the need for a horizontal stabilizer while extending the cabin rearwards. A control surface at the back (25) provides pitch control.

You might not want to go quite that far, but still use a H-tail so the vertical stabilizers provide noise shielding for the engines. The problem is an H-tail adds drag from the additional surface area and weight from the beefier structure needed to support vertical surfaces at the tips of the horizontal stabilizer. Airbus has a concept (US patent application 20120298795) for an articulating tail (above right) that pivots up or down to provide the trimming force with less tail area, drag and weight.

Or you might want a tail that can accommodate a third engine. In the concept above (US patent 8,196,861), a so-called "APTU" enclosed within the tailcone can provide auxiliary power like an APU when required or additional propulsion like a jet engine when necessary. The unit is fed by one or both of two lateral inlets either side of the fuselage. These inlets close when engine is not operating to reduce drag.

And if you want a full-size engine back there, then the Airbus designers have come up with what they call the "tailcoat" tail (above). This is claimed (US patent application 20120138736) to reduce the complexity, drag and weight of the third-engine installation, compared with a DC-10 or L-1011, by semi-burying the engine in the broad aft fuselage. I suspect the "tailcoat" name comes from the two flaps (6a/b) at the back.

And if you have engines mounted above the fuselage, you might want to enable the aircraft to replace one autonomously if it fails in flight. That's the crux of the design above (US patent 8,113,464), for a freighter with rear loading ramp - and a cargo compartment large enough not only to carry high-bypass turbofans, but also to allow a failed engine to be lowered through a hatch (7) into the cargo hold and another raised into position in its place.

And, finally, even if your tail looks conventional, it can work differently, as the concept below does (US patent 8,342,446). Vertical tails are sized by the need to provide directional stability and control in the worst case - engine failure and maximum asymmetric thrust at low airspeed on take-off. That makes the fin bigger and draggier than it needs to be in the cruise. So in the design below (left to right) the fin rotates (left between an extended position for maximum effectiveness at low speed and a retracted or "returned" position with less exposed area for minimum drag at high speed.



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