Outboard tail
A split tail mounted outboard of wingtips to reduce drag.
An outboard tail is an aircraft empennage design where the tail is divided into two halves, each attached to a short boom located just behind and outside each wingtip. These outboard horizontal stabilizers (OHS) may also be paired with boom-mounted vertical fins. The key feature of this layout is that the tail surfaces are placed to work with the wingtip vortices, reducing drag noticeably while avoiding major structural or handling problems.
The outboard tail sits beyond the main wingtips. Though sometimes called tailless, it differs from a true tailless wing because the horizontal stabilizer is separate from the wing, set back and supported by a short boom. On a swept wing, the boom can be very short, allowing the tail’s front to overlap the wing’s rear. The tail consists of outboard horizontal stabilizers, with or without additional vertical fins on the booms.
A normal wingtip produces a strong trailing vortex as air spills upward from below and moves inward toward the low-pressure area above the wing. These vortices waste energy and increase drag. In the outboard tail configuration, the tail surfaces are placed to interact usefully with the upwash behind the wingtips. With careful design, this can cut drag and boost efficiency without harming handling or adding excessive structural loads.
Compared to a tailless swept wing of similar span, the outboard tail offers a longer moment arm than if the stabilizer were attached directly to the wing. This means the wing does not need as sharp a sweep, avoiding unwanted side effects. The outboard elevators can also act as secondary ailerons, increasing effective control area. This reduces wing loading variation during critical maneuvers like landing, allowing a higher design load and greater safety margins.
Vertical tail fins normally provide directional stability and control. In an outboard layout, they can also improve airflow and contribute to overall efficiency, much like winglets.
The configuration was first patented in 1942 by French designer Robert Aimé Robert. During World War II, German designers Richard Vogt and George Haag at Blohm & Voss developed it. To test the control system, Škoda-Kauba adapted the V-6 design as the SK SL6 in 1944. Blohm & Voss then used it in the P 208 pusher-engined fighter proposal, which was not built but later analysis showed it would have been viable.
- First patented
- 1942
- First patented by
- Robert Aimé Robert
- Developed during
- World War II
- Developed by
- Richard Vogt and George Haag at Blohm & Voss
- Test aircraft
- Škoda-Kauba SK SL6 (1944)
- Reinvestigated by
- NASA (formerly NACA) in the late 1950s and early 1960s
- First prototype flown
- Scaled Composites SpaceShipOne (2003)
Lore & Background
The outboard tail configuration was first patented in 1942 by French designer Robert Aimé Robert. During World War II, German designers Richard Vogt and George Haag at Blohm & Voss developed the concept. To test the proposed control system, Škoda-Kauba adapted the V-6 design as the SK SL6 in 1944. Blohm & Voss then incorporated the design into the P 208 pusher-engined fighter proposal; although not taken up, modern analysis has shown it would have been viable. B&V produced a series of design studies for jet fighters, including the P 209.01, P 210, and P 212, and received an order for the P 215 all-weather fighter just weeks before the war ended, so it was never built.
In the late 1950s and early 1960s, NASA (formerly NACA) engineers re-investigated the principles as part of research into supersonic flight configurations, conducting analyses and wind tunnel tests at both subsonic and supersonic speeds. Vought studied the layout for their ADAM V/STOL design project. Further extensive studies were made by J. A. C. Kentfield and colleagues at the University of Calgary, Canada, from the 1990s on. Burt Rutan at Scaled Composites became interested in the configuration for its potential as a variable-geometry stabiliser on a spaceplane; during atmospheric re-entry, the tail would rotate vertically to stabilise the plane. His first prototype, SpaceShipOne, took to the air in 2003, and it has since flown on SpaceShipTwo and forthcoming SpaceShip III. The outboard tail is also seen as a natural partner to the FanWing concept, and a model was flown in 2011.
Reader's Guide
The outboard tail configuration offers a significant reduction in drag by positioning the tail surfaces to interact constructively with wingtip vortices, which otherwise carry away energy and increase drag. Compared to a tailless swept wing of similar overall span, the outboard tail provides a greater moment arm than if attached directly to the wing, allowing the wing to avoid sharp sweepback and its undesirable side effects. The outboard elevators can also serve as secondary ailerons, increasing effective control area and reducing loading variation on the wing during critical manoeuvres such as landing, which allows greater safety margins. Vertical tail fins in this configuration can enhance airflow and contribute to overall efficiency, analogous to winglets. The design was explored by NASA for supersonic flight and by Vought for V/STOL, and later by Kentfield at the University of Calgary. Its legacy includes practical application in Scaled Composites' spaceplane series, where the tail's variable geometry aids stability during re-entry, and its association with the FanWing concept.
Did You Know?
- Blohm & Voss received an order for the P 215 all-weather fighter just weeks before World War II ended, so it was never built.
- Burt Rutan's SpaceShipOne, which used an outboard tail, first flew in 2003.
- The outboard elevators can be used as secondary ailerons, increasing effective control area.
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