Aerodynamics: Wing shapes: part 2

With the performance of jet aircraft increasing at the expense of manoeuvrability and airfield performance, designers sought a compromise. Aerodynamic devices as simple as the 'dogtooth', or as complex as the variable-geometry wing, have all been syccessful.

 

1. Variable-sweep wing

In the late 1950s, designers found a neat way of mounting wings on pivots, so that the sweep angle could be varied. Previous variable-sweep (also, very loosely, known as variable geometry, or VG) arrangements had been clumsy, involving the wingroots sliding along slots. Now, by moving the pivots outboard to the ends of fixed triangular positions called gloves, the variable-sweep wing became attractive.

The Tornado employs variable-geometry wings to allow it to achieve maximum performance throughout its flight envelope. The aircraft is illustrated above with its wings in the forward position and below with them fully-swept. The above configuration allows maximum manoeuvraibility at low speeds.

 

2. Leading edges

In the 1950s, designers sometimes needed to modify the leading edge or other parts of a swept wing in order to obtain good flying qualities at all speeds. One palliative was to curve the leading edge down, while increasing the chord.

 

3. Conical camber

For the Convair F-102, the optimum leading-edge configuration was found to be the surface of a cone, hence it was termed 'conical camber'.

The F-102 Delta Dagger's carefully designed conical wing leading-edge shape resulted in a distinctively downturned wine profile.

 

4.'Dogtooth' wings

The Hunter was one of many aircraft whose behaviour was improved by the use of a 'dogtooth' in the wing leading edge.

The dogtooth creates a rapidly spiralling wake (vortex), which becomes more intense at high AoA in hard manoeuvres at high altitude. This energises the air over the wing, keeping the air attached and flowing directly to the rear.

A leading edge 'dogtooth' was added to later Hunters, improving their manoeuvrability at all speeds. A similar effect was achieved on the delta-winged Mirage III (right), by the addition of a leading-edge 'saw cut' which works in exactly the same way as the 'dogtooth'.

 

5. LERX

From the 1960s, Northrop investigated how a sharp-edged Leading- Edge Root Extension (LERX) just ahead of the root of the wing could be made to improve manoeuvrability. The extra surface creates a vortex which improves wing performance by preventing the airflow from breaking away. If the pilot pulls out of a dive, or into a tight turn, the LERX becomes far more powerful. It enables pitch rate and turn radius to be greatly improved, prevents wing rock (uncommanded roll) and maintains the effectiveness of the ailerons. LERX power is roughly proportional to area, and the spanwise position of the vortex is dictated by where the LERX joins the original leading edge.

Northrop created simple LERX surfaces for the F-5E, and cleverer ones (slightly curved downwards) for the YF-17, predecessor of the Hornet. British Aerospace riveted crude flat-sheet LERX onto the Harrier GR.Mk 5 and its successors (not the Sea Harrier), and these were later taken up by the US Marine Corps on its AV-8B Harrier II. A development of the LERX is the SMURF (Slde-Mounted Under Root Fin) which is seen on the Boeing/BAe T-45 Goshawk (bottom right). This aircraft suffered tailplane stall on overshoot with gear up and flaps down - the SMURF was able to prevent this by creating a vortex under the tailplane.

Left: Vortices stream from the LERX of a BAe Harrier GR.Mk 7 as it pulls around a tight turn at low level. The presence of LERX considerably increases the aircraft's manoeuvrability Left and above: The most recent and unusual application of the LERX concept is embodied in the SMURFs that were added to the rear fuselage of fhe T-45A.

 

This page was borrowed from the World Aircraft Information Files, which is produced by Areospace Publishing Ltd. and published by Bright Star Publishing plc. www.airpower.co.uk