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A braided composite spar or preform for a braided composite spar with a plurality of tubular plies of braided fibers. Each ply has a first set of fibers which wind in a clockwise direction in a first series of turns with a pitch between each adjacent pair of turns, and a second set of fibers which w

A braided composite spar or preform for a braided composite spar with a plurality of tubular plies of braided fibers. Each ply has a first set of fibers which wind in a clockwise direction in a first series of turns with a pitch between each adjacent pair of turns, and a second set of fibers which wind in an anti-clockwise direction in a second series of turns with a pitch between each adjacent pair of turns. The first and second sets of fibers in each ply are intertwined to form a braided structure. The spar or preform extends lengthwise from a root to a tip and has a tapered portion which tapers inwardly towards the tip. Each ply has a circumference in the tapered portion which reduces as it tapers inwardly. For at least one of the plies the pitches of the first and second sets of fibers increase continuously as the ply tapers inwardly in the tapered portion. The spar or preform can be used to provide a tubular main spar for a winglet. The winglet also has a front spar with a front spar web, an upper front spar cap, and a lower front spar cap. An upper skin of the winglet is joined to the braided spar and the upper front spar cap. A lower skin of the winglet is joined to the braided spar and the lower front spar cap.

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1. A braided composite spar or preform for a braided composite spar, comprising: a plurality of tubular plies of braided fibres, each ply comprises a first set of fibres which wind in a clockwise direction in a first series of turns with a pitch between each adjacent pair of turns, anda second set o

1. A braided composite spar or preform for a braided composite spar, comprising: a plurality of tubular plies of braided fibres, each ply comprises a first set of fibres which wind in a clockwise direction in a first series of turns with a pitch between each adjacent pair of turns, anda second set of fibres which wind in an anti-clockwise direction in a second series of turns with a pitch between each adjacent pair of turns, the first and second sets of fibres in each ply being intertwined to form a braided structure;wherein the spar or preform extends lengthwise from a first end to a second end, the spar or preform has a tapered portion which tapers inwardly towards the second end, each ply has a circumference in the tapered portion which reduces as each ply tapers inwardly, and for at least one of the plies the pitches of the first and second sets of fibres increase as the ply tapers inwardly in the tapered portion. 2. The spar or preform of claim 1 wherein each ply has an areal weight or thickness in the tapered portion which does not change by more than 10% within the tapered portion. 3. The spar or preform of claim 2 wherein each ply has an areal weight or thickness which does not change by more than 10% over the entire length of the spar or preform. 4. The spar or preform of claim 1 wherein the spar or preform has a first portion between the tapered portion and the first end. 5. The spar or preform of claim 4 further comprising one or more fastener holes in the first portion. 6. The spar or preform of claim 1 wherein the spar or preform has a second portion between the tapered portion and the second end. 7. The spar or preform of claim 1 wherein the first and second sets of fibres have a fibre angle which changes by more than 10° in the tapered portion. 8. The spar or preform of claim 1 wherein the spar or preform has a higher bending stiffness at the second end than at the first end. 9. The spar or preform of claim 1 wherein the spar or preform has a higher torsional stiffness at the first end than at the second end. 10. The spar or preform of claim 1 wherein the spar or preform has a centre line which extends lengthwise from the first end to the second end, and at least part of the centre line follows a curved path which does not lie in a single plane. 11. The spar or preform of claim 1 wherein each ply in the tapered portion has a height which reduces and a width which increases as each ply extends toward the second end. 12. A winglet comprising a composite spar according to claim 1, wherein the composite spar comprises forward and aft webs joined by upper and lower caps, and the winglet comprises an upper skin joined to the upper cap of the composite spar and a lower skin joined to the lower cap of the composite spar. 13. An aircraft wing comprising a main wing element with a tip; and a winglet according to claim 12 attached to the tip of the main wing element. 14. The wing of claim 13 wherein the composite spar is canted up or down relative to the main wing element. 15. The wing of claim 13 wherein the composite spar is swept forward or aft relative to the main wing element. 16. The wing of claim 13 wherein the main wing element comprises a spar; and the composite spar of the winglet is attached to the spar of the main wing element. 17. A method of manufacturing a braided preform for a composite spar, the method comprising: forming a plurality of tubular plies of braided fibres, each ply being formed by feeding a first set of fibres from a first set of bobbins onto a mandrel,wherein the mandrel has a tapered portion which tapers inwardly so as to reduce the mandrel outer circumference as the mandrel extends in an outboard direction along a length of the mandrel;rotating the mandrel and/or the first set of bobbins to generate a clockwise relative rotation between the first set of bobbins and the mandrel at a rotation rate ω1; feeding a second set of fibres from a second set of bobbins onto the mandrel; rotating the mandrel and/or the second set of bobbins to generate an anti-clockwise relative rotation between the second set of bobbins and the mandrel at a rotation rate ω2;traversing the bobbins and/or the mandrel to generate a relative motion between the bobbins and the mandrel at a speed S so that the first and second sets of fibres are wound onto the mandrel and become intertwined to form a braided structure;wherein the method further comprises for at least one of the braided plies varying a ratio S/ω1 between the speed S and the rotation rate ω1 as the first set of fibres are wound onto the tapered portion of the mandrel so that the ratio S/ω1 increases as the mandrel tapers inwardly, and also varying a ratio S/ω2 between the speed S and the rotation rate ω2 as the second set of fibres are wound onto the tapered portion of the mandrel so that the ratio S/ω2increases as the mandrel tapers inwardly. 18. The method of claim 17 wherein the ratios S/ω1 and S/ω2 are controlled so that each ply has an areal weight or thickness in the tapered portion which does not change by more than 10% within the tapered portion. 19. The method of claim 18 wherein the ratios S/ω1 and S/ω2 are controlled so that each ply has an areal weight or thickness which does not change by more than 10% over the entire length of the preform. 20. The method of claim 17 wherein the preform extends lengthwise from a first end to a second end, the tapered portion tapers inwardly towards the second end, the preform has a first portion between the tapered portion and the first end, and the method further comprises forming one or more fastener holes in the first portion.