A rare watercolour by Sir William Russell Flint, to be sold at auction next month, has been revealed as a fascinating piece of Scottish aviation history.

The portrait, painted in 1947, was commissioned by Scottish industrialist John Cuthill Sword, founder of Midland & Scottish Air Ferries, based at the former Renfrew airport.

However, it is the subject of the picture, air ace Winnie Drinkwater, which is expected to create most interest.

She was celebrated as the world's youngest - and Scotland's first - female pilot and aviation engineer.

Sword, who was head of the company which went on to become British Midlands, had harboured serious doubts that anyone would want to fly in an aircraft with a woman at the controls. However, he desperately needed a pilot.

The teenager, who grew up with a love for flying and spent many happy hours at Renfrew aerodrome, went solo in a Gypsy Moth a month after her 17th birthday, following just 15 hours' instruction at a cost of #34 10s, paid by her father.

By the time she was 19, she had become the youngest woman to hold a commercial pilot's licence and began giving joy rides from Prestwick beach, bringing in #100 a day from her ''five shillings a flip'' flights for holidaymakers.

Drinkwater died at 83 in New Zealand in 1996. In her later years, she revealed that Sword had serious reservations about her wish to make a living from flying.

''I was told if I could make a go of it, the job was mine,'' she said. ''Soon they were queuing up to fly with me.''

Early charter work included several press assignments.

She once flew to Lossiemouth at night and landed, by the aid of flares, to collect photos of Ramsay MacDonald for one title.

Flying solo, she had to swing her own propeller to start the engine, pull the chocks away, and leap aboard before taking off from makeshift airstrips.

Her exploits fired the popular imagination and she received fan mail, some simply addressed to Winnie Drinkwater, Air Ace, Scotland.

During the Glasgow Fair fortnight, she was up at 3am each day to fly newspapers to the Isle of Man for holidaying Glaswegians.

Then, piloting a twin-engined Dragon, she became the first woman airline pilot in the UK, flying the inaugural Renfrew-London service.

Her commercial flying career was as short as it was colourful, and ended after four years on a romantic note.

One day, she was dismantling an engine in one of the sheds at Renfrew Aerodrome, wearing overalls and covered in grease, when Francis Short, director of Short Brothers, the aircraft manufacturers, paid a visit.

He asked to be introduced to her and within four months they were married.

Edinburgh auctioneers, Shapes, describe the Russell Flint portrait as ''very special''.

Mr Paul Howard, auctioneer and valuer for the company said: ''Because of war rationing in 1947, materials were in short supply and eggs, used for the tempera paint, were given to the artist by Sword himself to ensure it was completed.''

The work is signed by the artist with the inscription: ''Winnie - for my friend John Sword with affectionate good wishes and gratitude''.

Work by Flint, whose female studies have made him one of the most sought after Scottish artists of the twentieth century, has recently been commanding bigger and bigger prices. An inscription by the artist adds to the desirability of the piece.

The Winnie Drinkwater study shows her in relaxed mode and, unlike most Flint studies, well covered, albeit in a flowing satin robe.

Mr Howard expects a lot of interest in the watercolour, because of all its associations.

''We have estimated it at between #10,000 and #15,000,'' he said.