a YORKSHIRE company has developed a rooftop wind turbine that it claims could produce up to half a family's energy needs without being an eyesore.  The Ridgeblade is a narrow box that sits along the ridge of a pitched roof. Inside are turbines that turn like the reel of a combine harvester. Its developers say it benefits from the natural acceleration of wind speeds at the top of pitched roofs, will work even in mild breezes, and can cope with most wind directions.  It would cost about ?5,000 to install and could, its makers claim, generate up to 2,000 kilowatt hours a year, 10 times more than conventional domestic turbines. The average British household uses 4,000kWh.  also, the device is likely to be less of a headache for planners as the box is unobtrusive, unlike traditional pinwheel turbines.  The Ridgeblade is being developed by the Power Collective and was designed by one of its directors, Win Keech, a former RaF pilot and Rolls-Royce turbine engineer.  Traditional rooftop turbines can be hampered by airflow disruption in built-up areas. They may also encounter noise and planning objections. Research from the Energy Saving Trust found some even used more energy than they produced.  "Win has found a way of realising the benefits of the urban environment," said Simon Pringle, a fellow director at the Power Collective.  "a pitched roof acts like an aircraft wing. as the wind hits, it