ESDU 97011 provides an empirical method for estimating delta C sub L0tw, the increment in lift coefficient at zero angle attack due to the deployment of full-span or part-span plain flaps on wings at low speeds. The increment delta C sub L0tw is derived by using the method of ESDU 94028, but with a modified efficiency factor, to establish the characteristics of a plain flap on an aerofoil section that is representative of the wing. Factors are applied to allow for planform geometry and part-span effects. The method has been found to be accurate to within /-15% for 98% of the data studied.