ESDU 98001 considers only the drag due to skin-friction which acts on surfaces that are more or less aligned with the aircraft fore-and-aft axis and is the primary source of airframe drag due to spray impingement on the fuselage and wing. The method uses tyre side- and centre-spray plumes predicted by the method of ESDU 83042 and so does not deal with tyre forward spray impingement. Comments are included on forces due to impingement on surfaces normal to the aircraft fore-and aft axis but they are thought to be small unless large flap deflections are used. The method requires the calculation of the wetted area of the airframe, and a simplified approach assuming constant fuselage and wing heights was adopted that also helped in defining readily regions where parts of the airframe were sheltered from the spray plumes. Predictions using that method, the Karman-Schoenherr equation for skin-friction drag coefficient as a function of Reynolds number, and a constant value of kinematic viscosity compared well with experimental results for a wide range of aircraft extracted from the literature. Examples of the comparisons are given and certain discrepancies are discussed. It is noted that the prediction of spray impingement for a given aircraft configuration requires the study of ranges of operating conditions such as speed, tyre normal load (which are interrelated because of the variation of lift with speed), and contaminant depth and the 'worst' case may not be obvious.

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