A method calculating a drag coefficient Cxmeasure of an airplane, said method comprising a first process and a second process,
- the first process comprising a correction of a lift model of the airplane to provide a corrected lift model, and
- the second process comprising use of a measured pitch angle and of the corrected lift model to calculate the drag coefficient Cxpressure by taking into account a calculated vertical wind,
- said first and second processes being based on measurements, including at least geometrical speed, airspeed and pitch angle, taken in flight by measurement apparatuses and on values calculated from said measurements taken in flight and during at least one flight, the measurement apparatuses producing measurements that might suffer bias, the measurements in flight being taken under at least one determined flight condition defining a determined flight point, each flight condition being defined by particular flight parameters values, each flight point corresponding to a determined combination of flight parameters values and thus to a determined flight condition, one of the flight parameters being a lift coefficient Cz, said measurements and values comprising firstly, θmeasure, the measured pitch angle of the airplane, and secondly, αmodel, a calculated angle of attack of the airplane, the measured pitch angle θmeasure being corrected by a pitch angle correction term Δθ0 and the angle of attack αmodel being corrected by an angle of attack correction term Δα(Cz, . . . );
- wherein the angle of attack αmodel of the airplane is calculated by solving a lift equation and utilizing a lift model associating the angle of attack α of the airplane with at least one flight parameter which is the lift coefficient Cz of the airplane, and
- wherein the pitch angle correction term Δθ0 and the attack correction term Δα(Cz, . . . ) are calculated utilizing a first assumption and a second assumption, the first assumption stating that an average vertical wind is practically zero during one or more flights, and the second assumption stating an independence between the vertical wind and the flight parameters Xi, the first and second assumptions being implemented according to the following calculation operations:
- the pitch angle correction term Δθ0 is calculated with the assumption that the mean vertical wind WZ is zero for all of the determined flight points that belong to one or plural flights having the same measurement bias;
- the angle of attack correction term Δα(Cz, . . . ) is calculated with the assumption that the mean vertical wind WZ for each determined flight condition is zero;
- the vertical wind WZ to which the airplane is subjected is calculated by:
- WZcalculated=VZgeom−TAS·sin(θmeasure−αmodel+(Δθ0−Δα(Cz, . . . )))
- where VZgeom is the vertical speed of the airplane in the terrestrial reference frame, which is positive going upwards, and TAS is the true air speed of the airplane in the aerodynamic reference frame,
- the drag coefficient Cxmeasure of the airplane being calculated by means of a calculation formula taking account of the calculated vertical wind, said calculation formula including an aerodynamic slope term γaero with
- γaero=(θmeasure+Δθ0)−(αmodel+Δα(Cz, . . . )).