Abstract:
Flight tests have been carried out on a Typhoon aircraft to compare the values of the aerodynamic side forces and yawing moments, during take-off, with the wind-tunnel measurements, and to compare various methods of estimating the rudder angles required to trim during a take-off run. The side forces can be checked fairly simply by estimating the various component side forces acting at each instant during the run and comparing the summation with the resultant side force measured by an accelerometer. The aerodynamic side forces were evaluated from the wind-tunnel tests under the corresponding conditions and the side force from the undercarriage was estimated from the load on the wheels and the angle of crab of the wheels to their instantaneous direction of motion. It is more difficult to compare the yawing moments operating as there is no direct method, at present, of measuring an angular acceleration. Angular accelerations are difficult to obtain by differentiation of the observed angular displacements of the aircraft, due to the rapid variations in angle produced by the pilot's over-corrections on the rudder. Nevertheless, it was possible in some of the runs to evolve the resultant yawing moment from double differentiation of the heading angles and where this could be done successfully, good agreement was obtained between this resultant moment and the summation of the estimated components. By integrating the summation of the estimated yawing moments along a take-off run, which should be approximately zero, a further check on the comparison of the flight and wind-tunnel yawing moments can be made. The results show very good agreement with the wind-tunnel tests. As runs have been done under various crosswind conditions on the aerodrome (i.e., different angles of sideslip) the order of each of the aerodynamic components was verified. A method of evaluating the rudder angles required to trim is suggested, by solving the side-force and yawing moment equations simultaneously, using the wind-tunnel measurements for the aerodynamic components and introducing the side force from the undercarriage, in terms of the crab angle of the wheels. In the yawing-moment equation, the second-order differential inertia terms is neglected as the changes of angle in the theoretical calculations (representing a straight take-off run) are very small. The effect of the tail wheel has been disregarded as it is only in operation during the initial stages of the run. Due to considerable over-correction by the pilot, it is desirable to design for a rudder range at least 20 per cent in excess of that required to trim.
