Figure 3 Progression of residual lift and net thrust throughout the modelled take-off run of Archaeopteryx. An instant after starting the take-off run, the thrust generated by flapping is 24 times higher than the residual lift (Table 1).
Although for much of the take-off run the lift exerted on the bird is higher than thrust, thrust is the only force exerting work on the bird and increasing continuously the level of its kinetic energy. The cumulative power exerted by the thrust on the bird at a certain time during the take-off run can be calculated by the area under the thrust curve, from the beginning of the run until that time. At lift-off (the intersection of the residual lift and the weight curve), Archaeopteryx has reached the kinetic energy necessary for flight; residual lift cancels out weight. Residual lift becomes useful lift at lift-off.