In order to examine the ability of a helicopter design to perform a given mission, the calculations described – where engine power and fuel consumption can be determined at any flight speed – can be assembled so the helicopter model can ‘fly’ a mission in a computer. The ease and immediacy of this procedure make it of direct use to a project assessment. There is one important consideration to be made. As the helicopter consumes fuel, the weight changes, which in turn affects the fuel consumption itself. So, in an ideal world, the calculation becomes circular. However, if the mission is ‘flown’ in small time steps, or mission legs, then an iterative scheme can be used to obtain an estimate of the fuel consumption rate over the leg/time period. The mission is then assembled by linking these individual mission legs where the value of the helicopter weight at the end of a particular leg becomes the start value for the succeeding leg. (Each leg will be defined by a fixed flight condition, particularly forward speed. However, if a climb or descent – at constant speed – is required the iterative scheme can still be used.) The mission may contain discrete weight changes of payload, such as changes in passenger/cargo payload or the deployment of ordnance. These can be incorporated by placing any such occurrence at the join of two mission legs and the weight change made in moving from one to its successor.
The iterative scheme to account for the changing helicopter weight is now described. For each leg the procedure begins with the calculation of the power and fuel consumption at the start weight of the leg. The duration of the leg can be obtained either by the time being explicitly stated or by dividing the range by the speed to give the time. This enables a first estimate of the weight change over the leg to be calculated. By subtracting half of that fuel usage from the start weight, a revised helicopter weight is thus obtained. Taking this new value of aircraft weight the calculation process is repeated and a new estimate for fuel usage is thus obtained. The two values of the fuel usage, over the leg, at the two weights are then compared. If they differ within a specified tolerance, the process is seen to have converged and the final estimate is adopted. If the fuel usage values do not lie within the tolerance, a revised mean aircraft weight is adopted
(by subtracting half of the latest fuel usage value from the start weight of the particular mission leg) and the process is repeated. This iteration continues until convergence to within the required tolerance is achieved.