CONDUCTING THE TRIAL
1.3.1 Establishing the test conditions
For the results of a trial to have any meaning the conditions which pertained at the time of the test will be of critical importance. Thus, for example, if a performance trial is being conducted the test pilot will have to fly the aircraft at exactly the right speed, with the right value of sideslip, and at the correct altitude. The environmental conditions such as temperature, winds, turbulence levels, etc. will also have to be appropriate. In many trials a variety of conditions will be required and often at opposite extremes. For example, very still air conditions may be needed to obtain accurate results but high levels of turbulence may also be required to give the test pilot conditions representative of operational flying to allow him or her to make qualitative judgements of the handling qualities. In all cases the test conditions need to be recorded for inclusion in the flight report.
1.3.2 Moving into the unknown
Nearly all test flying involves going from a known condition into an unknown one and it is how this is done which is important. Although there will always be an element of risk in this situation, provided the correct methodology is applied nasty surprises can mostly be avoided. The main part of this methodology is to adopt the incremental approach, i. e. only small changes are made in a single parameter when moving from a known condition to the next test point. As an example if control response tests are being conducted by making step cyclic inputs into a control fixture the size of the inputs would be increased very gradually. After each input the change in aircraft response is compared with the previous results with the aim of determining the relationship between changes in the input parameter and the aircraft output. Once a relationship has been established this can be used to predict the output for the next test point to determine if the test should continue. Any change in the relationship can also serve as a warning that extra caution is needed. To establish this input/output relationship a ‘how goes it plot’ is often constructed which in its simplest form consists of a graph where the two parameters are cross-plotted. Also marked on the graph will be the pre-determined test limits. Each test point adds to the trend line and the line is extrapolated prior to each subsequent point to ensure that the predicted result lies inside the test limits. For this system to work it is vital that only one parameter is changed at a time. Thus an input size change should not be made at the same time as the aircraft speed is changed. In addition to establishing trend lines for the test results the team also need to have a sound idea of the expected results prior to the trial taking place. This information can come from a variety of sources, such as results of previous trials under similar circumstances or as a result of computer modelling. Again the predicted trend information is compared with the actual results and major differences serve as a warning that all factors have not been accounted for.