Vertical climbs

Both reduced power verticals and maximum power verticals can be conveniently combined with OGE hover tests. Once the hover data has been taken, the desired torque increment can be added and the aircraft timed through an altitude band of around 400 ft, or for 20 seconds. Split times are recorded at intermediate altitudes so that a mean, smoothed ROC can be determined. A more accurate result can be obtained by adjusting the mean altitude to ensure that the referred weight remains essentially constant for each hover/RPV/MPV combination. If static droop is present, Nr will change as power is added and so an adjustment will have to be made to regain the desired referred rotor speed once the climb has been stabilized. It is easy to underestimate the altitude that will be required to achieve this. A further complication arises if the test team attempts to target a constant value of referred power available as this will involve adjustment of the collective as altitude increases and air temperature and density fall. In practice such adjustments are both difficult to make accurately and are very time-consuming. Consequently, an average torque value is recorded at a fixed collective position. If desired, TE can be evaluated using similar techniques to those applicable to the hover case.

A plot of referred ROC versus referred power for a given referred weight reveals that the relationship between power increment and ROC is approximately linear and that the lines are broadly parallel. A considerable amount of time and effort can therefore be saved by conducting only hover and MPV points at a given referred weight. With careful planning a very efficient use of time can be achieved as follows: [2]

This method relies on the expeditious establishment of successive test conditions. Only about 5 minutes can elapse between the hover and MPV points for a given referred weight otherwise the fuel burn will be excessive and more than 100 ft will be required to compensate for it. It is important to ensure that m/^9 is kept constant. In particular the effects of static droop must be accounted for when conducting MPVs by setting a slightly higher Nr in the hover before power is applied. During the MPV, adjustments are made to the NR to achieve the required value of m/^9 at the point where data is taken.

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