Planning a level flight performance trial
It is important that flight trials are carefully planned and that certain essential conditions are fulfilled if the maximum information is to be obtained from the minimum flight hours. The planning task may be summarized by the following questions:
• What test conditions are required?
• What characteristics (temperature and altitude) prevail at the proposed test site(s)?
• What test conditions can be obtained at the proposed test site(s)?
• What range of ballast and fuel states is required to complete the matrix of test conditions?
• How many flight hours are required?
188.8.131.52 Example: planning flight tests at constant W/сю2 and ю /^0
Suppose a helicopter is to be tested with an AUM range of 4000 kg to 5000 kg that is cleared to operate between sea level and 10000 ft pressure altitude The rotor speed can be varied in flight and has a power-on RRPM range of between 95% and 105% (standard RRPM is 100%). It is expected that in service the helicopter will experience temperature profiles between ISA — 30°C and ISA + 30°C. At the home establishment the trials will be conducted within the following constraints:
• Although the test aircraft can be ballasted to 5000 kg, a sensible minimum start mass is 4200 kg (2 crew and 45 minutes endurance).
• The helicopter has been authorized to operate at pressure altitudes up to 12000 ft for short periods to complete data runs as required.
• Average temperature profiles experienced at the home establishment are between ISA — 20°C and ISA + 15°C.
• Data gathering will not start below pressure altitudes of 2000 ft or above pressure altitudes of 10 000 ft.
• Referred rotor speeds will be selected using an ISA atmosphere as this is most representative of summer conditions at the test establishment.
From the test specification, it is possible to determine the range of referred parameters required to address the test objectives, see Table 3.2 and Table 3.3. Likewise the range of referred parameters available at the test site can be determined, see Tables
3.4 and 3.5. Comparing the test requirements with the available referred parameters reminds us that since rotor speed affects referred weight it will be impossible to target the maximum possible referred weight and rotor speed simultaneously. Therefore, a lower referred weight will have to be targeted for tip effects testing.
The interdependency of the test parameters requires an integrated approach to the selection of test parameters. Start by selecting a maximum referred weight of 7000 kgf for tip effects evaluation. This weight is the approximate mean referred weight available at 10000 ft. From the variation of referred rotor speed at 10000 ft, 1.00, 1.04 and 1.08 can be selected as the target referred rotor speeds. By assuming a fuel burn per data run (say 450 kgf) and an ISA atmosphere, the altitudes required to set the desired W/am2 can be identified, see Table 3.6. (Note that W/am2 multiplied by (m/V9)2 equals W/8, therefore a pressure altitude can be identified for any actual mass.) It can be seen that these targets are impractical since a pressure altitude above 12 000 ft is required to reduce the relative density as the rotor speed is increased to give the higher referred RRPM. Likewise as the altitude is raised to account for fuel burn the rotor
Table 3.3 Operational variation of referred rotor speed (%).
speed must be reduced in order to keep the desired referred RRPM. A revised test matrix can be developed but it will inevitably be more conservative, see Table 3.7.
From Table 3.7 it can be seen that a referred weight of 6300 kgf can be tested at three referred rotor speeds (1.01, 1.03, 1.05). On the assumption that tip effects are not present, additional referred weights to test at a fixed mean referred RRPM, say 1.03, can be selected. Table 3.8 shows a suitable range of referred weights.