The W/8 method
Suppose that a test flight is to be conducted using an aircraft with a take-off weight of WTO and that data is required at a referred weight (W/8) of WREF. At any point in the sortie if the fuel burn is WFUEL then the relative pressure required for the desired W/8 is given by:
WTO ~ WF1
The pressure altitude (in feet) associated with this relative pressure can be found using an inverse form of the ISA atmospheric equation [3.3]:
1 _ 8l/5.2559
6.8756 x 10 -6
Figure 3.1 shows an example of a planning chart based on these equations. Determining the effect of outside air temperature on the rotor speed required for a given referred rotor speed can be addressed in a similar manner. Suppose the standard rotor speed is )0, the referred rotor speed required (m/ V0) is mREF and the outside air temperature at the test altitude is T °С, then the relative rotor speed (m) required is given by:
T + 273.15 288.15
The actual rotor speed associated with this relative RRPM is obtained from:
) = m x )0
Figure 3.2 shows an example of a planning chart based on these equations. Note that the maximum and minimum power-on rotor speed limits might prevent the establishment of referred rotorspeeds significantly different from that depicted.
Fig. 3.1 Planning chart – W/S method.
-10 -8 -6 -4 -2 0 2 4 6 8 10
OUTSIDE AIR TEMPERATURE (°С)
Fig. 3.2 Planning chart – ra/V6 method.