Control of the referred groups

The use of the experimental method of data reduction depends on the ability to maintain at least one of the groups constant while varying the others, for example, if the relationship between P/cm3 and V/m is to be investigated whilst maintaining Wlam2 and m/70 constant. In any of the referred groups there is usually at least one parameter which is under our control and which can therefore be used to control the whole group. Table 3.1 shows each group in its referred or normalized form and the appropriate controlling parameter(s).

Table 3.1 Control of the referred groups.

Group

Referred form

Controlling parameter(s)

Forward speed

V

m

V or )

Rotor speed

m

70

) or HP

Weight

W

8

W or HP

Weight

W

cm2

W HD or )

Vertical speed

V

m

Vc or )

Hover height

Z

Z

3.2 PERFORMANCE TEST PLANNING

A typical requirement for a test team is to determine the performance of a rotorcraft under a range of environmental conditions. These are usually quoted as an altitude (X ft Hp) with a given standard atmosphere pertaining (ISA+F°С). The basic function of any performance test plan is therefore to establish sets of test points at the same referred weight (W/8 or W/am2) and/or the same referred rotor speed (m/^0). Assuming that the take-off weight is known and that the helicopter is fitted with accurate fuel-gone indications it is possible to draw up a series of tables or charts to enable in-flight adjustment of altitude to maintain the referred weight constant as fuel is burnt. In addition, if the W/8 method is being used and an accurate rotor speed indicator is fitted, it is also possible to tabulate the rotor speed adjustments required to maintain mQ0 constant as the outside air temperature changes.

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