Page 36 - Electric Drives and Electromechanical Systems

28 Electric Drives and Electromechanical Systems

Suppling the aircraft with a variable frequency supply (normally in the range
300e800Hz) and convert a proportion to either a fixed frequency supply or d.c. as
required. In any aircraft a substantial proportion of the electrical supply goes on
heating (galley ovens, de-icing), which does not require a constant frequency
supply.
Flying surfaces actuators of civil aircraft are conventionally powered through three
independent and segregated hydraulic systems. In general, these systems are heavy,
complex to install and costly to maintain. Electrically powered flight systems are not
new: several aircraft developed in the 1950’s and 60’s incorporated electrically actuated
control functions, however they were exceptions to the general design philosophy of the
time.
In the more- or all-electric-aircraft the distribution of power for flight actuation will
be through the electrical system, as opposed to the currently used bulk hydraulic system.
It has been estimated that the all-electric-aircraft could have a weight reduction of over
5000 kg over existing designs, which could be converted into an increase in range or a
reduction in fuel costs. To implement power-by-wire, high-performance electrically
powered actuators and related systems are required, (Howse, 2003). Electrically powered
flight actuators can take one of two principal configurations, the electromechanical
actuator with mechanical gearing, and the electrohydrostatic actuator, or EHA, with
fluidic gearing, between the motor and the actuated surface.
In an EHA, hydraulic fluid is used to move a conventional hydraulic actuator, the
speed and direction of which are controlled by the fluid flow from an electric motor
driven hydraulic pump, (Crowder and Maxwell, 2002). If a displacement pump (see
Fig. 1.16) is used, where the piston’s diameter is d p and the pitch diameter is d pp , the flow
rate Q(t) as a function of the pump speed, u p (t) can be determined to be,

QðtÞ¼ Du p ðtÞ (1.8)
where the pump constant, D, is given by,
2
pd d pp tana
p
D ¼ (1.9)
4
Hence the flow rate in a variable-displacement pump unit can controlled by
adjustment of the swash plate angle, a, and hence piston displacement. In this approach
two motor-drives are required, a fixed speed drive for the pump, and a small variable-
speed drive for positioning the swash plate. A different approach is just to control the
rotational speed of a displacement pump, u p , where a is fixed this design only requires
the use of a single variable-speed motor drive.
Fig. 1.17 shows a possible concept for an electrohydrostatic actuator suitable for medium
power surfaces, such as the ailerons. In most future designs the fixed pump option will be
consideredfor the rudder, whichrequiresa farhigherpoweroutput.Inthe actuatorthebasic
hydraulic system consists of the pump, actuator, and accumulator. Valve A ensures that the
low-pressure sides of the pump and actuator are maintained at the accumulator’s pressure,