The Art of the Helicopter John Watkinson - Karatunov.net

302 The Art of the Helicopter
Fig. 7.39 Two’s complement addition of bipolar signals for control mixing.
extra count will result in 10 000 (16 decimal) requiring five bits. Clearly there is no
facility to handle the leading 1 in a four-bit system and so the remaining four bits will
all be zero. Consequently the count starts again. This gives a finite word length counter
a circular structure allowing it to represent the operation of an angular sensor. Using
finite word length binary, angular parameters such as magnetic heading, variation, and
track angle can be incorporated in navigational computations.
7.21 Power-assisted controls
Figure 7.40 shows the interior of an hydraulic power-assisted ram. The output rod
carries a swinging arm operating the error valve. The input is connected to the arm
by a ball joint. The error valve contains a pair of discs and is called a spool because
it resembles a film spool. The spool controls the flow of fluid to the cylinder down
passages in the pushrod. If the pilot moves a control such that the input moves to the
left, the spool will slide left and the left disc will admit fluid under pressure to the
right-hand side of the piston. The right disc will allow fluid from the left-hand side of
the piston to return to the reservoir. The piston will move to the left under hydraulic
pressure, but in doing so it will move the upper end of the input lever to the left, and
will close the spool valve. In servo terminology, the linkage driving the spool valve is
comparing the actual load position with the desired position to produce an error.
As a result of the error the output of the ram is made to follow the input. If the
input reverses, the spool valve moves the other way and admits fluid to the other side
of the piston. As the pressure is so high, only tiny discrepancies need exist between
the input and output position for fluid flow to occur and so there is considerable loop