Extracts from JAR 25 - what is described here is what I have heard is the main reason for mach trims - there must be at least one pound of control wheel force to change the speed by 6 knots, i.e. pitch up to reduce speed 6 knots takes one pound.
If it takes LESS, one needs a mach trim to compensate for it.
STABILITY
JAR 25.171 General
The aeroplane must be longitudinally,
directionally k d laterally stable in accordance
with the provisions of JAR 25.173 to 25.177. In
addition, suitable stability and control feel (static
stability) is required in any condition normally
encountered in service, if flight tests show it is
necessary for safe operation.
JAR 25.173 Static longitudinal stability
Under the conditions specified in JAR 25.175,
the characteristics of the elevator control forces
(including friction) must be as follows:
A pull must be required to obtain and
maintain speeds below the specified trim speed,
and a push must be required to obtain and
maintain speeds above the specified trim speed.
(a)
This must be shown at any speed that can be
obtained except speeds higher than the landing
gear or wing flap operating limit speeds or
V„/MFc, whichever is appropriate, or lower than
the minimum speed for steady unstalled flight.
The airspeed must return to within 10%
of the original trim speed for the climb, approach
and landing conditions specified in JAR 25.175
(a), (c) and (d), and must return to within 75% of
the original trim speed for the cruising condition
specified in JAR 25.175 (b), when the control
force is slowly released from any speed within the
range specified in sub-paragraph (a) of this
paragraph.
The average gradient of the stable slope
of the stick force versus speed curve may not be
less than 1 pound for each 6 knots. (See ACJ
25.1 73 (c).)
(d) Within the free return speed range
specified in sub-paragraph (b) of this paragraph,
it is permissible for the aeroplane, without control
forces, to stabilise on speeds above or below the
desired trim speeds if exceptional attention on the
part of the pilot is not required to return to and
maintain the desired trim speed and altitude.
ACJ 25.173(c)
Static Longitudinal Stability (Interpretative Material)
See JAR 25.173(c)
The average gradient is taken over each half of the speed range between 0.85 and 1.15 Vtrim.
When manoeuvring at a constant airspeed
or Mach number (up to V„/M„), the stick forces
and the gradient of the stick force versus
manoeuvring load factor must lie within
satisfactory limits. The stick forces must not be so
great as to make excessive demands on the pilot's
strength when manoeuvring the aeroplane (see
ACJ No. 1 to JAR 25.143 (f)), and must not be so
low that the aeroplane can easily be overstressed
inadvertently. Changes of gradient that occur with
changes of load factor must not cause undue
difficulty in maintaining control of the aeroplane,
and local gradients must not be so low as to result
( f )
in a danger of over-controlling. (See ACJ No. 2 to
JAR 25.143 (0). 1
'[(g) (See ACJ 25.143(g)). The manoeuvring
capabilities in a constant speed coordinated turn
at forward centre of gravity, as specified in the
following table, must be free of stall warning or
other characteristics that might interfere with
normal manoeuvring1