1921-22.] Gryroscope and “ Vertical” Problem on Aircraft. 295 
plane. When the masses and are in the positions shown — that is, 
when they are perpendicular to P 2 P 2 > nearly so — the rods remain against 
the checks c^, c^, provided that the aeroplane is moving in a path which 
is strictly straight. When the aeroplane is turning slowly the condition 
that the rods should not move from to d^, respectively, is that 
oy^a should be greater than vip, where co is the angular speed of the erector, 
a the distance of one of the masses from the centre of the erector, v the 
speed of the aeroplane, and i[; its speed of turning. If vi[f exceeds co^a, one 
of the rods, say g-^^, will move over from to d^, and the corresponding lamp 
will light up. When leaves the lamp goes out, but is again switched 
on when 65 is passing over , and so on. When the value of vijj exceeds 
with the direction of reversed, the other lamp will be switched on 
and off as long as the turning motion of the aeroplane continues. Thus, 
as the erector rotates, if vij; exceeds cd^a one or other of the lamps is seen 
to flash in and out, and consequently the device may be utilised as a turn 
indicator of great delicacy. If n=10 revolutions per minute, a = 4 inches, 
'y = 100 feet per second, we have for the limiting case 100«^ = tt^ X 57*3/27, 
where i[t is in degrees per second. Hence, with the dimensions and speeds 
specified, the device is sensitive to all rates of turning exceeding ^th of a 
degree per second. This rate of turning corresponds to a banking angle of 
about frds of a degree ; the stabiliser is correct to within of a degree. 
The motors and are operated by a commutator, the component 
parts of which are attached to one of the uprights u (fig. 12 ) and the 
frame / respectively. So long as the relative shift of the uprights and 
the frame does not exceed a few degrees (the amount of the relative shift 
is, of course, determined by the construction of the commutator), current is 
supplied to the motor . When the aeroplane is on a banked turn the 
current supply is transferred to the motor m.^. Further, when the plane 
is banked in one direction a red lamp is lit up, and when the plane is 
banked in the other direction current is supplied to a green lamp. Thus 
during night flying the aeroplane may be trimmed by means of the aero- 
plane. Similarly, by utilising the relative shift of the pivoted system and 
the frame / to operate lamps, the aeroplane, during night flying, or when in 
clouds, may be trimmed in the fore-and-aft direction. 
A further method of shutting off the erector, in the presence of curved 
flight of the aeroplane, is shown in fig. 23, which illustrates an erector 
provided with four slots and four holes. Carried by the plate i (fig. 23) 
are four small electro-magnets, one for each ball. When the aeroplane is on 
a banked curve, current is passed through the coils of the electro-magnets, and 
the attraction of the cores for the steel balls locks the balls in the slots. 
