1921-22.] Gyroscope and “ Vertical” Problem on Aircraft. 269' 
small in comparison with the periodic time of the gyroscopic pendulum. 
It will be clear that if the pendulum is to meet with success it must have 
a real periodic time of precessional motion, or virtual periodic time in 
the presence of curvilinear motion on the part of the aeroplane, which 
is upwards of 1 hour. The view is widely held (see various papers 
published in the Proceedings of the Advisory Committee for Aeronautics 
and the Proceedings of the Aeronautical Research Committee) that pro- 
vided the periodic time of a gyroscopic pendulum exceeds 4 minutes, the 
aeroplane may manoeuvre 
without serious errors being 
introduced. Such pendti- 
lums are tiseless. 
If such a pendulum as 
that described above is 
placed on an aeroplane we 
see that it is deflected from 
the vertical by motion in a 
curved path, assuming it to 
be upright initially. When 
the aeroplane resumes 
straight flight, following 
on a turn, the gyroscopic 
pendulum precesses so that 
its axis of spin traces out 
a cone, the semi-vertical 
angle of which diminishes 
gradually as a result of pivot friction. If the pivots are good the axis will 
approach the vertical very slowly indeed, and consequently much time 
must elapse before the axis may be regarded as lying even approximately 
in the true vertical. 
To get over this difficulty many exponents of gyroscopics have advo- 
cated the provision of a dashpot, or the equivalent. One member of the 
dashpot is attached to the aeroplane and the other to the|pendulum ; or a 
vessel containing oil, or other viscous liquid, is mounted on the pendulum, 
and the construction is such that, as the pendulum precesses, the potential 
energy possessed by the pivoted system, when deflected, is dissipated in 
heat within the oil as a result of the precessional motion. Attached to 
the casing of the gyroscope g (flg. 1) are two pairs of reservoirs, each pair 
being connected together by means of a tube having a narrow bore. One 
pair of tubes lies in the line of P 2 P 2 ^i^d the other in the line of PiP^. The 
7” 
7 ^ 
