408 Alex Goodman 
Fig. 23. Digital indicator 
Instrumentation for Static and Dynamic Stability Tests — A comparison of the old and 
new measuring and recording systems, presented in Figs. 21 and 22, respectively, shows 
that both systems are essentially the same for the static stability tests. The major differ- 
ence is in the method and equipment used in the dynamic tests. For completeness, how- 
ever, the measuring and recording equipment used in the static and dynamic tests, shown by 
the block diagram in Fig. 22, will be described. 
The recording equipment for the static stability tests is a digital system designed to 
display and read out the unique steady-state value of each force and moment sensed by the 
transducers for any given test condition. The system is made up of seven channels to con- 
form to the number of gages in the model. Each channel is separate in all respects except 
for the power supply that it shares in common. 
Briefly, each channel is essentially an automatic null-balancing system; the transducer 
in the gage and the digital indicator combine together in a servo system. ‘The transducer 
output is balanced by a potentiometer. When a gage in the model is deflected, the resulting 
error signal from the transducer is amplified and drives a servo motor which positions a 
potentiometer to restore electrical balance, or null, to the system. The amount that the 
potentiometer is moved is then a measure of the force or moment applied at the gage. The 
various components and circuitry which constitute the recording systems are shown by the 
block diagram in Fig. 22. 
The upper path of Fig. 22 applies to the digital system used for static stability tests. 
A Brown recorder could be used in place of the digital system. The term gage is used to 
denote the variable reluctance transducer whether it be the magnigage used with the modular 
force gages or the magnitorque used with the roll gages. The 400-cycle power source sup- 
plies a 4.5-volt carrier to the gage in such a manner that the current divides into two paths, 
one about each coil. If the core of the gage is electrically centered, the impedances of the 
gage halves are equal, and consequently the voltages are equal. As the core is displaced, 
the impedance of one gage half increases and that of the other decreases with corresponding 
voltage changes. 
