Dynamites of Naval Craft - System Identtftcatton 
Various procedures were used to carry out the required integrations 
of heave acceleration to obtain velocity and displacement records, 
and the sampling times, integration time step, and numerical inte- 
gration techniques used had to satisfy certain requirements for use 
in system identification work. Similarly it was found necessary to 
have proper accelerometers that provided data relative toa ''true" 
vertical, rather than with reference to the craft body axes, which 
requires a properly aligned accelerometer system, i.e. a ''stable 
table''. In addition, accurate data on the craft weight, CG location, 
moment of inertia, etc. appropriate to the actual test condition in 
full scale must be available so that the precision of system identifi- 
cation can be fully realized using the representative mathematical 
model. 
Asa result of the analyses illustrated here, it appears 
to be possible to determine the numerical values of certain stability 
derivative coefficients in a simple linear mathematical model charac- 
terizing vertical plane motions of SES craft. Some coefficients 
have been shown to have negligible influence on different modes of 
motion as a result of the identification analyses, thereby verifying 
similar indications from separate simulation studies. While the 
more simple hydrostatic-type stability derivatives have been found 
with consistent values close to those from theoretical predictions, 
the damping-type terms are shown to be affected by force contribu - 
tions that probably arise from the seals, It is thus necessary to 
include additionel degrees of freedom in the mathematical model 
for vertical plane motion to represent seal motion and seal forces 
that are transmitted to the craft. 
Other requirements for successful system identification 
are that the data should be sampled, for use in digital processing, 
ata rate equal to 10 times as fast as the highest frequency of in- 
terest in the expected vehicle response, and possibly faster depend- 
ing upon the number of integration stages to be applied for determin- 
ing state variable trajectories. On this basis, and with the proper 
mathematical model (including seal dynamics) it can be anticipated 
that successful estimation of the important stability derivatives for 
SES craft vertical motion can be achieved using measured data ob- 
tained during tests in a random seaway. 
Another application of this particular technique was made 
to the case of a hydrofoil craft, using data generated on a digital 
computer. In that case the equations were exactly those given by 
Equations (14) - (20), with the addition of random forcing functions 
on the right hand side of Equations (14) and (15), which were 
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