ae ee en ee a 
392 Alex Goodman 
vertical plane will be used to describe the principles of the system. The system of axes as 
well as the symbols and coefficients used in this section have been defined in the notation 
at the beginning of this paper. 
The kind of motion for static coefficients is commonly used by wind tunnel and model 
basin facilities and, therefore, dces not need to be explained in detail. Figure 2 schemati- 
cally represents this type of motion. The components are given with respect to a body-axis 
system with the origin at the center of gravity, CG. 
0 (@=a) 
Fig. 2. Straight-line pitched motion for steady-state tests 
The system produces this motion by using a towing carriage to tow the model in a 
straight path at constant velocity. Discrete pitch angles for each run are set by a tilt table 
which supports the model through a pair of twin struts. Control surface angles are also set 
discretely for each run. Forces are measured by internal balances at each of the two struts 
to obtain static forces and moments. 
The unique feature of the DTMB Planar Motion Mechanism is the kinds of motions pro- 
duced to enable the explicit determination of the rotary and acceleration coefficients. Si- 
nusoidal motions are imparted to the model at the point of attachment of each of the two 
towing struts while the model is being towed through the water by the carriage. The mo- 
tions are phased in such a manner as to produce the desired conditions of hydrodynamically 
“pure heaving” and “pure pitching.” It is possible also, if required for any reason, to pro- 
duce various combinations of pitching and heaving. Figure 3 illustrates various types of 
motions including (a) the type of motion usually associated with oscillators, (b) pure heav- 
ing, and (c) pure pitching. The latter two are the basic motions associated with the DTMB 
Planar Motion Mechanism. The pure rolling motion is not illustrated but consists simply of 
oscillations about the longitudinal body axis. 
The oscillator motion depicted by Fig. 3a is actually a combination of pure pitching and 
heaving motions. Since the CG is constrained to move in a straight path while the model, 
which oscillates in a see-saw fashion, assumes sinusoidally varying angles of attack and 
pitch angles. As a result a mixture of static, rotary, and acceleration forces and moments 
is produced. It becomes necessary, therefore, to perform a similar oscillation about a second 
reference point. The two oscillation conditions together with the static tests provide data 
which can be used to separate the hydrodynamic coefficients. The solution of simultaneous 
equations involved in this process, however, could lead to errors because of the wide differ- 
ences in magnitude between the various individual coefficients. The oscillator type of 
