416 Alex Goodman 
x 2UEBig i 2g] 
In-Phase Component of 
Normal Force Coefficient. 
O.l 0.2 0.3 04 06 O7 08 
Angular Acceleration Parameter q, 
Fig. 30. Typical curves of forces versus angular acceleration am- 
plitude from pure pitching tests used to obtain added moment of 
inertia (and associated force) 
is the rotating arm technique [12]. In this case, the body is towed at uniform angular veloc- 
ity in a circular path at different radii and the resultant forces and moments are determined 
for various body angles. The rotating arm technique has been used also to obtain the static- 
stability derivatives. In this case, the data for the various body angles are extrapolated to 
infinite radius and cross-plotted against the body angle. This is an indirect procedure and 
is generally not recommended. 
The DTMB Planar-Motion-Mechanism System determines only the linear rotary deriva- 
tives. The DTMB rotating arm, using the same models, will supplement these results. and 
provide a measure of the nonlinearities which are presently being estimated theoretically. 
A detailed description of the DTMB rotating arm facility is presented in Ref. 13. For 
the purposes of the present paper a brief description of the main components, shown in Figs. 
31 to 33, is presented herein. 
The DTMB rotating arm basin is 260 feet in diameter and 21 feet deep. The arm pivots 
in the center on tapered roller bearings designed for centrifugal forces of 145,000 pounds. 
The drive system is located at the extreme end of the arm and consists of two 250-horsepower 
direct-current electric motors, directly coupled to two steel wheels which support the arm and 
run on an outer peripheral track. Kach wheel is preloaded against the track by nested com- 
pression springs which provide a normal force of 61,000 pounds. A maximum steady-state 
speed of 30 knots can be achieved at the 120-foot radius for runs restricted to one turn. The 
arm, shown in Fig. 31, is a tabular aluminum structure weighing about 37,500 pounds and 
having natural frequencies in the vertical, horizontal, and torsional modes of greater than 3 
cycles per second. 
Submerged models are attached by a pair of towing struts to a model positioning appa- 
ratus, shown in Fig. 32. The positioning apparatus is attached to a carriage (Fig. 31) which 
can be remotely positioned along the arm to any radius from 12.5 feet to 120 feet from the 
