Wind Tunnel Test

Last revision: May 26, 1998

Responsible Individual:
Cislunar Aerospace, Inc.: J. J. Pallis, K. K. Okamoto

Test Description: Two models of a tennis ball (one 11 inches in diameter, weighing 1 pound with fuzz and one 8.83 inches in diameter, weighing 1 pound without fuzz) have been mounted on an assembly for installation in the 48x32 wind tunnel. The model is bonded to a 1" steel rod, which is in turn mounted through a steel tube which is attached to the vertical I-beam supports outside of the wind tunnel. A hand crank is mounted at the end of the steel rod to allow turning of the ball. The estimated maximum aerodynamic load is 7 lbs.

The steel tube (mild steel, seamed, 1.25" OD, 1.010" ID) (Note - for our readers, OD means outside diameter and ID means inside diameter), acting as a cantilever, will take virtually the entire bending load from the ball subjected to the wind pressure. A simple analysis was performed to determine the bending of the tube under various loads. From the attached table the calculated load of 7 lbf will produce a maximum deflection of 0.035" in the direction of the airflow. Stresses in this section will be due to bending and are an order of magnitude below the yield strength in the material.

The shearing and compressive strength of the rigid polyurethane foam used (TAP Plastics X-30 Polyurethane Foam) to fill the ball is 20-25 psi, the tensile strength is between 25-30 psi. Conservatively, using 20 psi as the maximum permissible shearing stress, the maximum permissible load on the sting before failure would be 691 lbf. This is far above any possible load on the ball.

Therefore, the loading on the sting due to the airflow against the ball produce loads and deflections which are not significant and neither will cause failure in the materials or deflections which could affect results.

At times during the testing the tennis balls will be manually rotated at a rate of 1-3 rps, via a crank handle attached to the end of the mounting rod. At this low rotational speed there should be no additional danger of a failure in the tennis ball model setup.