GEM Research in the U.S. 305 
forward motion. At low velocities a half-moon-shaped depression, concave rearward, was 
beneath the front part of the base plate, the deepest parts being situated on either side of 
the nozzle center. This deep, very clearly visible depression moved to the rear with 
increasing speed and decreasing jet momentum. A high wave directly to the rear, which at 
times almost touched the nozzle, behaved similarly. In general, it can be said that all 
observed surface phenomena moved to the rear and became less pronounced in magnitude 
with increasing forward speed and decreasing jet momentum. 
Base-plate pressures were measured at 14 piezometer holes, the number of locations 
being tripled for some runs by making two 45-degree rotations of the base plate, and isopi- 
estic lines were drawn. In the stationary cases, especially for the higher altitude, a dip in 
pressure was noticed at about 0.7 nozzle radius, the same location as an elevation in the 
water surface. For all cases of forward speed the pressures were larger in the rear than in 
front, pressures at the center and extreme rear tending to be the highest. A roughly circular 
pressure valley at about 0.7 nozzle radius was clearly discernible, culminating in a saddle 
point between the two peak pressures. The high pressures to the rear and low pressures to 
the front combined to give nose-down pitching moments acting on the base plate for all 
cases of forward motion. 
The total base-plate lift, and hence the lift-augmentation factor, for each run was 
obtained by integration of the pressure distribution. As expected, the augmentation factor 
was found to increase with decreasing altitude for all conditions. For the stationary runs 
the augmentation increased with decreasing jet momentum for a given altitude, in qualitative 
agreement with the theoretical predictions of Mack and Yen (first footnote) even though the 
surface-configuration data did not support certain of their assumptions. The numerical 
magnitudes of augmentation, however, were less than their idealized predictions. For con- 
stant altitude and jet momentum, forward speed, within the experimental range, improved the 
augmentation initially, then caused a decrease at still higher speed. This feature, particu- 
larly the increase of augmentation with increasing speed at low speeds, was more pronounced 
for the lower altitude tested. It thus appears that there is an optimum forward speed for 
each combination of altitude and jet momentum. 
The same basic 7-inch annular nozzle, but with interchangeable mouthpieces, was also 
tested statically over a ground board. Discharge angles of 0, 15, and 30 degrees inward 
with a 1/8-inch annulus width were used, as were 0- and 15-degree angles with gaps of 
5/16 and 1/2 inch. As expected, the 30-degree discharge angle gave the highest augmenta- 
tion; no significant distinction in augmentation was noted, however, between the 0- and 
15-degree angles. It was found that, within the gap range tested, the augmentation factor 
and the radial uniformity of pressure both increased with increasing gap width. This 
behavior of the former is in qualitative agreement with the theoretical prediction of Mack 
and Yen (first footnote, Fig. 1) within its range of validity (altitude less than the nozzle 
radius). 
The results summarized herein of tests conducted at the Iowa Institute of Hydraulic 
Research on annular jets moving over water and stationary over land are described more 
fully in the report to the Office of Naval Research already referred to. 
E. C. Tupper (Admiralty Experiment Works) 
Mr. Chaplin did a very good job in condensing this research work into such a relatively 
short paper and I hate to suggest more work for him, but I would like to make two points. 
