4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



Comparison of the model results with the actual performance of 

 full sized machines, show a fair analogy exists, confirming satis- 

 factorily the behavior of the models. Certain experiments indi- 

 cate that up to about 15 or 20 m. p. h. the aeroplane controls have 

 very little influence on the change of trim of the full size machines, 

 and thus practically require the full size machines to follow the trim 

 imposed by the flow of the water about the hulls, and the models 

 were set to closely approximate the " natural trim." Once planing 

 IS attained, or the same thing once " suction " is broken, the controls 

 become efifective and may be used to modify the trim. In the case of 

 D-i, however, this condition is not reached till about 39 m. p. h. 

 These experiments have also shown that the " planing " effect is 

 very sensible to improvement if the angle of the bottom is increased, 

 and as this can be brought about once planing is attained this shows 

 a further advantage for those models which plane early. 



The conclusions drawn from these and previous experiments 

 are as follows : 



a. The step should be close to the position of the center of gravity, 

 to eliminate a nosing tendency, to facilitate change of trim while 

 planing, to avoid change of balance when getting away or landing. 



b. Hollow V sections keep the spray down, cut the water more 

 easily and cleanly, plane better, and greatly reduce shock on land- 

 ing or when ploughing through broken water, and practically elimin- 

 ate the necessity of shock absorbers. 



c. A shallow step is sufficient, but ventilation is essential to facili- 

 tate the breaking of suction effects. 



d. The bottom forward of the step should be inclined to the axis 

 of the machine, but 



e. The inclination must not be so great as to cause planing before 

 the controls are effective, and this is particularly necessary when 

 running before the wind. If the planing of the hull is too pro- 

 nounced, the machine rises to the surface with but very little control 

 available to maintain balance, and when running before the wind 

 this is more apt to occur due to the higher water speed necessary 

 before the machine can take the air. 



f . The bottom abaft the step should rise strongly as this favors a 

 steepening of the planing bow before suction is eliminated, and gets 

 the tail well clear when planing begins. 



PLATES 2 AND 3 

 Plate 2 shows the logarithmic plots of the resistance of the pre- 

 ceding models towed submerged at speeds up to 15 knots; and also 

 for model 1350-15, a quarter sized model of the original Curtiss 



