Comparison of Theory and Experiment on Ducted Propellers 



pressure jump to represent the propeller. The experimental scatter shown in 

 Fig. 16 was also present in the data for Fig. 17, but only the average value is 

 plotted in this figure. 



Figure 16 presents the theoretical and experimental duct pressure distribu- 

 tions for a propeller thrust coefficient Cj. of 0.220 and a speed coefficient j of 

 0.617, and Fig. 17 presents data for c^^ = 0.276 and / = 0.628. Generally, the 

 agreement between theory and experiment is satisfactory for the outside of the 

 duct but not satisfactory for the inside. This conclusion also holds for other 

 speeds of advance for this ducted system (19, 21). The calculations of Menden- 

 hall and Kriebel give a slightly better prediction on the inside of the duct than 

 those of Hough and Kaskel. It can be observed in Fig. 16 that the nonlinear cor- 

 rection to the duct surface velocity used by Mendenhall and Kriebel overcor- 

 rects the velocities on the inside of the duct forward of the propeller. Hough and 

 Kaskel observed in making their calculations (21) that a better prediction could 

 be made if a different cylinder diameter were used for the inside of the duct 

 from that used for the outside. 



For the ducted propeller in static operation, experimental and theoretical 

 pressure distributions are presented in Fig. 18 as obtained by Hess and Smith 

 (23), using a nonlinear theory. This figure compares calculated and experimen- 

 tal distributions on the forward portion of the duct only, as the pressure distri- 

 butions behind the propeller cannot be calculated by the Douglas method (23), 

 The predicted pressures are very satisfactory indeed. Kriebel and Mendenhall 

 (19) have made theoretical predictions of duct pressure distribution for a ducted 

 propeller in static operation which was a model of the Doak V2-4DA ducted sys- 

 tem. This unit consists of a duct with a chord-diameter ratio of 0.608 and a 

 profile -thickness ratio of 0.158, an eight-bladed propeller with pitch of 15° at 

 the tip, a set of seven inlet guide vanes, and a set of nine stators aft of the pro- 

 peller. At the static condition, the pressure distribution predictions from lin- 

 earized theory (19), Fig. 19, are very satisfactory for the outside of the duct, 

 are marginal for the inside of the duct forward of the propeller, and are unsat- 

 isfactory aft of the propeller. It is apparent that the contributions of the propel- 

 ler, guide vanes, and stator blades are not adequately considered. 



Fig. 18 - Pressure distribution 

 on the Douglas duct for the static 

 condition (23) 



1331 



