Experimental Determination of Unsteady Propeller Forces 

 0.8 



0.5 0.6 0.7 0.8 0.9 1.0 I.I 1.2 1.3 



Advance Coefficient, J = — f: 

 ' nD 



Fig. 22 - Blade -frequency torque 



thrust and torque should lead the velocity by 180° for a narrow-bladed propeller. 

 The effect of blade width shown on these figures is apparently due to the effec- 

 tive line of encounter being shifted forward for the wider blades and aft for the 

 skewed blades. 



The tandem set was tested in the three-cycle wake, so that the only signifi- 

 cant unsteady components were thrust and torque. Titoff and Biskup (13) have 

 reported bending moments for tandem propellers. The type of flow they used 

 was apparently more complex and included frequency components that excited 

 these moments. 



Figure 29 shows the unsteady thrust developed by the tandem propellers in 

 the three-cycle wake at design loading. When the blades of the two propellers 

 are aligned with each other, a strong blade-frequency thrust component is pro- 

 duced. As the angle between the two propellers is changed, the blade-frequency 

 thrust is reduced and has a minimum value when the forward propeller lags the 

 after by approximately 60° or half the blade spacing of one propeller. The 

 higher harmonics of blade frequency also show an effect of blade position. This 

 blade -position effect is mostly due to the fact that the three -cycle wake also has 

 small sixth-, ninth-, and twelfth-harmonic components. This figure also shows 

 the blade -frequency thrust component for the single three-bladed propeller, 

 which has an EAR of 0.60, equal to the total EAR of the tandem set which was 

 designed for the same operating conditions. The curves in this figure have been 

 drawn through the points obtained from the computer analysis. They are con- 

 sidered more reliable than the on-the-spot analysis, since they represent aver- 

 ages for 200 shaft revolutions. Also, the values read from the on-the-spot 



275 



