Davis and English 



developing 45,100 Ibf thrust. Scaling this stress directly with thrust then gives 

 29,200 Ibf/in^ for 40,000 Ibf thrust as compared with 22,000 Ibf/in^ from the De 

 Havilland static tests. Again applying the thickness correction reduces the T95 

 stress to 24,400 Ibf/in^ as compared with 22,000 Ibf/in^. This comparison is 

 slightly closer than that in the flying condition, as might be expected, since the 

 static load distribution used was more appropriate to the takeoff condition. The 

 closeness of this T95 result and the static test result then tends to confirm the 

 view that leading edge sweepback is not as highly effective in reducing the steady 

 stress level in the leading edge vicinity as predicted in the earlier simple static 

 tests on the plane blades. It is noticeable from the NPL tests that the stresses 

 in the takeoff condition are higher than those in the flying conditions by nearly 

 30 percent. 



In concluding this section on structural considerations, it would appear that 

 blades with relatively thick leading edges are essential not only for ensuring 

 structural integrity but also for reducing the leading edge deflection and the at- 

 tendant efficiency loss. High values of the elastic modulus of the propeller ma- 

 terial are also desirable for this purpose. Blade-root stresses can also be 

 high, but it is relatively easier to control these by thickening the blades. With 

 a fixed diameter the additional factors that influence the leading edge deflection 

 and stress levels are the blade pressure distribution and to some extent the 

 geometrical blade shape, although it appears that a large improvement due to 

 sweepback is not evident. In the calm -water flying condition when the sections 

 should be operating at relatively low values of incidence, the advantage of the 

 rearward centre of pressure position of the Johnson 3 -term section, say, over 

 the circular-arc section is desirable. However, in the takeoff and more heavily 

 loaded flying conditions when the incidences are relatively large, this advantage 

 will diminish in value. 



In the Bras d'Or application when 4-ft diameter screws developing thrusts 

 of about 40,000 Ibf are used, it is expected that steady maximum principal 

 stresses no greater than 25,000 to 30,000 Ibf/in^, including centrifugal effects, 

 will be experienced. It is also anticipated that due to the relatively moderate 

 wake caused by the strut/foil assembly, the fluctuating stresses liable to cause 

 fatigue will also be well within the capabilities of the material being used. Fluc- 

 tuating propeller loading assumptions as used for design purposes are shown in 

 Table 4. 



CONSIDERATION OF FUTURE METHODS OF PROPULSION 



In 1963, at the time when it was decided to proceed with the design and con- 

 struction of the Bras d'Or, the only practical means of propelling the vehicle in 

 the foilborne mode was by means of fully cavitating propellers. Despite the ad- 

 vances that have been made with alternative means of propulsion in the inter- 

 vening period, it is doubtful if a different decision would be made today, since 

 the system that has been adopted has required the least development effort in 

 return for a relatively high propulsive efficiency. 



The propulsion system may be divided into the three components— the en- 

 gine, the Zed drive transmission, and the propeller. Considerable operational 

 experience has been obtained with marinised gas turbines, but the requirement 



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