Requisite strength and stiffness, of course, set a limit on the final narrowness of the blades, apart from other considerations.
§ 15. The velocity with which the propeller is rotated has also an important bearing on this point; but a higher speed than 900 r.p.m. does not appear desirable, and even 700 or less is generally preferable.[32] In case of twin-screw propellers, with an angle at the tips of 40° to 45°, as low a velocity of 500 or even less would be still better.[33]
§ 16. Shrouding.—No improvement whatever is obtained by the use of any kind of shrouding or ring round the propeller tips, or by corrugating the surface of the propeller, or by using cylindrical or cone-shaped propeller chamber or any kind of air guide either before or after the propeller; allow it to revolve in as free an air-feed as possible, the air does not fly off under centrifugal force, but is powerfully sucked inwards in a well-designed propeller.
| Fig. 25. A Tube of Air. | Fig. 26. A Cylinder of Air. |
§ 17. General Design.—The propeller should be so constructed as to act upon a tube and not a "cylinder" of air. Many flying toys (especially the French ones) are constructed with propellers of the cylinder type. Ease of manufacture and the contention that those portions of the blades adjacent to the boss do little work, and a slight saving in weight, are arguments that can be urged in their favour. But all the central cut away part offers resistance in the line of travel, instead of exerting its proportionate propulsive power, and their efficiency is affected by such a practice.
§ 18. A good Shape for the blades[34] is rectangular with rounded corners; the radius of the circle for rounding off the corners may be taken as about one-quarter of the width of the blade. The shape is not truly rectangular, for the width of this rectangular at (near) the boss should be one-half the width at the tip.
The thickness should diminish uniformly from the boss to the tip. (In models the thickness should be as little as is consistent with strength to keep down the weight). The pitch uniform and large.
Fig. 27.—O T = 1/3 O P.