Aeroplane Construction and Operation

by John B. Rathbun

A uniform pitch propeller has a varying blade angle, smallest at the tip and increasing toward the hub. With a uniform pitch propeller, every part of the blade travels through the same forward distance in one revolution, hence it is necessary to increase the angle toward the hub as the innermost portions travel a smaller distance around the circle of rotation. Theoretically, the angle at the exact center would be 90 degrees. The blade angles at the different points in the length of a uniform pitch propeller are obtained as follows: Draw a right angle triangle in which the altitude is made equal to the pitch, and the base is equal to 3.1416 times the propeller diameter. The angle made by the hypotenuse with the base is the blade angle at the tip. Divide the base into any number of equal spaces and connect the division points with the upper angle. The angles made by these lines with the base are the angles of the different blade sections.

Blade Form. The blade may be either straight-sided or curved. In the latter case the most deeply curved edge is generally the entering edge, and the maximum width is about one-third from the tip. Much care is exercised in arranging the outline so that the center of pressure will not be located in an eccentric position and thus harmfully distort the blade when loaded. If this is not attended to, the pitch will vary according to the load. In one make of propeller the blade is purposely made flexible so that the pitch will accommodate itself correctly to different flight speeds and conditions. This, however, is carefully laid out so that the flexure is proportional throughout the blade to the changes in the load.

The Lang Propeller, Having Straight Edges, Slightly Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for Protection Against Spray. This Outline Is Often Known as the "Normale." Type From the French Propeller First Using This Outline.

A "Paragon" Propeller With a Curved Leading Edge. The Maximum Width Is About One-Third the Blade Length from the Tip and §o. Toward the Tip So That It Is Very Narrow at the Outer End. The Steel Propeller Flange Is Shown in Place on the Hub.

Propeller Diameter. The largest propellers are the most efficient. The propeller should be as large as can be safely swung on the aeroplane. Large, slow revolution propellers are far superior to the small high speed type. It is more economical to accelerate a large mass of air slowly with a large diameter than to speed up a small mass to a high velocity. The diameter used on any aeroplane depends upon the power plant, propeller clearance, height of chassis and many other considerations. Approximately the diameter varies from about 1/3 the span on small speed scouts, to 1/5 or 1/6 of the span on the larger machines.

Air Flow. The greater part of the air is taken in through the tips, and is then expelled to the rear. This condition prevails until the blade angle is above 45 degrees, and from this point the flow is outward. Owing to the great angles at the hub, there is little thrust given by the inner third of the blade, the air in this region being simply churned up in a directionless mass of eddies. At the tips the angle is small and the velocity high, which results in about 80 per cent of the useful work being performed by the outer third of the blade. In some aeroplanes a spinner cap is placed around the hub to reduce the churning loss and to streamline the hub. The blade section is very thick at the hub for structural reasons.

The "Disc area" of a propeller is the area of the circle swept out by the blades. It is the pressure over this area that gives the thrust, and in some methods of calculation the thrust is based on the mean pressure per square foot of disc area. The pressure is not uniformly distributed over the disc, being many times greater at the outer circumference than at the hub. The average pressure per square foot depends upon the blade section and angle. Because of the great intensity of pressure at the circumference, the effective stream is in the form of a hollow tube.

Number of Blades. For training, and ordinary work, two-bladed propellers are preferable, but for large motors where the swing is limited, three or four blades are often used. A multiple-bladed propeller absorbs more horsepower with a given diameter than the two-blade type. In general, a four-bladed propeller revolving slowly may be considered more efficient than the two-blade revolving rapidly. Where the swing and clearance are small, a small four-blade may give better results than a larger and faster two-blade. A three-blade often shows marked superiority over a two-blade even when of smaller diameter, and the hub of the three-blade is much stronger than the four-blade, although neither the three or four is as strong as the two-blade type.