Aeroplane Construction and Operation

by John B. Rathbun

Weight and Power. The weight lifted per horsepower varies in the different types of aeroplanes, this difference lying principally in the reserve allowed for climbing and horizontal speed. A speed scout may carry as little as 8 pounds per horsepower, while a slow two-seater may exceed 20 pounds per horsepower. A rough estimate of the horsepower required may be had by dividing the total weight by the weight per horsepower ratio for that particular type. Thus if the unit H. P. loading is 16 pounds and the total weight is 3200, then the horsepower will equal 3200/16 = 200 horsepower. Assuming that the live load w' is 0.32 of the total weight W, then W = w'/0.32. If m = lbs. per H.P., then H = W/m or H = w'/0.32m. Taking the case of a training machine where m = 20, and the live load is 640 pounds, the approximate horsepower will be: H = w'/0.32m = 640/0.32 x 20 = 100 horsepower. A speed scout carrying 320 pounds useful load, with m = 10, will require H = 320/0.32 x 10 = 100 horsepower.

CHAPTER XVIII. PROPELLERS.

Principles and Use of Propellers. A propeller converts the energy of the engine into the thrust required to overcome the resistance of the aeroplane. To maintain flight the thrust, or force exerted by the propeller, must always equal the total resistance of the aeroplane. A total resistance of 400 pounds requires a propeller thrust of 400, and as the resistance varies with the speed, the engine revolutions must be altered correspondingly. The propeller is the most complicated and least understood element of the aeroplane, and we can but touch only on the most elementary features. The inclined blades of the propeller throw back an airstream, the reaction of which produces the thrust. The blades can also be considered as aerofoils moving in a circular path, the lift of the aerofoils corresponding to the thrust of the propeller. The reactions in any case are quite complicated and require the use of higher mathematics for a full understanding.

Pitch and Velocity. When in action the propeller rotates, and at the same time advances along a straight line parallel to its axis. As a result, the tips of the propeller blades describe a curve known as "Helix" or screw-thread curve. The action is very similar to that of a screw being turned in a nut. For clearness in explanation we will call the velocity in the aeroplane path the "Translational velocity," and the speed of the tips in their circular path as the "Rotational velocity." When a screw works in a rigid nut it advances a distance equal to the "Pitch" in each revolution, the pitch of a single threaded screw being equal to the distance between the threads. Since the propeller or "Air screw" works in a fluid, there is some slip and the actual advance does not correspond to the "Pitch" of the propeller blades. The effective pitch is the distance traveled by the propeller in one revolution. The actual pitch or the angle of the blades must be greater than the angle of the effective helix by the amount of slip.

If N = Revolutions per minute, P = effective pitch in feet and V = translational velocity in miles per hour, then V= NP/88. With an effective pitch of 5 feet, and 1200 revolutions per minute, the translational velocity of the aeroplane will be: V = 1200 x 5/88=68.2 miles per hour.

Excelsior Propeller, an Example of American Propeller Construction. This Propeller Is Built Up of Laminations of Ash.

The actual pitch of the blades would be from 15 to 25 per cent greater than the effective pitch because of the slip. To have thrust we must have slip. With the translational velocity equal to the blade-pitch velocity, there is no airstream accelerated by the blades, and consequently there is no thrust due to reaction. The air thrown to the rear of a propeller moves at a greater speed than the translation when thrust is developed, and this stream is known as the "slipstream." The difference between the translational and slipstream velocity is the slip.

The angle of the blade face determines the pitch. The greater the angle of the blade with the plane of propeller rotation, the greater is the pitch. This angle is measured from the chord of the working face of the table, or from that side faced to the rear of the blade. In the majority of cases the working face is flat. The front face is always heavily cambered like a wing section, with the greatest thickness about one-third the chord from the entering edge. As in the case of the wing, the camber is of the greatest importance.