Propeller

“ The propellers were swung into action, hesitated for a moment, then wheezily subsided.
The incantation was repeated; the propeller blades coughed, and leaped into a deafening roar. The mechanics sprang aside, and the machine, stumbling forward for a few yards, turned into the wind. There was a sudden acceleration of the propeller, a crescendo from the engines, and the machine made swiftly across the field, rising as it attained flying speed, and disappearing into the night. ”

“ But when it’s cloudy we have to figure our position by dead reckoning. We know by our compass which way we are heading. We can tell by the number of revolutions of our propeller how fast we ought to be moving. We have an apparatus fastened to the taffrail that drags in the sea a good many fathoms behind us, and that turns like a propeller. ”

“ The Propeller.—The propeller or “screw,” by screwing its way forward through the air, is able to propel the airplane at the desired velocity. Regarding principles of propeller action the matter can be hastily summarized in the following brief lines. The propeller blades may be regarded as little wings moving in a circular path about the shaft; and they have a lift and drift as do the regular wings. ”

“ The propeller was beating hard and he opened the throttle wide. He felt her move and tremble, as if she struggled in the grip of the sand, and then lift buoyantly. The water that pressed her down had rolled off the deck and the oncoming comber had picked her up and was carrying her along. ”

“ This propeller was a patent of the late Sir Edward Harland, senior partner of the firm by whom the Britannic was built. In long ships the pitching in a heavy sea, and the vertical motion of the waves, tend to expose the upper portion of the propeller, the evil effects arising from this being the “racing” of the engines and its attendant dangers, together with a diminished speed. ”

“ On the Marine Propeller.—The marine propeller, which has already been made the subject of comparison, is of especial interest as representing in a concentrated form the experience of over half a century. Beyond the advantages to be gained from an examination of established practice as a quantitative check on our investigation, there is some reasonable probability that we may arrive at facts having a bearing on the future evolution of screw propeller design. ”

“ It is not necessary to remark that the propeller’s function is, by means of its revolutions, to drive the ship ahead, and to overcome the resistance which encounters the hull. Besides the skin friction, the eddy-making, and the wave-making, there is also the resistance of the air. Now let us suppose for a moment that instead of propelling itself ahead by its own engines and screws, a liner were to be taken in tow by a powerful tug-boat. ”

“ A, B, blank from which propeller is shaped; P, P, pencil lines at centre of bend; C, D, sections of blade at points opposite; E, G, propeller after twisting; H, view of propeller endwise, showing outward twist of tips; also shaft.
As the propeller is the most difficult part to make, it is best to begin with it. The flat blank is cut out of the whitewood, and subjected to the action of steam issuing from the spout of an actively boiling tea-kettle. The steam must be hot; mere vapor will not do the work. ”

“ A screw propeller operates on exactly the same principle as an electric fan, and ships may have one or more of these [Pg 103] propellers, which are fastened to shafts projecting through the hull beneath the water at the stern. If the ship were tied up strongly to a pier, so that it could not move, and the propellers were turned by the engines, the result would be to set in motion a column of water away from the propeller just as an electric fan sets in motion a column of air. ”

“ This propeller is driven in the ordinary manner by steam-engines of ordinary construction. Near the bow of the boat I place another steam-engine, driving what I call the 'mud-fan,' which projects from and in front of the bow of the boat. This is formed by a set of revolving blades shown at A, turned, like the propellers, by a shaft passing through a stuffing-box, D. The blades are shaped somewhat like those of a propeller, but they are sharper on their fronts and less inclined on their faces. ”

“ The fact is that propeller results depend largely upon the nature of the aeroplanes on which they are used. A propeller, for instance, which gives excellent results on one type of aeroplane, will not work satisfactorily on another. ”

“ For instance, the number of revolutions may be increased for the smaller propeller, thus engaging more air than the larger one at a lower speed; and, in practice, it is possible to run a small propeller at a speed that would not be safe for a large one. Another factor is the pitch of the propeller, which may be described as the distance the hub of the propeller would advance in one complete revolution if the blades moved in an unyielding medium, as a section of the thread of an ordinary bolt moves in its nut. ”

“ This angle of the blades causes them to force water back as they cut through it when the propeller is revolving. This forcing of the water back tends to produce a forward motion of the propeller, and in this way the boat on which the propeller is mounted moves through the water. The propeller is [45] caused to revolve by a steam-engine, steam-turbine, or gasolene-engine, as shown in Fig. 35 . Longer boats have more than one propeller. A boat that has two propellers is called a twin-screw boat. A boat driven with four propellers is called a quadruple-screw boat. ”

“ Place the propeller in a horizontal position, and it should remain in that position. If a weight of a trifle over an ounce placed in a bolt-hole on one side of the boss fails to disturb the balance, then the propeller is usually regarded as unfit for use. ”

“ On being dragged through the water the propeller spins round and the twisting action is communicated by the cord to the clock-work machinery which counts the miles. In the case of powerful steamers and in ordinary weather dead-reckoning is very accurately calculated by the number of revolutions of the propellers recorded in the engine-room ”

“ The making of the propeller is practicable for a careful workman. The illustrations will give a better idea than words of how it should be done. It should be remembered, however, that the safety of the aviator depends as much upon the propeller as upon any other part of the machine. The splitting of the blades when in motion has been the cause of serious accidents. The utmost care, therefore, should be exercised in the selection of the wood, and in the glueing of the several sections into one solid mass, allowing the work to dry thoroughly under heavy pressure. ”

“ A net which covered the envelope of the balloon supported a spar, 66 feet in length, at the end of which a triangular sail was placed vertically to act as rudder. The car, slung 20 feet below the spar, carried the engine and propeller. Engine and boiler together weighed 350 lbs., and drove the 11 foot propeller at 110 revolutions per minute. ”

“ Although an accident to my rudder at the last moment prevented me from making a free ascent before these men of science I, nevertheless, held my own against a very strong wind that was blowing at the time, and gave what they were good enough to proclaim a satisfying demonstration of the effectiveness of an aerial propeller driven by a petroleum motor. ”

“ PROPELLERS WITH VARYING PITCH.—It is growing more apparent each day, that a new type of propeller must be devised which will enable the pilot to change the pitch, as the speed increases, and to give a greater pitch, when alighting, so as to make the power output conform to the conditions. ”

“ Q.--Seeing that the vessels are the same in all respects except as regards the propellers, and that one of them exhibits a superiority, does not this circumstance show that one propeller must be more powerful than the other? ”

“ Her propeller is of a novel pattern, serving as steering-gear and propelling power. This vessel was originally fitted with a propelling apparatus known as the Fowler steering-wheel, a novel propeller which both gave the vessel headway and steered her. The blades of the propeller were worked by means of a simple steam-valve arrangement manipulated by the helmsman. The steering qualities with this apparatus were little short of marvellous, as she could be driven and steered with as great facility astern as ahead, and could even be worked sideways. ”

“ The motor, which was air-cooled, was located forward, and was just above the heads of the operator and the passenger who sat beside him. The single propeller, which was ten feet in diameter, gave a minimum thrust of one thousand pounds at two thousand revolutions per minute. ”

“ Fig. 59.—Casting Cylinder of a Marine Steam Engine.
The next figure, 60, exhibits a very common form of the screw propeller, and shows the position which it occupies in the ship. The reader may not at once understand how a comparatively small two-armed wheel revolving in a plane perpendicular to the direction of the vessel’s motion is able to propel the vessel forward. In order to understand the action of the propeller, he should recall to mind the manner in which a screw-nail in a piece of wood advances by a distance equal to its pitch at every turn. ”

“ A small propeller revolving very rapidly, or racing, is likely to give the model a torque, even if it be otherwise well proportioned. Don’t try to remedy this with rudder surfaces, but change your propeller, or your motor, or both. ”

“ When approaching shallow water, the propeller was raised by steam-power to the ordinary position without any necessity for stopping the engines during the operation. Although there was an increase of speed by this means through the uniform revolutions of the machinery in the heaviest sea, yet there was an objectionable amount of vibration at certain parts of the vessel, so that we found it necessary to return to the ordinary fixed propeller, working in the line of direction of the vessel. Comfort at sea is of even more importance than speed ”

“ The propeller is a particularly difficult piece of laminated work, being built up of from twelve to eighteen layers of thinly cut wood, while the upright posts of the central section are made up of ash and spruce, the heavier and more flexible wood forming the core. A feature of strength is to be found in the double trussing which is placed in all of the vital parts of the aeroplane, where the greatest strength is required. All this trussing is made with a cable of galvanised steel wire tested to withstand a pulling strain of nearly half a ton. ”

“ The stern also, as well as the stem, is iron-plated, and the rudder post, which is the Achilles’s heel of an Arctic ship, is of unusually strong construction. The rudder is so arranged that it can be hoisted on deck out of the way of the ice if necessary. The propeller is so arranged that it can be used either as a two-bladed or a four-bladed propeller, and is made of unusual strength. Powerful deck appliances in the shape of windlass, steam capstans and winch, enable the ship to warp herself out of a dangerous place, or pull herself off the bottom should she get aground. ”

“ Every professional aviator has his own ideas as to the design of the propeller, one of the most important features of flying-machine construction. While in many instances the propeller, at a casual glance, may appear to be identical, close inspection will develop the fact that in nearly every case some individual idea of the designer has been incorporated. ”

“ People supply the power for the propeller. They push handles back and forth. Even on this most modern ship in the world, there are boats that move in the oldest way—by muscle power.
The four propellers of the liner herself are each as tall as a two-storey house. They are turned by enormous steam turbine engines. Smoke from the boilers goes out through unusual-looking stacks. Inside each one are giant filters that take away most of the soot. Besides, there are wings called vanes at the top of the stacks to help keep the smoke from swirling down onto the deck. ”

“ When your propeller is too small the machine will not rise from the ground, or, if launched in the air, will quickly flutter to earth. If the model on leaving your hand, with the propeller in full motion, fails to keep its position from the very start, the blade should be made larger. ”

“ Forward, almost exactly like the front set of wings of the dragon fly, was the large, main plane, with the concave turn toward the ground. There was the usual propeller in front, operated by a four cylinder motor, the cylinders being air cooled, and set like the spokes of a wheel around the motor box. ”

“ The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. ”

“ Of these, (1) alone is to be regarded as a propeller loss; (2) is more correctly to be considered as a propulsion loss, for it is inevitable in the theory of propulsion and is not dependent on the instrument, whether it be a screw propeller or what not. ”

“ Thus we find that in aviation man has designed the propeller, which, by its rapid revolutions derived from the motive power of the aerial engine, cuts a spiral pathway through the air and drives the light craft rapidly forward. The chief use of the planes is for support to the machine, and the chief duty of the pilot is to balance and steer the craft by the manipulation of the rudder, elevation and warping controls. ”

“ Pitch (Propeller) .—The distance forward that the propeller would travel in one revolution, if there were no slip, that is, if it were moving in a thread cut at the same inclination as the blade. Pitch angle refers to the angle of inclination of the propeller blade.
29. Slip.—Slip is the difference between the actual travel forward of a screw propeller in one revolution and its pitch.
30. Dope.—A general term applied to the material used in treating the cloth surface of airplane members to increase strength, produce tautness, and act as a filler to maintain air and moisture tightness. ”