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SOME BASIC POINTS ABOUT PROPS



In the office of a colleague sits an industrial-sized fan with short and stubby blades that are as wide as they are long. Turn it on, and this electric-powered device, switched to its highest setting, pushes so much air you’d swear it wants to take flight. In fact, it actually sounds like a Cessna 150 starting its takeoff roll. Its blades are an interesting shape (like what you’d find on a marine vessel) and are so designed so as to push a large volume of air from the compacted area that its design permits. The reason any propeller, be it for an aeroplane or an office fan, is designed a given way is the result of a multitude of factors worthy of a doctoral dissertation. Some of the basics about propellers, though, can be easily overlooked. Here’s a refresher as to what some of it’s about.

The purpose of a propeller is to convert the torque, or rotational motion, of an engine’s crankshaft into forward motion and speed. Fundamentally, its design is that of a rotating airfoil that generates thrust in the same way that a wing’s airfoil section generates lift. As it rotates, a propeller advances through the air like a corkscrew and, in so doing, it pushes air backward causing an opposite reaction that propels the aircraft forward.

Like wings, propellers are designed for particular flight conditions. Look at any typical propeller and you’ll notice it tapers from root to tip. This twist is selected to optimize performance. Just like an aircraft’s wing, a propeller meets on-coming air at an angle of attack. Since the tangential velocities of the propeller’s airfoil section increase with distance from its hub, thrust must be made to be uniform along the length of the propeller. Thus, the twist in a propeller’s airfoil section progressively reduces its angles of attack from root to tip to achieve this desired uniformity.

The distance a propeller travels forward in one revolution is known as pitch. The angle at which the propeller blade is set – generally referenced to be at 75% of its radius – governs the pitch. A coarse pitch means that the blade is set at a large angle. A fine pitch means that the blade is set at a small one. A propeller set with a coarse pitch will travel forward a greater distance with each revolution then will one set with a fine pitch. A propeller with a coarse pitch will also move forward at greater speed for a given rpm. A propeller set with a fine pitch develops higher rpm, thereby enabling the engine to develop more power.

Aeroplanes with fixed pitch propellers cannot have their blade angles adjusted by the pilot. Thus, their blade angle is always a compromise for all flight conditions. Aircraft with variable pitch propellers may have their blade angle altered to meet varying conditions of flight. Of these variable pitch varieties, there are three types: adjustable, controllable, and constant speed. Blade angles for adjustable pitch propellers can be adjusted only while on the ground. Blades of controllable pitch propellers can be adjusted by the pilot during flight. Blades of constant speed propellers automatically adjust themselves to maintain a constant rpm as set by the pilot during flight.

Setting a propeller to a coarse pitch allows it to scoop more air. Through a combination of manifold pressure, carburetor and mixture settings, it improves the aircraft’s high speed cruise while achieving so with lower engine rpm and better fuel economy. Setting a propeller to a fine pitch gives the aircraft its best performance during take-off and climb though its efficiency is compromised in level flight owing to the smaller bite it takes from the air.

Generally speaking, the larger the propeller diameter, the more efficient the propeller will be. Its limitation in length – one obvious source being the height of the aircraft’s undercarriage – is the propeller tip speed which should be kept sub-sonic. Unlike the jet engine which moves a small mass of air backward at a high speed, a propeller moves a large mass of air backward at a slow speed. In some respects, then, our colleague’s office fan has more to do with jet engine principles than might otherwise be thought. By contrast, the propeller on the Gossamer Albatross – peddled as it was across the English Channel in 1979 – might be the most striking example of the efficiency inherent in a long propeller as it moves a large mass of air backward at a very slow speed.

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