TAKING A SPIN

Perhaps, one of the most critical devices in making powered flight possible was the invention of the propeller. The aircraft flown by the Wright brothers utilized two propeller-driven engines. Later in the history of flight, propeller-driven aircraft have set a number of records, to include Lindberg's successful transatlantic flight, as well a number of polar flights in the early days of aviation. In more recent history, the highest altitude flight of a propellered aircraft was performed by the unmanned, solar-powered Helios flying over the Hawaiian island of Kauai in 2001, reaching an altitude of 96,863 feet. During the course of this blog, we'll follow the development of propellers and their impact on aviation as a whole.

The earliest use of the of the propeller concept dates back to about 400 BC, when Archytas of Tarentum invented the first practical screw. By 200 BC the Greek scientist Archimedes had improved upon the original design with the new screw designs used to extract oils from olives and to move water up from wells with less effort. Before long this technology spread to Egypt and other areas of the Mediterranean. By the mid 1400's, Leonardo DaVinci, the great scientist and inventor sketched early helicopter designs with an upward facing airscrew, which DaVinci believed could lift the craft off the ground with enough rotation. Though progress in the development of screws appeared dormant for nearly three-hundred years, propeller technology began to be applied to the propulsion of boats. This technology evolved rapidly and by the early 1800's, boat propellers were used on a variety of marine vessels. In the 1840's Sir George Cayley expanded upon the propeller concept by designing a flying machine which included twin propellers. Alberto Santos Dumont, a well known aviator of the early 20th century, began to use aluminum for his airship propeller designs in 1906.

With the advent of their first flight in 1903, the Wright brothers took paper concepts of the propeller to the real. Using data from wind tunnel tests, the Wrights realized that aircraft propellers should be shaped similar to a wing, or airfoil, as opposed to a screw. The Wrights found that propeller blades act much like a rotating wing. However, instead of creating lift, the spinning blades displaced air backward to produce forward thrust. The Wrights also developed a concept of adding a twist along the length of each propeller blade, which gave the blades a uniform angle of attack with each rotation. The first propellers were made of wood and had a fixed pitch, which limited the performance of the aircraft. A propeller may be defined as a device with a rotating hub and radiating blades that are set at a pitch, which, when rotated, exerts a linear thrust upon an active fluid, such as water or air. In 1929, Wallace Turnball patented his design for a variable pitch propeller, which allowed the pilot to manually adjust the pitch of the blades, as well as enabling a more efficient operational control of the plane. By the 1940's, wider rectangular blades became more prevalent as aircraft engines increased in power, the primary advantage of rectangular blades was a greater absorption of energy in comparison to the more traditional round-tipped blades. The development of rectangular blades led to that of constant speed propellers. Constant speed propellers originated in the late 1930's and were put into service in 1940 on the British Spitfire and Hurricane fighters. Constant speed propellers are variable pitch propellers, which automatically adjust their blade pitch angle to maintain constant revolutions per minute, regardless of engine power, airspeed or altitude. Optimal efficiency is obtained with a fixed pitch propeller only at a certain airspeed and power setting. With a variable pitch propeller, peak efficiency can be obtained at a wide range of airspeeds and power settings, for example, upon landing the propellers may be reversed to have a breaking effect on the plane. A constant speed propeller is controlled by a hydraulic system which uses the aircraft's engine oil. This hydraulic system is operated by the use of a blue lever inside the cockpit, referred to as the propeller or pitch lever. Propeller aircraft of today employ constant speed propellers, due to their performance and fuel efficiency.

While the engineering governing how the propeller blades work has not changed drastically over the years, there have been a number of modifications in the materials from which they are made. The updates in materials coupled with improvements in engine technology has resulted in greater operating efficiency than that of a few decades ago. The early propellers were made almost entirely of wood while todays aircraft propellers are also made of aluminum or composite materials with the leading edge of some blades reinforced with nickel for strength and durability. Current aircraft propellers may have a blade count of from two to six or more blades, as opposed to the two bladed props of the pre 1930's era. The blade count of a propeller is dependent upon a number of factors, such as the propeller's diameter limitations, aircraft performance requirements (cruise, takeoff, loiter and high speed), noise requirements, the engine power and operating rpm of the propeller. The engine power of a plane has a direct relationship to the number of blades on its propellers. The more blades on it's propeller, the more efficient the aircraft operates. The most efficient mode of a propeller-driven aircraft is when the tips of it's blades are approximating the speed of sound. Though the perfect propeller design seeks to convert the plane's engine energy into forward thrust, the blade angle of a propeller varies as one moves from the base to the blade's tip. The angle difference along the blade ensures that all of the thrust generated is about equal across the entire blade.