Propeller engines have been present throughout the history of aeronautics since the very first aeroplane took to the air (the Flyer 1 invented by the Wright brothers in 1903). Even today, they constitute an optimal solution to propel aeroplanes as modern as the A400M.
Propeller engines still have some advantages over turbofan engines for some uses and this is why they are still used today.
Propeller engines are based on Bernoulli’s principle. This principle states how a perpendicular thrust force can be obtained due to the difference in pressures thanks to the rotation of a couple of blades having a curved surface. In helicopters, this force is applied in a vertical direction to overcome gravity while in aeroplanes advantage is taken of it in a horizontal direction to generate a lift force in the wings, which really sustains the aeroplane in the air. In the case of the autogyro, the engine’s function is similar to that of an aeroplane. In any case, I will focus on the use of propeller engines in aeroplanes in this article.
The vast majority of aeroplanes have between one and four engines. A larger number would turn out to be strange, since more engines would increase consumption. For their part, propellers can have a variable number of blades, normally 2, 3, 4, 5, 6 or 8. Engines usually make the blades rotate in them same direction (dextrorotary), though there are also some aeroplanes like the A400M which make them rotate in opposite directions, achieving greater efficiency at the cost of considerably increasing technical complexity.
Since lift depends on thrust, aircraft should by definition be quick in a longitudinal direction. The above-mentioned Flyer 1 flew at the considerable speed of 48 km/h. The frenetic pace of innovation achieved by the early engineers ensured that the maximum speed of aircraft increased rapidly. Thus, the first commercial flight reached 100 km/h in 1910 and a speed of up to 250 km/h was reached in the First World War. During said war, the challenge of mounting weapons on aeroplanes arose. Seeing as the propeller was usually placed at the nose of the aircraft, the bullets had to go through it. The solution found by the allied forces was to place deflecting wedges so the blades could resist the impact, while the German solution was more elaborate and consisted of creating a synchronisation mechanism for the blades’ rotation and the firing of the weapons.
With the coming of peace, racing aeroplanes were built with sole aim of beating any speed records that had been set. To mention just a few, Curtiss aeroplanes reached 430 km/h in 1923 and Howard Hughes, the eccentric millionaire, reached 566 km/h in 1935 in an aeroplane he had designed himself. Curiously enough, speed records were held by hydroplanes, which reached 700 km/h, though it was in 1939, a few months before the Second World War broke out, that an experimental aeroplane, the Me 209, set a speed record that would finally last for a few decades: 755 km/h.
During the war, jet engines were developed with a view to increasing the maximum available speed, which was achieved right from the start with speeds of 800 km/h. Propeller engines, which up to then had used piston technology, took advantage of the new technology and turboprop engines were developed, which are still in use to this day.
In any event, the limitation of engines equipped with blades resides in the fact that their efficiency no longer increases once they reach the speed of sound. However, it is still possible to improve their efficiency a little bit thanks to the use of counter-rotating blades, which consists of placing two propellers, one behind the other, which are made to rotate in opposite directions. Efficiency can be increased by 15% in this way. The Russian Tupolev Tu-95 aircraft, which uses this technique, holds the world speed record for propeller aircraft at 920 km/h.
Despite this constraint, propeller engines still have some advantages over turbofan engines for some uses and this is why they are still used today. For example, they allow a great deal of power to be obtained right from the start of the take-off, making it possible to use short runways. This makes them ideal for use as military aircraft. Their fuel consumption is lower for short distances, because turbofan engines become more efficient only when they can reach greater heights and speeds.
So, the next time you fly on a propeller aeroplane, remember to enjoy the ride. Despite their long history, they are still a totally up-to-date and practical technology. Enjoy your flight.