Aerodynamics: Wing Profiles and Thickness

Once the basic wing had been designed, engineers and designers raced to perfect it and, within a few years, aircraft had become radically different from their predecessors. The monoplane had arrived and, with it, came a host of new challenges for aircraft designers.

 

1. Cantilever monoplanes

By the late 1920s, designers were beginning to realise that the forest of struts and wires needed on biplanes was preventing them from increasing aircraft speed. Even the fastest racers, such as the Schneider Trophy seaplanes, still had numerous bracing wires. One designer who hardly ever used bracing wires was Dutchman Anthony Fokker, who designed cantilever (unbraced) monoplanes. He was able to design aircraft in this way by making their wooden wings thick enough to have the necessary bending strength. He frequently ensured that the wing's t/c ratio was as high as 20 per cent The t/c ratio - a key factor in wing design is the ratio of the thickness (at the thickest part) to the chord (distance from leading to trailing edge at the same spanwise location).

 

2. Duralumin

Designers in Germany and the USA learned how to use a new aluminium alloy called Duralumin. This was as light as aluminium, but much stronger, and it opened the way to the design of cantilever monoplanes with wings that did not require such a high t/c ratio as demanded by those of wooden construction. This made it possible to design faster-flying aircraft with thinner wings, such as the DC-1.

 

3. Wing thickness

In 1937, designer Sydney Camm at Hawker Aircraft began work on the Tornado and Typhoon. These aircraft were intended to have engines of 2.000 hp (1492 kW) and thus to exceed 400 mph (644 km/h). When these aircraft were put into steep dives, the pilots noticed unexpected violent buffeting (vibration), and even loss of control. It was obvious that something was very wrong with the aerodynamics. Gradually, and not without many accidents to these and other fast fighters, it was realised that the air flow over the top of the wing was reaching the speed of sound. This caused the formation of shock waves and the breakdown of smooth air flow. The answer was to make the wing thinner, even though (to preserve strength) it had to be muchstronger, and thus heavier. Whereas the Typhoon had a t/c ratio of over 18 per cent, the Tempest t/c ratio varied from 145 at the root to only 10 per cent at the tip. Thus, even though it had the same engine and was, in fact, a heavier aircraft, the Tempest was about 30 mph (48 km/h) faster than the Typhoon. Even more importantly, the air flow over the wings did not break down, so the pilot could go into combat without being bothered by buffeting or loss of control.

 

4. The laminar-flow profile

There was another important difference between the wings of the Typhoon and Tempest. The maximum thickness of the former was situated not far behind the leading edge, so the air surffered extremely violent acceleration over the front part of The wing. In the late 1930s, workers at the US National Advisory Committee for Aeronautics arrived at what they called a laminar- flow profile. The maximum thickness of this wing was located much further back, at 35-40 per cent of the chord. This enabled the air to accelerate over a much greater portion of the wing. While the air was acceleraing, the pressure gradient was favourable, in other words the air was moving towards a region of lower pressure. This kept the flow 'attached' to the wing, and extremely smooth. Once the air began to slow down, its pressure gradient became unfavourabie (moving towards higher pressure), and the flow began to separate from the surface and become turbulent, causing higher drag. One of the first aircraft to have a so-called laminar-profile wing was the North American Mustang. With the same engine as the Spitfire IX, it was 34 mph (55 km/h) faster. despite being larger and having three times the fuel capacity. Illustrated here is a comparison between the wing of the Typhoon (a) and that of the Mustang (bi). The wing profile of the Mustang was similar to that of the Tempest.

 

5. Vortices

Wing pressure varies in that it is lower above the wing than below it. At the wingtips, the air from underneath the aircraft tends to spill round to the lower pressure region above the wing, and the result is a violent spiralling vortex (air in rapid rotation).

 

6. The effects of vortices

The vortices trail behind the winqtips, and can persist for several minutes. In violent manoeuvres, the low pressure inside a vortex call cause moisture in the air to condense, leaving a vivid white trail.

 

7. The use of winglets

If a small aircraft comes in to land behind a large one -- even minutes later -- and flies into the predecessor's tip vortices, it can be flung upside down, and even into the ground. For this reason, there are strict rules about separation between landing aircraft, especially behind very large aircraft such as the 747. In the late 1960s, designers began experimenting with the wingtips, adding winglets (sometimes called tip fences) to cut down the formation of vortices. Even though the winglets added weight, they did not increase drag, but actually reduced it, enabling the aircrait to fly further.

The Beech 1900D was fitted with winglets to improve the aircraft's hot-and-high performance. Finlets and other auxiliary tail surfaces were added to improve longitudinal stabilify.

 

This page was borrowed from the World Aircraft Information Files, which is produced by Areospace Publishing Ltd. and published by Bright Star Publishing plc. www.airpower.co.uk