Aerodynamics: Wing shapes: part 1

As the aerodynamic properties of wings became better understood, aircraft designers began looking for ways to improve wing efficiency by reducing drag. This was particularly important with the advent of the jet engine, which promised higher speeds than had ever been possible before.

 

1. Before the jet engine: typical thickness/chord ratio

Until the invention of the jet engine, wing design followed well established rules. In general, monoplane fighters had a thickness/chord (t/c) ratio of about 13 per cent:

While heavy bombers might have a t/c ratio of 20 per cent:

 

2. Wing planform

In plan view, wings might be tapered on the leading edge (DC-3) or on the trailing edge (Boeing 247), or equally on both. Aspect ratio varied from about 4 for a fighter to 8 for a/bomber or airliner (the B-24 Liberator had an exceptional aspect ratio of 11.5) and up to at least 20 for a high-performance glider or motor glider.

 

3. Experiments

A few experimental 'tailless' aircraft had elevators or elevons (surfaces serving as ailerons for control in roll and as elevators for control in pitch) at the trailing edge of a wing either inclined backwards (e.g. Westland-Hill Pterodactyl) or with very large chord.

 

4. Supersonic airflow

The advent of the turbojet opened up a wholly new prospect of increased speed, up to, and possibly even beyond, the speed of sound. Designers had to learn completely fresh ideas. Not least of the new problems was that when airflow is supersonic (Mach number greater than 1) the relationship between the speed of the air and the air pressure is reversed! The diagram below shows this, as supersonic air flows through a venturi and accelerates through the expanding section.

 

5. Supersonic sections

The only thing designers understood from the outset was that everything possible had to be done to reduce drag. Thick wings were out! Many saw that the best supersonic wings would have completely new aerofoil profiles like those below.

However, these were all difficult to use, because they were inefficient at low speeds, such as those encountered during take-off and landing.

 

6. Increased drag near Mach 1

Research showed that, when Mach number (airspeed expressed as a decimal fraction of the local speed of sound) exceeded about 0.7, the drag began to increase extremely rapidly. By 1943 testing in wind tunnels had shown wing drag to vary as shown on the graph below:

There was a gap in the middle because near the speed of sound wind tunnels choked (became blocked by shockwaves, the intense pressure waves formed by a solid body in a supersonic airflow). Eventually the gap was filled (broken line). This showed how much engine thrust would have to be increased in order to overcome the increased drag and make aircraft supersonic. One answer was to use rocket engines. Another was to add an afterburner to a turbojet (an enlarged jetpipe in which extra fuel is burned, to increase the jet velocity).

 

7. Swept wings

Back in 1935 German aerodynamicists Büsemann and Betz had shown that Mach drag rise could be reduced by reducing t/c ratio, and that if this was done by making the wings lean back, then the wing would lie entirety behind the shockwave formed by the nose, and its own shockwaves would be weaker. Designers called such wings 'swept' or 'sweptback':

 

8. 35º compromise

By 1950 it had been agreed that 35º (measured along the 25 per cent or quarter-chord line) was a fair compromise between transonic drag (drag near the speed of sound), take-off/landing distance and flying qualities. This was the sweep angle of most fighters and jetliners of the 1955-75 period:

 

9. Delta planform

An alternative type of wing is the delta, so called because the Greek capital D (delta) is a triangle. By increasing the leading-edge sweep, typically to 60º, and filling in the gap at the back, designers found it structurally possible to reduce t/c ratio to 3.5 per cent. By fitting elevons it was possible to eliminate the horizontal tail.

 

10. Moving away from sweepback

Today sweepback is less in evidence, even on aircraft capable of sustained supersonic flight:

 

11. Advanced technology aerofoils

This is because stronger materials enable t/c to be brought below 5 per cent without sharp sweepback, and wing efficiency is further improved by varying camber. Here are the aerofoil sections possible with the F-16:

 

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