Introduction:
Airfoil design and selection is an important deciding factor in aircraft performance. Airfoil is a term used to describe the cross section of the aircraft wings. By observing birds in flight, man has understood that flat plates, resembling bird’s wings can produce the necessary lift to set the aircraft flying. However the actual importance of airfoil in designing the wings of the aircraft was recognized only in 1800. Since then many engineers have designed, tested and patented a number of variations in airfoil pattern. Over the years, there has been a remarkable advancement in technology of airfoil construction and also in our understanding on its efficiency. This has helped to replace the initial erroneous belief of using thin and highly chambered airfoils for high efficiency, with thick and zero chambered airfoil in many of the current day commercial jetliners. Even today, many variations are seen in the airfoil design and designers find it difficult to settle on one type of airfoil. This is because; the choice of the best airfoil for an aircraft depends largely on its application. This article gives an idea about the type of airfoil most commonly used in commercial jetliners and their efficiency.
Airfoil:
As the plane moves, its wings moves in the air, splitting the wind in its path and causing the split wind to move in upward and downward direction of the wing’s plane. The airfoil is designed such that the wind above the wing is at a low pressure, than the pressure of the wind below the wing. The difference in pressure causes the air to move from below the wing to meet the air above the wing, and it this process, lifting the wings which are in the middle of these two streams of air (Anon, 2014). This phenomenon is best explained by Bernoulli’s law, which states that fast moving air is at low pressure than slow moving air. The fundamental aim of airfoil design has been to create this low pressure and fast moving air current above the wing. The lift from below the plane should help balance the aircrafts weights. Thus it is necessary that the size of the wings be proportional to the size of the aircraft. When the aircraft moves faster there is more lift, and when the force of lift exceeds the force of gravity, the plane flys. While a aircrafts engines helps it to move forward at a faster speed, the wings of the plane help it to move upward. For efficient functioning, the airfoil is designed such that the hot air from the propeller fan is pushed downward by the wings. The airfoil in most commercial aircrafts has a curved upper surface and a lower flatter surface. Figure: 1 cited from Anon (2014): is a diagrammatic representation of the airfoils and its critical component, while Figure 2 is a diagrammatic representation of different shapes of airfoil seen in commercial jetliners. Picking up the right airfoil for the aircraft is largely based on experience and discretion of the modelers. The airfoil geometry (Fig: 1) has a few important terms (Web.mit.edu, 2014): Leading edge is the point at the front of airfoil which has the maximum curvature. Trailing edge is the point at the rear of the airfoil which has the maximum curvature. The chord line is the straight line connecting the leading edge and trailing edge. The thickness of the airfoil varies along the chord line. The mean line or the chamber line is the locus of the point’s midway between the upper and lower surfaces of airfoil. The chamber is very complex to calculate, and it is the average of the distance between the mean and the chord line, divided by the chord line.
Wing shape, airfoil design and efficiency:
The amount of lift generated by the wings depends on i. the shape of the air foil, ii. angle of attack, iii. size or area of the wing and iv. the dynamic air pressure (Abbott and Von Doennoff, 1959). The lifting efficiency is usually calculated as the ratio of the lift/drag. The wings of the aircraft are not completely flat, but tilted slightly at an angle called the angel of attack. This angle helps to push the air downwards (Collins, 2013). Additionally some planes have flaps on their wings to push the wind down during take off and landing. The bigger the wing, the more is the lift (Collins, 2013). The shape of the wing determines the performance of the aircraft. Most commercial jetliners have rounded straight wings with a moderate sweep (Fig: 3).This design gives the aircraft, less drag and helps to maintain stability at low speed. More the sweep in the wings more is the lift. Very high sweep wings are seen in jet fighters which take off and land at high speed. The thickness of the airfoil or the wing thickness is a compromise between the speed and the lift. A thick wing, produces more lift but more drag, and thus slows the speed. A thin wing produces less lift, but is faster. Most commercial aircrafts use 12 to 18% thick wings.
Conclusions:
Airfoil designing is a science in itself. A number of improvisation are happening in this area, which is making aircraft maneuvering efficient and assisting shorter takeoff at low speed.
References:
- Abbot, I. and Von Doenhoff, A. (1959).Theory of wing sections, including a summary of airfoil data. New York. Dover Publication.
- Anon, (2014). [online] Available at : http://www.boeing.com/company offices/about us/ wonder-of flight/airfoil.html [Accessed 31 Dec.2014].
- Collins, J. (2013). The new world champion paper aircraft book. New York. Ten Speed Publications.
- Web.mit.edu, (2014).2.972. How An Airfoil Works. [online] Available at: http://web.mit.edu/2.972/www/reports/airfoil/airfoil.html [Accessed 31Dec.2014].
