Aerofoils (Technical General)


AEROFOILS

General

  1. A thin flat plate is unsatisfactory as an aeroplane wing for two reasons. Firstly, it is necessary from structural considerations to have a certain depth of the wing and secondly, the plate is far from being the most efficient type of lifting surface. By curving the plate it is possible to increase the lift, while by increasing its thickness and giving it a streamlined shape, it is possible to reduce its drag. This is roughly the manner in which an aerofoil is designed. The amount of curving and thickness is determined by the particular requirement of the aeroplane for which it is intended.
  1. Aerofoil Terminology. Since the shape of an aerofoil and the inclination to the air stream are so important in determining the pressure distribution, it is necessary to properly define the aerofoil terminology. Fig below shows a typical aerofoil and illustrates the various items of aerofoil terminology.
  • Chord line is a straight line connecting the leading and trailing edges of the aerofoil.
  • Chord length is the characteristic dimension of the aerofoil.
  • The mean camber line is a line drawn halfway between the upper and lower surfaces. Actually, the chord line connects the ends of the mean-camber line.

Typical Aerofoil Terminology

 

 

 

 

 

 

 

 

 

Typical Aerofoil Terminology

 

 

 

The shape of the mean-camber line is very important in determining the aerodynamic characteristics of an aerofoil section. Maximum camber (displacement of the mean line from the chord line) and the location of the maximum camber help to define the shape of the mean-camber line. These quantities are expressed as fractions or percent of the basic chord dimension. A typical low speed aerofoil may have a maximum camber of 4 percent, located 40 percent aft of the leading edge.

The maximum thickness and thickness distribution of the profile are important properties of a section. The maximum thickness and location of maximum thickness are expressed as fractions or percent of the chord. A typical low speed aerofoil may have a maximum thickness of 12 percent, located 30 percent aft of the leading edge.

The leading edge radius of an aerofoil is the radius of curvature giving the leading edge shape. It is the radius of the circle centred on a line tangent to the leading edge camber and connecting tangency points of upper and lower surfaces with the leading edge. Typical leading edge radii are zero (knife edge) to 1 or 2 percent.

The lift produced by an aerofoil is the net force produced perpendicular to the relative airflow (Fig  below).

The drag incurred by an aerofoil is the net force produced parallel to the relative air flow.

The angle of attack is the angle between the chord line and the relative airflow. Angle of attack is given the shorthand notation (alpha). It is important to differentiate between pitch attitude angle and angle of attack. Regardless of the condition of flight, the instantaneous flight path of the surface determines the direction of the oncoming relative airflow and the angle of attack is the angle between the instantaneous relative airflow and the chord line. To understand the definition of angle of attack, visualize the flight path of the aircraft during a loop and appreciate that the relative airflow is defined by the flight path at any point during the manoeuvre.

Aerodynamic Forces

 

 

 

 

 

 

 

 

Aerodynamic Forces

 

 

 

Type of Aerofoils

The performance of an aerofoil is governed by its contour. Generally, aerofoils can be divided into three classes:

  • High lift aerofoils.
  • General purpose conventional aerofoils.
  • High speed aerofoils

 

Aerofoil Section

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Aerofoil Sections

 

 

 

 

High Lift Aerofoils. A typical high lift aerofoil section is shown in Fig above (A) and its general properties are:

  • High lift sections employ a high t/c ratio, a pronounced camber, and a well-rounded leading edge. Their maximum thickness is at about 25% – 30% of the chord aft of the leading edge.
  • The greater the camber, i.e., the amount of curvature of the mean camber line, the greater the shift of centre of pressure for a given change in the angle of attack. The range of movement of the CP is therefore large on a high lift section. This movement can be greatly decreased by reflexing the trailing edge of the wing upwards, but some lift is lost as a result.
  • Sections of this type are used mainly on gliders and other aircraft where a high CL is all-important and speed is a secondary consideration.
  1. General Purpose Conventional Aerofoils. Typical general-purpose aerofoil sections are shown in Fig above (B) and their general properties are:

 

  • General purpose sections employ a lower t/c ratio, less camber and sharper leading

 

edge than those of the high lift type, but their maximum thickness is still at about 25%-30% of the chord aft of the leading edge. The lower t/c ratio results in less drag and a lower CL than those of a high lift aerofoil.

 

  • Sections of this type are used on aircraft whose duties require speeds which, although higher than those mentioned in the previous type, are not high enough to subject the aerofoil to the effects of compressibility.

High Speed Aerofoils. Typical high-speed aerofoil sections are shown in Fig above (C) and their general properties are:

  • High speed sections employ a very low t/c ratio, low camber and a sharper leading edge. Their maximum thickness is at about the 50% chord point.
  • Most of these sections lie in the 5% -10% t / c ratio band, but even thinner sections have been used on research aircraft. The reason for this is the overriding requirement for low drag. Naturally, the thinner sections have low maximum lift co-efficient.
  • High speed aerofoils are usually symmetrical about the chord line. Some sections are wedge shaped whilst others consist of arcs of a circle placed symmetrically about the chord line, called the symmetrical bi-convex aerofoil. The behaviour and aerodynamics of these sections at supersonic speeds are dealt with in detail in the chapter on Design for High-Speed Flight.

Laminar Flow Aerofoils. There is another type of aerofoil in common use on modern aeroplanes. It is a fairly recent development and is known as the laminar flow aerofoil. Laminar flow aerofoils were originally developed for the purpose of making an aeroplane fly faster. The laminar flow wing is usually thinner than the conventional wing, the leading edge is more pointed and its upper and lower surfaces are nearly symmetrical. The major and most important difference between the two types of aerofoil is that the thickest part of a laminar wing conventional design the thickest part is at 25% chord.

The effect achieved by this design of the wing is to maintain the laminar flow of air through a greater percentage of the chord of the wing and to control the transition point. Drag is, therefore, considerably reduced since the laminar aerofoil takes less energy to slide through the air. The pressure distribution on the laminar flow wing is much more even since the camber of the wing from the leading edge to the point of maximum camber is more gradual.

 The performance of all aerofoils is sensitive to small changes in contour. Increasing or decreasing the thickness by as little as 1% of the chord, or moving the point of maximum camber an inch or so in either direction, will alter all the characteristics. In particular, changes in the shape of the leading edge have a marked effect on the maximum lift and drag obtained, and the behaviour at stall. A sharp leading edge stalls more readily than one that is well rounded. Also it is important that the finish of wing surfaces be carefully preserved if the aircraft is expected to attain its maximum performance. Any dents or scratches in the surface bring about deterioration in the general performance. These points are of particular importance on high performance aircraft where a poor finish can result in a drastic reduction not only in performance, but also in control at high Mach numbers.

Laminar Aerofoil

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The performance of all aerofoils is sensitive to small changes in contour. Increasing or decreasing the thickness by as little as 1% of the chord, or moving the point of maximum camber an inch or so in either direction, will alter all the characteristics. In particular, changes in the shape of the leading edge have a marked effect on the maximum lift and drag obtained, and the behaviour at stall. A sharp leading edge stalls more readily than one that is well rounded. Also it is important that the finish of wing surfaces be carefully preserved if the aircraft is expected to attain its maximum performance. Any dents or scratches in the surface bring about deterioration in the general performance. These points are of particular importance on high performance aircraft where a poor finish can result in a drastic reduction not only in performance, but also in control at high Mach numbers.

 

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