1.3 THE AEROFOIL AS AN AERODYNAMIC SURFACE
1.3.1 General: In its simplest sense an aero foil section may be defined as that profile designed to obtain a desirable reaction from the air through which it moves. In other words, an aerofoil is able to convert air resistance into a useful force that produces lift for flight.
The cross-section of an aircraft wing is a good example of an aerofoil section, where the top surface usually has greater curvature than the bottom surface.
The air travelling over the cambered top surface of the aerofoil shown in Figure 1.4, which is split as it passes around the aerofoil, will speed up, because it must reach the trailing edge of the aerofoil at the same time as the air that flows underneath the section. In doing so, there must be a decrease in the pressure of the airflow over the top surface that results from its increase in velocity (Bernoulli’s principle).
1.3.2 Aerofoil terminology:
We have started to talk about such terms as: camber, trailing edge and AOA without defining them fully. Set out below are a few useful terms and definitions about airflow and aero foil sections that are frequently used frequently throughout the discussion of generation of aerodynamic forces on aerofoil sections. (Figure 1.5, 1.6)
Camber is the term used for the upper and lower curved surfaces of the aerofoil section. Where the mean camber line is that line drawn halfway between the upper and lower cambers.
edges. Note that this line may fall outside the aerofoil section dependent on the amount of camber of the aerofoil being considered.
Leading and trailing edge are those points on the centre of curvature of the leading and trailing part o f the aero foil section that intersect with the chord line as shown in Figure1.6
Angle of incidence (AOI) is the angle between the relative airflow and the longitudinal axis of the aircraft. It is a built-in feature of the aircraft and is a fixed "rigging angle". On conventional aircraft, the AOI is designed to minimize drag during cruise thus maximizing fuel consumption!
Angle of attack (AOA) is the angle between the chord line and the relative airflow. This will vary, dependent on the longitudinal attitude of the aircraft, with respect to the relative airflow as you will see later.
Thickness/chord ratio (t/c) is simply the ratio o f the maximum thickness o f the aerofoil section to its chord length normally expressed as a percentage. It is sometimes referred to as the fineness ratio and is a measure of the aerodynamic thickness of the aerofoil.
The aerofoil shape is also defined in terms of its t/c ratio. The aircraft designer chooses that shape which best fits the aerodynamic requirements of the aircraft.
Light aircraft and other aircraft that may fly at low velocity are likely to have a highly
cambered thick aerofoil section; where the air flowing over the upper camber is forced to travel a significantly longer distance than the airflow travelling over the lower camber. This results in a large acceleration of the upper airflow significantly increasing speed and correspondingly reducing the pressure over the upper surface.
These high lift aerofoil sections may have a t/c ratio of around 15%, although the point of maximum thickness for these high lift aero foils can be as high as 25-30%. The design will depend on whether forward speeds are of more importance compared to maximum lift. Since, it must be remembered that accompanying the large increase in lift that thick aerofoil sections bring, there is also a significant increase drag. However, thick aerofoil sections allow the use of deep spars and have other
advantages, such as more room for fuel storage and for the stowage of the undercarriage assemblies.
Thin aerofoil sections are preferred on high speed aircraft that spend time flying at transonic and supersonic speeds. The reason for choosing slim wings is to reduce the time spent flying in the transonic range, where at these speeds the build up of shockwaves create stability and control problems. We need not concern ourselves here with the details of high speed flight; this will be addressed comprehensively in outcome 3.
However, it is worth knowing that the thinner the aerofoil sections then the nearer to sonic speed an aircraft can fly before the effects of shockwave formation take effect. What limit the fineness ratio of aerofoil sections is their structural strength and rigidity, as well as providing sufficient room for fuel and the stowage of the undercarriage. A selection of aerofoil sections is shown in Figure 1.7.
Concorde has an exceptional fineness ratio (3-4%) because of its very long chord length resulting from its delta wings. It can therefore alleviate the problems of flying in the transonic range as well as providing sufficient room for fuel and the stowage of its undercarriage assemblies. In general, fineness ratios (t/c ratios) of less than 7% are unusual (Figure 1.8).
With regard to the under surface alterations in the camber have less effect. A slightly concave camber will tend to increase lift, but convex cambers give the necessary thickness to allow for the fitment of deeper and lighter spars. The convex sections are also noted for limiting the movement of the centre of pressure (CP).
1.3.1 General: In its simplest sense an aero foil section may be defined as that profile designed to obtain a desirable reaction from the air through which it moves. In other words, an aerofoil is able to convert air resistance into a useful force that produces lift for flight.
The cross-section of an aircraft wing is a good example of an aerofoil section, where the top surface usually has greater curvature than the bottom surface.
The air travelling over the cambered top surface of the aerofoil shown in Figure 1.4, which is split as it passes around the aerofoil, will speed up, because it must reach the trailing edge of the aerofoil at the same time as the air that flows underneath the section. In doing so, there must be a decrease in the pressure of the airflow over the top surface that results from its increase in velocity (Bernoulli’s principle).
1.3.2 Aerofoil terminology:
We have started to talk about such terms as: camber, trailing edge and AOA without defining them fully. Set out below are a few useful terms and definitions about airflow and aero foil sections that are frequently used frequently throughout the discussion of generation of aerodynamic forces on aerofoil sections. (Figure 1.5, 1.6)
Camber is the term used for the upper and lower curved surfaces of the aerofoil section. Where the mean camber line is that line drawn halfway between the upper and lower cambers.
edges. Note that this line may fall outside the aerofoil section dependent on the amount of camber of the aerofoil being considered.
Leading and trailing edge are those points on the centre of curvature of the leading and trailing part o f the aero foil section that intersect with the chord line as shown in Figure1.6
Angle of incidence (AOI) is the angle between the relative airflow and the longitudinal axis of the aircraft. It is a built-in feature of the aircraft and is a fixed "rigging angle". On conventional aircraft, the AOI is designed to minimize drag during cruise thus maximizing fuel consumption!
Angle of attack (AOA) is the angle between the chord line and the relative airflow. This will vary, dependent on the longitudinal attitude of the aircraft, with respect to the relative airflow as you will see later.
Thickness/chord ratio (t/c) is simply the ratio o f the maximum thickness o f the aerofoil section to its chord length normally expressed as a percentage. It is sometimes referred to as the fineness ratio and is a measure of the aerodynamic thickness of the aerofoil.
The aerofoil shape is also defined in terms of its t/c ratio. The aircraft designer chooses that shape which best fits the aerodynamic requirements of the aircraft.
Light aircraft and other aircraft that may fly at low velocity are likely to have a highly
cambered thick aerofoil section; where the air flowing over the upper camber is forced to travel a significantly longer distance than the airflow travelling over the lower camber. This results in a large acceleration of the upper airflow significantly increasing speed and correspondingly reducing the pressure over the upper surface.
These high lift aerofoil sections may have a t/c ratio of around 15%, although the point of maximum thickness for these high lift aero foils can be as high as 25-30%. The design will depend on whether forward speeds are of more importance compared to maximum lift. Since, it must be remembered that accompanying the large increase in lift that thick aerofoil sections bring, there is also a significant increase drag. However, thick aerofoil sections allow the use of deep spars and have other
advantages, such as more room for fuel storage and for the stowage of the undercarriage assemblies.
Thin aerofoil sections are preferred on high speed aircraft that spend time flying at transonic and supersonic speeds. The reason for choosing slim wings is to reduce the time spent flying in the transonic range, where at these speeds the build up of shockwaves create stability and control problems. We need not concern ourselves here with the details of high speed flight; this will be addressed comprehensively in outcome 3.
However, it is worth knowing that the thinner the aerofoil sections then the nearer to sonic speed an aircraft can fly before the effects of shockwave formation take effect. What limit the fineness ratio of aerofoil sections is their structural strength and rigidity, as well as providing sufficient room for fuel and the stowage of the undercarriage. A selection of aerofoil sections is shown in Figure 1.7.
Concorde has an exceptional fineness ratio (3-4%) because of its very long chord length resulting from its delta wings. It can therefore alleviate the problems of flying in the transonic range as well as providing sufficient room for fuel and the stowage of its undercarriage assemblies. In general, fineness ratios (t/c ratios) of less than 7% are unusual (Figure 1.8).
With regard to the under surface alterations in the camber have less effect. A slightly concave camber will tend to increase lift, but convex cambers give the necessary thickness to allow for the fitment of deeper and lighter spars. The convex sections are also noted for limiting the movement of the centre of pressure (CP).
This limitation is most marked where the lower camber is identical to that of the upper camber giving a symmetrical section. Such sections have been adopted for medium and high speed main aerofoil sections and for some tail plane sections.
1.3.3 Aerofoil Profiles: There are different Profiles & Shapes of aerofoil. Profile of an aerofoil gives a shape to an aerofoil. According to the profile, shapes may be:
• Double convex symmetrical
• Double convex non-symmetrical
• Convex upper and concave lower surface.
• Convex Upper and flat lower surface
Figure 1.9 shows different profiles/shapes of aerofoil.
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