ppl_05_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail] Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4

MORE ABOUT AIRFLOW

AND AEROFOILS.

67

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

68

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

AIRFLOW.

The Viscosity of Air.

Now that we have investigated the nature of lift, and before going on to examine another of the principal forces which acts on an aircraft in fight, the force of drag, we must take a second look at airfow around an aerofoil.

In our examination of lift, one of the major assumptions we made was that air is inviscid; in other words, we assumed that air has no viscosity. That assumption enabled us to treat air as an ideal fuid and to use Bernoulli’s Principle, concerning ideal fuids, as one of the scientifc principles which explain how lift is generated by a wing.

Assuming air to be without viscosity is a valid assumption when investigating lift at low airspeeds because although, in reality, air does possess viscosity, the viscosity of air is so low that it can be discounted.

However, in order to explain drag, we must put aside the assumption that air is inviscid and acknowledge that air is, in reality, viscous, even though its viscosity is very low. The reason why we must now take into account the viscosity of air is that scientists have shown that if air were not viscous, no drag force would act on an aircraft moving through the air, whatever its shape. However, as aerodynamicists and aircraft designers know only too well, and as you will already have discovered from your fying lessons, there is such a thing as drag.

Drag is, of course, in most circumstances, a great disadvantage to a pilot. This is the case if a pilot wishes to fy as far as possible and as fast as possible or to obtain the best possible glide performance from his aircraft. On the other hand, drag enables a pilot to exert control over his aircraft: for instance, when he wishes to modify the aircraft’s lift/drag ratio in order to maintain a desired glide-slope at an appropriate approach speed when landing. You will learn more about the lift/drag ratio later in this book, and in the section dealing with Aircraft Performance.

For the moment, however, let us just accept that although in our examination of lift we assumed the air to be inviscid, air must, in reality, possess some degree of viscosity if we are to account for the force of drag which acts on an aircraft.

The viscosity of air also accounts for the true nature of airfow around a wing of aerofoil cross section.

Airflow and Friction.

We must now, then, put aside the purely streamlined, non-turbulent view of the fow of air around an aerofoil that we spoke of and depicted in Chapter 3 where we were assuming that air possessed no viscosity.

Because air does possess viscosity, we must now take into account that when one layer of air fows over the layers of air lying next to it, both above and below, the layers rub together causing a frictional force to be generated between them which acts parallel to the direction of fow in such a way as to slow down the faster-moving layers of air and to speed up the slower-moving layers. (See Figure 4.1, overleaf.) The same type of frictional force will be generated between the airfow and the surfaces over which the air is fowing, such as the upper and lower surface of a wing.

Air possesses viscosity, even though its viscosity is very low.

If the air possessed no viscosity, there would be no drag.

69

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

The depth of the airflow

near the wing where the

relative velocity of the flow reduces from its free-stream value to zero is called the boundary layer.

The boundary layer contains

airflow which is both laminar

and turbulent.

The free-stream airfow at a given distance above, below and in front of the aerofoil, which is unaffected by the presence of the aerofoil, will fow past the aerofoil at a relative velocity equal and opposite to the aerofoil’s own velocity through the air. But, wind tunnel experiments show, from the streamlines made visible by smoke, that the relative velocity of the airfow nearer to the aerofoil begins to reduce, because of the frictional forces which exist between the layers of air, until the particles of air which are actually in contact with the surface of the aerofoil are actually at rest on the aerofoil’s surface. (See Figure 4.1.) You may have noticed that fne impurities on the surface of an aircraft’s wing are not “blown away” during fight.

The depth of airfow within which the frictional forces generated by the viscosity of air cause the airfow’s relative velocity to reduce from its free-stream value to zero on the aerofoil’s upper surface constitutes what aerodynamicists call the boundary layer. The most common way of representing the changing velocity of the airfow within a boundary layer is to use the velocity profle shown in Figure 4.1. The depth of the boundary layer depicted in Figure 4.1 is greatly exaggerated.

Figure 4.1 The friction forces in real airflow, due to the air’s viscosity, cause the relative velocity of the airflow to reduce as it approaches the surface of the wing.

The Boundary Layer.

Depending on several factors which will be mentioned later in this book, the airfow in the boundary layer may be either laminar or turbulent. In normal fight conditions, at an angle of attack well below the stall angle, the boundary layer on a wing is typically only a millimetre or so thick.

The frictional forces that we have described, and the consequent presence of the boundary layer, account for what is known as skin-friction drag, about which you will read more later. You will not be surprised to learn that the smoother the surface of an aerofoil, the lower will be the skin-friction drag. However, because of air viscosity, skin-friction drag can never be eliminated altogether. The best that can be done - and much research has gone into this - is to try to arrange that the airfow within the boundary layer remains laminar over as much as the aerofoil as possible, so that the layers of air are kept sliding smoothly over each other. Once the boundary layer becomes turbulent, the energy losses within the turbulent fow cause skin-friction drag to increase signifcantly.

70

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

For our revised view of airfow around a wing we will consider airfow meeting an aerofoil and a very small positive angle of attack of, say, 4°, as shown in Figure 4.2. The angle of attack is not indicated on the diagram, and the diagram, for the sake of clarity, is not to scale. In order to keep things simple, we will consider airfow over the upper surface of the aerofoil only. (Remember that the pressure distribution curve above the wing indicates the lowest static pressure at its highest point, and the highest static pressure at its lowest points.)

The transition

point is the point where the

airflow within

the boundary layer changes from laminar to turbulent.

Figure 4.2 Airflow in the Boundary Layer and Separation (NB: the depth of boundary layer is greatly exaggerated) .

As depicted in Figure 4.2, the boundary layer over the leading edge of an aerofoil is laminar and produces only little drag. The boundary layer tends to remain laminar as long as the airfow continues to accelerate towards a region of lower pressure.

Because, as you have learnt, the camber of an aerofoil increases the turning effect that a wing has on the airfow, thus generating greater velocity change, the airfow will generally accelerate, and static pressure continue to fall, up to the point of maximum camber. Up to the point of maximum camber, then, the boundary layer has a good chance of remaining laminar because the pressure gradient is favourable – in other words, the air is fowing from a region of higher static pressure to a region of lower static pressure.

The Transition Point.

At a certain point on the aerofoil, however, normally aft of the point of maximum camber, the pressure gradient becomes adverse causing the airfow in the boundary layer to slow down and the laminar fow to become turbulent, though still attached to the surface. The point at which boundary layer fow changes from laminar to turbulent is called the transition point.

Though the boundary layer remains very thin for the whole extent of its fow over a wing, it does increase in thickness as it moves towards the trailing edge, especially after the transition point.

The Separation Point.

Towards the trailing edge, as the aerofoil cross-section reduces signifcantly in area, the pressure gradient may become so adverse that the boundary layer actually separates from the aerofoil’s surface. This is called the point of separation. Aft of the separation point, the boundary layer is replaced by a completely unpredictable

The point at

which the pressure

gradient

becomes so adverse that the boundary layer separates from the wing’s surface is called the separation point.

71

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

The forward movement of

the separation point with

increasing angle of attack is an important factor in the stall.

and haphazard region of airfow, sometimes referred to as the “dead air” region. The dead air region is a region whose thickness and extent is of a vastly different scale than that of the boundary layer.

Following separation, we may consider that the streamlined fow of air over the wing, represented by the thin boundary layer, with its laminar and turbulent regions, has completely broken down.

The principal effects of the separation of the boundary layer from the surface of the wing are as follows:

•an area of reduced pressure is established to the rear of the wing which greatly increases form drag, a type of drag which will be explained in more detail, later in this book.

•there is an abrupt decrease in the lift force.

•the air fow becomes erratic and violent.

As angle of attack increases, the point of separation moves forward. A wing will eventually stall when the angle of attack has reached a value where the point of separation moves so far forward that the lift force decreases abruptly over the whole wing. Because of the violent and erratic fow in the dead air region, the aircraft is often subject to pronounced buffeting just before the stall occurs.

Control of the Boundary Layer.

The turbulent section of the boundary layer is much thicker than the laminar boundary layer. Because of the energy expended in the change of the airfow from laminar to turbulent, aerodynamicists estimate that a turbulent boundary layer causes in the region of fve times as much skin friction drag as a laminar boundary layer. It is the turbulent boundary layer which hastens the complete separation of the airfow from the surface of a wing.

Because of the adverse effects of separation and because of the relationship between the turbulent boundary layer and separation, aerodynamicists put a lot of thought into how the boundary layer can be infuenced in such a way as to maintain laminar fow within the boundary layer, and to avoid or delay separation, in order to improve aircraft performance. You will learn that certain types of devices may be ftted to the wing of an aircraft in order to infuence the boundary layer in the Chapter in this book entitled ‘Lift Augmentation’.

DIFFERENT TYPES OF AEROFOIL.

One defnition that aerodynamicists have for aerofoil is: “any body which, when set at a suitable angle to a given airfow, produces much more lift than drag.”

Having examined airfow around the aerofoil of a typical light aircraft wing, let us now look at some different types of aerofoil to discover how their characteristics infuence the airfow around them and how they are each designed to meet certain performance criteria and to suit different aircraft roles.

You have learnt that the pioneers of aviation discovered, very early on, that wings

72

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

produced greater lift for lower drag when the wing was of curved cross section, with a positively cambered upper surface, rather than being just fat. This “curved” type of wing cross section was given the name aerofoil, in Britain. (In the United States, the word airfoil is used.)

Aerofoils have developed, over the years, from thin cross-sections resembling the wings of a bird (used by Wright and Bleriot) to the deeper type of aerofoil which is still used on light aircraft today. The modern light-aircraft type of aerofoil has been in use since the 1930s. They are not only effcient lift-generators but also can be constructed to incorporate load-bearing spars so that they possess the necessary structural strength required by the unbraced wings of cantilever monoplanes.

Figure 4.3 The development of the aerofoil cross-section. The wing of the Hawker Hurricane was a Clark Y Section aerofoil

Figure 4.3 depicts three representative aerofoils which show how the aerofoil has developed over the years. The lower aerofoil cross section, is a depiction of the

Clark Y Section used for the wing of the WW2 fghter aircraft, the Hawker Hurricane, and is typical of the type of aerofoil still used on light aircraft.

A wing of aerofoil cross section is more effcient than a fat wing primarily because it is much more effective in producing the downwards turning of the airfow at equal angle of attack, which, as you have learnt, is the key factor in the generation of lift. Air also fows over a wing of aerofoil cross section much more smoothly (with less turbulence) than over a fat plate wing. This greater extent of laminar fow also contributes to effcient lift generation and to keeping drag as low as possible. (See Figure 4.4.)

Figure 4.4 Comparison of airflow around flat plate and aerofoil with indication of the lift and drag forces.

73

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

You see from Figure 4.4 that, for the aerofoil, the ratio of the length of the lift vector to that of the drag vector is much greater than for the fat plate. This more favourable lift drag ratio, together with the more laminar airfow and the greater downwash, indicates why a wing of aerofoil cross section is a much more effcient lifting surface than a fat-plate type of wing.

AEROFOIL TERMINOLOGY.

Figure 4.5 Aerofoil terminology.

Certain technical terms are used to refer to the various characteristics of the aerofoil.

These terms are defned by the pictures and words below.

•The straight line drawn from the centre of curvature of the leading edge to the trailing edge is called the chord line.

•The chord (c) is the distance between the leading edge and trailing edge measured along the chord line.

•The line joining the leading and trailing edges of the aerofoil which is, at all points, equidistant from the upper and lower surfaces is known as the mean Camber Line.

•The maximum camber of an aerofoil is the point at which the distance

(d)between the mean camber line and the chord line is maximum. The

maximum camber is expressed as a percentage of the chord: d X 100 . c

Typical maximum camber for a subsonic aerofoil is 3%.

Figure 4.6 A symmetrical aerofoil has no camber

74

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

•Symmetrical aerofoils are aerofoils with a symmetrical cross section. They have no camber since the chord line and the mean camber line are coincidental. See Figure 4.6. The fn and tailplane of an aircraft are often symmetrical aerofoils.

•The thickness (sometimes called maximum thickness) is the greatest distance between the upper and lower surfaces of the aerofoil. Be aware that the position of maximum thickness is not necessarily the same as that of maximum camber.

Figure 4.7 Most light aircraft have a thickness-chord ratio of 12% to 15%, with the point of maximum thickness at about 25% chord.

•The thickness of a wing is usually expressed as a fraction of the chord. This fraction is called the thickness/chord ratio. For subsonic aircraft, the thickness/chord ratio is between 12% and 15%.

•The position of the points of maximum thickness and maximum camber are expressed as being a fraction of the chord, aft of the leading edge. In the diagram, maximum thickness is shown at about 25% chord.

The Significance of the Aerofoil in Wing Design.

Aerodynamicists, when designing an aircraft, choose an aerofoil section which has the optimum characteristics for the aircraft’s role.

The main differences to be observed between the various types of aerofoil used for the wings of modern aircraft are in the extent and position of a wing’s maximum camber and maximum thickness, and in the thickness/chord ratio of a wing.

As you have learned, any factor which affects the overall shape of a wing and, especially, its ability to turn or defect the airfow will also affect the wing’s Coeffcient of Lift at any given angle of attack. For instance, at zero degrees angle of attack, the symmetrical aerofoil cross section shown in Figure 4.6 will not turn the airfow at all and will, consequently, not generate any lift.

On the other hand, an aerofoil with a pronounced upper surface camber will cause a marked turning of the airfow, even at zero degrees angle of attack, and, therefore, generate a measurable amount of lift. This point is illustrated in Figure 4.8 which is

A symmetrical

aerofoil will not cause

downwash at

0° angle of attack and, thus, generates no aerodynamic force at 0° AoA.

75

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS

Laminar flow wings

generate good lift for low

drag, but they may have poor characteristics in the stall.

Figure 4.8 Symmetrical and cambered aerofoils at 0° Angle of Attack showing how they affect airflow.

a highly simplifed representation of the airfow around symmetrical and cambered aerofoils at 0° angle of attack.

You have seen in Figure 4.3 how camber, thickness and thickness/chord ratio of the wings of early aircraft evolved rapidly. Figure 4.9 below, shows the four basic aerofoil sections which are met in relatively modern types of wing design.

Figure 4.9 Typical aerofoil sections used in modern wing design.

A typical light training aircraft might have a wing with a thickness/chord ratio of 10%, with its maximum camber at about 40% chord.

The Clark-Y aerofoil, for instance, which is the top left aerofoil of Figure 4.9 and is typical of a modern, light-aircraft, general-purpose wing, has a thickness/chord ratio of just under 12%, a camber of 3.55% with a maximum camber at about 35 to 40% chord. This type of wing has reasonably high lift for low drag, at typical light-aircraft speeds, and possesses smooth stall characteristics.

Relatively thick aerofoils (thickness/chord ratio about 15%) are high-lift aerofoils, best for low speed operations and, in larger aircraft, for carrying heavy payloads at low speeds.

76