basic aerodynamic part two

BASIC AERODYNAMICS

AIRFLOW AROUND THE BODY

FREE STREAM FLOW

•Air flow is the movement of the air particles in a certain direction.

•To be able to observe air flows around objects, the flows have to be made visible first. Thee are two ways of doing this:• By using smoke or vaporized talcum powder• By using pieces of thread connected to the surface of the object

that indicate the flow direction at that particular point.

LAMINAR FLOW AND TURBULENT FLOW

•The flow pattern of laminar flows consists of layers.

•Each air particle follows a certain course and is not influenced by a particle from another layer.

•There is no difference in velocity between the layers.

Laminar Flow

•Turbulent flows are flows in which the velocity and direction change continuously.

•Some air particles move opposite to others in the air flow.

•At a certain velocity, laminar flow becomes turbulent flow. This velocity is called the critical velocity.

•Reaching the critical velocity means that the drag increases suddenly.

Turbulent Flow

At the front face part of the air is brought to rest, this area is called stagnation.

Airflow around a smooth plate

•. A vortex is an energetic swirling mass of air or water like a tornado or whirlpool.

• All aircraft, and birds for that matter, generate vortices off their wingtips. These vortices form because of the difference in pressure that exists between the upper and lower surfaces of the wing.

BOUNDARY LAYER

•Air is viscous and clings to the surface over which it flows. At the surface, the air particles are slowed to near-zero relative velocity. Above the surface the retarding forces lessen progressively, until slightly above the surface the particles have the full velocity of the main airstream. The air immediately above the surface is called the Boundary Layer. It begin at the stagnation area.

•The laminar boundary layer increases in thickness with increased distance from the font face edge. Some distance back from the leading edge, the laminar flow begins an oscillatory disturbance which is unstable.

•A waviness occurs in the laminar boundary layer which ultimately grows larger and more severe and destroys the smooth laminar flow. Thus, a transition takes place in which the laminar boundary layer decays into a turbulent boundary layer.

•The area at the boundary layer changes from the laminar to turbulent is called the Transition Region.

RELATIVE AIR FLOW

•Because aeroplanes move themselves in the atmosphere and the air also moves (wind). It is necessary to describe the meaning of relative airflow.

•Relative airflow is the direction of the airflow with respect to an object. If an object moves forward horizontally the relative airflow moves backward and horizontally. If an object moves forward and upward, the relative airflow moves backward and downward.

•Applying this to movement of an aircraft, the flight path and relative airflow are parallel but travel in opposite directions. Relative airflow may be affected by several factors including wind speed, direction and aircraft altitude.

•Technically speaking, the relative airflow is the velocity of the with respect to a body in it. When the wind blows, the path of the aircraft referred to the ground usually is not the same as the path referred to the moving air. The aerodynamic forces on the aircraft are always considered as a function of the relative airflow and not of the ground wind.

BERNOULLI’S PRINCIPLE

•The Swiss mathematician and physicist Daniel Bernoulli explains the relationship between potential and kinetic energy in a fluid. This principle is extremely important in helping us understand the way an aeroplane wing or a helicopter rotor produces lift as it passes through the air.

•Bernoulli’s principle explains that if we neither add nor take away energy from a flow of fluid, any increase in the velocity (kinetic energy) of the flow will cause a corresponding decease in its pressure (potential energy).

•A venturi is a specially shaped tube through which a fluid, either a liquid or a gas, flows. There is a restriction in the tube where the area gets smaller, and then it returns to its original size. Fluid flows through this tube, and, at position 1, the pressure is the value P1. the speed, or velocity, is shown v1. if we do not add or take away any energy from the fluid, it must speed up when it reaches the restriction to allow the same volume of fluid to move the same distance through the tube in the same length of time.

When it speeds up, the kinetic energy (velocity) increases. But since no energy was added, the potential energy (pressure) had to decrease. We see this on the gages P2 and v2. as soon as the tube returns to its original size, at 3, the fluid slows down so the same volume will move the same distance in the same time. When it slows down, the velocity decreases, but the pressure increases to the value it had originally.

Venturi Tube

FREE STREAM FLOW

AIRFOIL SECTIONS

INTRODUCTION TO LIFT

Aerodynamic lift depends on the shape of the airfoil section and on the airfoil surface area. The figure on next slide show a typical subsonic airfoil section and some of the more important terms related to its shape.

Airfoil Sections

Leading edge – this part of the airfoil meets the airflow first.

Trailing edge – this is the portion of the airfoil where the airflow over the upper surface rejoins the lower surface airflow.

Chord line – the camber of an airfoil is the characteristic curve of its upper and lower surfaces. The upper camber is more pronounced, while the lower camber is comparatively flat. This causes the velocity of the airflow immediately above the wing to be much higher than that below the wing.

Relative wind – this is the direction of the airflow with respect to the wing. If a wing moves forward horizontally, the relative wind moves backward horizontally. Relative wind is parallel to and opposite the flight path of the airplane.

Angle of attack – this is the angle between the chord line of the airfoil and the direction of the relative wind. It is important in the production of lift. The angle formed by the wing chord line and relative wind is called the angle of attack.

FINENESS RATIO

•Is the ratio of length to breadth.

•Based on previous figure, applying this to airfoils, the length is L and the breath is E (maximum thickness).

•For best result it should be about 4 to 1, but it really depends on the air speed, the higher the speed, the greater should be the fineness ratio, but experiments have shown that there is not much variation in the drag for quite a large range of fineness ratio.

HISTORY OF AIRFOILS

AIRFOIL REVOLUTION

As an airplane moves through the air it experiences several forces acting on its structure. The pattern of the airflow around the aircraft and the magnitude of the aerodynamic force depend on various variables which have to be taken into account. These variables are:

•shape of the aircraft

•angle of attack (attitude to the free stream airflow)

•speed of the aircraft (true airspeed)

•size of the aircraft (frontal area)

•air density

•viscosity (stickiness or thickness)

AERODYNAMIC FORCES

DRAG THRUST

WEIGHT

•Lift is the upward force created by the effect of airflow as it passes over and under the wings.

•Lift is the key aerodynamic force. It is the force that opposes weight. In straight and level, un-accelerated flight, when weight and lift are equal, an airplane is in a state of equilibrium. If other aerodynamic factors remain constant, the airplane neither gains or loses altitude.

ICS •Upwash is the deflection of the oncoming

airstream upward and over the wing.

•Downwash is the downward deflection of the airstream as it passes over the wing and past the trailing edge.

FLIGHT PATH

RELATVE WIND

As the angle of attack increases, lift also increases. Notice that lift acts perpendicular to the relative wind, regardless of angle of attack.

If the maximum lift angle is exceeded, lift decreases rapidly and the wing stalls.

Washout and wash in are terms normally used to describe the twisting of the wing either leading edge down and trailing edge up, washout or leading edge up and trailing edge down, wash in.

WEIGHT

•The weight of the airplane is not a constant.

•It varies with the equipment installed, passengers, cargo, and fuel load.

•During the course of a flight, the total weight of the airplane decreases as fuel is consumed.

•Additional weight reduction may also occur during some specialized flight activities, such as crop dusting, fire fighting, or sky diving flights.

•In contrast, the direction in which the force of weight acts is constant. It always acts straight down toward the center of the earth.

THRUST

•Thrust is the forward acting force which opposes drag and propels the airplane.

• In some airplanes, this force is provided when the engine turns the propeller.

•Each propeller blade is cambered like the airfoil shape of a wing. This shape, plus the angle of attack of the blades, produces reduced pressure in front of the propeller and increased pressure behind it. As is the case with the wing, this produces a reaction force in the direction of the lesser pressure. This is how the propeller produces thrust, the force which moves the airplane forward.

•Jet engines produce thrust by accelerating a relatively small mass of air to a high velocity. A jet engine draws air into its intake, compresses it, then mixes it with fuel. When this mixture burns, the resulting heat expands the gas, which is expelled at high velocity from the engine’s exhaust, producing thrust.

•Drag is caused by any aircraft surface that deflects or interferes with the smooth airflow around the airplane.

•A highly cambered, large surface area wing creates more drag and lift than a small, moderately cambered wing.

•If you increase airspeed, or angle of attack, you increase drag and lift. Drag acts in opposition to the direction of flight, opposes the forward acting force of thrust, and limits the forward speed of the airplane.

•Drag is broadly classified as either parasite or induced.

PARASITE DRAG

•Parasite drag includes all drag created by the airplane, except that drag directly associated with the production of lift.

•It is created by the disruption of the flow of air around the airplane’s surfaces.

•Parasite drag normally is divided into three types: form drag, skin friction drag, and interference drag.

FORM DRAG

•Form drag is created by any structure which protrudes into the relative wind.

•The amount of drag created is related to both the size and shape of the structure.

•For example, a square strut creates substantially more drag than a smooth or rounded strut. Streamlining reduces form drag.

SKIN FRICTION DRAG

•Skin friction drag is caused by the roughness of the airplane’s surfaces.

•Even though these surfaces may appear smooth, under a microscope they may be quite rough.

•A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag.

INTERFERENCE DRAG

•Interference drag occurs when varied currents of air over an airplane meet and interact.

•This interaction creates additional drag. One example of this type of drag is the mixing of the air where the wing and fuselage join.

A wing root can cause interference drag.

INDUCED DRAG

•Induced drag is the main by product of the production of lift. It is directly related to the angle of attack of the wing. The greater the angle, the greater the induced drag. Since the wing usually is at a low angle of attack at high speed, and a high angle of attack at low speed, the relationship of induced drag to speed also can be plotted.

•Over the past several years the winglet has been developed and used to reduce induced drag. As discussed earlier in this section, the high pressure air beneath the wing tends to spill over to the low pressure area above the wing, producing a strong secondary flow. If a winglet of the correct orientation and design is fitted to a wing tip, a rise in both total lift and drag is produced. However, with a properly designed than the additional drag, resulting in a net reduction in total drag.

The trailing vortex of a wing is attached to the wing by friction and is in effect acting as a brake as it is being dragged along by the aircraft. This is induced drag.

TOTAL DRAG

•Total drag for an airplane is the sum of parasite and induced. The total drag curve represents these combined forces and is plotted against airspeed.

•If we fly slower, we need a larger angle of attack, and the induced drag goes up. If we fly faster, the parasite drag predominates and increases the total drag.

INDUCED DRAG CALCULATION

DEVELOPMENT OF LIFT AND DRAG

Five factor affect aerodynamic lift and induced drag

• shape of the airfoil section• area of the airfoil• air density• speed of the air relative to the airfoil surface• angle between the airfoil and the relative airflow (angle of attack)

The direction of the lift produced by the wing is always perpendicular to the direction of the relative airflow.

The centre of pressure is the point on the chord line of an airfoil at which all of the aerodynamic forces are concentrated.

The lift vector acts from the centre of pressure in a direction that is perpendicular to the relative airflow, and the drag vector acts from this same point in a direction parallel to the relative airflow.

On an airfoil, the centre of pressure moves forward as the angle of attack increases, and backward as it decreases.

LIFT CALCULATION

ASSIGNMENT

SLIDE PRESENTATION

TITLE – ATMOSPHERE

CONTAINS - ARRANGEMENT OF ATMOSPHERE AND CHARACTERICS OF EACH LAYER.

- ATMOSPHERIC CONDITIONS

PRESENTATION DATE – THURSDAY

THE WING

Wing Sections

•Span (b) - is the maximum distance from tip to tip, not depending on the planform.

•Area (S) - is the surface of the planview of the apparent wing, including the parts covered from the fuselage, engines and others.

Wing Description

ANGLE OF INCIDENCE

The acute angle formed between the chord of the wing and a line drawn parallel to the longitudinal axis of the aircraft.

Angle of incidence and angle of attack

MEAN CHORD (c)

•Normally the airfoils which form a wing vary along the wing span. A good example is a tapered wing with the longest chord (cr) at the root and the shortest at the tip (ct) .

•The mean chord is the one which multiplied with the span results in wing area.

C = mean chord (m)B = spanS = wing area (m2)

MEAN AERODYNAMIC CHORD (MAC)

•Is the mean chord of an equivalent unswept rectangular wing which would produce the same lift moment and lift force like the considered wing.

•The position of the MAC (respective to the longitudinal axle) is very important considering aeroplane’s stability.

•The MAC of such wings is normally described in the maintenance manual.

AREA AND LIFT

•One factor in the total lift of an airfoil is the area of the surface exposed to the air flow.

•If the area is small, the region of pressure differential is small and there is little lift.

•On the other hand, if the area is great, the region of pressure differential is great and there is a large amount of lift.

PLANFORM

•The wing may have an almost infinite variety of planforms built not only for aaerodynamic reasons but for structural reasons as well.

•The way in which the lift per metre of span caries along the span, depends on among other things the way in which the chord varies along the span.

•For untapered rectangular, planform wings, most of the trailing vortices are shed near the tips. In the case, the down wash will be greatest near the tips.

•If a tapered wing is used, the lift is increased at the centre, and trailing vortices are produced more evenly along the span.

EXAGGERATED WING PLANFORMS

•Theoretical analysis indicate that for a given amount of lift, the smallest amount of trailing vortex induced drag will be produced when the down wash is constant along the span.

•The same analysis also shows that the constant down wash condition is obtained if the lift per metre of span varies from zero at the tips to a maximum at the centre, following an elliptical relationship.

ASPECT RATIO

•This is defined as the ratio of SPAN of an airfoil to its MEAN CHORD.

•The magnitudes of the losses at the wing tip obviously depend upon the length of the chord of the wing tip.

•The smaller the chord, the smaller are the losses, owing to the decrease in area involved, a decrease in the chord at the tip may be obtained by increasing the aspect ratio and the effect of this is shown.

•For a given amount of lift, the chord required reduces as the wing span is increased.

•Thus, wing designed to minimize trailing-vortex induced drag, have a long span with a small chord in other words, the aspect ratio is high.

•For a glider or sail plane, the aspect ratio may be as high as 40.

•Commercial aircraft have aspect ratios between five and eight, while jet fighters that operate at transonic and low supersonic speeds have wings of aspect ratios near 3.5

EFFECTS OF ASPECT RATIO

•The effect of increasing aspect ratio is principally to reduce drag for any given amount of lift. The effect of higher aspect ratio is to increases the range of an aircraft because there is less drag to overcome.

•Subject to certain limitations, the most efficient airfoil has the greatest aspect ratio.

•A true cantilever wing has no external bracing. If it has a high aspect ratio, it is long and narrow. It is obvious that when the length is increased too much, the wing will droop from its own weight.

•Furthermore, the root section where it joints the fuselage must be of heavy construction, and this added weight may in itself offset some or all of the gain in lift.

•When a wing reaches a certain length, it requires external bracing which adds to the weight and causes increased drag.

RELATIONSHIP ASPECT RATIO VS. WING TIP VORTEX

WASHOUT AND WASHIN

•Many aircraft are designed with a greater angle of incidence at the root of the wing than at the tip; this characteristic of wing is called washout.

•The purpose of washout is to improve the stability of the aircraft as it approaches a stall condition. The section of the wing near the fuselage will stall before the outer section, thus enabling the pilot to maintain good control and reducing the tendency of the aircraft to “fall off” on one wing.

•If a wing is designed so that the angle of incidence is greater at the tip than at the root, the characteristic is called washin.

•A difference in the washout and washin of the right and left wings of an aircraft is used to compensate for propeller torque. Propeller torque causes the aircraft to roll in a direction opposite that of the propeller rotation.

•To compensate for this, the right wing is rigged or designed with a smaller angle of incidence at the tip than that of the left wing. Thus, the wing is washed out more than the left.

AIRFOIL CONTAMINATION

ICE ACCUMULATION

•Rain, snow, and ice are transportation’s ancient enemies. Flying has added a new dimension, particularly with respect to ice.

•Under certain atmospheric conditions, ice can build rapidly on airfoils and air inlets. The accumulation of ice on the leading edge of wing can significantly alter the geometry of the wing and cause drag penalty and performance degradation and in the worst case, safety can be affected. The two types of ice encountered during flight are “rime” and “glaze”.

•Rime ice forms a rough surface on the aircraft leading edges. It is rough because the temperature of the air is very low and freezes the water before it has time to spread.

•Glaze ice (clear ice) forms a smooth, thick coating over the leading edges of the aircraft. When the temperature is just slightly below freezing, the water has more time to flow before it freezes.

Ice may be expected to form whenever there is visible moisture in the air and the temperature is near or below freezing. An exception is carburetor icing which can occur during warm weather with no visible moisture present. If ice is allowed to accumulate on the wings and empennage leading edges, it destroys the lift characteristics of the airfoil. Ice or rain accumulations on the windshield interfere with vision.

Effect Of Ice Formation On Leading Edges

CONTAMINATION EFFECTS

Ice on an aircraft affects its performance and efficiency in many ways. Ice build up increases drag and reduces lift. It causes destructive vibration, and hampers true instrument readings.

Control surfaces become unbalanced or frozen. Fixed slots are filled and movable slots jammed. Radio reception is hampered and engine performance is affected.

Ice, snow, rime formation or deteriorations of other kind having a thickness and surface roughness of a medium sandpaper on the leading edge and upper surface of the wing can reduce wing lift by around 30% and increase drag by 40%.

It is important to consider that other kind of contaminations or deteriorations also affect wing lift and drag in the same way like explained for ice, rime and snow.

Some examples of contamination and deterioration are:

•Misrigging of control surfaces•Absence of seals on movable sections•Dents on surfaces (bird strikes, accidental ground damages)•External repair (doubler)•Paint peeling•Lack of cleanless

The first 20% of the wing chord is particularly sensitive, because disturbancy of air flow passing it can cause separation resulting in early stall.

These changes in lift and drag significantly increase stall speed, reduce controlability and alter flight characteristics.

Wing Contamination Effects

basic aerodynamic part two

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