Aerofoils and Lifting Surfaces

Aerofoils - Subsonic Speeds

• Focus shifts from minimizing drag to generating lift for aircraft weight support.
• Wings or aerofoils, inclined at a small angle, provide lift in conventional airplanes.
• Forward motion comes from rotating airscrews, jets, or rockets.
• Aerofoils are usually slightly curved.
• Flat surfaces were occasionally used in early flight attempts.

Lifting Surfaces

• Air flowing past an aerofoil (flat plate, or shape) causes reduced pressure on the top surface.
• Higher pressure underneath creates a net force pushing the aerofoil upwards and backwards.
• Tangential force from pressure differences on the small leading and trailing edge faces is not negligible.
• Pressure at the leading edge is typically very low.
• At small inclination angles, tangential force acts in a specific direction.
• Though tangential force may point forward, the resultant of tangential and normal forces must be tilted back relative to local flow direction.

Lift and Drag

• Resultant or net force on a lifting surface splits into two airflow-relative components.
• Lift is at right angles to the direction of airflow.
• Drag is parallel to the direction of the airflow.
• The term "lift" can be misleading since it can act horizontally or even downwards.

Airflow and Pressure over Aerofoil

• Curved surfaces produce greater lift, especially relative to drag, compared to flat surfaces.
• Curved surfaces offer the benefit of thickness essential for structural integrity.
• Air flows more smoothly over an aerofoil than a flat plate according to experiments.
• There is a slight upflow before air reaches the aerofoil.
• There is a downflow after air passes the aerofoil.
• Air doesn't hit the aerofoil nose cleanly, but divides behind it on the underside.
• Streamlines are closer together above the aerofoil where pressure decreases.
• Increased kinetic energy due to higher velocity accompanies decreased static pressure, illustrating Bernoulli's Theorem.
• Air acts as if passing through a bottleneck, increasing velocity at the curved aerofoil's highest points.
• Pressure decreases towards the top surface of the aerofoil which emphasizes the essential importance of curving streamlines.

Chord Line and Angle of Attack

• Inclination angle to airflow has great importance.
• Some straight line in the aerofoil section must first be decided to measure the angle to the airflow.
• Chord line definition must suit all aerofoils due to shape variety.
• The Chord is the line joining the leading edge to the trailing edge in most modern aerofoils with a convex under-surface.
• This is the centre for symmetrical aerofoils.
• Angle of attack refers to the angle between the chord of the aerofoil and the airflow direction.
• The term 'angle of incidence' can be used but its important to be precise about the definition.
• Airflow direction changes due to the aerofoil's presence, so the airflow direction over the surface isn't the same as that at a distance.
• The direction of the airflow at such a distance that it is undisturbed by the presence of the aerofoil is considered.

Line of Zero Lift

• Aerofoil may provide lift even when slightly negatively inclined to airflow.
• Chord may be at a negative angle, but the curved surfaces of the aerofoil are inclined at various positive and negative angles, which produce lift.
• Tilting the aerofoil's nose downwards until creating no lift, positions it similarly to a flat plate placed edgewise.
• A straight line through the aerofoil parallel to the airflow would settle whether the aerofoil provides lift or not.
• The 'line of zero lift' or 'neutral lift line' serves as a superior chord line definition.
• Determining this line requires wind tunnel experiments, which is awkward for practical measurements.
• It isn't significant in practical flight, except in a dive when angle of attack may approach the no lift condition.
• For an aerofoil of symmetrical shape zero lift corresponds to zero angle of attack.

Pressure Plotting

• Lift and drag change rapidly as angle of attack alters, with experiments showing changes in the pressure distribution over the aerofoil.
• Experiments are done by the method known as 'pressure plotting'.
• Number of small holes in the aerofoil surface are connected to glass manometer tubes containing water or other liquid that are connected to a common reservoir.
• Suction on the aerofoil causes liquid to rise in the corresponding tubes.
• Increased liquid pressure causes depression.
• This is like using U-tube manometers connected to a common reservoir, with experiments on models in wind tunnels and on airplanes in flight.
• Results are very interesting and instructive.
• Multiple manometers give a good visual indication of the form of pressure distribution.
• For recording, pressure transducers coupled to a computer are more convenient.

Pressure Distribution

• Pressure distribution is obtained over an aerofoil at a 4° angle of attack.
• The decrease in pressure on the upper surface is greater than the increase on the lower surface.
• Pressure isn't evenly distributed, with decreased and increased pressure being most marked over the aerofoil's front portion.
• Upper surface's decreased pressure provides the greater part of the lift(sometimes four-fifths).
• Greatest height to which water rises in a manometer with ordinary flight speed of an ordinary aerofoil is 120-150 mm.
• If there was a 'vacuum' the water would rise 10m, ie 10 000mm.
• Reason changes of pressure can not be represented by pressure distribution diagram around leading edge is they are very sudden.
• The increased pressure continues on the underside until the air is brought to rest at the point head-on into the wind, where the pressure is 1/2 pV2, or q.
• The point at which this happens is called the stagnation point.
• After the stagnation point there is a very sudden drop to zero, to the decreased pressure of the upper surface.

Centre of Pressure

• Decreases and increases of pressure are greatest near the aerofoil's leading edge.
• If all distributed pressure forces would be replaced all the forces would act less than halfway back along the chord.
• Point where this resultant force acts is called the center of pressure.
• A decreased pressure is above the aerofoil and an increased pressure is below.
• The decrease of pressure above is greater than the increase below.
• Effect is greatest near the leading edge
• All this is important when we consider the structure of the wing: the top surface must be held down, while the bottom skin is pressed up against the ribs.

Total Resultant Force on an Aerofoil

• Summing distributed forces due to pressure, along with total resultant force acting at the center of pressure finds that the force is not at right angles to the chord line.
• The center is also not at right angles to the flight direction.
• Near the tips of swept wings, it can sometimes be inclined forward relative to the latter line due to rather complicated three dimensional effects
• The force must on average be inclined backwards relative to the flight direction, otherwise we would have a forwards component, or negative drag.

Movement of Centre of Pressure

• Pressure plotting experiments show pressure distribution over the aerofoil changes considerably as angle of attack is altered.
• This changes the center of pressure.
• It is defined as being a certain proportion of the chord from the leading edge.
• At a negative angle the pressure on the upper surface near the leading edge is increased.
• At a negative angle the pressure on the lower surface is decreased, causing a loop of the pressure diagram.
• As angle of attack is increased up to 16°, the center of pressure gradually moves forward until close to the leading edge.
• Above this angle it begins to move backwards again.
• At angles between 2° and 8° is very rarely below 0° or above 16°, the center of pressure tends to move forward as the angle of attack of the aerofoil is increased
• The forward movement of the center of pressure the trailing edge of the aerofoil.
• This is called instability.
• In the case of a flat plate, an increase of the angle of attack causes the center of pressure to move backwards making the flat plate stable.

Lift, Drag and Pitching Moment of an Aerofoil

• The goal is to attain necessary lift to keep the airplane airborne which requires the airplane be propelled through the air.

• Lift can only be attained with the expense of a certain amount of drag reduced to the minimal level possible.

• The objective is to analyze how much lift, and how much drag, is able to obtained from different aerofoils at different velocities.

• Measurements are taken in wind-tunnel work rather than measuring the components separately.

• Aerofoil is set at different angles and then are measured with a balance.

• Lift, drag, and pitching moment of aerofoil depend on:

• The shape of the aerofoil

• The plan area of the aerofoil

• The square of the velocity

• The density of air

• On aerofoils the plan area is recorded but for drag the frontal area is considered.

• The symbol used for the plan area of a wing is S in place of the frontal areas used when considering drag.

• Lift = CL.p.V2.S or CL.q.S

• Drag = CD. pV2.S or CD.q.S

• Pitching moment = CM . pV2.Sc or CM.q.Sc

• The ' lift coefficient', the drag coefficient', and the 'pitching moment coefficient' of the aerofoil are called the symbols CL, CD, and CM.

• These symbols depend on the shape of the aerofoil, which is also altered by alterations to angle of attack.

Aerofoil Characteristics

• Experiments results should be set using curves the following:
• The lift coefficient
• The drag coefficient
• Lift-to-drag ratio
• Position of the center of pressure/pitching moment coefficient
• All of the above alter as an angle of attack is increased over given angles of flight.
• Coefficients are virtually separate from air density,the aerofoil scale and the velocity of the experiment.
• If the formula of a lift is C_{L} \cdot \frac{1}{2} \rho V^{2} \cdot S then the theories are not necessarily true.

Lift Curve

• Angle of attack has to have reach 0° and for there to be a defitinite coefficient and therefore a derfire lift.
• Maximum lift coefficient is gained adter it begins to reduce the chart and then is curved downwards.

Stalling of Aerofoil

• Any increase the angle at which the aerofoil strikes the air will lead in a rise in aerofoil, the angle will lead to a loss of lift when a certain angle is reached.
• The shape of an aerofoil makes small distinction to an angle at which the stalling takes place but effects amount of lift obtained.
• The angle is called the stalling angle of the aerofoil.
• When critical ange of attack is reached a complete alteration occurs.
• The airflow sepeates then forms the same way as those beinghind a plate being at wind is the airfow breaks way from top surfacae and forms vortices similar to those those behind a flat plate .
• In 'stalling' and seperation takes, resulting in more lift.
• Considerable suction has been built up mostly near the leading adge unitl stlaaing is reached.

Drag Curve

• Expected to be as high an angle is at the drag is at on this curve.
• Drag is increases rapidl up to around 6° however the increase in drag isn't as fast.

The Lift/Drag Ratio Curve

• Most lift is about 15° and the least drag is at 0°.
• As a result both on the extremes.
• Greatest angle of attack of about 3° or 4° otherwise, this is thhat the angle the aerofoil provides it's result.

Centre of Pressure and Moment Coefficient

• Curves the the center of pressure moves by and the effect on the patching moment coefficients.
• Center of pressure the tends to be unstable.

Aerodynamic Centre

• There must be a point where where there is no chafe in pti g moment when the angel of attack increasr is there is a point of considerable importance that arises from the differential effctas if different reference opints.
• That point is called the aerodynamic center the winsf.

Aspect Ratio

• Wings with larger spans have small advantages at the point of view of drag/lift.
• The ratio span/chord is aspect ratio, and the aspect ratios of those wings and the last one gives the best results.

Induced Drag

• The air flows upwards outside the wingspan.
• Net direction flows past a wing it puills down.

The Ideal Aerofil

• A high maximum lift coefficient
• A good lift/drag ratio
• a a high maximum value
• a low minim drag coefficien

Composites

• The modern aerofoil comes in a way where you half do you an which half to do that.
• the final aeroplance the shape of aerofoil is the last part with the composit.

camber

• The form is the change shape the aierfil secgion to opation btete results.
• This is made by the curvatur or cambre o th center e.
• A linequidsitant forms the upper and lower sugaes.
• There is variable section of commonky used aerfils .
• The thickness for pure waight is that than that is what comes down the cord.
• the concord the lower the is may b lightly convex.

Laminar flow aerofils.

• Attainenmtn of really high sped sdees.
• In general the may say far ha to be cosnding.

Can you answer these?

• How foes pressure destribution oeve an aierfil .