Aircraft Lift and drag.pptx

8/14/2019 Aircraft Lift and drag.pptx

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Aircraft Lift and Drag

Resultantforce


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Airflow over an aerofoilFollowing observations can be made from airflow over an aerofoil

There is a slight upflow before reaching the aerofoil There is a downflow after passing the aerofoil The air does not strike the aerofoil exactly on the nose, but actually

divides it at a point just behind it on the underside The streamlines are closer together above the aerofoil where the

pressure is decreased.


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Chord line and angle of attack The angle of attack to the airflow

has a big influence on generation

of lift. Since the aerofoil surfaces are

curved, it is not easy to define adatum with reference to whichthe angle of attack can bemeasured. Therefore, a chord lineis defined to serve this purpose.

Chord line is an imaginary line joining the leading edge andtrailing edge of an aerofoil.

The angle between the chord lineof the aerofoil and the directionof airflow is called angle ofattack.

Aerofoil with concave pressure surface

Aerofoil with flat pressure surface

Aerofoil with convex pressure surface


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Line of zero lift An aerofoil may provide lift even with a negative angle of

attack. In such a case, though the angle of attack may be

negative, the curved surfaces of the aerofoil may be inclinedat various angles, positive and negative, the net effect being aslightly positive angle which produces lift.

If the nose of the aerofoil is tilted downwards until itproduces no lift and if a straight line is drawn through theaerofoil parallel to the airflow, such a line is called a line ofzero lift or a neutral lift line.

For a symmetric aerofoil, zero lift corresponds to zero angle ofattack.


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Centre of pressure

If all the distributed forces due topressure were replaced by asingle resultant force, then theposition on the chord at whichthis resultant force acts is called

the centre of pressure . This is an important consideration

for the wing. The top surface ofthe wing must be held down on

to the ribs, while the bottom skinwill simply be pressed up to theribs. Thus, the forces to be borneby fasteners can be evaluated.


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Movement of centre of pressure

If the angle of attack isaltered, the pressuredistribution over the aerofoilchanges considerably.

Plot of pressure distributionwith angle of attack shows

that for this particularaerofoil, as the angle ofattack is increased up to 16,the centre of pressure (and

hence lift) gradually movesforward until it is about 1/3chord from the leading edge;above this angle, it begins tomove backwards.

Change of lift distribution with angle of attack


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Lift, drag and pitching moment of an aerofoil Ultimate objective of the aerofoil is to obtain the lift necessary

to keep the aircraft in the air. In order to do so, the aerofoilmust be propelled through the air at a definite velocity and itmust be set at a definite angle of attack to the airflow.

We know that a purely vertical force can not be obtained, or,we can obtain lift at the expense of a certain amount of drag.

Drag is unwanted and so, it must be as small as possible toreduce the power required to pull the aerofoil through the air.

Therefore, before selecting an optimum aerofoil for an aircraft,different aerofoil shapes should be investigated to know the

lift-to-drag ratios for various velocities and angles of attack. The lift, drag and pitching moments primarily depend on:

(a) Shape of the aerofoil (c) Square of the velocity

(b) Plan area of the aerofoil (d) Density of the air


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Lift, drag and pitching moment of an aerofoil Accordingly, the lift, drag and pitching moments are expressed

as:

Here,C L , C D and C M are coefficients of lift, drag and pitching moment respectively.(These depend on the shape of the aerofoil)

is the density of air

S is the area of the aerofoil projected on the plane of the chord

c is the chord length of the aerofoil

V is the air speed

ScqC ScV C

S qC S V C

S qC S V C

D M

D D

L L

..or .21.momentPitching

..or .21

.Drag

..or .21.Lift

2

2

2

Pitching moment is positivewhen it tends to push the noseupwards and negative when thenose tends to go downwards.


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Aerofoil characteristics

The aerofoil characteristics therefore depend on: The lift coefficient The drag coefficient The ratio of lift to drag, and

The position of the centre of pressure or the pitchingmoment coefficient. The aerofoil characteristics are analysed with the use of

The lift curve The drag curve The lift/drag ratio curve, and Centre of pressure and moment coefficient curve


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Lift curve

Ordinary angles of flight

Figure shows typical variation of lift coefficient with angle ofattack for a particular aerofoil

When the angle of attack is zero, there is a definite liftcoefficient and hence a definite lift.

Between 0 and 12 the graph ispractically a straight line indicating

that the lift increases steadily withincrease in angle of attack

Above 12, although the lift stillincreases for a few degrees, the

curve starts to peak. At about 15 the lift coefficient

reaches a maximum and thendrops. The angle at which C L peaks

is called the stalling angle.


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Stalling of aerofoil For this particular aerofoil, as long as the angle of attack is less

than 15, the air is defelected by the aerofoil and flow is steady Suddenly, when the angle of attack reaches the critical value of

15, there is complete change in the nature of flow. The airflowbreaks away or separates from the top surface forming vorticesthus destroying the low pressure distribution. This causes

severe loss of thrust. Sudden loss of thrust due to high angle of attack is called

stalling of aerofoil


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Drag curve


Figure shows typical variation of

drag coefficient with angle of attack The drag is minimum at zero or

slight negative angle of attack andincreases on both sides of this

angle Initially, the increase in drag

coefficient is not rapid with angle ofattack. Later, the increase becomesmore and more rapid

Especially after the stalling anglewhen the air separates, theincrease in drag coefficient is veryhigh meaning very high thrust to fly


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Lift/Drag curve


For optimum performance, anaerofoil should be operated atmaximum lift and minimum drag.

The lift curve showed that themaximum lift occurs at 15 andthe drag curve showed thatminimum drag occurs at 0 angleof attack. But both of them are atthe extreme ends of the range ofpossible angles.

Therefore, it is better to considerthe best lift/drag ratio, which isthe same as best ratio of C L/C D

We notice that the best C L/C D ratio occurs at an angle of attack

of 3 or 4 which is the best operating point for this aerofoil


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Centre of pressure and moment coefficient curve



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Centre of pressure and moment coefficient curve

We know that the centre of pressure gradually moves

forward as the angle of attack increases; and this trendstowards instability. After 12, it begins to move back againbut this angle is not often used in flight.

It should be noted that the pitching moment and itscoefficient depend not only on the lift (or resultant force) andthe centre of pressure, but also on the point about which themoment is taken (or the reference point)

Aircraft Lift and drag.pptx

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