chap i - airfoils

A- Definition:An Airfoil is an aerodynamic shape that

provokes a reaction in order to generate primarily lift with minimum drag or air resistance

B. Airfoil characteristicsAny airfoil has two types of characteristics:

i. Design characteristicsii. Performance characteristics

i. Design characteristics

The leading edge L/E: it is the part of an airfoil that meets the air first.

Its shape depends on the operational function of the airfoil.

If the airfoil is used at high speeds then the L/E is thin and sharp

If the airfoil is used at low speeds then the L/E is fat and thick

The trailing edge T/E: it is the back part of an airfoil with the purpose of causing minimum turbulence when the airflow leaves the airfoil

Upper camber: is the upper curvature of an airfoil and plays an important role in generating lift, its is generally 1.5 – 2.5 % longer than the lower camber.

lower camber: is the lower curvature of an airfoil that encounters higher pressure than the upper surface and plays a major role in creating lift.

Chordline: is an imaginary line joining the leading and trailing edges of an airfoil, it is used as reference in some important calculations.

ii. performance characteristics

Coefficient of Lift Cl : is a performance number that indicates how much lift the airfoil will generate with respect to Angle Of Attack

Note: the airfoil will generate lift up to the critical angle of attack which is the maximum allowable angle not to exceed.

If you exceed this critical angle, the airfoil will stall.

Center of pressure Cp : is the point through which the lift and drag act.

It is usually located behind the center of gravity.

With increase in angle of attack the center of pressure moves forward and may surpass the C.G.

Its location is measured with reference to the mean chord

Lift to drag ratio L/D: is an indicative ratio that evaluates an airfoil’s efficiency.

i.e. The higher the value of this ratio, the more efficient is the airfoil

This means that when lift increases greatly, drag only increases with very small increments.

Coefficient of Drag CD : is an indicative value of air

resistance that an airfoil generates when moving through the air, and with changes in angle of attack.

C. ANGLES RELATED TO THE AIRFOILi. ANGLE OF ATTACK - AOA

Is the angle formed between the chord and the direction of relative wind, it can be positive or negative, it is not a fixed angle, it depends on the attitude of the aircraft.

V

ii. ANGLE OF INCIDENCE

Is the angle formed between the chord and the longitudinal axis or a line parallel to the longitudinal axis, it is a fixed design value, and does not change with aircraft attitude.

Note: The wing is usually designed with a profile twist where the

angle of incidence along the wing from tip to root is not the same.(ONLY BY A FEW DEGREES 2 TO 30)

When the angle of incidence at the root is larger than at the tip, this is called WASHOUT

When the angle of incidence at the root is smaller, this is called WASHIN

WASHOUT

iii- Dihedral Angle

Is the angle formed between the upward or downward inclination of the wing in relation to the lateral axis

Note: Dihedral affects stability and has an important role in

minimizing sideslip.

A positive upward inclination = dihedral angleA negative downward inclination = Anhedral

D. AIRFOIL CENTERS

i. Center of Pressure Cp

Is the point through which the lift and drag act.On a cambered airfoil, It is always located behind the

center of gravity and aerodynamic centerWith increase in angle of attack the center of pressure

moves forward and may surpass the C.G.But never quite reaches the AC but only in stall.

Note:When the aircraft approaches stall, the Cp moves

forward towards the AC, and once the aircraft stalls the Cp moves quickly backwards and the plane’s nose pitched down.

ii. Aerodynamic Center - AC

Is the point around which all changes in lift EFFECTIVELY ACT.

In other words, when the angle of attack increase or decrease, the total lift changes and the center of pressure moves, but the net effect is as if the change in moment and lift occurs at the AC.

Note:On all airfoils the AC is at quarter chord, meaning

25% from the leading edge.

iii. Center of Gravity

Is an imaginary point in which all mass/weight is said to be concentrated.

It is normally located between the AC and the CpThe thrust and weight act through the CG.

Note:For location purposes, the CG is referenced in all

calculations against the mean aerodynamic Chord.