Aerofoils

Question 1

the shape of an aerofoil is designed to what?

Answer 1

produce aerodynamic forces conducive to flight.

Question 2

what aerofoil is mainly used now for training aircraft?

Answer 2

wing section resembling general-purpose type.

A moderate amount of curvature on the upper surface and less on the lower surface and a rounded leading edge. Empennage section is more symmetrical

Question 3

what does leading-edge mean?

Answer 3

edge facing upcoming air.

Question 4

what does trailing edge mean?

Answer 4

edge at rear of aerofoil

Question 5

what does chord line mean?

Answer 5

straight line joining the trailing and leading edges

Question 6

what does chord mean?

Answer 6

length of chord line for any given section

Question 7

what does camber mean?

Answer 7

average curve of the wing, a line drawn from leading to the trailing edge.

Equivalent distance between upper and lower surfaces.

Question 8

what does thickness mean?

Answer 8

depth of aerofoil at any given position along chord line.

max thickness usually occurs closer to leading-edge, approx 30-40% of chord value back from leading edge.

Question 9

what does thickness/chord ratio mean?

Answer 9

max thickness of aerofoil, expressed as a percentage of chord value.

range from 15-17% for high lift aerofoil, general purposes 10-12%, high speed 7%.

Question 10

the angle of attack is defined as what? and what is relative airflow defined as?

Answer 10

the angle between the chord line and relative airflow.

the airflow which is remote from aircraft and is unaffected by aircraft motion through it. (freestream airflow)

Question 11

how is the angle of attack reported?

Answer 11

graphically as a vector thus giving it velocity and direction, RAF is always of some magnitude but opposite in direction to aircraft flight path vector.

Question 12

In the practice of aerodynamic theory, its conventional to treat aerofoil and airflow as what?

Answer 12

aerofoils as stationary and airflow as moving.

no difference in results of gathered in this way compared to reality of moving aircraft in stationary air, relative motion between two.

Question 13

AoA is commonly denoted as what?

Answer 13

alpha or the alpha symbol.

Question 14

what is total reaction?

Answer 14

the sum of all the pressures, such as the places with regions of pressure both above and below freestream static pressure.

a single vector.

Question 15

the effects of all pressure acting on the aerofoil combined to form what?

Answer 15

a single resultant force that acts through the centre of pressure.

as pressure pattern around aerofoil change with speed and AoA, so too will total reaction vector and CoP location.

Question 16

general-purpose aerofoils have operating AoA of what angle?

Answer 16

2-15 degrees. 17 degrees is stall.

Question 17

AoA increase equals what?

Answer 17

TR increase in magnitude and tilts slightly rearwards, and CoP moves forward.

Question 18

at stall what happens to TR?

Answer 18

TR reduces in magnitude and tilts significantly rearwards and CoP moves rapidly rearwards.

Question 19

Normal cruising AoA for cambered aerofoil is usually what?

Answer 19

30-40% of the chord and moves forward to approx 15-20% of the chord as AoA increases then rapidly move backwards at the stall point.

Question 20

symmetrical aerofoils have virtually no what?

Answer 20

CoP movement within normal operating ranges of speed and AoA.

Question 21

because the TR vector varies in direction, it’s normal to resolve it into what two useful components?

Answer 21

one acting perpendicular to RAF (lift) and one parallel to RAF (drag).

Question 22

TR can never be perpendicular to what?

Answer 22

relative airflow, always some drag produced as a by-product of lift thus TR always tilted at least slightly rearwards.

Question 23

errors by compression of air can be ignored to what speed?

Answer 23

250kts

Question 24

indicated airspeed = what?

Answer 24

dynamic pressure

Question 25

lift formula is what?

Answer 25

L = Cl1/2pv^2S

Where Cl = coefficient of lift
1/2 = constant coming from the kinetic energy formula
P = rho representing air density
V = true airspeed
S = wing area

Question 26

what is CL?

Answer 26

coefficient of lift, number acting as a multiplier in equations depending on aerofoil shape and AoA

Question 27

1/2pV^2 = what?

Answer 27

dynamic pressure / IAS

Question 28

what are the few factors to consider in the lift formula?

Answer 28

freestream air density and velocity are both combined in 1/2pv^2 to give dynamic pressure and aerofoil surface which is in CL.

Question 29

how is the coefficient of lift best described?

Answer 29

the lifting capability of the wing that is dependent on aerofoil shape, and AoA presented to the relative airflow.

Question 30

AoA is readily variable by what?

Answer 30

the pilot and as well as the IAS and thus the lifting surface, whereas plane form surface area isn’t.

Question 31

what is the formula consisting of variables controlled by pilots and thus control lift generated by a particular lifting surface?

Answer 31

Lift = AoA x IAS

Question 32

typical CL curve for general purpose aerofoil feature what? (6)

Answer 32

• aerofoil cambered since CL curve shows lift generated at 0 degrees AoA.
• Zero lift AoA occurs at -4 degrees
• straight line from 0 to 12 degrees angle
• shows Cl varies more or less proportionally to AoA.
• beyond 12 degrees Cl curve begins to level off indicating a reduction in the increase of Cl with AoA up to critical AoA.
• any further increase after max results in Cl degreasing at a greater rate than it had increeasing

Question 33

Max Cl occurs when?

Answer 33

at critical or stalling AoA.

value of Cl at this point and the shape of the curve at the apex give a good indication of the performance of a particular aerofoil.

Question 34

normal operating range of the AoA is what?

Answer 34

from just above zero to just below the critical angle.

using abbreviated lift formula, for level flight low airspeed are associated with high AoA and increasing airspeed with lower AoA.

Question 35

higher the Cl max greater the lifting capability which means what?

Answer 35

aerofoil with higher Cl max will generate more lift at given airspeed and AoA, be able to maintain flight at lower airspeed without stall and more manoeuvrability.

Question 36

shown by reduction in the rate of Cl increase towards the peak of the curve. As AoA increases beyond the critical angle what happens?

Answer 36

the breakdown of streamline flow increases significantly, separation point moves forward to cover most of the upper surface and aerofoil is said to be stalled.

in a stall, region of low pressure above wing has collapsed, Cl and lift fall away rapidly but lift is still produced.

Question 37

A curve with a sharp peak indicates what? and what does a curve indicate?

Answer 37

a rapid breakaway process and a relatively sudden stall.

more progressive breakaway resulting in a more docile stall.

Question 38

the camber of an aerofoil has a significant effect on what? and why?

Answer 38

Cl

increasing camber increases Cl for all AoA on an otherwise identical aerofoil while reducing the camber decreases Cl.

Question 39

the max Cl is particularly susceptible to what?

Answer 39

changes in surface direction of lifting surface especially on the leading edge (up to approx 30% of the chord)

Question 40

the smoother the surface of an aerofoil means what?

Answer 40

the more the airflow can adhere to the surface at higher AoA, increasing Cl.

dirty or damaged surfaces can reduce Cl, breakaway is encouraged and premature stalling is encountered.

Question 41

is the aspect ratio (ratio of wingspan: chord length) a factor for Cl?

Answer 41

yes