Aerodynamics Flashcards
Lift and drag
How does an airfoil generate lift?
Air flows across a curved upper surface and is accelerated –> a low pressure region is formed perpendicular to the relative wind –> lift is the reaction
Lift and drag
The angle of attack of a helicopter rotor blade is defined as the angle between the:
Blade’s chord line and the relative airflow
Lift and drag
State the lift formula
FL = CL * 1/2 * p * v^2 * S
Lift and drag
The lift coefficient of an airfoil section increases with what?
Increases with an increase in angle of attack up to the stall
Lift and drag
What is the Magnus effect?
Superposition of translational and rotational velocities f a rotating body (e.g. drum) with the result of pressure differences which cause a lift force
Lift and drag
The blade pitch angle of a rotor blade element is
The angle between the chord line and the tip path plane
Lift and drag
Which factors determine the magnitude and direction of the relative airflow in a still air hover?
Induced airflow velocity and rotational velocity of the blade element
Lift and drag
The chord line of an airfoil section is the line drawn between:
The leading and the trailing edges
Lift and drag
The center of pressure of an airfoil element
Is the point where the total aerodynamic force is acting
Lift and drag
The centre of pressure of a symmetrical airfoil section is behind the leading edge approximately at the following % of the section chord?
0,25 = 25%
Lift and drag
The force which acts at right angles to the relative airflow is
Lift
Lift and drag
The centre of pressure of an airfoil section is
The point on the chord line through which the resultant of all aerodynamic forces acts
Lift and drag
The chord line of a blade section is
A straight line from leading to trailing edge
Lift and drag
The camber line of a symmetrical airfoil section is
Common with the chord line
Lift and drag
In the case of a symmetrical aerofoil
Pitching moment variations due to centre of pressure movement are small
Lift and drag
Thickness/chord ratio of an aerofoil section is expressed in percentage of
Chord
Lift and drag
The resultant force from pressure envelopes around an aerofoil can be described as
The total reaction
Lift and drag
That point where airflow leaves the surface of an aerofoil is known as
The seperation point
Lift and drag
A current requirement for the main rotor blade section is that
Changes in angle of attack produce minimum centre of pressure movement
Lift and drag
The total rotor thrust is
A component of total reaction acting at right angles of the aerodynamic forces on the blades, and perpendicular to the plane of rotation
Lift and drag
State the drag formula
FD = CD * 1/2 * p * v^2 * S
Lift and drag
CL varies with
Angle of attack
Lift and drag
What is the advantages a symmetrical aerofoil section as related to helicopter blade design?
The centre of pressure moves little in the normal range of attack range
Lift and drag
An increase in angle of attack of a rotor blade would cause an increase in
Drag and lift forces
Lift and drag
On a symmetrical blade element with a positive angle of attack lift is produced by
Airflow velocity increasing over upper surface giving decreased pressure and velocity decreasing over lower surface giving increased pressure
Lift and drag
Rotor blades profile drag is
A component of total reaction the aerodynamic forces, acting parallel to the plane of rotation and backwards at 90 degrees to total rotor thrust
Lift and drag
The amount of lift produced by a given helicopter rotor blade element is dependent upon
Angle of attack of the blade, the square of the air velocity relative to the blade element and the air density
Lift and drag
The technical term “geometric twist” can be described as
A reduction in blade angle towards the tip to give a more equal distribution of lift along the span
Lift and drag
Rotor blade sections are designed so that the center of pressure
Is normally positioned close to the feathering axis to reduce control system loads
Lift and drag
The term “washout” means
That the used airfoil varies in design (f.e. thickness, camber) from blade root towards blade tip
Lift and drag
