Aerodynamics Flashcards
How does an airfoil generate lift?
Air flows across a curved upper surface and is accelerated
–> a low pressure region is formed; ⊥ to the relative wind
–> lift is the reaction
The angle of attack of a helicopter rotor blade is defined as the angle between the:
Blade’s chord line and the relative airflow.
State the lift formula!
FL = CL ● 1/2 ● p ● v² ● S
The lift coefficient of an airfoil section:
Increases with an increase in angle of attack up to the stall.
What is the magnus effect?
Superposition of translational and rotational velocities of a rotating body (e.g. drum) with the result of pressure differences which cause a lift force.
The blade pitch angle of a rotor blade element is:
The angle between the chord line and the tip path plane.
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.
The chord line of an airfoil section is the line:
Drawn between the leading edge and the trailing edges.
The center of pressure of an airfoil element:
Is the point where total aerodynamic force is acting.
The centre of pressure of a symmetrical airfoil section is behind the leading edge approximately at the following % of the section chord:
0,25
Explanation:
Means 25 %
The force which acts at right angles to the relative airflow is.
Lift.
The Centre of Pressure of an aerofoil section is:
The point on the chord line through which the resultant of all aerodynamic forces acts.
The chord line of a blade section is:
A straight line from leading to trailing edge.
The camber line of a symmetrical airfoil section is:
Common with the chord line.
In the case of a symmetrical aerofoil:
Pitching moment variations due to centre of pressure movement are small.
Thickness/chord ratio of an aerofoil section is expressed in percentage of:
Chord.
The resultant force from pressure envelopes around an aerofoil can be described as:
The total reaction.
That point where airflow leaves the surface of an aerofoil is known as:
The separation point.
A current requirement for the main rotor blade section is that:
Changes in angle of attack produce minimum centre of pressure movement.
The total rotor thrust is:
A component of total reaction acting at right angles of the aerodynamic forces on the blade, and perpendicular to the plane of rotation.
State the drag formula!
FD = CD ● 1/2 ● p ● v² ● S
CL varies with:
Angle of attack.
What is the advantage of a symmetrical aerofoil section as related to helicopter blade design?
The centre of pressure moves little in the normal angle of attack range.
An increase in angle of attack of a rotor blade would cause an increase in:
Drag and lift forces.
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.
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.
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 ti the blade element and the air density.
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.
Rotor blade sections are designed so that the center of pressure:
Is normally positioned close to the feathering axis to reduce control system loads.
The term “washout” means:
That the used airfoil varies in design (f.e. thickness, camber) from blade root towards blade tip.
Don’t drink water while studying… Why?
Because chemistry says that concentration decreases while adding water.
