StudyGuide-AerodynamicPrinciples Flashcards
What are the four aerodynamic forces acting on a helicopter?
Thrust: overcomes drag to move the aircraft forward
Drag: opposes thrust, keeping the aircraft from moving forward
Weight: gravity acting on the mass of the aircraft-opposes lift
Lift: keeps the aircraft flying and opposes weight
How is lift created?
Lift is created through a combination of Bernoulli’s principle (the faster a fluid moves, the lower the pressure generated) and Newton’s third law of motion-equal and opposite reaction
What is the lift formula (equation)?
C^L (1/2)pV^2*S
C: coefficient I’d Lift
p: air density
V: velocity
S: surface area
What is the coefficient of Lift?
The angle of attack on the rotor blade: the higher the coefficient of Lift, the more Lift is produced up until the critical angle of attack is reached..
What is V^2 in the lift formula?
V is the velocity of the airfoil (the rotor RPM) with regards to helicopter flight (the fact that it is squares in the equation indicates that a slight change in velocity produces a significant impact on Lift.
What is S in the lift formula?
Surface area: in helicopters, this is a fixed value (constant variable) that does not change (surface area of the rotor blades)
What does (1/2)p*V^2 mean, in the lift equation?
This represents the Dynamic Energy (also known as Kinetic Energy):
this energy is derived from the movement of the air.
p is the pressure (or air density)
The greater the air density, the lower the pressure altitude.
What is induced flow?
This is what we call downwash. It is the downward, vertical movement of air through the rotor system due by the production of Lift. It is directly proportional to the angle of attack.
What is relative wind?
Angle of airflow as it impacts the airfoil.
The flow of air in relation to the airfoil
What is rotational relative wind?
Relative wind produced by the rotation of the rotor blades (the rotational relative wind is greatest at the rotor tips.
What is resultant relative wind?
Angle of airflow at the rotor blades, with consideration to rotational relative wind and induced flow. (Flow of air, relative to the rotor blade, modified by induced flow.)
What is dissymmetry of Lift?
Lift differential (unequal rotor thrust) produced by the rotor disc due to forward flight or wind. (One blade advances into the wind and the other blade retreats- or goes with the wind)
What is flapping?
The vertical movement of a blade up or down to increase or decrease Lift in order to compensate for dissymmetry of Lift.
Flapping modifies the resultant relative wind by moving with (retr) or against (adv) the induced flow, which changes the angle of attack and therefore the coefficient of Lift.
What is coning?
Upward sweeping angle of the rotor blades as a result of Lift and centrifugal force.
What is the Coriolis effect?
The Coriolis effect (also known as the law of conservation of angular momentum) states that an object will have the same rotational momentum unless acted upon by an outside force.
This effect can be observed when the rotor blades speed up or slow down as the center of gravity moves further from or closer to the axis of rotation. (Like an ice skater spinning, arms out vs arms in)
What is lead-lag?
The horizontal movement of the rotor blades forward and backward along a vertical hinge, allowing the forces to equalize and rove undue stress on the system. (Not needed with a semi-rigid system: the underslung hinge causes the blade to move outward when flapping up, keeping the center of mass from changing significantly)
What is cyclic feathering?
The changing of the blade angle separately for individual blades through pilot input via the cyclic.
(Swashplate moves pitch links up and down)
How does gyroscopic precession affect helicopter flight?
Due to gyroscopic precession, changes to the rotor system are felt 90° later in the plane of rotation
What is center of pressure?
The point along the chord line of an airfoil where all aerodynamic forces are considered to act
