Aerodynamics

Question 1

Forces Acting on an Aircraft

Answer 1

• Lift
• Weight
• Thrust
• Drag

Question 2

Angle of Attack

Answer 2

AOA
The angle at which the airfoil meets the oncoming airflow. The angle measured between the resultant relative wind and chord line.

Question 3

How is lift generated, and what specific principles are involved?

Answer 3

Lift is generated when an object changes the direction of flow of a fluid or when the fluid is forced to move by the object passing through it.

1) Bernoulli’s Principle/ Venturi Flow
2) Newton’s 3rd law of motion

Question 4

What kinds of drag exist?

Answer 4

• Induced
• Profile
• Parasite
• Total

Question 5

Induced Drag

Answer 5

Induced drag is generated by the airflow circulation around the rotor blade as it creates lift.

Question 6

Profile Drag

Answer 6

Profile drag develops from the frictional resistance of the blades passing through the air.

Question 7

Parasite Drag

Answer 7

Parasite drag is caused by non-lifting components as the aircraft moves through the air, and it increases with airspeed.

Question 8

Total drag

Answer 8

Total drag is the sum of the three drag forces

Question 9

Blade Span

Answer 9

the length of the rotor blade from center of rotation to the tip of the blade

Question 10

Chord Line

Answer 10

a straight line intersecting leading and trailing edges of the airfoil

Question 11

Chord

Answer 11

the length of the chord line from the leading edge to the trailing edge

Question 12

Mean camber line

Answer 12

a line drawn halfway between the upper and lower surfaces of the airfoil (follows the shape of the airfoil)

Question 13

Leading Edge

Answer 13

the front edge of an airfoil

Question 14

Flightpath Velocity

Answer 14

the speed and direction of the airfoil passing through the air

Question 15

Relative wind

Answer 15

the airflow relative to an airfoil and is created by movement of an airfoil through the air.

Question 16

Trailing Edge

Answer 16

rearmost edge of an airfoil

Question 17

Induced Flow

Answer 17

the downward flow of air through the rotor disk

Question 18

Resultant Relative Wind

Answer 18

Relative wind modified by induced flow

Question 19

Angle of Incidence

Answer 19

AOI

the angle between the chord line of a blade and rotor hub. Also commonly referred to as blade pitch angle.

Question 20

Center of pressure

Answer 20

the point along the chord line of an airfoil through which all aerodynamic forces are considered to act. As AOA changes, these pressures change and center of pressure moves along the chord line.

Question 21

Cyclic Feathering

Answer 21

Changes the Angle of Incidence differently around the rotor sytem, creating differential lift by changing the AOA.

It is a means to control rearward tilt (blowback) caused by flapping action and counteract dissymmetry of lift.

Question 22

Translating Tendency

Answer 22

The tendency of the a single main rotor helicopter to drift in the direction of tail rotor thrust (right in US).

This tendency is counteracted in Robinsons by angling the main rotor 1degree to the left. This leaves us with 9deg of movement left and 11 deg of movement to the right.

Question 23

How is translating tendency counteracted in Robinson?

Answer 23

The main rotor is angled 1 degree to the left.

Question 24

What is coning?

Answer 24

As the rotors spin faster, centrifugal force increases, forcing the blades out and making them more ridgid. As lift increases, the blades are forced up and out, creating a conical path known as coning.

Question 25

Why does blade stall happen?

Answer 25

If rotor RPM drops below the minimum power-on RPM, the lift being generated is stronger than the centrifugal force, causing the blades to fold upward with no chance of recovery.

Question 26

Coriolis Effect

Answer 26

AKA The Law of Conservation of Angular Momentum - The value of angular momentum of a rotating body does not change unless an external force is applied. - As the mass of a rotating body is moved closer or farther from the axis of rotation, the speed of the rotating mass varies proportionately with the square of the radius - As the rotor begins to cone, the diameter of the rotor disk shrinks. This results in an increase in rotor rpm, which causes an increase in lift. Most pilots arrest this RPM increase with slight collective increase.

Question 27

Gyroscopic Precession

Answer 27

The resultant action or deflection of a spinning object when a force is applied. This action occurs approximately 90 degrees in the direction of rotation (counter-clockwise in US).

Question 28

Advancing Blade

Answer 28

Is the blade that is moving in the opposite direction of the direction of flight/relative wind. This increase of velocity causes more lift, making the blade flap up in this position. In Robinsons in CCW systems, this happens at 3 o'clock

Question 29

Retreating Blade

Answer 29

is the blade that is moving in the same direction as the direction of flight/relative wind. This decrease in velocity causes less lift, making the blade flap down in this position. In Robinson in CCW systems, this happens at 9 o'clock.

Question 30

Dissymmetry of Lift

Answer 30

is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half. - Issue is alleviated by flapping.

Question 31

Retreating Blade Stall

Answer 31

- At high forward airspeed, the retreating blade stalls because of a high AOA and slow relative wind speed. - The combination of blade flapping and slow relative wind limits the max forward airspeed. Symptoms - nose pitch up - vibration - rolling tendency (left in CCW systems)

Question 32

Translational Lift

Answer 32

- Improved rotor efficiency resulting from directional flight. - Causes nose to rise. - If nose is allowed to rise, in CCW systems, you can expect a roll to the right

Question 33

ETL

Answer 33

Effective Translational Lift The state between hover and forward flight where the rotor system begins to outrun it's vortecies, generating more lift on the front of the rotor system, causing the nose to pitch up and a roll to the right. These effects are the combination of dissymmetry of lift, gyroscopic precession, and transverse flow effect.

Question 34

Translational Thrust

Answer 34

Occurs when the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight. As the tail rotor works in progressively less turbuelnt air, more antitorque thrust, resulting in nose left yaw.

Question 35

Induced Flow

Answer 35

The downward movement of air through the rotor system. (Downwash)

Question 36

Transverse Flow Effect

Answer 36

- As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disk area, and increases at the aft area, causing increased lift in the front section and decreased lift in the aft. - Due to gyroscopic precession, the forces cause a roll to the right.