Aerodynamics of Flight

Question 1

What are the four forces acting on the aircraft?

Answer 1

Thrust, Drag,

Weight, Lift

Question 2

What are the three types of drag?

Answer 2

Profile drag, induced drag, parasite drag - PIP

Question 3

What is profile drag?

Answer 3

Frictional resistance of the blades passing through the air

Question 4

What is induced drag?

Answer 4

The component of lift that is acting in a rearward direction, generated by the airflow around the rotor blade as it creates lift.

Question 5

What is parasite drag

Answer 5

Drag caused by the non-lifting components of the helicopter such as the cabin, rotor mast, tail, and landing gear.

Question 6

What is induced flow?

Answer 6

The downward flow of air through the rotor disk.

Question 7

What is the resultant relative wind?

Answer 7

Airflow, relative to the rotor blade, modified by induced flow.

Question 8

What is the angle of attack?

Answer 8

Angle measured between the resultant relative wind and the chord line of the rotor

Question 9

What is the angle of incidence?

Answer 9

The angle between the chord line and the rotor hub – also known as the blade pitch angle.

Question 10

Is the angle of incidence mechanical or aerodynamic?

Answer 10

Mechanical

Question 11

Is the angle of attack aerodynamic or mechanical?

Answer 11

Aerodynamic

Question 12

What changes the angle of incidence?

Answer 12

Collective input and cyclic feathering.

Question 13

What is the translating tendency?

Answer 13

The tendency of the helicopter to move in the direction of the tail rotor thrust in hovering flight.

Question 14

How is the translating tendency counteracted?

Answer 14

1. Main transmission is mounted at a slight angle to the left
2. Flight controls can be rigged so the rotor disk is tilted to the left slightly when the cyclic is centered

Question 15

What is the danger of rearward flight?

Answer 15

The horizontal stabilizer can press the tail downward, resulting in tail strike if the helicopter is moved into the wind.

Question 16

What is coning?

Answer 16

Rotation of the rotor system creates centrifugal force which tends to pull the blades straight outward from the main rotor hub. The faster the motion, the greater the centrifugal force. This force gives the rotor blades their rigidity and strength to support the weight of the helicopter. Blade assumes a conical path instead of remaining perpendicular.

Question 17

What is the danger if the rotor RPM is allowed to go too low?

Answer 17

Should it decrease too much, the rotor blades would fold up with no chance recovery.

Question 18

What is the Coriolis Effect?

Answer 18

Also known as the Law of Conservation of Angular Momentum. Rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of the rotation.

Question 19

What is gyroscopic precession?

Answer 19

The resultant action or deflection of a spinning object when a force is applied to this object, which occurs approximately 90 degrees in the direction of rotation from the point where the force is applied.

Question 20

What is dissymetry of lift?

Answer 20

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

Question 21

What is retreating blade stall?

Answer 21

When the critical angle of attack is reached on the retreating blade.

Question 22

What indicates retreating blade stall?

Answer 22

Nose pitch up, vibration, and a rolling tendency usually to the left.

Question 23

What is translational lift?

Answer 23

Improved rotor efficiency resulting from directional flight.

Question 24

What is Effective Translational Lift (ETL)?

Answer 24

Between 16 and 24 knots, the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air. Flow through the air is more horizontal.

Question 25

What is the transverse flow effect?

Answer 25

As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disk area and increases at the aft disk area. The result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots.