Helicopter Aerodynamics

Question 1

Translating Tendency

Answer 1

Tendency of single rotor helicopters to translate in the direction of tail rotor thrust.

Question 2

Transverse Flow

Answer 2

A condition of increased drag and decreased lift in the aft portion of the rotor disk compared to the front portion. (Caused by the air having a greater induced velocity and reduced angle of attack in the aft portion of the disk)

Question 3

Translational Lift

Answer 3

Improved rotor efficiency resulting from directional flight is called translational lift.

Question 4

Effective Translational Lift (ETL)

Answer 4

A flight condition where a helicopter starts to fly out of its own rotor downwash and moves into clean, undisturbed air that reduces wing tip vortices and causes the rotor to become more efficient (usually between 16 – 24 knots).

Question 5

Dissymmetry of Lift

Answer 5

The difference in lift between the advancing half of the rotor disk and the retreating half.

Question 6

Compressibility

Answer 6

Phenomenon resulting from the advancing blade approaching Mach 1 (because of excessive forward speed).

Question 7

Loss of Tail Rotor Effectiveness (LTE

Answer 7

Aerodynamic phenomenon that is characterized by an uncommanded yaw which does not subside of its own accord. (if not corrected can result in the loss of aircraft control)

Question 8

Vortex Ring State

Answer 8

A powered flight condition in which the helicopter settles in its own downwash (resulting in a descent that is aggravated by the increase in power).

Question 9

Dynamic Rollover

Answer 9

When a helicopter exceeds its critical rollover angle with one skid or wheel on the ground. (Requirements: pivot point, roll rate, exceed critical angle – TH-1 is 150)

Question 10

Retreating Blade Stall

Answer 10

When the retreating blade exceeds the critical angle of attack and stalls (causing the aircraft’s nose to pitch up and roll in the direction of the stall).

Question 11

Rotational Relative Wind

Answer 11

Wind that flows opposite the physical flight path of the airfoil, striking the blade at 90 degrees to the leading edge and parallel to the plane of rotation, and is constantly changing in direction during rotation.

Question 12

Induced Flow

Answer 12

The downward flow of air through the rotor blades caused by the increased blade pitch angle. (this downward component of air is added to the rotational relative wind.)

Question 13

Resultant Relative Wind

Answer 13

Rotational relative wind modified by induced flow.

Question 14

Drag

Answer 14

The component of the airfoil’s TAF parallel to the resultant relative wind.

Question 15

Induced Drag

Answer 15

Drag incurred as a result of the production of lift.

Question 16

Parasite Drag

Answer 16

Drag that is contributed by non-lifting surfaces.

Question 17

Profile Drag

Answer 17

The sum of surface friction drag (skin) and form drag. (Example: frictional resistance of the rotor blades passing through the air).