ACAT.08 Aerodynamics

Question 1

Define tranlating tendency

Answer 1

The tendency of a single rotor helicopter with a counterclockwise rotating rotor system to drift laterally to the right while at a hover

Question 2

What causes Translating tendency

Answer 2

Thrust produced by the tail rotor to compensate for main rotor torque.

Question 3

Translating tendency compensation

Answer 3

1. Rigging of the flight control system
2. Tilting of the mast
3. Stabilization augmentation systems
4. Left cyclic input by the pilot

Question 4

Define Dissymmetry of lift

Answer 4

Dissymmetry of lift is the unequal lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half. If the helicopter is at a steady hover there is no dissymmetry of lift (with no wind)

Question 5

Relative wind velocity of advancing blade is blade speed ____ airspeed

Answer 5

Plus

Question 6

Relative wind velocity of retreating blade is blade speed ____ airspeed

Answer 6

Minus

Question 7

Main rotor method to overcome dissymmetry of lift

Answer 7

Flapping

Question 8

When blade flapping has compensated for dissymmetry of lift, the rotor disk is tilted to the rear is called?

Answer 8

Blowback

Question 9

What can the pilot do to compensate for dissymmetry of lift?

Answer 9

Cyclic feathering

Question 10

Explain Effective tranlational lift (ETL)

Answer 10

It is when the A/C completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air which occurs between 16 to 24 knots. As translational lift becomes more effective, the A/C will increase in altitude without any increase of power.

Question 11

Airspeed for ETL

Answer 11

16-24 kts

Question 12

ETL causes relative wind to become more horizontal, resulting in:

Answer 12

1. More vertical lift component
2. Less induced drag
3. Increased angle of attack

Question 13

Define Settling with power

Answer 13

A condition of powered flight in which the helicopter settles in it’s own downwash

Question 14

What conditions must exist simultaneously for settling with power to occur

Answer 14

1. Vertical or near vertical descent rate of at least 300 ft/min depending upon: - Gross weight- Rotor RPM- Density altitude
2. 20% - 100% of available engine power with insufficient power remaining to arrest the descent
3. Slow forward airspeed, less than ETL

Question 15

Settling with power recovery

Answer 15

Increase airspeed with forward cyclic. Reduce collective pitch as altitude permits

Question 16

Define Dynamic rollover

Answer 16

A helicopters susceptibility to a lateral rolling tendency

Question 17

Dynamic rollover conditions

Answer 17

Pivot point, Rolling motion, Exceed critical angle

Question 18

Physical factors for dynamic rollover

Answer 18

• Main rotor thrust - CG - Tail rotor thrust - Crosswind component - Ground surface - Sloped landing area - Low fuel condition

Question 19

Human factors

Answer 19

• Inattention - Inexperience - Inappropriate control input - Failure to take timely corrective action - Loss of visual reference

Question 20

Recovery of dynamic rollover

Answer 20

A smooth moderate collective reduction

Question 21

Primary factor for retreating blade stall

Answer 21

Excessive airspeed

Question 22

Contributing factors for retreating blade stall

Answer 22

1. High gross weight
2. High DA
3. High “G” maneuvers
4. Low rotor RPM
5. Turbulence

Question 23

Symptoms of retreating blade stall

Answer 23

1. Abnormal vibrations
2. Pitch-up of nose
3. Tendency to roll toward the stalled (left) side
4. Loss of control (if corrective action is not applied)

Question 24

Corrective actions for retreating blade stall

Answer 24

1. Reduce collective pitch
2. Regain aircraft control
3. Reduce airspeed
4. Increase rotor RPM to normal operating range
5. Minimize maneuvering
6. Descent to a lower altitude.

Question 25

Explain why hovering OGE requires more power than hovering IGE

Answer 25

Induced flow velocity is increased, causing a decrease in AOA. A higher blade pitch angle is required to maintain the same AOA as in IGE hover. The increased pitch angle also creates more drag. More power to hover OGE than IGE is required by this increased pitch angle and drag

Question 26

What is translational lift?

Answer 26

The efficiency of the rotor system is improved with each knot of incomming wind gained by movement of the A/C or surface wind. As the incomming wind enters the rotor system, turbulence and vortexes are left behind and the airflow is more horizontal. The improved rotor efficiency resulting from directional flight / wind is translational lift.

Question 27

What is transverse flow effect?

Answer 27

Occurs between 10 and 20 knots. Airflow over the forward half of the rotor is more horizontal, and more vertical over the aft half resulting in greater lift in the forward area and greater drag in the rearward. This effect causes unequal drag in the fore and aft portions of the rotor disk. Adding gyroscopic precession results in a lateral vibration easily recognisable by the aviator during takeoff, and to a lesser degree during approach. Because of the greater lift in the front and gyroscopic precession this effect results in a right rolling motion.