Aerodynamics

Question 1

Transverse flow effect

Definition:

Answer 1

Differences in lift, drag and induced flow between the fore and aft portions of the rotor disk, occurring from 10-20 knots In forward flight, air passing through the forward portion of the rotor disk is more horizontal (increased AOA and more lift) and air passing through the rear portion of the rotor disk is more vertical (reduced AOA and less lift) The greater the distance the air flows over the rotor disk, the longer the disk has to work on it and the greater the deflection.

Question 2

Transverse flow effect

Indications:

Answer 2

Vibrations - This causes unequal drag and results in vibration, noticeable during takeoff and during deceleration for landing.

Right rolling motion - Gyroscopic precession causes the effects to be manifested 90 degrees in the direction of rotation

Question 3

Transverse flow effect

Corrective Action:

Answer 3

Cyclic Feathering - Apply left cyclic until after 10-20 knots, then move cyclic back to center.

Question 4

Dissymmetry of lift (Main rotor and Tail rotor)

Definition:

Answer 4

Differential lift between the advancing and retreating halves of the rotor disk caused by different wind velocities across each half. On the advancing half, add rotational velocity to rotational relative wind On the retreating half, subtract rotational velocity from rotational relative wind

Question 5

Dissymmetry of lift (Main rotor and Tail rotor)

Indications:

Answer 5

Pitch up (blow back) during takeoff

Question 6

Dissymmetry of lift (Main rotor and Tail rotor)

Corrective Action:

Answer 6

Blade flapping (aerodynamic) Upflap - increases induced flow/drag, less lift. Downflap - decreases induced flow/drag, increases lift.

Cyclic feathering (mechanical) FWD cyclic applied when blades are right and left, manifested forward and backward.

Question 7

Effective translational lift (ETL)

Definition:

Answer 7

Effective translational lift (ETL) is when the rotor completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air and occurs at about 16 to 24 knots, Efficiency continues with increased airspeed until the best climb airspeed is reached, when total drag is at its’ lowest point. Greater airspeeds result in lower efficiency due to increased parasite drag.

Question 8

Effective translational lift (ETL)

Indications:

Answer 8

As translational lift becomes more effective, the combined effects of gyroscopic precession, dissymmetry of lift, and transverse flow effect cause both the nose to pitch up (blowback) and the aircraft to roll to the right.

Question 9

Effective translational lift (ETL)

Corrective action:

Answer 9

Aviators must correct with additional forward and left lateral cyclic input to maintain a constant rotor-disk attitude.

*Four types of drag are induced drag, profile drag, parasite drag, and total drag.

Question 10

Settling with power

Definition:

Answer 10

A condition of powered flight in which the helicopter settles in its own downwash, also referred to as Vortex Ring State.

Question 11

Settling with power

Conditions required for settling with power

Answer 11

1. Vertical/near vertical rate of descent of 300FPM or more
2. Slow forward airspeed (less than ETL)
3. Rotor system using 20-100% of available engine power

Question 12

Settling with power

Conditions conducive for settling with power (SFDOOM)

Answer 12

1. Steep approach at a high rate of descent
2. Formation flight approach
3. Downwind approach
4. OGE hovering above max hover ceiling
5. OGE hove with not-constant altitude
6. Masking/unmasking

Question 13

Settling with power

Corrective action:

Answer 13

1. Cyclic – Forward, to gain airspeed [preferred method] (if forward cyclic not available, use lateral)
2. Collective - A large application during the initial stage

Question 14

Dynamic rollover

Definition:

Answer 14

Susceptibility of a helicopter to a lateral-rolling tendency.

Question 15

Dynamic rollover

Conditions required for dynamic rollover:

Answer 15

1. Pivot Point - contact with the ground
2. Rolling Motion - More rolling motion means critical angle is exceeded sooner
3. Exceeding the Critical Angle – angle which, if exceeded, recovery is impossible

Question 16

Dynamic rollover

Types of dynamic rollover:

Answer 16

1. Rolling over on Level ground (takeoff) (most common)
2. Rolling upslope (takeoff)
3. Rolling downslope (takeoff or landing)

Question 17

Dynamic rollover

Physical Factors (MAST-C)

Answer 17

Main rotor thrust

Aircraft CG/ low fuel

Sloped landing area/ Ground surface

Tail-rotor thrust

Crosswind component

Question 18

Dynamic rollover

Human Factors (FLIII)

Answer 18

Failure to make timely control movements

Loss of visual reference points

Inexperience

Inattention

Inappropriate control inputs

Question 19

Dynamic rollover

General Factors (CHLLL)

Answer 19

Crosswind 
High roll  rates 
Left  pedal  inputs 
Lateral  loading 
Left  wheel high/Right wheel down

Question 20

Retreating blade stall (critical airspeed)

Definition:

Answer 20

Retreating blade eventually stalls in high speed flight because of the high AoA needed to compensate for dissymmetry of lift. Decreasing velocity of airflow on the retreating blade demands a higher AoA to generate the same lift as the advancing blade. The stall will begin at the tip of the blade and advance inboard.

Question 21

Retreating blade stall (critical airspeed)

Indications:

Answer 21

Rotor Vibration followed by Left Roll and Pitch Up

Question 22

Retreating blade stall (critical airspeed)

Conditions that produce Retreating Blade Stall
3 HIGHS + 1 LOW = TURBULENCE

Answer 22

1. High Gross Weight
2. High Density Altitude
3. High “G” Maneuvers
4. Low Rotor RPM (rotor droop)
5. Turbulent Air

Question 23

Retreating blade stall (critical airspeed)

Corrective Actions: (INCREASE, 4 REDUCTIONS)

Answer 23

1. Reduce airspeed
2. Reduce collective
3. Reduce altitude
4. Reduce severity of the maneuver
5. Increase Rotor RPM to normal limits