Aerodynamic Factors

Question 1

Transverse Flow Effect

Answer 1

Is the difference of lift / airflow that exist between the FWD and Rear 1/2 of the rotor disk between 10-20 knots.

This will result in a more horizontal airflow over the FWD 1/2 and a more downward / induced flow (IF) of air over the rear 1/2.

INDICATIONS: Pilot senses a right roll and vibrations.

CORRECTIVE ACTION: Left cyclic (causing cyclic feathering).

Question 2

Dissymmetry of Lift

Answer 2

Dissymmetry of lift is the differential of lift that exist between advancing and retreating halves of the rotor disk caused by the different air flow velocities across each half.

INDICATIONS: Pilot will experience nose pitch up during take-off (blow back).

CORRECTIVE ACTION: Dissymmetry of lift is corrected through Blade Flapping and Cyclic Feathering.

TWO FACTORS THAT CAUSE DISSYMMETRY OF LIFT:

1. Airflow Velocity
2. No lift areas (3): Rev. Flow, Neg. Stall, Neg Lift

Question 3

Retreating Blade Stall

Answer 3

Is when the retreating blade on a helicopter tends to stall at high airspeed because of the high angle of attack needed to compensate for the advanced stage of Dissymmetry of Lift.

Pilot will experience Rotor Vibration followed by Left Roll and Nose Pitch Up.

Conditions likely to produce Blade Stall at high Airspeed: (LT LAG)

 - Low Rotor RPM (Rotor Droop)
 - Turbulent Air
 - Loads (High Gross Weight)
 - Altitude (High Density Altitude)
 - High "G" Maneuvers

RECOVERY: (Inc-RPM + Reduce CAMA)

 - Increase RPM (to normal limits)
 - Reduce Collective / Airspeed / Severity of Maneuver / Altitude (if possible)

NOTE: The blade tip will stall first (area of highest AOA) and then move inboard towards the mast.

Question 4

Settling With Power

Answer 4

Is a condition of powered flight in which the helicopter settles in its own downwash. (Also Known As Vortex Ring State).

Three things must be present to experience Settling With Power:

 1. Low Airspeed (below ETL)
 2. 20% - 100% of available PWR applied with sufficient PWR to stop descent.
 3. Vertical Descent > = 300 fpm

Flight conditions conducive to Settling With Power:

 1. Downwind Approach
 2. Formation Approach / Landing
 3. Steep Approach (>30 degrees)
 4. Masking / Unmasking
 5. OGE
 6. Hovering above Max hovering ceiling

RECOVERY: If in the early stage of Settling With Power, increase collective sufficiently to stop descent. If enough power is not available move cyclic to move out of the column of air. Decreasing collective may also help if time permits.

Question 5

Dynamic Rollover

Answer 5

Is the susceptibility of a helicopter to a lateral-rolling tendency.

3 Conditions required to experience Dynamic Rollover:
Pivot Point
Rolling Motion
Exceed the dynamic / critical rollover angle

Dynamic Rollover can occur on:
Level Ground
Upslope - Slope Takeoff
Downslope - Slope Landing

PHYSICAL FACTORS (MAST-C)
M - Main Rotor Thrust
A - Aircraft CG /Low Fuel
S - Surface Area / Slopes
T - Tail rotor Thrust
C - Crosswind Component

HUMAN FACTORS (SWIIL)
S - Slow Corrective Inputs
W - Wrong Corrective Inputs
I - Inattention
I - Inexperience
L - Loss of Visual Reference

CORRECTIVE ACTION: Smooth and moderate collective reduction is most effective in preventing dynamic rollover.

Question 6

Airflow During A Hover

Answer 6

1. There are two types of airflow at a hover - Induced Flow and Wing Tip Vortexes.
2. Both types of airflow are smaller IGE due to ground interference compared to OGE where both types of airflow are larger due to no ground interference.
3. This will require more power to hover OGE than IGE.
4. Angle of Attack will remain the same but Angle of Incidence will be larger OGE.

A. Rotor Tip Vortexes: Air swirls from High pressure to Low pressure. These rotor tip vortexes reduce the efficiency of the outer portion of the rotor tip and causes the following blades to operate in turbulent air. Ground interference will cause rotor tip vortexes to be less during IGE compared to OGE. Rotor Tip Vortexes is the primary cause of high power requirements during a hover.
B. Downwash / Induce Flow (IF): A large volume of air will move downward (60-100 mph) through the rotor system. Ground interference will cause (IF) to be less during IGE compared to OGE. The increase velocity of (IF) during OGE would normally result in a decrease in AOA. Therefore Collective / Power must be increased to achieve a higher blade pitch angle to maintain the same AOA during OGE as IGE. (requiring more power).

Why does it take more power to hover OGE than IGE?
During OGE there is a much higher velocity of IF affecting AOA. Therefore Collective / Power must be increased (Increasing Angle of Incidence) to maintain the same AOA during OGE as in IGE.

Question 7

Effective Translational Lift (ETL)

Answer 7

1. Effective Translational Lift (16-24 knots) occurs when the Main Rotor System completely outruns the recirculation of old vortexes and begins to work in relatively clean and undisturbed air and become much more efficient.

EXPLANATION / INDICATIONS:

1. The flow of air passing through the rotor system is more horizontal resulting in a decrease in Induce Flow (IF) and an increase in angle of attack. The rotor system becomes more efficient and this increased efficiency continues as airspeed increases to Best Rate of Climb Airspeed, (Total drag is at lowest point). Airspeeds above Best Rate of Climb Airspeed will result in lower efficiency due to increased parasite drag.
2. Because of the combined effects of Dissymmetry of Lift & Transverse Flow Effect the nose of the helicopter will pitch-up (Blowback) and experience slight vibrations with a right roll.