Aerodynamics (Fundamentals Of Flight)

Question 1

FEATHERING

Answer 1

Feathering is the action that changes the pitch angle of the rotor blades by rotating them around their feathering (spanwise) axis. (FAA-H-8083-21B) (figure 1-18).

Question 2

Flapping

Answer 2

When blade flapping compensates for dissymmetry of lift, the upward and downward flapping motion changes induced flow velocity. This changes AOA on the advancing and retreating blades

Advancing side - minimum blade pitch angle and AoA (flap up)
Retreating side - high blade pitch angle and AoA and is climbing towards the highest position at the back of the disk. (Flap down)

Question 3

GYROSCOPIC PRECESSION

Answer 3

The phenomenon of precession occurs in rotating bodies that manifest an applied force 90 degrees after application in the direction of rotation.

Question 4

Parasite Drag

Answer 4

Parasite drag is incurred from the non-lifting portions of the aircraft.

Question 5

Profile Drag

Answer 5

Profile drag is incurred from frictional resistance of the blades passing through the air.

Question 6

Induced Drag

Answer 6

Induced drag is incurred as a result of production of lift.

Question 7

IN GROUND EFFECT

Answer 7

Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters. Induced flow reduced this increase in AOA requires a reduced blade pitch angle. This reduces the power required to hover IGE. Less wing tip vortices.
Ground slows down induced flow velocity

Question 8

Out-of-Ground Effect

Answer 8

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. Large wing tip vortices.
Induced flow velocity is increased

Question 9

TRANSLATING TENDENCY

Answer 9

During hovering flight, the counterclockwise rotating, single-rotor helicopter has a tendency to drift laterally to the right. The translating tendency (figure 1-52, page 1-36) results from right lateral tail-rotor thrust exerted to compensate for main rotor torque (main rotor turning in a counterclockwise direction).

Question 10

Dissymmetry of lift

Answer 10

is the unequal lift across the rotor disk resulting from the difference in the velocity of air over the advancing blade half and the velocity of air over the retreating blade half of the rotor disk area.

Question 11

TRANSLATIONAL LIFT

Answer 11

Improved rotor efficiency resulting from directional flight is translational lift. In addition, the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight.

Question 12

TRANSVERSE FLOW EFFECT

Answer 12

In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. This is due to the fact the greater the distance air flows over the rotor disk, the longer the disk has to work on it and the greater the deflection on the aft portion. Downward flow at the rear of the rotor disk causes a reduced AOA, resulting in less lift. The front portion of the disk produces an increased AOA and more lift because airflow is more horizontal. These differences in lift between the fore and aft portions of the rotor disk are called transverse flow effect (figure 1-60, page 1-42). This effect causes unequal drag in the fore and aft portions of the rotor disk and results in vibration easily recognizable by the aviator. It occurs between 10 and 20 knots.

Question 13

EFFECTIVE TRANSLATIONAL LIFT

Answer 13

Occurs with the helicopter at about 16 to 24 knots, when the rotor—depending on size, blade area, and RPM of the rotor system completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air.

Question 14

BUCKET SPEED

Answer 14

Bucket speed is the airspeed range providing the best power margin for maneuvering flight. Using the cruise chart for current conditions, enter at 50 percent of maximum torque available, go up to gross weight, over to the lowest and highest airspeed intersecting the aircraft gross weight, and note speeds between which there is the greatest power margin for maneuvering flight.

Question 15

MUSHING

Answer 15

Mushing is a temporary stall condition occurring in helicopters when rapid aft cyclic is applied at high forward airspeeds. Normally associated with dive recoveries, which result in a significant loss of altitude, this phenomenon can also occur in a steep turn resulting in an increased turn radius. Mushing results during high G-maneuvers when at high forward airspeeds aft cyclic is abruptly applied. This results in a change in the airflow pattern on the rotor exacerbated by total lift area reduction as a result of rotor disc coning. High aircraft gross weight and high density altitude are conditions conducive to and can aggravate mushing.

Question 16

SETTLING WITH POWER

Answer 16

Settling with power is a condition of powered flight in which the helicopter settles in its own downwash.

Question 17

The following conditions must exist simultaneously for settling with power to occur:

Answer 17

A vertical or near-vertical descent of at least 300 feet per minute (FPM). Actual critical rate depends on gross weight, rotor RPM, density altitude, and other pertinent factors.

Slow forward airspeed (less than ETL).

Rotor system must be using 20 to 100 percent of the available engine power with insufficient power remaining to arrest the descent. Low rotor RPM could aggravate this.

Question 18

DYNAMIC ROLLOVER

Answer 18

A helicopter is susceptible to a lateral-rolling tendency called dynamic rollover. Dynamic rollover can occur on level ground as well as during a slope or crosswind landing and takeoff.
Three conditions are required for dynamic rollover—pivot point, rolling motion, and exceed critical angle.

Question 19

Three conditions are required for dynamic rollover

Answer 19

pivot point
rolling motion
exceed critical angle

Question 20

Human factors considered in the prevention of dynamic rollover include

Answer 20

Inattention

Inexperience

Failure to take timely corrective action

Inappropriate control input

Loss of visual reference

Question 21

RETREATING BLADE STALL

Answer 21

In forward flight, decreasing velocity of airflow on the retreating blade demands a higher AOA to generate the same lift as the advancing blade.

Question 22

CONDITIONS PRODUCING BLADE STALL

Answer 22

High blade loading (high gross weight).

Low rotor RPM.

High- density altitude.

High G-maneuvers.

Turbulent air.

Recovering from blade stall

Question 23

The following steps enable the aviator to recover from retreating blade stall:

Answer 23

Reduces collective.

Reduces airspeed.

Descends to a lower altitude (if possible).

Increases rotor RPM to normal limits.

Reduces severity of the maneuver.

Question 24

COMPRESSIBLE AND INCOMPRESSIBLE FLOW

Answer 24

At low airspeeds, air is incompressible. Incompressible airflow is similar to the flow of water, hydraulic fluid, or any other incompressible fluid. At low speeds, air experiences relatively small changes in pressure with little change in density. However, at high speeds greater pressure changes occur causing compression of air which results in significant changes to air density. This compressible flow occurs when there is a transonic or supersonic flow of air across the airfoil.

Question 25

RELATIVE WIND

Answer 25

Relative wind is the direction of the airflow produced by an object moving through the air.

Question 26

ROTATIONAL RELATIVE WIND

Answer 26

The rotation of rotor blades as they turn about the mast produces rotational relative wind. Rotational relative wind 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. Rotational relative wind velocity is highest at blade tips, decreasing uniformly to zero at axis of rotation (center of the mast).

Question 27

INDUCED FLOW (DOWNWASH)

Answer 27

At flat pitch, air leaves the trailing edge of the rotor blade in the same direction it moved across the leading edge; no lift or induced flow is being produced. As blade pitch angle is increased, the rotor system induces a downward flow of air through the rotor blades creating a downward component of air that is added to the rotational relative wind. Because the blades are moving horizontally, some of the air is displaced downward.

Question 28

RESULTANT RELATIVE WIND

Answer 28

at a hover is airflow from rotation that is modified by induced flow. This wind is inclined downward at some angle and opposite the effective flight path of the airfoil, rather than the physical flight path (rotational relative wind). The resultant relative wind also serves as the reference plane for development of lift, drag, and total aerodynamic force (TAF) vectors on the airfoil.

Question 29

ANGLE OF INCIDENCE

Answer 29

Angle of incidence is the angle between the chord line of a main or tail rotor blade and the rotational relative wind of the rotor system (tip-path plane)

Question 30

ANGLE OF ATTACK

Answer 30

AOA is the angle between the airfoil’s chord line and relative wind. The relative wind associated with AOA is the resultant relative wind. AOA is an aerodynamic angle (figure 1-6). It can change with no change in angle of incidence.

Question 31

TRANSIENT TORQUE

Answer 31

Transient torque is a phenomenon occurring in single-rotor helicopters when lateral cyclic is applied and is caused by aerodynamic forces.
Left turn increases TQ
Right turn decreases TA

Question 32

TOTAL AERODYNAMIC FORCE

Answer 32

As air flows around an airfoil, a pressure differential develops between the upper and lower surfaces. The differential, combined with air resistance to passage of the airfoil, creates a force on the airfoil. This is known as TAF (figure 1-39). TAF acts at the center of pressure on the airfoil and is normally inclined up and rear. TAF, sometimes called resultant force, may be divided into two components, lift and drag.

Question 33

CONSERVATION OF ANGULAR MOMENTUM

Answer 33

The law of conservation of angular momentum states the value of angular momentum of a rotating body will not change unless external torques are applied. In other words, a rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of rotation.

Question 34

Transient Torque

Answer 34

Left turn - torque increase
Right turn - torque decrease

Factors:
faster the cyclic moves
High cyclic displacements
higher power=higher spike
High weight

Question 35

Transient Torque

Answer 35

Increases or decreases in torque due to lateral movement of the cyclic

Left turn - torque increase
Right turn - torque decrease

Factors:
faster the cyclic moves
High cyclic displacements
higher power=higher spike
High weight