Aerodynamics Helicopter

Question 1

Bernoulli’s Principle?

Answer 1

Is a statement of the law of conservation of energy states energy cannot be created or destroyed and the amount of energy entering a system must also exit. A simple tube with a constricted portion near the center of its length illustrates this principle. Fluid speeds up in direct proportion to the reduction in area. Venturi effect is the term used to describe this phenomenon.

Question 2

Venturi Flow?

Answer 2

While the amount of total energy within a closed system (the tube) does not change, the form of the energy may be altered. Fluid flow pressure has two components—static and dynamic pressure. Static pressure is the pressure component measured in the flow but not moving with the flow as pressure is measured. Static pressure is also known as the force per unit area acting on a surface. Dynamic pressure of flow is that component existing as a result of movement of the air. The sum of these two pressures is total pressure. As air flows through a constriction, static pressure decreases as velocity (dynamic pressure) increases.

Question 3

Angle of Attack?

Answer 3

The AOA is the angle at which the airfoil meets the oncoming air flow. When the AOA is increased, air flowing over the airfoil is diverted over a greater distance, resulting in an increase of air velocity and more lift.

Question 4

Forces Acting on the Aircraft?

Answer 4

Thrust—the forward force produced by the power plant/propeller or rotor.
Drag—a rearward, retarding force caused by disruption of air flow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind.
Weight—the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the aircraft downward because of the force of gravity.
Lift—opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flightpath through the center of lift.

Question 5

What are the three types of Drag?

Answer 5

Profile

Parasitic

Induced

Question 6

Profile Drag?

Answer 6

Profile drag develops from the frictional resistance of the blades passing through the air. Profile drag is composed of form drag and skin friction. Form drag results from the turbulent wake caused by the separation of air flow from the surface of a structure. The amount of drag is related to both the size and shape of the structure that protrudes into the relative wind. Skin friction is caused by surface roughness.

Question 7

Induced Drag?

Answer 7

Induced drag is generated by circulation around the rotor blade as it creates lift. The high pressure area beneath the blade joins the low pressure area above the blade at the trailing edge and at the rotor tips. This causes a spiral, or vortex, which trails behind each blade whenever lift is being produced. These vortices deflect the airstream downward in the vicinity of the blade, creating an increase in downwash. Therefore, the blade operates in an average relative wind that is inclined downward and rearward near the blade. Because the lift produced by the blade is perpendicular to the relative wind, the lift is inclined aft by the same amount. The component of lift that is acting in a rearward direction is induced drag.

Question 8

Parasitic Drag?

Answer 8

Created by anything moving through the air. Cowlings, openings, windscreens….

Question 9

Airfoil?

Answer 9

An airfoil is any surface producing more lift than drag when passing through the air at a suitable angle. Airfoils are used for stability (fin), control (elevator), and thrust or propulsion (propeller or rotor). Certain airfoils, such as rotor blades, combine some of these functions. In some conditions, parts of the fuselage, such as the vertical and horizontal stabilizers, can become airfoils. Airfoils are carefully structured to accommodate a specific set of flight characteristics.

Question 10

Blade span?

Answer 10

The length of the rotor blade from center of rotation to tip of the blade.

Question 11

Chord line?

Answer 11

A straight line intersecting leading and trailing edges of the airfoil.

Question 12

Chord?

Answer 12

The length of the chord line from leading edge to trailing edge; it is the characteristic longitudinal dimension of the airfoil section.

Question 13

Mean camber line?

Answer 13

A line drawn halfway between the upper and lower surfaces of the airfoil. The chord line connects the ends of the mean camber line. Camber refers to curvature of the airfoil and may be considered curvature of the mean camber line. The shape of the mean camber is important for determining aerodynamic characteristics of an airfoil section.

Question 14

Flightpath velocity?

Answer 14

The speed and direction of the airfoil passing through the air. For airfoils on an airplane, the flightpath velocity is equal to true airspeed (TAS). For helicopter rotor blades, flightpath velocity is equal to rotational velocity, plus or minus a component of directional airspeed.

Question 15

Relative wind?

Answer 15

Defined as the airflow relative to an airfoil and is created by movement of an airfoil through the air.

There are two parts to wind passing a rotor blade:
• Horizontal part—caused by the blades turning plus movement of the helicopter through the air.
• Vertical part—caused by the air being forced down through the rotor blades plus any movement of the air relative to the blades caused by the helicopter climbing or descending.

Question 16

Induced flow?

Answer 16

The downward flow of air through the rotor disk. 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.

Question 17

Resultant relative wind?

Answer 17

Relative wind modified by induced flow. This is inclined downward at some angle and opposite the effective flightpath of the airfoil, rather than the physical flightpath (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. When the helicopter has horizontal motion, airspeed further modifes the resultant relative wind. The airspeed component of relative wind results from the helicopter moving through the air. This airspeed component is added to, or subtracted from, the rotational relative wind depending on whether the blade is advancing or retreating in relation to helicopter movement.

Question 18

Angle of incidence?

Answer 18

The angle between the chord line of a blade and rotor hub. It is usually referred to as blade pitch angle. Collective input and cyclic feathering change the angle of incidence.

Question 19

Center of pressure?

Answer 19

The point along the chord line of an airfoil through which all aerodynamic forces are considered to act.

Question 20

Airfoil types?

Answer 20

Symmetrical- is distinguished by having identical upper and lower surfaces.

Non-symmetrical- has different upper and lower surfaces, with a greater curvature of the airfoil above the chord line than below.

Question 21

Blade Twist?

Answer 21

Because of lift differential due to differing rotational relative wind values along the blade, the blade should be designed with a twist to alleviate internal blade stress and distribute the lifting force more evenly along the blade. Blade twist provides higher pitch angles at the root where velocity is low and lower pitch angles nearer the tip where velocity is higher. This increases the induced air velocity and blade loading near the inboard section of the blade.

Question 22

Rotational Relative Wind?

Answer 22

The rotation of rotor blades as they turn about the mast produces rotational relative wind (tip-path plane). The term rotational refers to the method of producing relative wind. Rotational relative wind flows opposite the physical flightpath of the airfoil, striking the blade at 90° to the leading edge and parallel to the plane of rotation; and it is constantly changing in direction during rotation. Rotational relative wind velocity is highest at blade tips, decreasing uniformly to zero at the axis of rotation (center of the mast).

Question 23

In Ground Effect (IGE)?

Answer 23

Ground effect is the increased efficiency of the rotor system caused by interference of the air flow when near the ground. The air pressure or density is increased, which acts to decrease the downward velocity of air. Ground effect permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced.

Question 24

Out of Ground Effect (OGE)?

Answer 24

Induced flow velocity is increased. A higher blade pitch angle is required to maintain the same AOA as in IGE hover. The increased pitch angle also creates more drag. This increased pitch angle and drag requires more power to hover OGE than IG.

Question 25

Cyclic Feathering?

Answer 25

Cyclic feathering creates a differential lift in the rotor system by changing the AOA differentially across the rotor system. Aviators use cyclic feathering to control the attitude of the rotor system but this does not change the amount of lift being produced by the rotor system.

Question 26

Flapping?

Answer 26

Flapping along with cyclic feathering is the primary means of compensating for dissymmetry of lift. It does not occur when the tip-path plane is perpendicular to the mast. Flapping is the up and down movement of rotor blades about a hinge on a fully articulated rotor system. The semi-rigid systems flap as a unit. Rigid rotors have no hinges. So the blades cannot flap or drag, but they can flex. By flexing, the blades themselves compensate for the forces which previously required rugged hinges. It occurs in response to changes in lift due to changing velocity or cyclic feathering.

Question 27

Primary drag acting on a helicopter while hovering?

Answer 27

Induced drag incurred while the blades are producing lift

Question 28

Translating Tendency? How is it counteracted?

Answer 28

Drift by a single main rotor helicopter in the direction of tail rotor thrust. Counterclockwise aircraft drift right. If the mast is perpendicular to fuselage the aircraft hangs left side low. Counteracted by:

• Transmission tilted to left when mounted.
• Flight control rigging.
• Tail rotor being mounted further up the vertical fin in the horizontal plane of rotor reducing body roll.

Question 29

Coning?

Answer 29

Blades tend to cone upward due to lifting forces and is counteracted by centrifugal forces due to rotor RPM. As the rotor begins to cone due to G-loading maneuvers, the diameter or the rotor disk shrinks. Due to conservation of angular momentum, the blades continue to travel the same speed even though the blade tips have a shorter distance to travel due to reduced disk diameter. The action results in an increase in rotor rpm which causes a slight increase in lift. Most pilots arrest this increase of rpm with an increase in collective pitch. This increase in blade rpm lift is somewhat negated by the slightly smaller disk area as the blades cone upward.

Question 30

Coriolis Effect?

Answer 30

The Coriolis Effect- the law of conservation of angular momentum states that the value of angular momentum of a rotating body does not change unless an external force is 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. Angular momentum is the moment of inertia (mass times distance from the center of rotation squared) multiplied by the speed of rotation.

Question 31

Gyroscopic precession?

Answer 31

Gyroscopic precession is the resultant action or deflection of a spinning object when a force is applied to this object. This action occurs approximately 90° in the direction of rotation from the point where the force is applied. Blade inputs take place 90 degrees in the direction of rotation.

Question 32

Translational Lift?

Answer 32

Improved rotor efficiency resulting from directional flight is called translational lift. As the incoming wind produced by aircraft movement or surface wind enters the rotor system, turbulence and vortices are left behind and the flow of air becomes more horizontal. In addition, the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight.

Question 33

Effective Translational Lift (ETL)?

Answer 33

Between 16 and 24 knots, the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air. The flow of air through the rotor system is more horizontal; therefore, induced flow and induced drag are reduced. The AOA is effectively increased, which makes the rotor system operate more efficiently. This increased efficiency continues with increased airspeed until the best climb airspeed is reached, and total drag is at its lowest point.

Question 34

Characteristics of Effective Translational Lift (ETL)?

Corrected for by?

Answer 34

As speed increases, translational lift becomes more effective, nose rises or pitches up, and aircraft rolls to the right. The combined effects of dissymmetry of lift, gyroscopic precession, and transverse flow effect cause this tendency.

Applying forward and left lateral cyclic input to maintain a constant rotor-disk attitude.

Question 35

Translational Thrust?

Answer 35

Tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight. As the tail rotor works in progressively less turbulent air, this improved efficiency produces more anti-torque thrust, causing the nose of the aircraft to yaw left.

Question 36

Transverse Flow Effect?

Answer 36

As the helicopter accelerates in forward flight, induced flow drops at the forward disk area and increases at the aft disk area. These differences in lift called transverse flow increase the AOA at the front disk area causing the rotor blade to flap up, and reduces AOA at the aft disk area causing the rotor blade to flap down. Because of gyroscopic procession the result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots.Transverse flow is recognized by increased vibrations of the helicopter at airspeeds just below ETL on takeoff and after passing through ETL during landing.

Question 37

Three authoritative regions of a blade?

Answer 37

Driving- also called the propeller region, is nearest the blade tips. Normally, it consists of about 30 percent of the radius.

Driven- the total aerodynamic force (TAF) acts behind the axis of rotation, resulting in an overall drag force and produces some lift, but that lift is offset by drag. The result is a deceleration in the rotation of the blade. The size of this region varies with the blade pitch, rate of descent, and rotor rpm.

Stall- The inner 25 percent of the rotor blade operates above its maximum AOA.