Aerodynamics

Components of an Airfoil

- Leading Edge - Trailing Edge - Mean Camber Line - Chord Line

Bernoulli's Principle

An increase in the speed of a fluid produces a decrease in pressure and a decrease in the speed produces an increase in pressure

Airfoil

Any surface producing more lift than drag when passing through the air at a suitable angle. Two Types: - Symmetrical - Asymmetrical

Types of Drag:

- Profile Drag - Induced Drag - Parasite Drag - Total Drag: sum of all three

Weight

A known, fixed value, such as the weight of the helicopter, fuel, and occupants. Can also be influenced by aerodynamic loads (banking left/right at steep angles add G's)

Tip-Path Plane

Also known as Rotational Relative Wind, is the imaginary circular plane outlined by the rotor blade tips as they make a cycle of rotation. • Flows opposite to the physical flightpath of the airfoil • Strike the blade perpendicular to the leading edge • Always parallel to the plane of rotation • Constantly changing in direction during rotation • Highest velocity a the blade tip, where blade speed is the highest • Decreases uniformly to zero a the axis of rotation

Translating Tendency

Also known as drift. Tendency of the helicopter at a hover to move in the direction of tail rotor thrust

Three Axis of Rotation

An axis is defined as a line passing through a body about which the body revolves. All axis of flight ass thru the center of gravity • Y or Vertical Axis - Yaw • Z or Lateral Axis - Pitch • X or Longitudinal Axis - Roll

Asymmetrical Airfoil

Asymmetrical (Airplane wings) ○ Camber Line ≠ Chord Line ○ Center of pressure shifts as AOA changes ○ Positive lift @ 0 AOA ○ Improved Lift/Drag ratio ○ Better stall characteristics ○ More torque on airfoil structure ○ More expensive to make

Profile Drag

Develops from the frictional resistance of the blades passing through the air. ○ No significant change with the airfoil's AOA ○ Increases with airspeed ○ Composed of: - Form Drag: Related to both the size and shape of the airfoil - Skin Friction: Airfoil surface roughness

Newton's Third Law

For every action there is an equal and opposite reaction; The air that is deflected downward also produces an upward (lifting) reaction

Lift

Force generated when an object changes the direction of flow of a fluid or when the fluid is forced to move by the object passing through it. 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.

Drag

Force that resists the movement of a helicopter through the air and is produced when lift is developed

Induced Drag

Generated by the airflow circulation around the rotor blade as it creates lift ○ Increases with airfoil's AOA ○ Decreases when airspeed increases; Increases when airspeed decreases ○ Major cause of drag a low airspeeds

Thrust

Generated by the rotation of the main rotor system. In a helicopter, thrust can be forward, rearward, sideward, or vertical. The resultant lift and thrust determines the direction of movement of the helicopter

Out of Ground Effect (OGE)

Hovering at higher power when above the IGE altitude. Requires more power due to increased induced flow (downwash), which requires a higher blade pitch, which in turn creates more drag. High risk of settling with power.

In Ground Effect (IGE)

IGE - the increased efficiency of the rotor system caused by interference of the airflow when near the ground. The air density is increased, which acts to decrease the downward velocity of air • Maximum ground effect achieved over smooth hard surfaces • Reduced when hovering over tall grass, water, tree & bushes, rough terrain • Takes effect to a height of one rotor diameter measured from the ground to rotor disk

Transverse Flow Effect

In forward flight, the induced flow drops to near zero at the forward disk area and increases at the rear disk area. These differences in lift between the front and aft portions of the rotor disk are called transverse flow effect. This increases 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. The result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots. • Increased vibrations at airspeeds just below ETL on takeoff & after passing thru ETL during landings • Cyclic input to the left is needed to counteract it

Angle of Incidence

Mechanical angle between the chord line and the rotor hub. Also referred to as blade pitch angle • Angle of Incidence = AOA when no induced flow (downwash) is present

Retreating Blade Stall

Occurs at high forward speeds when the retreating blade stalls as a result of excessive blade flapping, increasing the AOA, and the slow relative wind acting on the retreating blade. • Nose pitches up • Low Frequency vibrations • Rolling to the direction of the stall (left for counter-clockwise rotors) • Avoided by not exceeding the Never-Exceed Speed (VNE)

Coriolis Effect

Or law of conservation of angular momentum. A rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of rotation. • The closer the mass gets to its center of rotation, the faster it will move • As the blades flap up, the move closer to the center of rotation and thus speed up • The lead-lag hinge on a fully articulated rotor system compensates for this effect

Pendular Action

Oscillating tendency of the helicopter to swing like a pendulum because the main body is suspended from a single point to the main rotor system. • It can be exaggerated by overcontrolling

Parasite Drag

Present any time the helicopter is moving through the air. Generated by non-lifitng components of helicopter: cabin, rotor mast, tail, landing gear ○ Increases with airspeed: PD = V^2 ○ Major cause of drag at high airspeeds

Solidity Ratio

Ratio of the total rotor blade area, which is the combined area of all the main rotor blades, to the total rotor disk area. Measures the potential for a rotor system to provide thrust and lift

Total Drag

Sum of all three drags; Profile, Induced, Parasite ○ Creates total drag curve ○ Lowest point in curve: airspeed where drag is minimized - L/Dmax = lift-to-drag ratio at the lowest point in curve

Symmetrical Airfoil

Symmetrical (Helicopter rotor blades) ○ Camber Line = Chord Line ○ No lift @ 0 Angle of Attack (AOA) ○ Center of pressure does not change as AOA changes

Coning

Tendency for the rotor blades to assume a conical path as a result of centrifugal forces (pulling the blades outward) and lift forces (pushing blades upward)

Relative Wind

The airflow relative to an airfoil • Always moves in a parallel, but opposite direction to the movement of the airfoil • Comprised of: ○ Horizontal component = blades turning + movement of helicopter thru air ○ Vertical component = air being forced down thru the rotor blades + movement of air from helicopter climbing or descending

Angle of Attack (AOA)

The angle between the airfoil's chord line and the resultant relative wind • As the AOA increases, the center of pressure on a non-symmetrical airfoil moves upward towards the leading edge • As the AOA decreases, the center of pressure on a non-symmetrical airfoil moves downward towards the trailing edge • The center of pressure stays the same in symmetrical airfoils regardless of AOA • Burbling occurs at the trailing edge of an airfoil. It increases as the AOA increases. Once 2/3 of the airfoil is affected by burbling, a stall condition occurs

Effective Translational Lift (ETL)

The point in which the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air. This increased efficiency continues with increased airspeed until the best climb airspeed is reached, and total drag is at its lowest point • Nose pitches up • Rolls to the right • Pilot needs to apply forward and left lateral cyclic input to maintain a constant rotor-disk attitude

Center of Gravity

The single point on the helicopter, regardless of its attitude, about which it can be balanced perfectly.

Dissymmetry of Lift

The unequal lift between the advancing and retreating blades of the main rotor caused by the different wind flow velocity across each blade - retreating & advancing. • It is compensated by blade flapping • Max forward airspeed is restricted by dissymmetry of lift

Gyroscopic Precession

When a force is applied to the outside of a rotating body, parallel to its axis of rotation, the rotating body tilts 90 degrees in the direction of rotation from the point where the force is applied

Translational Lift

When the helicopter accelerates to an airspeed of approximately 15 knots in forward flight, a rapid increase in excess power develops. This is due to an increase in the efficiency of the rotor system. • 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

Steady State Flight

When the helicopter is in straight-and-level, un-accelerated flight, the opposing forces balance each other in what is called steady state flight: Lift = Weight Thrust = Drag

Skid

When the helicopter slides sideways away from the center of the turn. Occurs when the rate of turn is too great for the amount of bank being used, and inertia exceeds the horizontal component of lift (HCL)

Slip

When the helicopter slides sideways toward the center of the turn. Occurs when the rate of turn is too low for the amount of bank being used, and the horizontal component of lift (HCL) exceeds inertia

Four Aerodynamic Forces

lift, weight, thrust, and drag

Axis of Rotation

the imaginary line about which the rotor rotates. It is represented by a line drawn through the center of, and perpendicular to, the tip-path plane. Not the same as rotor mast.

Lift depends on:

• Airflow Speed • Air Density • Airfoil Area • Angle of Attack (AOA) between air and airfoil

Weight Principles

• Any time a helicopter flies in a constant altitude curved flightpath, the load supported by the rotor blades is greater than the total weight of the helicopter • The tighter the curved flightpath is, the steeper the bank is; the more rapid the flare or pullout from a dive is, the greater the load supported by the rotor • Each rotor blade must support a percentage of the gross weight. ○ 2 blades: Each blade carries Total Gross Weight/2; 3 blades: Each blade carries Total Gross Weight/3

Two principles of Lift:

• Bernoulli's Principle • Newton's Third Law of Motion