Helicopter Principles, aerodynamics, and emergencies

Cyclic Feathering

The mechanical change of the angle of incidence, or pitch, of individual rotor blades, independent of other blades in the system. If one side pitches up, the other side will pitch down to balance lift.

Transverse flow effect

Right rolling motion and slight vibrations that occur in a helicopter as it begins to transition into forward flight.

This is because of the change of airflow from vertical to horizontal. It occurs between 10 and 20 knts of air speed. As the airflow hits the forward part of the rotor disk it's being transitioned more and more into vertical airflow as it approaches the aft part of the rotor disk.

Chord Line

An imaginary straight line connecting the leading edge (front) to the trailing edge (back) of an airfoil

Rotational Relative Wind

The path that the blade travels around

Resultant Relative Wind

Rotational relative wind modified for the up-flow or down-flow of airflow through the blade.

Dissymmetry of Lift

Unequal amount of lift on opposite sides of the rotor disc occurs in flight due to the difference in airflow over the advancing and retreating blades.

What causes Dissymmetry of Lift

The advancing blade experiences an increase in lift, while the retreating blade experiences a decrease in lift. In forward flight, the advancing blade has a higher airspeed than the retreating blade, creating uneven lift across the rotor disc.

Gyroscopic Procession

The phenomenon of a procession in rotating bodies manifests as an applied force that acts 90 degrees after application, in the direction of rotation.

On an aircraft, if there is an increase in pitch at the nose it will manifest 90 degrees later as an increase in pitch at the left side, inducing a right roll.

Compensation for dissymmetry of Lift

Blade flapping and Cyclic feathering

Blade Flapping

The automatic movement of the rotor blade to counteract the difference in lift. This self-correcting action, facilitated by flapping hinges, ensures consistent lift across the entire rotor disc, preventing rotor tilt and maintaining controlled flight.

Angle of Attack

The lifting region of the blade. The angle between the chord line and the resultant relative wind.

Angle of incidence

The angle between the rotational relative wind and the chord line. Mechanical angle controlled by the cyclic and collective. Sum of AoA and Induced Flow

Induced Flow

The continuous downward movement of air (downwash) pushed by the rotating main rotor blades as they generate lift

Translating Tendency/torque effect

The drifting to the right of the helicopter due to main rotor thrust and tail rotor compensation of main rotor thrust

Compensating for Translating Tendency

Rigging: you can manipulate the flight controls so you already have a slight increase in pitch to fight the right drift

Transmission: If the transmission is slighty left offset, shifting the center of gravity.

FMC: Flight Management Computer automatically makes the necessary flight control input

Pilot Input

Vortex Ring State

A series of vortices that are developing in a rotor system

Settling With Power

A condition where the helicopter tends to settle in its own down wash as a result of vortex ring state

What do you experience in a vortex ring state

reduction in efficiency of the rotor system

a lot of vibrations in early stages

reduced cyclic authority

loss of collective pitch effectiveness

Three requirements for entering a vortex ring state and settling with power

1- Air speed is less than ETL

2- Descent greater than 300'/min

3- 20%-100% power is applied

What conditions are conducive to enter vortex ring state

1- Steep approaches (30°)

2- Downwind/tailwind approach

3- Poor altitude control in an OGE hover

4- Hovering above max hover ceiling

5- Descent from an OGE hover

6- Formation approaches

Recovery from vortex ring state

1- Initial coll. increase

2- Establishing directional flight, if not, forward right cyclic with right pedal

3- Autorotation (Altitude permitting)

Autorotation

A condition where the rotor blades are being driven by an upward flow of air rather than the engine/turbine.

Phases of autorotation

Level flight

Entry into autorotation

Descent

Flare

Termination

Rotor disk during autorotation with 0 air speed

Stall region (root)- ~25%, turbulent air

Driving region- ~25%-70%, Driving the rotor due to upward flow of air, increasing rotor rpm

Driven region (tip)- l~30%, ift producing, absorbing rotor rpm to have auto-rotational/glide descent

Rotor disk during autorotation with forward air speed

Everything is shifted slightly towards the retreating blade. Stalling region increases, driving and driven region decrease due to lower AoA. Blade flapping and feathering compensate for dissymmetry of lift.

Dynamic rollover

The point at which a helicopter will continue to roll over despite pilot input

Static rollover

The point at which an object will continue to roll over from a stationary position

Pivot point

The point where the object pivots

Center of gravity

The point around which an object's weight is evenly distributed

Critical angle

The angle that determines the direction an object will roll

Causes for dynamic rollover

1- Check for tie-downs

2- Skid/wheel contact

3- Stuck landing gear

4- Improper technique for slope takeoffs or landings

5- landing or taking off with cyclic at its limits

Factors of dynamic rollover

Physical: Main rotor thrust, tail rotor thrust, aircrafts center of gravity, crosswind component, and ground surface conditions

Human: Inattentiveness, inexperience, Inappropriate corrective inputs, slow inputs, and loss of visual reference points

How to avoid dynamic rollover

Focus and slow down on takeoffs and landings

Count off landing gear when taking off and landing

Corrective actions for dynamic rollover

Best corrective action is to lower the collective

Types of rotor systems

Rigid, semi-rigid, fully articulated

Rigid rotor systems

Mast is attached to the hub and blades come off of that. Blades can only feather. Stresses were solely absorbed by the blades

Semi-rigid rotor systems

Mast with blades attached to the hub but it had a flapping hinge. Blades could flap and feather so the rotor systems absorbs the stress.

Subject to mast bumping which can cause rotor separation. Common during low G flight and slope landings.

Fully articulated rotor system

Allows each blade to flap and feather independently also has blade hunting which gives the blades the ability to lead and lag. The blade hunting prevents the blades from facing the Coriolis force.

Parasite drag

Air resistance on the object as it moves. Increases with surface area and speed. Basis of why you want to streamline aircraft. Increases exponentially as air speed increases.

Deals with non-lifting parts of the helicopter.

Profile drag

Caused by frictional resistance of the blades as they move through the air.

Increases at a gradual rate as air speed increases.

Deals with lifting parts of the helicopter.

Induced drag

The result of producing lift. Higher angle of attack where you make more lift and downward velocities and vortices which induces induced drag.

Angle between axis of rotation and lift vector.

Exponential decrease in drag as airspeed increases.

Total drag

Sum of parasite, profile, and induced drag.

The lift equation

Lift

ClSA1/2p*Vsquared

Coefficient of lift

Surface area

Air density

Velocity squared

Coefficient of lift in the lift equation

A measure of the amount of lift a particular airfoil shape can produce. Based on shape and angle of attack. Pilots can influence this through the angle of incidence

Surface area in the lift equation

general surface area of the rotor disc. larger the area the more lift you'll have. Rotor coning can be controlled by the pilot. As the rotor starts to cone, the diameter decreases in surface area. Thus producing less left since there is more lifting force than centripetal force.

Influenced by low rotor rpm, high gross weight, and excessive G-forces

Air density in the lift equation

The viscosity of the air. Effects how easily the rotors can move the air around it. As density increases, the easier it is for the rotor to move it. As atmospheric pressure increases, air becomes more dense and performance increases. Altitude increases, density decreases and performance decreases. Temperature warm air expands and reduces density, decreasing performance. Increased moisture content of the air decreases performance.

Velocity squared in the lift equation

Greatest effect on lift because it directly effects the speed of the air speed around the airfoil as it travels through the air. V has a multivariable influence on lift because if it decreases enough you can have a stall condition and/or low rotor rpm. Exponential impact on lift.

Dynamic pressure

1/2p*Vsquared

measures the airspeed through pitot tubes

Mast bumping

When the main rotor hub contacts or bumps the rotor mast. Only applies to semi-rigid and teetering rotor systems.

Airfoil

The surface that produces more lift than drag at a suitable angel. A surface that bends and manipulates the wind to produce lift.

Blade span

The length of the blade from the hub/point of rotation to the tip.

Leading edge

The rounded portion that projects into the relative wind

Trailing edge

Tapered edge that projects away from the relative wind

Camber

The curvature of the airfoil itsself

Mean camber line

Line halfway between upper and lower surfaces of the airfoil

symmetrical airfoil

Cheap and easy to produce, heavy and sturdy, very stable, mean camber line has equal parts above and below

asymmetrical airfoil

Better lift/drag ratios, better stall characteristics

As pressure changes differently around the airfoil, the chord line shifts which causes excessive flapping, lead, and lag.

Vertical stabilizer

Helps directional stability. Located at end of tail boom.

Horizontal stabilizer

Helps hold fuselage steady by counteracting the nose up tendency

Gurney flaps

Increase lift/drag ratio of an existing airfoil by improving the boundary layer flow across the airfoil.

Bernoulli's Principle

As fluid moves through a narrow space, it increases in velocity and decreases in pressure. Also known as the Venturi effect

How air acts around an airfoil

Above: Faster air, lower pressure

Below: Slower air, higher pressure

Newton's 3rd law

For every action there is an equal and opposite reaction. An

Inclined plane method of lift

Airfoil at an angle pushes air down which pushed the airfoil up.

Total aerodynamic force

Generally up and aft of the airfoil.

Conservation of Angular Momentum

Angular momentum of a rotating body will remain constant unless external forces are applied.

Coriolis Force

The tendency of a rotating body to increase in acceleration as the center of gravity gets closer to the axis of rotation (mast)

What you experience under conservational of angular momentum

G-loading

Blade diameter shrinks

center of gravity/mass shifts inward, causing rotor rpm to increase

Coriolis negative effects

Over speeding a rotor if it happens to quickly, under speeding a rotor if it slows down and the engine can't keep up in time, exceeding torque limit during rapid unloading G forces

Regions of a rotor disk

The driven region (tip), driving region (mid-span), and stalled region (root).

Reverse flow

Air flows from trailing edge to leading edge so it doesn't produce lift. Only happens on inboard retreating side.

Negative stall

Induced flow causes the airflow to push down on the airfoil outside the critical angle so it's useless and you aren't getting any lift. Only happens on inboard retreating side.

Negative lift

Airflow is impacting the airfoil because the above the chord line but it is considered lift because it is above the chord line but is inside the critical angle. Only happens on inboard retreating side.

Positive lift

More on the inboard advancing side than the inboard retreating side. The resultant airflow is inside the critical angel and below the chord line, pushing the airfoil up.

Positive stall

Forms on outboard retreating side because it is below the cord line and outside critical angle. No longer getting lift because it isn't impacting the airfoil at a beneficial angle.

Retreating blade stall

In high speed flight, positive stall region grows into mid section of blades on retreating side due to greater angles of attack during higher forward air speeds.

Retreating blade stall causes

Excessively high forward air speed

Low rotor rpm (low nr)

Too much forward cyclic

Excessively high collective angle

Vne

Velocity never exceed. Never fly at or above this speed to avoid retreating blade stall.

Retreating blade stall symptoms

Vibrations due to differences in lift and drag throughout the rotor systems

Vertical bounce

Flight control feedback I.e. stiffening or ineffectiveness

Uncommanded and violent pitch up of nose and roll to retreating side

Retreating blade stall recovery

Lower the collective, reducing AoA of blades, reducing air speed, and bringing blade rpm up

Reduce the severity of the movement you were doing and adjust flight controls for normal flight

Loss of tail rotor effectiveness (LTE)

Uncommanded rapid yaw rate that does not subside on it's own accord and which if not corrected can result in loss of control. Yaw rate is to the right in a counterclockwise rotor system.

LTE is a wind issue that occurs when operating at speeds less than ETL

Types of LTE

Weathercock stability

Tailrotor VRS

Main rotor disk interference

AoA reduction

Weathercock stability

Wind is impacting the helicopter from the aft position (4 to 8 o'clock). The helicopter tends to weathervane and point into the wind. Unless resisting pedal input is made, an uncommanded turn to the right or left will occur. Attempting to hover in this position will cause high pilot workload.

Tailrotor VRS

When wind impacts the tail rotor from the 8 to 11 o'clock position. The tail rotor essentially settles with power but when enough of a left cross wind occurs, it creates a vrs laterally reducing the tail rotors ability to counteract the forces produced by the main rotor

Main rotor disk interference

When wind impacts the helicopter from the 9 to 11 o'clock position. As a result of your main rotor disk vortex interfering with the tail rotor, causing the tail rotor to operate in disrupted air.

AoA reduction

When wind impacts the tail rotor from the 2 to 4 o'clock position. There is a right crosswind on the tail rotor that causes an AoA reduction. the cross wind increases induced flow of the tail rotor which decreases the thrust.

LTE recovery

Must recognize quickly.

Apply forward cyclic and pedal control to get out of it.

If altitude permits, a collective reduction can help get out of it.

Types of airspeed (least to most accurate)

Indicated

Calibrated

True

Indicated airspeed (IAS)

Difference in airspeed from ram air entering the pitot tube and static pressure measured by a static port.

Calibrated airspeed (CAS)

Based on flight testing and corrections for known errors on instruments.

True airspeed (TAS)

Calibrated air speed adjusted for temperature and pressure.

Crucial airspeeds

VBE- Velocity Best Endurance. Gives you the greatest air-born time per unit of fuel consumed.

VY- Maximum rate of climb. The speed where the aircraft climbs at the fastest rate per unit of time. Roughly the same airspeed as VBE.

VX- Best climb angle. Yields greatest altitude gain over distance.

VBR- Velocity best range/max range air speed. greatest range per unit of fuel.

VH- max speed the helicopter can fly

Man made wind indications

Wind socks: weathervane into the wind with at least 3kts of wind, if fully extended 15kts

Flags/banners if no wind sock:

ASOS, AWOS, AFIS: weather stations

Smoke and dust clouds 45 degrees is 10-15kts

Natural wind indications

Trees

Fields of tall grass. You can see grass move at 10-15kts

Waves in lakes. Water is smooth on upwind side and wavy on downwind side. White capping happens at >15kts

Docked boats point into the wind

Birds take off and land into the wind

Cockpit wind indications

Compare air speed to ground speed. If air speed > ground speed= into the wind, airspeed < ground speed= tailwind

Crab angle: anytime you're in aero dynamic trim, the aircraft want's to point towards the wind

Turn around a point: As you turn around a reference point on the ground, the wind will push the aircraft and elongate the circle showing you where the wind is coming from.

Rotor wash: Look to see where your rotor wash is longer around your aircraft and that will be your down wind

Height-Velocity/Deadman's Curve

The area of certain velocities and altitudes that pilots should avoid because recovery after an engine failure is unlikely.

Advancing blade compressibility

A high speed, low temp flight where advancing blade reaches a mach one or higher speed.

Calculating advancing blade compressibility

C = 2πr=ft

answer in ft*main rotor rpm= ft/min

then convert to kts

subtract speed of sound from answer in kts to find speed where you can begin experiencing compressibility

Advancing blade compressibility symptoms

Experience nose pitch down and right roll followed by extreme vibrations. Rotor blade will experience severe stress and can be damaged.