TC's passion is helicopters and he has been involved with them for many years.

 

 

 

 

 

READ the following first:

 

Purpose: I have a picture over my desk of a WW1 biplane crashed into the top branches of a large tree.  The caption reads: 'Aviation in itself is not inherently dangerous.  But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect.'   Most of us have seen the above picture, but for those of you who haven't, please reflect the statement for a few minutes. This site is intended to be always an unofficial guide to the safe and efficient operation of popular light helicopters based initially on Australian & New Zealand experience. This site makes available information, materials and products subject to the following terms and conditions. By accessing this site, you agree to the terms outlined in this legal notice.

 

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Terminology: DA                             Density Altitude    (Above sea level) PA                             Pressure Altitude   (Above sea level) MAP                          Manifold Pressure  (USA : Inches) OAT                          Outside Air Temperature (measured in degrees Centigrade)

Climatic Considerations  Australia: The local conditions in some areas can vary from 0°C to +45°C, 90% humidity in the Northern tropics (even at 500km from the coast!) with the DA approaching 7000 ft at 2000 ft PA. Combining this with the red highly erosive gritty dust that covers the ground in many places gives the pilot of any Helicopter an environment which requires good training and skills, especially operating at low level.  New Zealand: Hundreds of thousands of years younger than it’s close Aussie neighbour, NZ consists of two main large islands with a much colder climate.  It is ideally suited for turbo charged piston and turbine helicopter operations because it’s mountainous terrain (up to 12,000 ft) and lush vegetation in high rain fall areas reduce the number of areas suitable for fixed wing landing strips. As in Australia, the pilot requires good training and skills to operate in what usually is a daily high DA working environment.

 

Helicopter Flight controls names and Basic Descriptions and other Helicopter things that happen!!

 

Collective pitch lever:

·        Changes pitch of each blade an equal amount (relative to the Main Rotor hub)

·        An increase in pitch is accompanied by an increase in Main Rotor (MR) Thrust and a corresponding requirement to increase power to maintain MR RPM due to increased MR Blade drag

·        Where there is a decrease in pitch - It results in less MR thrust - therefore less MR Blade drag means less power is required to maintain MR RPM

 

Cyclic pitch stick:

·        Through the swash plate angle in relation to the MR mast - Alters pitch on each MR blade consecutively/independently of each other blade (relative to the mast)

·        In a nil wind hover - total MR thrust directly opposes gravity (weight)

 

Flapping:

·        Is a vertical (up or down) movement of the MR Disc or blades above and below the plane of rotation due to an increase or decrease in lift on the MR blades – usually caused by the forward airspeed increasing/decreasing (cyclic) or a change of MR Thrust at any airspeed.

·        The same effect happens in a hover held over a spot when the wind increases or decreases.

·        Flapping occurs around an axis – visualise the disc rotating above and at a right angle to the MR mast as an example.

·        A Delta 3 flapping hinge (to limit total flapping) is set at an angle (not 90 degrees) in relation to the chord to reduce pitch when flapping up and an increase in pitch when flapping down.

·        The Delta 3 hinge can be  on all the  individual MR  blades or can be built into the hub design.

·        Tail Rotor blades and the TR Disc is basically a small MR assembly on its side and has the same flapping characteristics as the MR Disc/Blades.

·        Many TR Assemblies have the Delta 3 hinge built into the hub – eg- Hiller 12E/Bell 47/206 and Robinson R22/44

 

Semi rigid rotors:

·        Flap as a unit

·        On a teetering hinge

·        First used by Juan De La Cierva on his gyro copters

 

Fully articulated rotors:

·        The main rotor hub is firmly attached to the MR mast and does not teeter.

·        Each blade flaps independently because it has its own flapping hinge

 

Dragging:

·        Only In a fully articulated rotor system

·        Freedom of each blade to physically move independently in the Disc plane of rotation.

·        The Blades lead and drag (go backwards a little – drag- and than back - lead -to the normal position) to overcome geometric imbalance

 

Soft in plane:

·        Used to describe rotor heads without physical dragging freedom

·        Soft in plane rotors use Elastomeric Rubber like synthetic polymer bearing rather than a dragging hinge. Visualise as each blade held in place by a bit of rubber and the rubber “squashing” a bit when required to allow the blade to slightly move back and forth to eliminate imbalance.

 

Gyroscopic precession:

·        Is a effect which occurs when a force is applied to the outer edge of a spinning disc

·        The effect with be felt 90 degrees later (in direction of rotation)

·        Visualise touching the outer edge of a kid’s spinning toy top which is turning clockwise. Touch it at the 3 o’clock position and it will move in a direction from 12 o’clock -  not the 9 o’clock direction

 

Advance angle:

·        Is the angular difference between the pitch operating arm position on the swash plate and the position where the blade pitch is actually changed.

 

Phase Lag:

·        Is the Angular difference Between the tilt of the swash plate and the tilt of the MR  disc.

 

Ground effect:

·       Using a stationary nil wind hovering reference point of about 2 ft from the bottom of the helicopter skids to the ground - a cushion of air from the rotor downwash builds up under the helicopter while it is hovering as the air being pushed downwards by the rotor disc slows down as it hits the ground and struggles to get away from under the helicopter rotor disc  as it has to change direction and flow sideways to escape.

·        This “Ground Cushion of air” builds up and opposes the high speed and volume of the airflow coming down through disc - reducing the amount of the normal induced flow (pitch) required to hold the helicopter at that height AGL. 

·        This equilibrium state (IGE) enables a decrease in power required to hover as compared to a higher hover (OGE)

·        For a less amount of power required.

 

Ground effect is reduced:

·        Whenever the ground cushion air can escape quickly or cannot build up (OGE) in conditions such as hovering over sloping ground, undulating surfaces, water surfaces, tall crops, thick timber or forest.

·        Ground effect is lost (left behind) in forward flight around at approx. 10-12 knots (Transition)

 

Tail rotor:

·        Counteracts Torque when energy is applied to the Main Rotor system by an engine

·        Torque reaction rotates the fuselage in the opposite direction of MR blade rotation

·        The Higher the torque input – the faster the reaction of the fuselage will be.

·        The Tail Rotor is situated some distance from the MR mast to gain leverage.

·        Tail rotor thrust is utilised to overcome  torque reaction.

·        The tail Rotor always produces positive thrust – either very little (Right pedal on the R22) or a lot (left pedal on an R22)

·        As the tail rotor iss virtually a small MR disc on its side – when high Thrust is required it requires approx 10% of engine power to overcome TR Blade drag and internal gearbox friction.

 

Tail rotor drift:

·        Is the result of a unbalanced couple between torque affect tail rotor thrust

·        The Helicopter tends to drift right.

Engineering overcomes  to a certain extent by:

·        Tilting the mast a few degrees to the left

Or

·        rigging the swash plate to tilt the disc left a degree or so.

 

Tail rotor roll:

·        This happens when the centre line of the tail rotor is positioned lower than the main rotor hub.

·       Visualise this when the helicopter is on level ground by mentally drawing a line parallel to the ground from the centre of the TRotor to a point in line with the MR mast. On an R22 this is about the same level and just in front of the LH fuel tank cap.

·       Then visualise and draw a mental line from the above point straight up to the middle of the MR Hub.

·       When airborne, the helicopter is a pendulum hanging from the rotor hub.

·       When too high a rate (distance and speed) of input is made by the pilot to change TR pitch, not only does the nose of the helicopter turn in the direction of the input – but as a result of the vertical unbalanced couple – the helicopter will also roll and pitch. Pilots new to helicopters then – instead of using correct slower pedal input to reduce the rate of turn or bring the nose back to where they want it – make cyclic inputs to correct the issue and it tends to get worse due overcontrolling.

·       Making measured, controlled light inputs of the TR pedals at all times, virtually eliminates this type of over controlling.

·       When in a stabilised hover in the R22 – TR Roll tends to make the helicopter hover left skid low.

 

Offsetting:

·        In some helicopters such as the Hiller 12E and B47G - the Main Rotor mast is offset to the left of helicopter’s centre line which reduces tail rotor roll.

 

 Re-circulation:

·        Occurs when induced flow of air through the MR disc – usually but not always in the hover - is unable to dissipate horizontally because it is close to and strikes a barrier such as a fence, cliff, clump of trees etc.

·        The induced airflow then interacts continually around the outside of the disc with the MR Blade tip vortices and is accelerated through the disc a number of times instead of building a ground cushion or hovering in “clean air”.

·        The resulting reduction in lift can cause an accident.

 

Vortex ring state:

VRS Has the following 3 requirements at the same time to commence (Most common on final approach with a tail wind.)

·        A high MR Blade Pitch angle

o   In a power on slow decent

or

o   A strong flare to arrest a fast descent

or

o   A strong high pitch pull during an autorotative flare

 

·        A high rate of decent – different for various helicopter designs and rotor systems but usually stated but not limited 300 ft/min

·        A low forward airspeed (above the hover) – through transition – usually stated but not limited to less than 30 Kts

 

DO NOT DELAY your response to the initial signs of VRS

 

Text book corrective actions – take at least 1 of the critical 3 requirements for VRS out of the equation and fly away.

        Cyclic

o   With some safe height AGL – FWD to gain airspeed

o   On approach – any direction 180 Degrees FWD of the Pilots Shoulders to increase airspeed and enable non contact with external objects such as rocks/trees etc

 

        Collective

                        immediately lower as required to reduce pitch (and MR Blade Tip vortices) and not make contact with external objects

·        If height not critical enter autorotation with no initial flare (to keep as much airspeed as possible) to reduce tip vortices ASAP

 

Flapping to equality: (dissymmetry of lift)

·        Occurs when advancing blade experiences increasing lift

·        Due to increased relative airflow

·        Which in turn reduces AOA

 

Flap back:

·        Occurs during accelerated flight or in fixed Fwd flight if the collective is raised.

·        Because the advancing blade experiences the highest relative airflow (more lift) at the 3 o’clock (approx.) position

·        Gyroscopic precession occurs  90 degrees later – although for practical purposes think: more lift happening causes the blade to rise up until the 12 o’clock position when it flaps down as the lift reduces.

 

Inflow roll (also causes flapback)

·        Occurs in forward flight

·        Due to more air being accelerated through rear of MR Disc than that of the front of the disc.

·        This results in increased induced flow at the rear of the disc - which in turn creates a lower Angle Of Attack and the blade flaps.

·        The maximum rate of flap down is when blade is in most rear position.

 

Airflow reversal:

·        Is created due to the forward speed being faster than the airflow which is created due to rotational velocity of the blade

 Most likely to occur:

                  At high forward airspeeds

                 And low RRPM

·        Occurs first near the root of retreating blade and then spreads outwards as forward speed increases or RRPM reduces.

 

  

Retreating blade stall:

·        Occurs at high forward airspeed (compressibility)

·        And high speed (but not necessarily VNE) with low rotor RPM

Symptoms:

·        On certain aircraft types - Sudden Shuddering or Vibration – with Nose pitch up (often sudden and strong) followed by a roll to the left while others roll to the right.

Compressibility:

·        Occurs when high rotor RPM – AND - high forward airspeed combine to create relative airflow faster than the speed of sound at the tip of the advancing blade

 

Hooks joint effect:

·        Occurs in the rotor system with MR Blade dragging Hinges/attachments

·        Due to coriolis affect - The C of G will be displaced.

         A semi rigid rotor system over comes this by

·        By under slinging the blades below the hub attachement.

 

Transition:

·        Is the change of state from hover to movement in horizontal direction Or vice versa

·        As speed increases (Fwd - usually in relation to the helicopter body but can be from any direction the MR Disc is travelling towards), a Horizontal airflow is created across the disc from the direction

·        Horizontal airflow reduces the amount of induced flow through the disc

Resulting in:

·        Reduced rotor drag

·        More efficiency

·        Less power required

 

      Flare effect:

·        Decelerates the helicopter in the direction as the disc is tilted (usually aft)

·        Horizontal airflow acting below the MR disc reduces induced flow increasing main rotor thrust which in turn increases disc loading – then causing the MR blades to cone up and Coriolis effect increases RRPM

 

Parasite drag:

·        Is caused by airflow around the fuselage

·        Parasite power (read engine in normal flight) is used to overcome it

·        Parasite drag increases with the square of the speed - So if the helicopter speed is doubled then the increase drag requires required by 4 times the power to balance the drag.

·        EG: If an R22 requires 100 Hp for 100 kts – it will require 10000 Hp for 200 kts.

 

Rotor profile power:

·        Is the power required to drive ancillary equipment and the rotor system at zero Angle of Attack to overcome rotor profile drag.

Rotor profile drag is fairly constant at all speeds

·        It increases slightly at high forward speed

 

Induced power:

·        Is the Power required to overcome rotor drag

·        The Induced Power required is at a maximum when hovering out of ground affect (hoge) Due to high pitch angles/high induced flow

Induced Power reduces to a minimum/ as forward airspeed increases – ie – Induced Airflow