helicopters

04/12/23 Author: Harry L. Whitehead 1

VI. Helicopters

A. History

B. Configurations

C. Types of Rotor Systems

D. Forces Acting on the Rotor

E. Flight Conditions

F. Controls

G. Stabilizer Controls

H. Vibrations

I. Power Systems


HelicoptersHistory

1483, DaVinci– Developed “Helix”– Kind of aerial screw– Shows basic

understanding that the atmosphere can support weight but no provisions for torque on fuselage

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1800s, Forlanini (Italy)– Used steam engine– Counter-rotating “butterfly” wings– Could ascend (without pilot) to 40 feet for about

20 minutes

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1907, Cornu (France)– First piloted helicopter– Flew for few seconds– Used internal combustion engine– No controls but well balanced

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory 1909, Igor Sikorsky

(Russia)– Small counter-rotating

coaxial rotors– First use of airfoil

shaped rotors

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1920s, Petroczy & Von Karmon (Austria)– Counter-rotating,

coaxial, airfoil rotors– 3 40HP engines– No controls, just made to

lift straight up

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1923, de Bothezat (U.S.)– 4 rotors– Complicated power

transmission system– Low power– Several flights of 1

minute @ 6 feet

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1923, de la Cierva (Spain)– Developed Autogyro– Solved some control

problems by allowing rotors to Flap

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1936, Focke-Wulfe (Germany)– FW-61 established

endurance & speed records

– Mostly flown by Hannah Reich

– Flown inside stadium for most of records

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersHistory

1939, Sikorsky (U.S.)– Developed VS-300– Broke all FW-61

records– Used 3-bladed main

rotor, vertical 2-bladed tail rotor & 2 horizontal 2-bladed outrigger rotors for stability and control

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersConfigurations

Autogyros– Developed by de la

Cierva– Uses free-spinning main

rotor with airplane-like engine/prop for forward motion

– No power to main rotor, spins from air action = can’t hover or ascend vertically

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Dual Rotor– 2 counter rotating main

rotors• No tail rotor needed • May be separate or

coaxial– Used extensively

through history, today few (Boeing, Kaman)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Single Rotor– Most used design– 1 main rotor for lift and

control– Tail rotor for anti-torque

• FAA calls it “Auxiliary Rotor”

• More precisely known as “Anti-torque Rotor”

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Single Rotor

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Hughes Helicopter H-17 Skycrane1952

Function: transportCrew: 2 Engines: 1 * G.E. J35Rotor Span: 130ft Length: Height: 30ft Disc Area: Empty Weight: Max.Weight: 46000lbSpeed: Ceiling: Range: 65kmLoad: 25000lbs

Hot Cycle Blades



Tilt Rotor– Bell V-22– Engines and main rotors

(“PropRotors”) mounted on wingtips

• Rotate so rotor is horizontal (on top) to takeoff and land like helicopter

• Rotate so rotor is vertical to act like prop for high speed forward flight

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersTypes of Rotors General

– All must change blade angle or Pitch for control actions

– Called “Feathering”– Is rotation around the

span axis of the blade

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Some also:• Flap or Teeter

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Some also:• Lead/Lag (Hunt

or Drag)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersTypes of Rotors Semi-Rigid Rotor

– 2-bladed– Blades Feather and

entire rotor Teeters– No Hunting action

allowed– Very popular in early

Bell designs (and others)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


Semi-Rigid– Bell 206

HelicoptersTypes of Rotors

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Fully Articulated Rotor– 3 or more blades– Blades can Feather, individually Flap, and Hunt

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

–Hunting limited by mechanical Dampers



Fully Articulated– Is most complicated but smoothest in flight– Problem: Ground Resonance potential

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Fully Articulated– Hughes 500

(McDonnell-Douglas, Boeing)



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Fully Articulated– Sikorsky S58



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Fully Articulated– AStar 350



Rigid– 2 or more blades– Blades Feather but all other forces absorbed by

bending of the blades– Strongest and most maneuverable but needs

composites to withstand fatigue

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

04/12/23 27

HelicoptersForces on the Rotors Static Forces

– Gravity pulls down and blades can bend relatively low

• Called Droop– All need some kind of

Droop (Static) Stop to prevent too low and possible Tail Boom strike

• Especially for Fully Articulated at low RPM

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersForces on the Rotors Turning Forces

– Centrifugal Force tries to hold the blades straight out but lift tries to bend up

– Result is Coning• Upward bending into Cone shape• More lift = more Coning

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersForces on the Rotors Torque

– From Newton’s 3rd Law

– Main rotor turns in one direction = fuselage tries to turn opposite (Torque)

– Is directly proportional to power applied to M/R

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Compensated for by Tail Rotor thrust

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

What happens if Tail Rotorfails during flight?



– Compensated for by Tail Rotor thrust or counter-rotating M/Rs

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersForces on the Rotors

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Torque– Problem: Tail Rotor

causes “Translating Tendency” or “Drift”

• Is movement of entire helicopter in direction of T/R thrust (to right in U.S.)

• Compensated by slight tilt of M/R mast to left


HelicoptersForces on the Rotors Gyroscopic Precession

– Any rotating body (M/R) acts like a Gyroscope and exhibits 2 characteristics:

• Rigidity• Precession

– Rigidity resists the change from it’s position in relation to space, not the Earth

– Precession is the fact that the effect of any upsetting force applied to the body is felt 90o later in direction of rotation

• Affects the design and rigging of the M/R

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– For flight = need to tilt “Rotor Disk” in direction of desired flight

• Changes lift & thrust vectors toward that direction = movement of helicopter

– To accomplish = need to make pitch change 90o earlier

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Desired direction of flight



– For flight = need to tilt “Rotor Disk” in direction of desired flight

• Changes lift & thrust vectors toward that direction = movement of helicopter

– To accomplish = need to make pitch change 90o earlier

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersForces on the Rotors Ground Effect

– Increased lift within ½ rotor diameter of ground– “Cushion of Air”– Comes from change in angle of attack near

ground because relative wind changes

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


Ground Effect– Out of Ground Effect (OGE)

• Rotor wash is free to accelerate straight down = given angle of attack and lift and large tip vortex

Rotation

Angle of Attack

Downwash

Relative Wind


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


Ground Effect– In Ground Effect (IGE)

• Rotor wash is forced to move outward as well as down = reduced down vector = increased angle of attack + smaller tip vortex

Downwash

Rotation

Angle of Attack

Relative Wind


HelicoptersForces on the Rotors Flight Forces

– Same as airplane:

• Lift up• Weight

(Gravity) down• Thrust forward

and up• Drag back and

down

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– In hover:• Lift and Thrust

both act up• Weight and

Drag act down

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Forward Flight:• Thrust vector

tilted in desired direction = overall loss of upward lift = need more power applied

• Similar to airplane in turn

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Dissymmetry of Lift

– At a hover with no wind the rotor blades are all traveling at the same speed in relation to the air around them

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Any relative air motion (wind or flight) = blade going into wind (Advancing Blade) travels faster than Retreating Blade

• Think in terms of Airspeed

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems100 mph


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


– Faster airfoil = more lift on Advancing side (and less lift on Retreating side)

– Lift not equal = Dissymmetry of Lift

– Without compensation = roll to left (and gets more severe with speed increase)


HelicoptersFlight Conditions

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Dissymmetry of Lift– Compensated for by allowing the blades to Flap or the rotor

to Teeter• Advancing blade Flaps (Teeters) up = decrease in angle

of attack due to upward vector of Relative Wind



Dissymmetry of Lift– Compensated for by allowing the blades to Flap or

the rotor to Teeter• Retreating blade Flaps (Teeters) down =

increase in angle of attack due to Relative Wind change

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Coriolis Effect

– Caused by Flapping or Teetering up

– Blade flaps up = Center of Mass moves closer to axis of rotation = RPM increases

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

04/12/23 49


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Coriolis Effect– The inertia of the

rotor stays constant so as the Axis of Rotation is reduced the Speed of Rotation must increase

– Is same as skater in spin with arms out then speeds up when arms are moved in to sides



– Creates force to accelerate the blade (Hunting action)

– Fully Articulated head allows limited Hunting action

• Uses hydraulic or composite dampers to minimize movement

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Semi-Rigid usually uses “UnderSlung Rotor Head”

• Teetering Axis is above Feathering Axis (“Delta Hinge” arrangement) = as teeters it also swings to high side

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Semi-Rigid usually uses “UnderSlung Rotor Head”

• Center of Mass of the Rotor then stays basically in line with driveshaft/mast

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Translational Lift

– Increased lift during the translation to forward flight from a hover

– Occurs between 16 and 24 knots airspeed

• Feel vibration and definite increase in lift (that point is called “Effective Translational Lift”)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Translational Lift– At hover and below 15 knots, the ground is forcing

the rotor downwash outward and creating some turbulence around rotor blades

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Translational Lift– At hover and below 15 knots, the ground is forcing

the rotor downwash outward and creating some turbulence around rotor blades

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

–Above 15 kts, the blades “bite” into undisturbed air = more efficient = less power needed



Translational Lift– Above about 50 knots, drag starts to increase

greatly and we need more power to further accelerate

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Transverse Flow

Effect– At slow airspeeds

(less than 20 kts.) = air through rear of rotor is accelerated downward longer than air at front = decrease in angle of attack in rear

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Effect– Effect felt 90o later =

drift to right– Pilot must

compensate with some left Cyclic to keep going in a straight line

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Effect– As airspeed increases

= entire rotor has basically undisturbed airflow = no Transverse Flow Effect is felt

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Autorotations– Flight with no engine power applied to the main

rotors– Air is normally drawn down through rotors but if

have engine failure = aircraft drops and wind goes up through rotors = keeps them rotating at near normal RPM

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Autorotations– When engine fails, pilot lowers Collective stick

to bottom = sets in minimal angle on all blades and adjusts Cyclic to certain forward airspeed

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Autorotations

– With Relative Wind from underneath and forward:

• Lift and Drag vectors are changed so Resultant is forward of Axis of Rotation = tries to accelerate rotor and is called Autorotative Force

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




• Occurs in middle 25 – 75% of rotor

• Is called the Autorotative (Autorotation) Region

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




• In outer 30% of rotor = blade twist makes the angle of attack low and the speed makes the drag high

• Resultant is behind the Axis of Rotation

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




• Is a Decelerating force (Anti-Autorotative Force) and is called the Driven (or Propeller) Region

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




• Inner 25% has an angle of attack higher than the Critical Angle of the airfoil = Stall Region and also creates an Anti-Autorotative Force

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Autorotations– At some forward airspeed these forces combine to

stabilize the RPM (achieve equilibrium)– RPM means Inertia = energy available to use when

near the ground• This Autorotation RPM is critical rigging

adjustment

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Autorotations– At about 50 feet above the ground, the pilot pulls

back on the Cyclic to flare the aircraft (pulls the nose up some = reduced airspeed)

• = momentary increase in airflow and higher RPM (= more inertia)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Autorotations– At about 10 feet above the ground, the pilot pulls

up on the Collective and starts to use that energy in the rotor to cushion the landing

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



Autorotations– Also leads

manufacturers to publish “Height-Velocity Diagram” in Flight Manual

– Also known as the “Dead Man’s Curve”

– If fly in shaded area combinations of Height (Altitude) and Velocity = can’t successfully Autorotate

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Retreating Blade Stall

– As we move forward = Retreating Blade flaps down to compensate for Dissymmetry of Lift by increasing the angle of attack

– At some high forward airspeed (especially if the rotor RPM is allowed to get low) a portion of the airfoil (rotor disk) will exceed the Critical Angle of Attack and Stall

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersFlight Conditions Retreating Blade Stall

– Generally occurs at the 7 – 9 o’clock position (looking down on the rotor = left rear of rotor) = vibrations + nose pitches up

• gyroscopic precession = loss of lift in rear of rotor

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Retreating Blade Stall– Nose pitch up =

excessive angle of attack in front (stall) = loss of lift on left and roll to left



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Vortex Ring State (Settling With Power)– If descending at 300

fpm or more + less than 10 mph forward airspeed + 20 to 100% power applied = can descend inside rotor downwash



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Vortex Ring State (Settling With Power)– Blades produce tip

vortices (like any airfoil) + upward flow of air in middle of rotor (from descent) = Vortex across entire rotor = loss of lift and increased descent rate



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Vortex Ring State (Settling With Power)– Increasing power

to control descent rate = increases problem by increasing the amount of vortex created

– Must accelerate out of it or descend below it (if there’s enough altitude)



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Vortex Ring State (Settling With Power)



A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Ground Resonance

http://www.chinook-helicopter.com/Fundamentals_of_Flight/Ground_Resonance/Ground_Resonance.html


HelicoptersControls Axes of Flight

– Same as airplane: Longitudinal Axis = Roll, Lateral Axis = Pitch, Vertical Axis = Yaw

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls Flight Controls

– 3 basic controls: Cyclic, Collective, Pedals

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls Flight Controls

– 3 basic controls: Cyclic, Collective, Pedals

– Cyclic:• Controls Pitch and

Roll• Tilts rotor disk in

desired direction of movement

• Is primary airspeed and flight path control (pitch & roll)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Axes of Flight– Cyclic:

• Uses Swashplate to do job– Is device with rotating component and

stationary component– Connected by double-row ball bearing– Lower (stationary) part connected to Cyclic

stick via push-pull tubes and/or hydraulics– Upper (rotating) part connected to main

blades and rotates with them

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

HelicoptersControls

Axes of Flight– Cyclic:

• Uses Swashplate to do job– Pilot pushes Cyclic stick in direction of desired

movement– Swashplate is tilted to change M/R blade pitch a

different amount depending on where it is in rotation

» The pitch changes cyclically as it rotates» Direction of tilt is designed to take Gyroscopic

Precession into account» May or may not tilt same as rotor disk action

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Axes of Flight– Cyclic:

• Example system: Huey (Bell UH-1) Fore &

Aft tubes

Lateraltubes


HelicoptersControls

Axes of Flight– Collective:

• Changes the pitch of all blades the same amount at the same time (collectively)

• Controls the overall lift generated by the rotors

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• Uses the Swashplate to do the job by raising or lowering it to change the pitch on all blades

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• Collective stick also has engine throttle(s) – Motorcycle style rotating throttle except must

rotate away from you to increase– Turbines usually governed so open throttle

wide open and let governor keep RPM steady

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Axes of Flight– Collective:

• Example system: Hughes (Schweizer) 269



– Pedals:• Control Yaw by controlling the thrust of the Tail

Rotor (on single-rotor helicopters) and driven by main transmission so will still work if engine quits

– Dual rotors = differential cyclic control by pedals

– Coaxial rotors = rudder in rotor downwash• Push left pedal to yaw to the left, right pedal to

yaw to the right – Left pedal increases T/R thrust

• Needed especially during slow and high power conditions (I.e. takeoff and landing)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Tail Rotor Types:• Semi-rigid

– Most common until recently– Usually 2-bladed– Has same Dissymmetry of Lift problems as

M/R so will teeter usually (some let blades flap)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Tail Rotor Types:• Semi-rigid

– Most common until recently– Usually 2-bladed– Has same Dissymmetry of Lift problems as

M/R so will teeter usually (some let blades flap)

– Most use Offset Hinges so pitch is physically changed as rotor teeters = minimal actual teetering action

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Tail Rotor Types:• Fenestron

– French design– Enclosed multi-bladed

variable-pitch fan – Safer and quieter

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Tail Rotor Types:• NOTAR

– Developed by Hughes Helicopters (then McDonnell-Douglas now Boeing)

– Uses fan inside tail boom with exhaust out side of boom through variable vent connected to pedals

– Also uses Coanda Effect from rotor downwash

» Air flowing over the curved surface “sticks” to that surface and creates lift sideways

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

04/12/23 94

HelicoptersControls

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems Miscellaneous

– Stabilizer surfaces• Fixed Horizontal

– Creates download on tail to keep fuselage more level during high speed flight

• Synchronized Elevator– Connected to Cyclic– Changes pitch to change tail down load for various

flight speeds• Fixed Vertical

– For directional stability


HelicoptersControls

Miscellaneous– Hydraulics

• For larger or heavier M/R systems• Mostly use Irreversible type systems to overcome

flight loads and dampen vibrations in sticks

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Miscellaneous– Example system:

• Bell 206

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls Stabilizer Controls

– Are inherently unstable

– As rotor lift/thrust vector tilts away from vertical = creates vector to pull away from center

– = negative stability

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

•Compensations•Bell Stabilizer Bar

–Bar below M/R @ 90o to blade span–Acts like gyroscope and uses Rigidity in Space characteristic to try and keep rotor and aircraft in one attitude–Worked too well so needs hydraulic damper to limit it’s effectiveness and allow reasonable maneuverability


HelicoptersControls

Compensations– Offset Flapping Hinge

• On fully-articulated rotor heads and on some tail rotors

• Hinge moved a distance from rotor’s rotation axis = acts like lever to provide restoring force

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Compensations• Stabilization Augmentation System (SAS)

– Like simple autopilot– One- or two-axis– Only to aid stability, not true autopilot

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls Vibrations

– Large number of moving and rotating parts = susceptible to vibrations

– Vibrations = abnormal wear, premature part failure, and uncomfortable ride for people

– Must minimize vibes

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Types• Low Frequency

– Feel as “beat” in structure and may be able to almost count the beats

– Comes from Main Rotor

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

–Vertical vibe»Up & down motion»Caused by blades being Out-of-Track




A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

–Lateral vibe»Side-to-side motion»Comes from blades being out of balance or spaced unequally



– Types• High Frequency

– Felt as “buzz” in structure– Comes from cooling fan, engine and/or

accessories, gearboxes, or (most commonly) Tail Rotor

– May only notice if some part of body goes to sleep

» Feet = Tail Rotor (through pedals)» Butt = others

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Measurement of vibes• Feel

– Adjust until feels OK (at minimum level)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Measurement of vibes• Electronic

– Use accelerometers to measure rate and strength accurately

– Use Strobe light or “Clock” to locate – Use above as coordinates on chart to

determine exactly where and how much weight to add or remove

– Can use to troubleshoot (narrow down vibe rate and look at those components operating at that rate)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Correction of vibes (M/R & T/R)• If out of balance condition

– May require Static or Dynamic procedures (or both depending on helicopter)

– Some require Static balancing after assembly» Put on balance stand and adjust until no

movement when released» T/R done like propeller (knife-edge stand)» M/R done on special stand with Bullseye

level

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


Vibrations– Correction of vibes (M/R & T/R)

• If out of balance condition» M/R also may require Blade Sweep to be

adjusted (for chordwise balance)» = stretch string between blades and adjust

until blades are exactly 180o apart (adjust by “sweeping” blades forward or aft as necessary)

HelicoptersControls

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• If out of balance condition– Dynamic balancing done during operations on

ground and in air– Uses Electronic gear to measure rate and

strength and charts to show adjustments• Some M/Rs don’t need dynamic after static but

all T/Rs do

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Example: Chadwick-Helmuth Vibrex® system


HelicoptersControls

Vibrations– Measurement of

vibes• Example chart:

–T/R balance

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls



–T/R balance

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls



–M/R balance

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Correction of vibes (M/R & T/R)• If out of Track condition

– Track = path Blade tips follow during rotation– In-Track = all tips follow same path (or Cone

the same amount) and = minimal vertical vibes

– All M/Rs need to be checked and adjusted and some T/Rs

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




– Ground check» Use marking stick or Flag» Marking Stick uses crayon or grease

pencil on end of long stick and carefully raise to bottom of blades to make mark on lowest one (adjust until marks all blades)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• If out of Track condition– Ground check

» Flag is strip of canvas suspended between F shaped pole + put crayon mark on blade tips (different color on each blade) then move Flag so just touches each blade to get a colored mark

» Use colors to determine which blade needs adjustment

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Vibrations– Flag Tracking

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Flag Tracking



– Correction of vibes (M/R & T/R)

• If out of Track condition

– Ground check

» All are adjusted by changing the length of the Pitch Links (controls Angle of Incidence of blades)

» Link between Swashplate and M/R blade

» Increase angle = more lift = blade flies higher

» Each manufacturer usually has standard adjustments (I.e. 1/6 turn = ½” blade movement)

» Limitation: can’t check in flight

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• If out of Track condition– Ground & Flight

» Use spotlight or strobe» Spotlight uses colored reflectors attached to blade» Light shows colored streaks and can see “altitude”

difference between them

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Ground & Flight» Strobe is keyed by pickup on swashplate» Flashes once for each blade » Has reflectors on each blade with different

angled “Target” line» Flashes ‘stop’ targets at one location and

can easily see difference and which blade to adjust




– Ground & Flight» For ground and hover adjustment = use

Pitch Links» For in-flight adjustment = most blades

have trailing edge fixed trim tabs to allow limited bending

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Power Systems & Other Components

– Powerplants• Reciprocating

– See all types: Horizontal and Vertically mounted Opposed engines & some Radials

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls



– Verticals and Radials usually are Dry-sump with M/R Transmission (GearBox) mounted on top and using same oil supply

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Power Systems & Other Components– Powerplants

• Reciprocating – Verticals and Radials

usually are Dry-sump with M/R Transmission (GearBox) mounted on top and using same oil supply

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Bell 47


HelicoptersControls



– Horizontals usually use some form of Belt Drive

» Multiple V-belts or one wide “timing” belt

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• Reciprocating – None have propeller for

cooling air blast and “fly wheel” for starting

– All use some form of Cooling Fan driven by engine to blow air across cylinders

– All are generally hard to start (no fly wheel to help process keep going)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls Power Systems & Other Components

– Powerplants• Reciprocating Instruments

– Since M/R is essentially a Variable-pitch Propeller = all use both Tachometer (RPM) and Manifold Pressure gauges for power measurement

– Engines must be operated at relatively constant RPM (to allow enough Lift & Thrust) and usually very near the manufacturer’s Overspeed limit

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• Reciprocating – Usually uses Correlated Throttle and

Collective» Pull up on collective = more blade pitch =

more lift/thrust generated = more drag» Need more engine power to keep RPM

constant» Correlation increases throttle automatically

as Collective is pulled up (may not do entire job, though)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• Reciprocating – Usually uses Correlated Throttle and

Collective» Pull up on collective = more blade pitch =

more lift/thrust generated = more drag» Need more engine power to keep RPM

constant» Correlation increases throttle automatically

as Collective is pulled up (may not do entire job, though)

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Bell 47


HelicoptersControls


• Turbines– Are ideal powerplants as operate most efficiently at

constant RPM and have very high power to weight ratio

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls


• Turbines– Are TurboShaft engines

» All output power is converted to rotating shaft power (Torque)

» Torque sent to Transmission to drive Main & Tail Rotors and other necessary components

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Powerplants• Turbines

– Are TurboShaft engines» Two basic types: Direct Shaft & Free

Turbine» Direct Shaft has PTO shaft connected to

all Compressor and Turbine section stages

» Are very hard to start as must turn all engine + Main and Tail rotors

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




– Are TurboShaft engines» Two basic types: Direct Shaft & Free

Turbine» Free Turbine has some Turbine stages

which only supply PTO power» Easier to start as rotors not mechanically

connected to main part of engine

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




– Are TurboShaft engines» Measure power output with Tachometers,

Torquemeters, and Turbine Temperature gauges

» Tachs measure RPM in % (due to high actual RPM)

» Free Turbine versions need to measure both main engine (N1) and Power Turbine (N2) and usually have separate gauges

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




– Are TurboShaft engines» Torquemeters measure power being

absorbed by M/Rs» Similar to MAP gauge on recips» Measures in % or in Pounds of Torque

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems




– Are TurboShaft engines» Turbine Temps very important as are

directly proportional to how hard the engine’s working and critical during the start cycle

» May be TIT, ITT, TOT, or EGT system (manufacturer’s choice)

» CAN NOT exceed max. limit or will damage Turbine section components

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems



– Transmissions• For speed and/or directional change of

rotating shaft(s)• May be Rack & Pinion or Planetary Gear

systems• Uses engine oil or has own supply

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls Power Systems & Other

Components– Transmissions

• For speed and/or directional change of rotating shaft(s)

• May be Rack & Pinion or Planetary Gear systems

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

Schweizer (Hughes) 269 Transmission: Rack (Ring Gear) and Pinion


Bell 47 Transmission:Planetary system


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

HelicoptersControls

Power Systems & Other Components– Transmissions

• Engine drives M/R Transmission which in turn drives the T/R, Hydraulic pumps, Electrical Generator, Cooling Fans (if appropriate for the aircraft), and Rotor Tach sending unit connected to (usually) Dual Tach (Rotor and Engine RPM on same gauge)


A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


– Clutch• USED TO RELIEVE THE ENGINE LOAD

DURING STARTING• May be Manual, Electrical, or Centrifugal

• Manual and Electrical pull Idler Pulley against Belt(s) to tighten them and connect engine with Transmission


HelicoptersControls

Power Systems & Other Components– Clutch

• Centrifugal uses hinged Shoes pushed against a Drum by Centrifugal Force

– Shoes on arms attached to engine crankshaft

– Drum attached to Transmission

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems


HelicoptersControls

Power Systems & Other Components– Freewheeling Unit

• FOR AUTOROTATION PURPOSES

• Disconnects M/R from engine if engine turns slower than M/R

• Usually either Roller or Sprag style

A. History

B. Configurations




F. Controls


H. Vibrations

I. Power Systems

helicopters

Business

rotor flight conditions

auxiliary rotor

bladed main rotor

control tail rotor

bladed tail rotor

j35 rotor span

coaxial rotors

airfoil rotors