PrinciplesOfGliderFlight-ControlStability

8/3/2019 PrinciplesOfGliderFlight-ControlStability

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Richard [email protected]

Principles of glider flight

[ Lecture 2: Control and stability ]

ASK-21 illustrations Copyright 1983 Alexander Schleicher GmbH & Co.All other content Copyright 2007 Richard Lancaster.


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[ Flight surfaces ]

Tailplane(Horizontal stabiliser)

Tail fin(Vertical stabiliser)

Main wings


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[ Control surfaces ]

Aileron

Flap

Rudder

Elevator

Airbrake


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[ Axes ]

Lateral axis

Longitudinalaxis

N

ormalaxis

(verticalaxis)


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[ How a control surface works ]

Deflecting a control surfacedown:

Increases aerofoil camber. Increases angle of attack.

Therefore lift is increased.

Deflecting a control surfaceup:

Decreases aerofoil camber. Decreases angle of attack.

Therefore lift is reduced.

Chord line

Chord line


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[ Equilibrium ]


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[ Linear equilibrium ]

To maintain steady wingslevel flight the lift, drag andweight forces must exactlybalance each other out.

Increase lift glider climbsReduce lift glider fallsIncrease drag glider decelerates

Reduce drag glider accelerates

LIFT

WEIGHT

DRAG


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[ Centre of gravity (CG) ]

A glider's Centre of Gravity(CG) is the point at which ifyou placed the glider on apivot it would balanceperfectly.

Increasing the weight of thepilot will move the CGforwards.

Decreasing the weight of thepilot will move the CGbackwards.

The CG can be considered asthe point on which the force ofgravity acts.

= Centre of Gravity (CG)

Pivot

WEIGHT


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Lift and drag forces areproduced across the entiresurface of the wing.

[ Centre of pressure (CP) ]

For the sake of simplicity, allof these individual forces can

be represented by a singleforce. The point on the wingat which this single force canbe considered to act is calledthe Centre of Pressure (CP).

The distribution of pressure

across a wing changes withangle of attack. On mostglider wings this causes thecentre of pressure to moveforward on the wing as theangle of attack is increasedand back as it is decreased.

Centre of Pressure

CP


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[ Unwanted pitching forces ]

The centre of gravity will notmove significantly during aflight as its position is set bythe weights of the pilots andballast.The centre of pressure willmove forward and backwardon the wing depending on theangle of attack.

Therefore although theaerodynamic force producedby the wing may be of the

correct size to balance thegravitational force, the point atwhich it acts on the aircraftmay be offset from the CG.

This produces a rotationalpitching torque on the aircraft.

CP

WEIGHT

Aerodynamic forceproduced by glider's

wings

Offset

Pitching torque


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[ Rotational equilibrium & trimming ]

To balance the rotationaltorque produced by thelocations of the CP and CG,the tailplane therefore needsto produce a counter torque.

If CP in front of CG:Tailplane needs to generatean up force.

If CP behind CG:Tailplane needs to generate adown force.

When you trim a glider youare setting the elevator to aposition that will cause thetailplane to generate the up ordown force required tocounteract the torqueproduced by the CP and CG.

Counter torque


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[ Control ]


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[ Effect of the elevator]

Consider an aircraft that istrimmed and flying wingslevel.

Page 1 of 7


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Stick is pushed forward whichdeflects the elevator down.

Page 2 of 7


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This produces a net upwardsforce on the tailplane.

Page 3 of 7


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Pitching torque

The torque produced by theupwards force on the tailplanepitches the nose down.

Page 4 of 7


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The pitching down of the nosereduces the angle of attack ofmain wing. Hence decreasingthe amount of lift it isproducing.

Page 5 of 7

This means that theaerodynamic force producedby the wing is no longer largeenough to counteract gravity.

The glider therefore starts tofall and accelerate.


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Once the desired speed hasbeen reached a slightadjustment of the position ofthe elevator might be requiredto correctly hold and trim theglider at the new speed.

Page 6 of 7


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Note: The effect that the elevator

has on the glider is the mostcomplex of all the control surfaces.The preceding explanation hastherefore been kept at a fairlysimplistic level.

Interested students may like to

lookup the Phugoid.

Page 7 of 7


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[ Primary effect of the ailerons ]

Aircraft is trimmed and flyingwings level. Stick is pushedto the right.

Left aileron deflected down. Angle of attack increased.Lift increased.

Right aileron deflected up. Angle of attack decreased.Lift reduced and possiblymade slightly negative.

Page 1 of 2


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[ Primary effect of the ailerons ]

Aircraft rolls and will continuerolling until the stick is centredand the ailerons return to theirneutral position.

Page 2 of 2


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[ Secondary effect of the ailerons ]

Aircraft is trimmed and flyingwings level. Stick is pushedto the right.

Aileron deflected down. Angle of attack increased. Lift increased.Drag increased.

Aileron deflected up. Angle of attack decreased. Lift reduced.Drag reduced.

1 of 2


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[ Secondary effect of the ailerons ] 2 of 2

Aircraft yaws in theopposite direction to that inwhich it is rolling. This istermed as adverse yaw.

The yaw does notcause the aircraft's

direction of flight tochange. Hence theyaw is preventedfrom building aboveabout 40 degrees bythe stabilisingweathercock effect ofthe vertical fin.

[ ]


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[ Primary effect of the rudder]

Aircraft is trimmed and flyingwings level. Right rudderpedal depressed.

Rudder deflected to the right Angle of attack of tail finincreased relative tooncoming airflow. Lateral aerodynamic forceproduced on tail fin directed tothe left.

Page 1 of 2

[ ]


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[ Primary effect of the rudder] Page 2 of 2

Aircraft yaws to the right but thedirection of flight doesn'tchange.

The yaw will therefore reach apoint at which despite thedeflection of the rudder, theangle of attack of the tail fin tothe oncoming airflow becomeszero. At this point the yaw willstop increasing.

[ ]


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[ Secondary effect of the rudder]

While the into wind wing isswinging forward its speed ismomentarily increased. Hencewhile yawing the into wind wingwill generate more lift.

Once yawed dihedral effects

increase the amount of lift the upwind wing produces.

While the down wind wing isswinging backward its speed ismomentarily decreased. Hencewhile yawing the down wind wingwill generate less lift.

Once yawed dihedral andfuselage shadowing effects

decreasing the amount of lift thedown wind wing produces.

These effects are small but causea gentle rolling of the glider in thedirection of the yaw.


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[ Manoeuvring ]

[ C t i t l f ]


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[ Centripetal force ]

The laws of physics state thatfor a glider to fly along acircular path (E.g. whenthermalling or simply turningthrough 90 degrees) theremust be a force acting on the

glider toward the centre of thecircle. This force is called acentripetal force.

The force required to turn aglider is exactly the same kind

of force as the tension in apiece of string if you havebucket attached to the end ofit and you are swinging itaround your head.

Centripetalforc

e

Centripetalforce

Bucket

[ T i ]


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[ Turning ]

There is only one forceproduced by a glider that islarge enough to provideenough centripetal force toturn the glider. This is the lift

force produced by its mainwings.

To use the main wing's liftforce as a centripetal force,the glider must be banked

over so that some of the liftforce is pushing the glidertoward the centre of the circle.

Centripetalforce

WEIGHT

LIFT

[ Sti k b k d i t ]


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[ Stick back pressure during turns ]

During a turn, if the mainwing's lift force is inclined overto the side to provide acentripetal force. Then less ofthe lift force will be actingvertically to counteract theforce of gravity.

Stick back pressure istherefore required during aturn to increase the mainwing's angle of attack, henceincreasing the size of the lift

force produced by the wing,such that the component ofthe wing's lift force that isacting vertically up is onceagain large enough tocounteract the force ofgravity.

WEIGHT

LIFT

Lossofvertica

llift

ifaircraftbank

ed

withoutstickb

ack

pressure

Increase in lift forceproduced by stick back

pressure

[ L d f t ( G) ]


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[ Load factor (n,G) ]

Load factor is a measure ofthe number of times greaterthe lift force being producedby a wing is than the weight

force produced by gravity.

Load factor (n) = GLift

Weight

The wings of a gliderperforming a loop and pulling2G will be generating a lift

force that is twice as large asthe weight force that is actingon the glider.

A glider in steady wingslevel flight will have a liftforce which approximatelycounterbalances the weight

force. It will therefore havea load factor of 1G.

[ Load factor's effect on stall speed ]


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[ Load factor's effect on stall speed ]

The speed at which a gliderstalls increases with loadfactor. The higher the loadfactor the higher the speed atwhich the glider will stall.

The stall speed at a particularload factor can be calculatedusing the equation:

n : Load factorv

SL-STALL: 1G stall speed

Note: The 1G stall speed isthe steady wings level flightstall speed.

Stall speed = (n) (vSL-STALL

)

10

Stalls

peed(knots)

0

20

30

40

50

60

70

80

0.0

Load factor (G)

1.0 2.0 3.0

Stall speed graph for a gliderwith a 1G stall speed of 40 knots:

[ Why does load factor increase stall speed? ]


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[Why does load factor increase stall speed?]

50knots2G

50knots

1G

Whilst the wing of the 1G

aircraft might be well below itsstall angle, the increasedangle of attack required bythe other aircraft to generate2G of lift may take the wingpast it's stalling angle andstall it.

An aircraft travelling at50knots and pulling 2G is bydefinition generating twice asmuch lift as a similar aircrafttravelling at 50knots andpulling 1G. As the airspeed isthe same in both cases the2G aircraft will require twicethe angle of attack on its wingto generate the 2G of lift asthe aircraft that is onlygenerating 1G of lift.

[ Load factor and stall speed in a turn ]


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[ Load factor and stall speed in a turn ]

In a turn, as the bank angleincreases, the lift forceproduced by the main wingneeds to be made larger andlarger by application of stickback pressure for it tomaintain a great enough

vertical component tocounteract gravity.

Therefore as bank angleincreases so does the loadfactor and hence the stallspeed.

Bank angle : 0Load factor : 1GStall speed : 40kts

Bank angle : 30Load factor : 1.15GStall speed : 43kts




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[ Stability ]

[ What is stability? ]


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[ What is stability? ]

Unstable Neutrally stable

Stable Over stable

An object is termed as beingstable if it tends to return to itsoriginal position after beingdisturbed.

We want a glider to be stable.

So that when it's flying alongand is disturbed by a gust, itwon't go into a dive or rollonto its back. Instead, withoutpilot intervention, it will tend toreturn to whatever it wasdoing before being disturbed.

However we don't want aglider to be over stableotherwise we won't be able tomanoeuvre it.

[ Yaw stability ]


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[ Yaw stability ]

Due to an unplanned push onthe rudder pedals the gliderbecomes yawed relative tothe direction in which it istravelling (E.g. The oncomingair flow).

The tail fin (vertical stabiliser)now has an angle of attackrelative to the oncomingairflow.

A lateral aerodynamic force istherefore produced on the tailfin that creates a torque whichwill tend to weathercock theglider back into line with theairflow.

Restoring torque

Angle of attack oftail fin

[ Roll damping ]


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[ Roll damping ]

Due to a gust of wind, a wing islifted and the glider starts to roll.

Downward going wing: Angle ofattack increased because thewing is moving down and hencethe air is moving up past it. Lift on

this wing is therefore increased.

Upward going wing: Angle ofattack reduced because the wingis moving up and hence the air ismoving down past it. Lift on thiswing is therefore reduced.

This produces a counter torquethat damps out the rolling motion.

However this will not roll theglider back to wings level as theeffect stops when the glider stopsrolling.

Directionofrotation

Angle of attackreduced, lift reduced

Angle of attackincreased, lift increased

[ Dihedral ]


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[ Dihedral ]

If the tips of glider's wingsare higher that their roots thenthe wings are said to havedihedral.

If the tips are below the rootsthen the wings are said tohave anhedral.

Dihedral angle

[ Dihedral and roll stability ]


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[ Dihedral and roll stability ]

If a glider is banked over butinsufficient stick back pressure isapplied to create enough lift tocounteract gravity, then the gliderwill slip sideways though the air.

If the glider has dihedral, then the

wing facing into the sideslip willhave its angle of attack increasedby the air flowing in from the sidedue to the sideslip. The lift on thiswing is therefore increased.

Conversely the wing facing awayfrom the sideslip will have its

angle of attack reduced andhence the amount of lift it isproducing will drop.

This produces a torque on theglider that will tend to roll it backto wings level.

Note: Anhedral will reduce roll stability.

Restoring torque

Sideslip

Wing has increasedangle of attack and hence

increased lift due tosideslip

Wing has decreasedangle of attack and hence

reduced lift due tosideslip

[ Pitch (longitudinal) stability ]


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[ Pitch (longitudinal) stability ]

Due to a gust of wind theglider's nose pitches uprelative to the direction inwhich it is travelling. (E.g.The oncoming air flow).

The tailplane (horizontalstabiliser) now has asignificant angle of attackrelative to the oncomingairflow.

An aerodynamic force istherefore produced on thetailplane that creates a torquewhich will tend to weathercockthe glider back into line withthe airflow.

Restoring torque

Angle of attack oftailplane

[ Pitch stability C of G limits ]


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[ Pitch stability C of G limits ]

For the tailplane to correctlyfunction as a pitch stabilisingdevice, the glider's centre ofgravity must be within the foreand aft limits specified by themanufacturer in the glider's flightmanual.

A heavy pilot will move the CGforward. A light pilot will move theCG backwards.

CG behind aft limit : Glider becomes unstable. Prone to spinning.

CG in front of forward limit : Glider becomes too stable. Elevator will not have enougheffectiveness to round out.

ForwardCofGl

imit

AftCofGl

imit

Glider unstableGlider too stable

Permitted range


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[ Any questions? ]

Document revision: 02 - 10/03/2007

All ill t ti f ASK 21 lid t i d i thi


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All illustrations of ASK-21 gliders contained in thispresentation are Copyright 1983 Alexander SchleicherGmbH & Co and are used here with their kind writtenpermission.

All other content is Copyright 2007 Richard Lancaster.

For non-commercial educational purposes permission isgranted to project, print as handouts, email anddistribute this presentation on physical digital media.

Permission is also granted to distribute this presentationbetween acquaintances on a non-commercial basis viaemail, physical digital media and hard copy printouts.

However this presentation may not be otherwiserepublished or redistributed by any means electronic orconventional without prior written permission of thecopyright holders. In particular this presentation may notbe used for any commercial purposes whatsoever.

This presentation may not be altered in any way withoutprior written permission of Richard Lancaster.

The latest revision of the presentation can always bedownloaded from:

www.carrotworks.com

Richard [email protected]

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