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Session Speaker M. SivapragasamSession 06Aircraft Longitudinal Static StabilityM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologySession ObjectivesAt the end of this session, student will be able to: Classify the various aircraft control surfacesDifferentiate handling qualities of conventional and fly-by-wire aircraftCategorise the forces and moments acting on an aircraftDerive the conditions for longitudinal static stability of an aircraftEstimate the hinge moment coefficientAnalyse the longitudinal static stability of an aircraft with canard configuration Analyse the longitudinal static stability of tailless aircraft

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyIntroductionWe study the mechanics and dynamics of flight so that :We can design an airplane to accomplish a specific task efficientlyWe can make the task of the pilot easier by ensuring good handling qualitiesWe can avoid unwanted or unexpected phenomena that can be encountered in flightM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyAircraft Control SurfacesThe elevator provides pitch control.It is used to provide pitching moment equilibrium and enables adjustment of incidence.Ailerons give roll control.They are used to bank the aircraft to initiate turns and to oppose disturbances in roll due to crosswinds or gusts.The rudder gives yaw control, its most important functions are in balancing engine-failed yawing moments,turn co-ordination, spin recovery and crosswind landing.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyAircraft Control SurfacesThe three controls are used to enable moment equilibrium in all three axes simultaneously.Controls may be operated mechanically by the pilot, or there might be aerodynamic assistance, power assistance or full power operation.Full power operation can be mechanically or electronically signaled (fly by wire).M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControl Deflection

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyHandling Qualities Conventional a/c

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyHandling Qualities Fly-by-wire a/c

The pilot has direct control only of the Flight Control System. However, he can tailor his inputs to the FCS by observing the airplanes response while always keeping an eye on the task at hand.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyForces and MomentsThe forces and moments on an aircraft arise from two sources:the mass of the aircraft and its aerodynamics.The mass and mass distribution give rise to inertial forces mass * linear acceleration, moment of inertia* angular acceleration and gravitational forces (mass* acceleration due to gravity).M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyForces and MomentsThe aerodynamic contributions are:Static forces and moments due to linear velocities;Damping forces and moments due to angular velocities;Control forces and moments produced by rudder, ailerons, elevators etc.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyReference CentresCentre Of Pressure (C.P) : The point through which the resultant aerodynamic force acts. There are no moments around C.P.Half Chord Point : The point through which aerodynamic force due to Camber actsQuarter Chord : The point through which aerodynamic forces due to AOA act M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyCentre Of Pressure (C.P) : The point through which the resultant aerodynamic force acts. There are no moments around C.P.Half Chord Point : The point through which aerodynamic force due to Camber actsQuarter Chord : The point through which aerodynamic forces due to AOA act Aerodynamic CentreA more useful concept is that of aerodynamic centre. The aerodynamic centre is generally defined as the location at which the pitching moment is unaffected by the incidence Linear aerodynamics approximation. For small change in incidence the changes can considered to act through a certain point their combined centre of pressure. This point is the aerodynamic centre of the whole aircraftHere d M/ d = 0M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyAerodynamic Centre and Neutral pointThe aerodynamic forces and moments acting on an aircraft of a given shape are independent of the position of the centre of gravity. Therefore, the aerodynamic loads can be considered separately from the gravitational loads.The aerodynamic loads can be considered to act through a point at which the aerodynamic moments are zero. This location is known as the Centre of Pressure.This location changes with angle of incidence, hence not much useM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyReference Centres : Aerofoil

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyEffect of 3D Wing on A.CCambered airfoil. Camber causes the value cmac of the individual airfoils to become more negative. So Cmac also becomes more negative. xac doesnt really change. Swept wing : When dealing with swept wings, the term (xac(y) x0) becomes important. Wings with high sweep angles tend to have a shifting AC at high angles of attack.Whether this improves the stability or not depends on other parameters as well.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & Technology3D EffectTapered wing: The taper ratio (= ct/cr) (the ratio of the tip chord and the root chord) slightly influences stability. For swept back wings, a low taper ratio tends to have a stabilizing influence.Slender wing: The aspect ratio A has only little influence on the position of the AC.However, a slender wing (high A) with a large sweep angle will become unstable at large angles of attack.Wing Twist : Applying a wing twist angle " causes the basic lift distribution clb to change. This causes Cmac to change as well. In what way Cmac changes, depends on the direction of the wing twist.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStatic StabilityMost aircraft (apart from high performance fighters) are statically stableStatic stability implies:All the forces and moments around the aircrafts cg at a fixed flight condition and attitude are balancedAfter any small perturbation in flight attitude the aircraft returns to its equilibrium positionThe equilibrium position is usually called the trim position and is adjusted using the trim tabsM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStability Review

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStability Review

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStability Review - Trim Point Location

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStability Review - Component Contribution

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPitching Moment Equation

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyEquilibrium EquationsSteady level flight is assumed, thrust balances drag and they both pass by the cgForce equilibrium: Pitching moment around cg equilibrium:

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStability Condition

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPitching Moment Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPitching Moment Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyWing Tail InterferenceFlow over wing and the horse-shoe Vortex modifies the flow over the tail.The angle seen by the tail is less than geometric

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPitching Moment Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPitching Moment Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyElevator Angle for Trim

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControls Fixed Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControls Fixed Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControls Fixed Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyMeasuring Stability Margin

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControl Surface DeflectionIf we deflect the control surface (say elevator) downwardsCoefficient of pressure distribution is altered (see fig)Additional lift is producedAlong with moment, notice Cp change near the TE

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyElevator + TabAssuming linear variation with , ,

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyNet Changed Force & Moment

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControls Free Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyElevator Hinge Moment

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyElevator Hinge Moment

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControl Free Stability Margin

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControl Free Neutral Point

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControl Free Neutral PointAs with the controls fixed stability margin, the controls free stability margin is positive when the aircraft is stable.Similarly, the centre of gravity position must be ahead of the controls free neutral point if the aircraft is to be stable.Usually, the constants of the elevator and tab are such that h / n>h n.An aircraft that is stable controls fixed will usually be also stable controls freeM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyHands Free TrimAssume that the aircraft is trimmed at a hands-off position (elevator is free)If the pilot changes elevator angle and then releases the control stick, the aircraft will return to the old trim position.In order to adopt a new hands off trim position the pilot must first move the elevator to the desired angle and then adjust the trim tab.The correct tab angle is the one that requires zero control force. The pilot can then take his hands off the control stick.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyTrim Tab Angle

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyControl Force for Trim

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyMeasuring Forces

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyMeasuring Forces

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyForces on Control SurfacesSince the control surfaces have to be deflected by positive and negative angles, they have no camberOnly symmetric sections are used for surface which have to be deflectedFixed surface like tail plane stabilizer can be camberedAs such standard equations used for wing also applyC L = L /( 0.5 * q * S ) Lift CoefficientC D = D /( 0.5 * q * S) Drag CoefficientC m = M /(0.5 * q* S * C) Moment CoefficientM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyHinge Moment

We have to compute the force and moment on the hingeThis is the force needed to move the surface or to hold it in placeFor large planes this is large, pilot effort alone not sufficientNeed to provide amplifiers and actuators

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyHinge MomentAssuming linear variations for derivatives

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyBalancing the control surfacesIn smaller aircraft the control surfaces are connected directly to the pilot's controls and to move the control surfaces he has to apply forces to overcome the hinge moments.In larger aircraft powered controls are used, but in either case it makes sense to reduce the hinge moments by aerodynamic means.This will reduce pilot fatigue incase of manual control Actuator sizes and hydraulic line pressures in powered case.M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyBalancing the control surfacesSeveral methods of balancing have been used before, only a couple will be described heremany of them are now out of useHorn Balance (see figure)

Set back hinge

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyC.G Travel and Stability limitsWe have seen that the cg position plays a significant role in the aircraft pitching characteristics.If the CG is well forwardmargins are large large forces and moments required.In a manually controlled aircraft it means excessive stick movementsforces that are larger than we can expect a pilot to exert for any length of time.Pilot fatigue and possible accidentsM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyC.G Travel and Stability limitsIf the CG is aft of a neutral point the aircraft will be unstable stick movements or forces to change will be in the 'wrong' sense.However, when instability level is small, aircraft will only diverge slowlyTime-to-double the amplitude is small Aircraft can be controlled by computer based controls.This explains the modern trends in combat aircraft towards Relaxed Static StabilityM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyVarious Stability Limits

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyHorizontal Tail Volume Ratios ratios

Vertical Tail Volume RatiosM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPitch-up during Stall

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyEffect of Sweep on Stall

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyEffect of Elevator on Stability

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyPhugoid Motion

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyCanard aircraft

European design trends is towards Tailless CanardM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyStability Analysis of Canard Configuration

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & Technology

Stability Analysis of Canard ConfigurationM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyLSS : Canard

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyLSS : Canard

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyLSS : Canard

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyLSS : Canard

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyLSS : Canard Controls Fixed

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyLSS : Canard Comments

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyTailless Aircraft

Indias own Tejas is a Tailless Cropped Delta M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyTypical Tailless Configuration

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyTailless Aircraft

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyTailless Aircraft

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyTailless Aircraft : Elevons

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyCanard + TailSome Russian aircraft in Indian Air Force have both

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyExample

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologyExample contd

M. S. Ramaiah University of Applied Sciences#Faculty of Engineering & TechnologySummaryIn this session following topics were discussed:Classification of various aircraft control surfacesHandling qualities of conventional and fly-by-wire aircraftForces and moments acting on an aircraftConditions for longitudinal static stability of an aircraftEstimation of the hinge moment coefficientLongitudinal static stability of an aircraft with canard configurationLongitudinal static stability of tailless aircraftM. S. Ramaiah University of Applied Sciences#Faculty of Engineering & Technology