POLITECNICO DI MILANO - DIPARTIMENTO DI INGEGNERIA AEROSPAZIALEAIRCRAFT SYSTEMS – LECTURE NOTES, VERSION 2004 Chapter 7 – Landing gear system Chapter 7 Landing gear system These lecture notes are available for the students of the Polytechnic of Milan for free download. No commercialisation allowed. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. E’ vietata la commercializzazione. 7.1 • extraction/retraction mechanism.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. • stability in braking.1 Introduction The landing gear system includes: • strut. which means in general that its mass is significant. the landing gear arrangement is tricycle. consisting of a main landing gear group located near the aircraft centre of gravity and a nose or tail landing gear (fig. the tail element is almost obsolete. which may have just a central fuselage wheel and tailskid. There is then a high influence of the landing gear on the local structure. • shock absorber. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. 7. No commercialisation allowed.1). taxi control. 7. The landing loads can reach factors of 2. As a matter of fact. • high pilot visibility during taxiing. With exception of gliders. 7.2 General layout and design The layout of the landing gear system determines the load transfer to the structure. • low drag during take-off acceleration. energy absorption at landing. 4. • brakes. this can range from 3 to 7% of the aircraft total mass. 3. VERSION 2004 Chapter 7 – Landing gear system 7. 2.5 for transport aircraft. braking also determines both vertical and horizontal loads that influence structural sizing. A second observation concerns the number of struts. Landing is the main sizing conditions for the system and its surrounding structure. The most common configuration has double main landing gear and single nose gear.1 – Nose and tail wheel configurations easy freight loading). • wheel. • horizontal floor (occupants’ comfort and Fig. so that all the ground loads are transmitted by it to the aircraft structure.POLITECNICO DI MILANO . 7. Taxi control includes steering and taxi stability. • tyre. This can apply for These lecture notes are available for the students of the Polytechnic of Milan for free download. E’ vietata la commercializzazione. Shock absorber and extraction/retraction mechanism may not be present in small airplanes. Depending on aircraft category. braking. The system must then have considerable mechanical resistance.5 for small general aviation vehicles and higher for combat aircraft. because the nose landing gear has a series of unquestioned advantages: • lateral stability in taxiing. The landing gear is the interface of airplane to ground. The main functions of the landing gear are as follows: 1. • steady touch down with no risk of aerodynamic bounce. ground stability and control. which must be taken into account since the initial design stage.2 . On the other hand the main wheels should not be too far from the aircraft centre line. The telescopic version is always lighter but requires higher ground clearance. E’ vietata la commercializzazione.5. to minimise roll and yaw instabilities during non levelled touch down and reduce wing root moment (if the landing gear is wing mounted). VERSION 2004 Chapter 7 – Landing gear system a wide range of aircraft weights. 7. 7.5 – Telescopic and articulated leg As far as the strut design is concerned. These lecture notes are available for the students of the Polytechnic of Milan for free download.POLITECNICO DI MILANO . 7.3). or two additional struts are located under wing root area (fig. 7. Fig. a condition that may easily occur Fig. 7.3 . acting on the centre of gravity must fall inside the WHEEL BASE area delimited by the landing gear Fig 7.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. CG RESULTANT WHEEL TRACK The lateral track of the main landing gear gives stability during taxing. No commercialisation allowed. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. two solutions are mainly adopted: the telescopic and articulated leg. The resultant force vector. then for small aircraft and helicopters the articulated version is more frequently adopted (from the figure it is clear that the piston stroke in the cylinder is lower than the wheel stroke). 7. Fig. due to weight and inertial forces. in such a way MAX REAR CG POSITION TAIL CONE CLEARANCE that during touch down a nose down moment is generated by the ground forces to the airplane (fig.3 – Main landing gear with respect to during take-off (in fact a aircraft CG position and tail cone shape reinforcement or skid is normally integrated in that part of the AIRCRAFT CG structure).4 – Taxing stability ground contact points.2). The struts must be aft enough with respect to the most rear position of the centre of gravity. preventing aerodynamic bounce. to prevent rollover. 7. Moreover the tail cone must not contact the ground. but normally for airliners beyond 300000 kg of maximum take off mass an additional main landing gear strut is located under the fuselage. shown in fig.2 – Landing gear typical layouts The longitudinal position of the main landing gear group depends on the centre of gravity position and the tail cone shape. the mechanism must be blocked (downlock and uplock respectively). normally of the hydraulic type. No commercialisation allowed. The area under the Fig. Many solutions are based on the four bar linkage (cases A to C). Actuators. while extraction can initiate by gravity and be completed by drag. 7. stroke will be of the type E shown in fig.6 – Some kinematic patterns load line represents the necessary work. 7. In other solutions (case D) one bar end B can slide along a slot.7 – Piston load-stroke diagram aircraft is taxiing.6. the extraction is possible with no power from the hydraulic system.7. In general the mechanism should be designed in such a way that D gravity and aerodynamic drag favour extraction. An uplock is also activated when the landing gear is fully retracted. There are different solutions for the mechanism to obtain suitable landing gear movement. VERSION 2004 Chapter 7 – Landing gear system 7. which commonly is in the range 70 – 80 %. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. if the conditions on gravity and drag are satisfied. E’ vietata la commercializzazione. If this is divided by the area of the rectangle defined by the max load and stroke. This means in all aircraft with exception of agricultural and small general aviation airplanes. Some A are schematically shown in fig. to prevent non-intentional These lecture notes are available for the students of the Polytechnic of Milan for free download. control the extraction/retraction operation. More complex kinematics include three-dimensional motion and the deflection of the bogie. with a constant sign: this means that retraction is obtained by applying a force to contrast drag and movable equipment weight.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. one obtains the efficiency of the kinematic mechanism. A kinematic lock at extraction can be obtained by making the four bar linkage to reach its dead centre at full extraction. 7. In both extracted and retracted configurations. Landing gear extraction is a primary operation and always its actuation has high redundancy.4 .POLITECNICO DI MILANO . where one bar is represented by the aircraft frame. In any case a downlock based on a hydraulic or electric device is activated to prevent any movement of the strut when the Fig.3 Extraction and retraction A retractable landing gear is installed whenever a drag improvement is worthy. that for the main landing gear of large airplanes is made of double tandem C wheels. where the installation of a movable landing gear would increase the costs beyond the requirements of the aircraft category. 7. a diagram reporting piston load vs. 7. A low cost alternative to this method is to use an absorber made of a package of rubber blocks. Uplocks and downlocks are normally provided for the landing gear doors too. when loaded. The hydraulic solution is anyway the mostly adopted one. Very small aircraft with fixed landing gear may rely on the elastic properties of the landing gear legs and the damping effect of tyre sideslip on the ground. with some hydraulic and mechanical solutions to obtain efficient energy absorption and comfortable taxiing. so that the Fig. The system elasticity is due to the gas transformation and the damping effect to the liquid pressure losses. 7. which are compressed A) DEFORMABLE LEG B) HYDRAULIC SYSTEMS by the landing gear. the total orifice area can be changed by inserting check valves in some orifices or valves that throttle the orifices in one flow direction. the second and further stages start compression when a high load is developed. The systems till now described are passive devices. in some application (small aircraft and helicopters). • for high landing vertical velocities. the orifices have maximum area. the shock absorber responds with high reaction forces due to oil viscosity. still under development. 7. to attenuate the load transfer to the airplane structure. then flattening the reaction curve. VERSION 2004 Chapter 7 – Landing gear system extraction during flight. when the leg extension is that of the aircraft is in normal ground operation. in this case also controllable orifices can be used. Its secondary requirement is to allow a comfortable taxiing. Different types of shock absorbers are available. No commercialisation allowed. • during taxiing the absorber should be softer. 7. are based on active control of damping. • to increase the energy absorption in crash conditions.POLITECNICO DI MILANO . Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. 7. and fig. The complexity of the system increases with the requirements. for instance the internal structure of the absorber can be shaped in such a way that. compresses a gas (nitrogen) in a cylinder and causes an oil flow through orifices. a hydraulic system is commonly used. the second stage is a composite cylinder in series with the main stage (crash cartridge). Since the damping force is function of the orifice geometry and oil These lecture notes are available for the students of the Polytechnic of Milan for free download.4 Shock absorber layouts The main role of the shock absorber is to zero the vertical component of the airplane velocity during landing.8A. A list of main requirements for an efficient and functional shock absorber follows: • damping characteristics should be different in compression and extension.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. relief valves may be installed on the absorber.8 – Some shock absorber solutions elastic effects is due to the rubber compression and damping to hysteresis and local friction.8B shows some of the many possible versions. but when costs and dimensions allow. E’ vietata la commercializzazione. with no rebound and limited load transfer to the vehicle structure (and occupants). the first stage works in normal operation. More advanced solutions. Substantially the system structure is made of a movable piston that. 7.5 . which also could be a dangerous operation at high velocity. multi-stage shock absorbers can be used. as schematically shown in fig. 10 – Shock absorber reaction the aircraft weight. Both the systems are controlled on the basis of inputs from sensors of vehicle acceleration and velocity. as follows: γ V0 A + kA3 x& 2 R = p ⋅ A = p A + kQ ⋅ A = p 0 V0 − A ⋅ x ( x R Fig. called efficiency: These lecture notes are available for the students of the Polytechnic of Milan for free download. 7. 7. An interesting indication is given by the ratio between this work and the work that could be ideally absorbed with a constant maximum force RMAX and the maximum stroke ∆. proportional to the stroke derivative x& .POLITECNICO DI MILANO . Q = oil flow rate. the shock absorber can be depicted as a hydraulic cylinder linked to an accumulator.10 shows an indicative plot of the reaction. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. 7.3 – 1.6 No commercialisation allowed. When the piston moves.4 can be used. and their peculiarity is to achieve quasi-plastic behaviour when the field intensity is increased. a control of the damping characteristics of the shock absorber can be obtained in two ways: 1.9. γ = polytropic exponent. The area under the reaction curve represents the work absorbed by the system. pA = accumulator pressure. The final part is related to the x piston return to static equilibrium under Fig.9 – Shock absorber schematic representation 2 ) where: p = pressure on piston. and shock absorber conditions. 7. as shown schematically in fig. the oil from the cylinder passes through an orifice and compresses the gas in the accumulator. 7. According to this approach the reaction R of the shock absorber is function of the piston position and its time derivative. ∆ Fig. 2. and a viscous term. p0 = initial accumulator pressure. . The resulting reaction is then the sum of a polytropic transformation. The first solution is possible with the use of micro actuators that throttle the orifices. elastic and RMAX R proportional to the stroke x.5 Shock absorber functioning principles In its most basic configuration. which normally is in the range 60 – 70 % of the maximum stroke ∆. As polytropic index γ. k = orifice pressure loss coefficient. control of fluid viscosity. x = piston stroke (x=0 for all extended shock absorber). V0 = initial accumulator gas volume. VERSION 2004 Chapter 7 – Landing gear system properties. A = piston area. E’ vietata la commercializzazione. 7. control of orifices area. Generating the field in the orifices sections allows changing significantly the damping behaviour of the shock absorber by controlling the characteristics of a small volume of fluid. a value of 1. The second solution is possible with the use of electrorheological or magnetorheological fluids. these oils have properties sensitive to electric or magnetic fields respectively.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. because the process is fast enough to be considered quasi-adiabatic. POLITECNICO DI MILANO . stroke. First of all the work absorbed by the landing gear can be written remembering eq. This allows approximating the integral as follows: ∫ ∆ 0 L ⋅ dx = 2 ⋅ Mg ⋅ ∆ . 7. L=L(x) = function of lift vs. The equation.7 .9. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES.6 Preliminary stroke estimation The max stroke of a landing gear shock absorber can be preliminarily estimated. VERSION 2004 Chapter 7 – Landing gear system ∫ η= ∆ 0 R ⋅ dx RMAX ⋅ ∆ (eq. even if many aircraft and landing gear characteristics are unknown. as follows: ∆ ∆ 1 Mv Z2 + Mg∆ = ∫ R ⋅ dx + ∫ L ⋅ dx . 0 0 2 (eq. 7. related to the vertical forces during shock absorber compression.2) where: M = aircraft mass. R=R(x) = function of shock absorber reaction vs. due to the change of trajectory of the airplane during shock absorber compression and consequent decrease of the angle of attack. vZ = aircraft vertical velocity.1) The ideal absorber (η = 1) is a perfectly stiff-plastic system. is not of immediate use. 7. ∆ = max shock absorber stroke. Lift function L(x) can be approximately considered to decrease linearly from its max value Mg to (1/3)Mg. 7. but a number of simplifying considerations can be done. g = gravity.8 – 0. as it is written. The contribution of the tyre is not included in this discussion. 7. 7. x = shock absorber stroke.1 and considering the definition of the landing load factor n: n= [R + L]MAX Mg ≤ RMAX + Mg Mg then obtaining: ∫ ∆ 0 R ⋅ dx = η ⋅ Mg ⋅ (n − 1) ⋅ ∆ . E’ vietata la commercializzazione. 3 Substituting into eq.2 and solving with respect to ∆ brings to: These lecture notes are available for the students of the Polytechnic of Milan for free download. A simple energy balance. can be applied to the landing aircraft considering kinetic and potential energy and the works developed by the shock absorber and lift force. stroke. No commercialisation allowed. A modern hydraulic shock absorber has and efficiency around 0. m&y& = − mg + F1 ( x − y ) + F2 ( x& − y& ) − FT 1 ( y ) − FT 2 ( y& ) x M F2 F1 y m FT1 FT2 Fig. FT1 ( y ) = tyre elastic reaction function. but contains a series of approximations that cannot be accepted in further development stages of the aircraft design. brakes and all parts connected to the moving equipment). No commercialisation allowed. wheels. The procedure above explained is still used at preliminary stage for the evaluation of the max shock absorber stroke. F2 (x& − y& ) = shock absorber viscous function. FT 2 ( y& ) = tyre viscous reaction function.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. VERSION 2004 Chapter 7 – Landing gear system ∆= v Z2 [η ⋅ (n − 1) − 13 ]⋅ 2 g .POLITECNICO DI MILANO . in fact lift. 2 ∂α where: ρ = air density. a system of two differential equations can be written. 7.11 – Multi-body representation of aircraft and landing gear where: M = aircraft mass less landing gear mass. is given by: L= 1 2 ∂C L ρv S α. 7. with Fig. of v). where one rigid body is the aircraft and the other is the landing gear (legs.12. Then. v = aircraft velocity. 7. 7. the angle of attack α. All the components of lift are constant during landing gear compression. following the indications in fig. L(x& ) = lift function. in its classic formula. is clearly given by the sum of the aircraft pitch orientation αA with respect to the horizon and the angle of its trajectory αT with respect to the horizon. A more detailed evaluation can be performed by a rough two rigid body model. pistons. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano.12 – Aircraft trajectory during landing exception of α (and. 7. F1 (x − y ) = shock absorber polytropic function. α = aircraft angle of attack.8 . S = aircraft reference surface. It is easy to find out that lift can be expressed as a function of the time derivative of x. g = gravity. m = landing gear mass. in a very minor extent.11. as follows: M&x& = − Mg − F1 ( x − y ) − F2 ( x& − y& ) + L( x& ) . E’ vietata la commercializzazione. as shown in fig. CL = aircraft lift coefficient. and this last one is easily related to the vertical velocity: These lecture notes are available for the students of the Polytechnic of Milan for free download. the discs are released by a series of springs. The stator discs are keyed to the axle. which is usually powered by the hydraulic system.13): • pressure plate. The main components of a disc brake. brings lining blocks on one side only and. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. or pressure plate. . They are alternated to the stator discs. rotating then with it and being free to move along its axis. ROTOR DISC BRAKE ASSEMBLY STATOR DISC WHEEL STATOR AXLE WHEEL KEY BACK PLATE KEY SLOTS TO AXLE KEY SLOTS TO WHEEL LINING BLOCKS ROTOR CYLINDERS PRESSURE PLATE Fig 7. Since the rotor and stator discs are in relative rotation. This action compresses the entire disc package. or pads. • rotor discs. • back plate. the higher the normal contact force and then the friction force. The stator disc at one extremity. on the two flat surfaces. which contrasts the pressure. but based on different sizing principles. during braking. or anyway constrained in such a way to be only free to move along its axis. These lecture notes are available for the students of the Polytechnic of Milan for free download. with a functioning principle similar to that of the automotive systems. The complete equipment is housed inside the wheel. is pushed against the first rotor disc of the assembly by a series of hydraulic pistons.7 Brakes Most airplanes are equipped with disc brakes. 7. because rotor and stator parts are all free to move along the wheel axis. made of a mixture of metallic and ceramic materials. etc. with exception of the back plate at one extremity. 7. are as follows (fig. the contact between the lining blocks and the rotor discs will generate a tangential friction. so that the assembly results a sandwich of rotor discs and stator discs packed together. Drum brakes are almost obsolete. then occupying a large part of the room between the axle and the wheel. v The above written system of differential equations must be numerically solved. The rotor discs are keyed to the wheel. E’ vietata la commercializzazione.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. The higher the hydraulic pressure. responsible for braking. stroke. a parametric analysis can be done by changing the shock absorber geometrical and mechanical characteristics and evaluating their influence on the load factor.13 – Disc brake The stator disc at the opposite extremity. or back plate. VERSION 2004 Chapter 7 – Landing gear system α = α A + αT = α A + x& . • stator discs.9 No commercialisation allowed. lining blocks.POLITECNICO DI MILANO . They bring. 7. When pressure is reduced. is fully constrained to the axle and brings lining blocks on one side only. because the lining friction surface increases with the number of discs. Brake sizing is based on heating during a single landing. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. 7. cv = heat sink specific heat. On the other hand.10 . For sizing. VERSION 2004 Chapter 7 – Landing gear system The rotor discs usually have radial slots. 7. k may often be approximated to 0. This means that a high part of the kinetic energy at landing will be converted into brakes heating. Large aircraft may have braking nose wheels. The stator disc is usually made of steel. RB RR TM NM TM NM NN Fig. The discs can be made of steel but. ∆T = temperature increment during braking. v the max landing velocity and ∆T the difference between the allowable disc material temperature and the highest possible initial temperature. M should be the max landing mass. Considering that anyway there is a drag contribution to braking. The lining is fragmented into sector blocks because it is made of a brittle compressed mixture of metals and ceramics. flaps and spoilers. to minimise disc deformation during heat up. For a short period in the 60’s they were made of beryllium. when affordable. the part of brakes that is involved is often referred to as heat sink. A multiple disc brake is used whenever a high braking power is necessary.8. No commercialisation allowed. discs must be sized in such a way to withstand the high tangential stress that is generated by friction. M = aircraft mass. Usually brakes are located in all the main landing gear wheels: the above-mentioned mass m is then related to all the brakes located in the wheels. E’ vietata la commercializzazione.POLITECNICO DI MILANO . but its manufacturing costs and difficulties excluded it from standard use. because they allow a reduced total disc mass. This event can be expressed by a simple formula of energy balance: 1 k ⋅ Mv 2 = m ⋅ cv ⋅ ∆T . 2 where: k = fraction of energy converted to brake heat. considering that ventilation has a limited effect and neglecting the contribution of possible thrust reverse. m = total heat sink mass. but would be rather cold at the core and overheat in periphery. Moreover a thick disc would not allow a suitable temperature distribution.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. v = landing velocity. Materials with high specific heat and high operating temperature are of course preferred. that may be broken if the pressure is not distributed uniformly on the entire surface.14 – Evaluation of the braking intensity These lecture notes are available for the students of the Polytechnic of Milan for free download. carbon discs allow lower weight. RR where RR is the rolling radius (that depends on the normal reaction NM. normally around 0. µ = disc friction coefficient. The wheels are equipped with speed sensors and their signal is transmitted to a processing unit (the older systems were of course based on analogue technology. 7. 7.4. risk of tyre explosion.3. in a minor extent. Then TM is limited by: TMMAX = µ G ⋅ N M .9 for dry runway and tyre and asphalt in good conditions to 0.POLITECNICO DI MILANO . If this tangential force exceeds the limit indicated in eq. (eq. a simple algebraic system in the two unknowns NM and TMMAX can be set up and solved if any drag contribution is neglected. In static conditions (airplane at rest) NM will be equal more or less to 90% of the aircraft weight. The braking force TM will be then given by: TM = C . as follows: C = p ⋅ A ⋅ µ ⋅ RB . The speed decrease of all braking wheels is monitored and compared to each other and with predetermined deceleration patterns.4) where NM is the normal reaction of the main landing gear. No commercialisation allowed. the wheel tends to block. and then also the tangential force between wheel and runway. 7. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. By increasing the braking system pressure.3. due to reduction of the friction coefficient. an increase of the stopping distance. a loss of guidance control. (eq.14 the braking torque C can be easily evaluated as a function of the hydraulic pressure and discs geometry. 7. 7.1 for iced runway. this is defined by a factor µG that ranges from 0.11 . Three effects result from the wheel blocking: 1. RB = radius of the lining block centroids. From fig. avoid wheel blocking by modulating the braking system pressure. but worked on the same principle). the braking torque increases according to eq.5 in case of wet runway and down to 0. The anti-skid braking systems. Depending on the longitudinal and vertical position of the centre of gravity. due to the loss of tyre grip.3) where: p = hydraulic pressure. 3. of the aircraft geometry. VERSION 2004 Chapter 7 – Landing gear system The braking intensity is a function of the brake geometry and. which were developed after the Second World War. If one wheel These lecture notes are available for the students of the Polytechnic of Milan for free download. 2. In braking conditions NM is lower and the inertia forces overload the nose landing gear. due to the conventional position of the aircraft centre of gravity. A = total lining friction area. but is limited by the tyre grip. tyre pressure and tyre geometry). Of course the braking force cannot increase indefinitely with pressure. 7. the wheel to ground friction coefficient switches from static to dynamic and decreases. The pilot controls the system pressure through the pedals.8 Anti-skid and auto-braking systems Deceleration is obtained through the friction between the braking wheels and ground. E’ vietata la commercializzazione. 7. capable of maintaining the wheel speed at a limited skidding level that maximises the ground friction. The normal force N is generated by the tyre pressure p and the intersection area A between tyre and ground: N = p⋅ A Braking force was already considered in the previous paragraph. 7. because they operated an intense reduction and increase of pressure. E’ vietata la commercializzazione. current systems have a smooth and refined control.POLITECNICO DI MILANO . or outside a predetermined boundary.4. 7. No commercialisation allowed. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. A servo-valve then releases the braking pressure of that specific wheel. longitudinal slip and vehicle velocity. by allowing it to spin-up to the speed level sampled prior to slippage deceleration. or operates the thrust reversers). because a pre-set braking deceleration is automatically applied. In autobraking the pilot does not need to use the pedals to brake. Older systems were actually on-off control systems. Fig. the event is interpreted as incipient wheel locking.15. 7. Now. and then the control system starts to find the new modulating pressure. this depends on different factors: tyre material and conditions. also during automatic braking. or moves the thrust lever back to idle. 7. With the auto-braking armed in landing mode.9 Tyres and wheels The tyres are the aircraft-to-ground interface and are then the regions where braking and steering forces are generated. if one defines the ground friction factor as the ratio between the longitudinal braking force and the normal force. the maximum value of the friction factor is obtained around a 10% longitudinal slip and the friction factor decreases when the vehicle velocity increases. its max value is given by the normal force and the ground-tyre grip factor as indicated in eq.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. The system works also in take-off mode: if the pilot makes any operation typical of a rejected take-off (extends the spoilers. ground material and conditions. The automatic braking system is often associated with anti-skid system. VERSION 2004 Chapter 7 – Landing gear system is decelerating more intensively than the others. the intensity is pre-set by the crew before landing.12 .15 –Friction factor vs. The longitudinal slip k is given by the ratio between the sliding speed vSL and the vehicle speed v: k= v SL v − ω ⋅ R R = v v where ω is the wheel rotational speed and RR the already mentioned rolling radius. In any case the pilot can overcome the auto-braking by pressing the pedals beyond a predetermined excursion. longitudinal slip and vehicle velocity These lecture notes are available for the students of the Polytechnic of Milan for free download. The anti-skid system is always operating. As shown in fig. then providing a constant deceleration. also contributing in a minor extent to shock absorption. or at spoiler extension. brakes will be automatically activated a few seconds after touch down. 7. auto-braking is triggered at maximum level. λ. These are the wheel parts that mostly require maintenance.17 – Tyre section reinforcement. Some wheels for military aircraft are equipped with fusible plugs.16 – Side friction coefficient This coefficient. is the sidewall. house the brakes.16. from the tread to the beads. These lecture notes are available for the students of the Polytechnic of Milan for free download. The external layer in contact with the ground is the tread.17 shows some details of the tyre structure. 7. The internal multiple layer structure is the TREAD SIDEWALL cord body. still made of rubber but smooth. the wires also serve as anchoring line of the cords. There are substantially two types of wheels: split wheel and demountable flange wheel. They contain a BEAD TOE ring of high strength steel wires covered by BEAD WIRES the rubber and by chafing strips for Fig.4. made of rubber. B and C represent respectively the overall outside diameter. is a function of the slip angle β and is linear for angles lower than 5°. CORD LAYERS The beads are the edges of the tyre. The first one is made of two halves bolted together and with the connection line sealed by an o-ring. 7. thicker than the automotive version. 7. in many cases. capable to withstand high radial and lateral loads. 0. where A. The wheel is then mounted on the axle by tapered roller bearing. Aircraft tyres can be both with internal tube or tubeless. The wheels have the double function to carry the tyre and.5 β 5 10 Fig. Tyre designation follows one of the two rules as follows: AxB and AxB-C.POLITECNICO DI MILANO . Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. activated by the overheating that may occur during the spin up in high velocity landing conditions. which give the body a uniform stiffness and strength. Since the cords are capable to withstand only tensile loads. 7. connected to the wheel rim. The side external layer. A law similar to eq. because it is subject to severe wearing during spin up at landing. then tending rather rapidly to the ground friction factor µG. Fig. Each layer may be considered a composite structure. 7.13 . E’ vietata la commercializzazione. basically consisting in cleaning and lubrication.DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS – LECTURE NOTES. VERSION 2004 Chapter 7 – Landing gear system A lateral force arises when the tyre has a slip angle with respect to ground: this is responsible for steering. made of parallel nylon cords imbedded in rubber. cross sectional width and rim diameter.0 λ coefficient. adjacent layers have orthogonal cord CORD BODY directions. The second one is made of a main wheel body closed on one side by a bolted flange. can approximate this force: S =λ⋅N . No commercialisation allowed. as indicated in fig. Aircraft wheels are made of aluminium or magnesium alloys. or pressure relief valves. measures are in inches. 7. given by the normal force and a side friction 1.