Advanced mechanical design report Essay Example

Designing Linear Actuators for Aircraft Landing Gear

TITLE: DESIGNING LINEAR ACTUATORS FOR AIRCRAFT LANDING GEAR

OBJECTIVES:

• To know the various parts and functions of a linear actuator as well as the landing gear

• To determine the working mechanism of a linear actuator

• To understand the working mechanism of the landing gear of an aircraft

INTRODUCTION

Aircraft which are designed to have shorter takeoffs, as well as landing operations, have some similar characteristics in the design of their landing gear. The surfaces are offering short landing usually are poorly prepared or deep sloped. This needs the development of a very efficient gearing system that will ensure smooth landing and takeoff of such planes similar to the smooth landing and takeoff of the planes with longer landing periods over a specifically prepared surface (Perkins et al. 2016).

To achieve such efficiency in poor landing conditions the landing gear is designed taking into consideration specific criteria making it a different unique and compact design. The design allows the transfer of all the landing loads to the structure of the aircraft. The wheels are made in a way that they operate under low pressure but also have a larger diameter which makes the load transfer process more effective compared to when a smaller wheel could have been used instead. The wheels are also equipped with a special feature that allows them to absorb large amounts of energy during landing. This allows for smooth distribution of the instant forces the wheels come into contact with during landing to other parts of the plane. As a result, the design criteria which helps in compensating for the rough short distances through which such planes are required to land or take off (Perkins et al. 2016).

However, when it comes to analyzing the utility of the plane specific considerations have to be taken into account. This is attributed to the large vehicles that load or unload the plane from the ramp at the main entrance. The roll angle of the aircraft, the pitch angle as well as the height above the surface which is adjusted for the sake of the optimal maximization of the headroom height. This also helps in minimizing the ramp angle through which the vehicles move cargo into and out of the loading bay at the landing site (Claeyssen et al. 2010).

Having most systems which balance the attitude of the plane while on the ground is a problem because they are hydraulic in nature. They come with some disadvantage. The first one is that it is very expensive to acquire hydraulic designs for airplanes landing. This makes it even difficult to repair them whenever they break down due to high maintenance and repair related costs. Secondly, hydraulic systems which need to support the plane are massive for them to be in a position to provide the required balancing force. This is because the force is a function of area and pressure. The larger the area, the bigger the force, the heavier the mechanism since force is directly proportional to mass and acceleration of the body involved. The other disadvantage of using hydraulic systems is that they are prone to failure. It is very difficult to determine the point through which they will break down. This leaves the company in a difficult situation in determining the exact time through which they should be promised (Claeyssen et al. 2010).

The same principle applies for the retractable gears as well as the landing gears of an aircraft. This has made the gears to be out of favor by most designers in the industry. They currently prefer to use electrical signal driven designs. The electrical designs have been preferred because of several advantages that it poses. It has a better power to weight ration meaning the heaviness of the mechanism is lower compared to the hydraulic design. The other advantage is that it reduces the essence of multiple redundancies as no one will want to have such a mechanism with an improved version currently available. This design also has a lower failure probability making it more reliable in the long run for aircraft managing companies. These advantages are also found in landing gears which should be appreciated for enabling the smaller planes to be able to land for a short time in rugged topography that may be steeply sloped. This is as a result of the great design m work done on the system ensuring its high performance under the extreme conditions it is meant to serve (Perkins et al. 2016).

The three main functions of the landing gear are the retraction, the plane suspension as well as its adjustment at different attitudes ensuring high levels of stability. All these functions have been compiled into a simple wheel assembly that is singly attached to the aircraft assembly helping reduce unnecessary weights to the airplane structure. This has also helped in improving the degree of safety of the aircraft during its landing and takeoff operations. The system has a comparatively higher efficiency of almost eighty percent this allows the reduction of its size further to more portable abut reliable design (Claeyssen et al. 2010).

Fig 1: side view of aircraft with a landing gear

Fig 2: Cross section of the plane about its landing gear

Fig 3: side view of the main landing gear

Fig 4: the gear at full retraction and extension positions

The landing gear is mainly made of the following parts the strut which is in the position of a length adjuster acting as a connector to the main aircraft through a pivot. The damper provides to the trailing arm with the help of a second pivot in the mechanism. A motor which is positioned between the two pivots is essential in driving the arms of the mechanism. The trailing arm provides the wheel, aircraft framework connection with the help of the third pivot. Therefore the three pivots are responsible for the lowering or raising of the wheel of the aircraft. This report looks at the design of the landing gear wheel of the aircraft some of its unique features as well as the forces that are distributed to various parts of the plane during Landing or takeoff in short distances, where the land is not even or smoothly prepared for the landing (Liscouët et al. 2012).

METHODOLOGY

During this session, various aspects of linear actuators were studied how they relate to each other. The design parameters such as tensile, shear stress as well as the pressure and area affecting the required designs were computed. These computations were cross-checked with required standards ensuring that the final measurements for the design of the mechanism were of high accuracy, precision as well as being reliable.

CALCULATIONS

Formula and Calculations

Fasteners design

d= nominal diameter of the bolt

The direct tension =F in Newton’s

The bolt is subject to a bending moment M as well as tension T

Hence the resulting forces as determined as follows.

Tensile stress at point A is given by

Where T =22/7

= stress

d =nominal bolt diameter

Bending stress at A is given by

Where M= moments of area

Y – Young’s modulus

Bending stress

Total stress at A

Substituting or design values we get total stress at A to be 20 MPa

Stress at point B

= —

Negative sign shows its compression

Design of the nut

Pb is the pressure exerted by the bearing= 1.5N/M²

Fs =6140N

do=24 mm, dc =19.5mm

n = no of threads in the screw is given by:

n=

Substituting our design values we get

The numbers of threads are approximately 27 to the nearest whole number.

Height of the nut = n* p

Therefore height = 27*3 = 81mm

Thickness of the screw =P/2 3/2=1.5mm

Checking of the design

The shear stress in the screw

Screw thread shear stress =

Substituting the values

Screw thread shear stress=

Screw thread shear stress = 2.47Mpa

Shear stress in the nut

Nut`s shear stress =

Substituting the values

Nut`s shear stress=

Nut`s shear stress = 2.0Mpa

Both the stress values are found within the limits required the design is a safe one

D1 = outside diameter of the nut

D2 = outside diameter of the nut collar

t = thickness of the nut collar

Determining the tearing stress of the nut

Fs= (3.142/4)*(D1²-DO²)*

is the permissible tearing stress of the nut

Substituting the respective values

6140= (3.142/4)*(D1²-24²)*

Making D1² subject of the formula and simplifying further you obtain the outside diameter value of the nut to be 29.8 mm.

Rounding off to the nearest diameter we find the outside diameter to be 30 mm.

Considering the crushing stress of the nut collar

Fs= (3.142/4)*(D2²-D1²)*

is the permissible tearing stress of the nut collar

Substituting the respective values

6140= (3.142/4)*(D2²-30²)*

Making D2² subject of the formula and simplifying further you obtain the outside diameter value of the nut to be 34 mm.

Considering the shearing of the nut collar

Fs= 3.142*D1*t1* shear stress

Substituting the values

6140= 3.142*30*t1* 30

Making t1 the subject of formula we find its value to be 2.17 mm this has been rounded off to have the value of our design t1=2.5 mm

Power calculations

Power = torque *angular velocity

Power =

Power = 2.068 Kw

Power screw calculation:

Drag force (FD) =

standard

= screw core diameter

= pitch thread of the screw

Concerning our source, the approximate value is = 21

Assuming
= 21 mm.

The mean diameter of screw =

Max length (L) = 1100mm

Nominal diameter =

Checking the design:

Shear stress in the screw,

Since the stress is within the limit, the design is safe.

Bearing design:

R=1250 mm Time =10s

Life of bearing:

Assuming for 20 years, the operation sequence of the bearing is 4 times/day:

=1460 times/year

Considering 29200 time/year

Amplification = 10,

Operation hours = 811 hrs of operation.

Expected life = 850 hrs of operation.

Adjustment factor catering for operating condition = 0.9

Adjustment factor catering for material condition = 0.8

Bearing dynamic load rating, reliability L99:

L99 = 60* Rpm*life in hours

L90= life corresponding to 90% reliability.

rating of the dynamic load ‘C’:

Bearing NO-304 was selected for the design

DISCUSSION

Functions of the various components of the complete design.

The mounts on fuselage are used to mount the design onto the framework of the aircraft. This device is made of tough steel material with high tensile strength. The durability of the whole component is dependent on the quality of this mounts.

Linkage pivot points provide the required flexible connection between the linkages, main strut and the framework of the body. There flexibility allows smooth landing retraction and suspension of the plane in a given attitude at various angles (Perkins et al. 2016).

The motor is the main source of power for the whole system as it provides the required energy that is converted into linear motion by the actuator. Its capacity is important during the determination of the capacity of the linear actuator for a given aircraft design. This is because the actuator should not at any point consume more power or energy than that which the motor can produce (Perkins et al. 2016).

The gearbox is responsible for shifting the different gear ratios thereby adjusting the speeds through which the linear actuator will move. This helps in varying the torque which is a product of the force of the moving body with its angular velocity. Thorough maintenance is required in this unit as it forms the main core of the functioning of the linear actuators.

The main strut acts as the connector of the motor and gearbox to the landing wheel of the aircraft. It is made of toughened steel that is corrosion proof for durability. The strut helps in distribution of the forces acting on the design during the landing suspension or retraction of the plane (Perkins et al. 2016).

Several forces that are considered when designing the linear actuator. The forces can be caused by the process of transmitting power from the source to the load. The second cause is through stresses caused by vibration of different elements of the design when it’s in operation. Static loads are caused by several factors such as tensile stress, buckling forces as well as compressive forces. These types of forces are mainly transmitted through three parts of the design the rod, the nuts, the screws component`s strut and finally its housing mechanism. With a high number of required cycles, the fatigue limits are given the priority. Shearing and Bending forces also fall into this category. They are mainly caused by the masses of the various parts of the design as well as their individual torques contributions to the design. These are found acting on most of the bearings and anchorage points of the design. Torsion stressed is mainly caused by the reaction torques of the motors as well as friction between any moving parts in the design. The dynamic stresses are caused by vibration movement that results into transversal vibrations causing the formation of mechanical bending stress. They are also caused by translational and rotational loads within the design. The most critical static loads considered during design are the tension buckling and compression forces. The others are assumed to have minimum impact on overall functioning of the whole system (Perkins et al. 2016).

The amount of energy that is found in the landing gear inertia is relatively smaller when compared to the energy as a result of the static loads acting in the opposite direction. This is the best explanation as to why static loads are usually by far greater than the dynamic loads. Hence for the sake of this design, the static loads were used to determine the size of the various components, since they had already taken into consideration the effects of dynamic loads which are comparatively lower (Perkins et al. 2016).

Design requirements

Design requirements for a landing gear actuation are found in different ways. High degree of reliability should be portrayed by design. This refers to the reduced risks of premature of the design. It can be attributed to use of strong material for such components that can be able to resist high static loads but at the same time be light in weight. More reliable design provides an economical way of utilization of the design in aircraft (Liscouët et al. 2012).

Safety of the design is also important. This refers to the level of harm by which human beings around the design are exposed to danger when in operation. A complete design takes into consideration a good housing that help minimize cases of external injury to human beings or animals. The other perception of safety of the device is to what extent it prematurely fail when in operation. If a design can fail at any time, then it may not be considered a safe component for use in the aircraft bearing in mind the number of lives planes transport at any given time (Liscouët et al. 2012).

Cost effectiveness refers to the affordability of the design it may be determined by the level of technology invested during its development or the materials used in making its functional units. Most of the people will prefer to use designs that are affordable but of good quality. Have such system allows the managers of the aircraft to easily replace the component or unit if it has failed beyond repair. On the other hand having those that are selling at high prices will not attract many buyers to the product no matter how best the quality of the product maybe (Liscouët et al. 2012).

A low maintenance cost is also another critical design aspect where the materials which will be used in this design are made of parts which can be easily afforded by the operators. This makes it easier for such people to be able to replace worn out parts of the design as well upgrade some of the parts to improve the performance of the design. A low maintenance cost for the design will increase its application in the market (Liscouët et al. 2012).

The design is made of special material that will ensure minimum weight being added to the aircraft but at the same time provided the same functions as heavier designs currently existing. Having them lighter helps reduce the overall additional weight added to the aircraft which allows the plane to be more stable during the landing, taking off and suspension at different attitudes (Liscouët et al. 2012).

To guarantee safety on the landing gear actuation procedure the following failure safety features have been incorporated into the design. Each of the motors has the required capability to raise or lower the gear wheel on its own. This helps in landing of the plane if the other motor malfunctions. Having two operational motor means the retraction time of the aircraft will always be achieved as per the required standards. In this design, more emphasis has been put on the retraction mode because it has higher level of risks as compared to the other operations of the system (Liscouët et al. 2012).

EMA system synergies

The system is made up of two motors which power one single-speed reduction gearbox. The clutch helps in freeing up the landing gear from the actuator in case of jamming. The utilization of two motors ensures the redundancy levels are at the minimum possible values. During calculation all the two types of motors for the design were considered as well as their power output. The roller and ball screws are essential in transforming the rotary motion into linear motion. The latter has better running speed essential during landing, whereas the former has got a good load bearing capacity with its small size (Perkins et al. 2016).

The main reason for using a linear actuator is to convert the available energy into a horizontal motion, there several specifications which should always be considered during its design. The first thing to consider is the quantity of force required to move the load in a given direction for our case the load is the aircraft. The required speed of movement for the actuator as well as the drive cycle through which the actuator will navigate. One has also to consider the type of energy source that will be responsible for powering of the actuator (Perkins et al. 2016).

Secondly one has to consider the aircraft conditions through which the actuator will be working. This mainly involves studying the type of aircraft the actuator will be used. The degree of safety of using the actuator as it should not be overloaded by the aircraft leading to premature failure as this can be fatal. The type of motor driving the actuator as well as space through which it will be applied is also a critical factor to be considered during the design. In this case, The application distance was a short one as this is meant for the planes that take off or land at a very short period (Liscouët et al. 2012).

The other important consideration is the power requirements which should be at a level that the source of power can effectively sustain for a long period. Failure to be sustained may lead to premature Failure which can easily crash the whole plane. The efficiency of the actuator regarding the power used in converting energy into a given motion in a given period is essential as a more efficient actuator will take considerably low power consumption levels to do the required job (Perkins et al. 2016).

Advantages of using this type of actuators for landing gears in aircraft

The designed actuator will have better precision in operation as compared to the other actuators that can do the same job. This design is portable being lighter as compared to the hydraulic type of actuators. Nature through which is has been designed allows it to be easily integrated with another component of the aircraft including the framework and the landing gear wheel. This is because of the three pivots that provide a quick easy and reliable connection. The design is also easy to install coming at a cheaper purchasing and maintenance costs as compared to the hydraulic type of actuators. The process of assembling this type of actuator is very easy and time-saving (Perkins et al. 2016).

Maintenance

The actuator alongside the whole gear landing system should be regularly checked for any wearing out. In case of damaged parts, they should be immediately be replaced with original ones. All the moving parts of the assembly should also be greased or oiled. The correct type of bearings should be used during overhaul and maintenance of the system. Carrying out of routine autonomous maintenance is essential in increasing the lifespan of the design while in operation. This is because of the reduced frequency of breakdowns in the system (Perkins et al. 2016).

From the design, a safe actuator was developed which met all the required design parameter standards. This was proved by the cross-checking of the values obtained during calculation and analysis of various forces, stresses acting at different points of the mechanism. During the design, some assumption were made when designing of the values of some of the dimensions. This by choice in design as long as they were within the required range of values (Liscouët et al. 2012).

CONCLUSION

At the end of the study, the various components of a linear actuator, as well as the landing gear in aircraft, had been understood together with their functions. The functioning of the components and how the actuator and the landing gear work together in ensuring safe landing take off and suspension of the aircraft had also been covered. The linear actuator has been designed according to the standards of designing aircraft equipment. The values precisions and accuracy were cross-checked throughout the calculations to minimize any sources of errors that may have occurred at the first round of calculation. The linear actuators provide more efficient, cheaper and reliable actuators for landing gears of an aircraft compared to the hydraulic type of design.

REFERENCE

Perkins, C., White, N., & Alexander, S. (2016). U.S. Patent Application No. 15/074,810.

Claeyssen, F., Jänker, P., LeLetty, R., Sosniki, O., Pages, A., Magnac, G., … & Dodds, G. (2010, June). New actuators for aircraft, space and military applications. In Proceedings of the 12th International Conference on New Actuators (pp. 324-330).

Liscouët, J., Maré, J. C., & Budinger, M. (2012). An integrated methodology for the preliminary design of highly reliable electromechanical actuators: search for architecture solutions. Aerospace Science and Technology, 22(1), 9-18.