1    

Design Of An Experimental Aircraft Main Landing Gear

Katarzyna Okulska Warsaw University of Technology

 

Among many aircraft designs, some of them step out greatly. While market competition is growing, it is important to seek out new solutions which would attract the attention of customers. There are many possibilities. This project is MOSUPS concerned with the design of a light aircraft with joined-wings and a pusher drive. However, unusual aircraft structures imply unusual requirements for landing gear. The design of such landing gear is the subject of this project.

Nomenclature αp – angle of the leg pitch lo – overall length lsafe – minimum safe distance from the ground α – angle of rotation hg – vertical deflection hb – horizontal deflection ho – deflection of tire l – length of pitch leg M – maximum mass of aircraft w – sink rate P – work done by the tire E – energy of descending L – lift W – gravity N – safety factor ηg – efficiency of vertical deflection ηb – efficiency of horizontal deflection ηo – efficiency of tire’s deflection η – friction factor  Fx – load of X-axis Fy – load on Y-axis (side load) Fz – vertical load TX’ – shear force on X-axis TZ’ – shear force on Z-axis MX’ – bending moment about X-axis MY’ – torque MZ’ – bending moment about Z-axis H – height of module h – height of core b – Width of module δ – thickness of the shear webs

2    

I. Introduction  In the recent years, the light aircraft market has developed significantly. The availability of the light aircrafts

as a means of transport, and not as a hobby, improved greatly. Airport and airstrip networks, which are adapted for take-off and landing of small aviation, is wider than ever before. Combined with better service of air traffic, an advancement in technology was noticed. The latest improvements, modern avionics and materials contributed to the capability of small aircrafts to fly faster and further. The market is also more competitive. Manufacturers are looking for new, profitable solutions which make their products the best choice for the client. Project MOSUPS fits well with this trend. It aims to design an experimental aircraft with joined-wings and a pusher drive.

In the moment of writing this article, there were two models of the MOSUPS aircraft: one to flight testing and another to wind tunnel testing. Designers are going to design two versions of the aircraft: a two-seater ultralight version and a four-seater one.

Joined-wings arrangement is not usually used in an aircraft design. Neutral point of stability is between front and aft wing. Due to best visibility from the cockpit, the pilot’s seat has to be in front of or below the front wing. In this case, application of the drive with a tractor propeller which is located in the front of fuselage (classic one-engine aircraft)

causes too large stability margin. There is a reasonable need to apply pusher drive in the back of the aircraft in order to place the center of gravity correctly. How could this be achieved? There are two options. The first option is to use a pusher propeller whose diameter is average for that type of aircraft. The great disadvantage of this solution is a quite high (and heavy) landing gear. The second one is to use a ducted fan (Fig.1). The fan guarantees smaller dimensions, but it is a novelty design which demands a greater number of tests. The goal of my project was to design a MOSUPS four-seater aircraft main landing gear in order to check its feasibility.

What is important to remember, there were not any dimensions, technical data and performance for four-seater version of the aircraft available. Most of them needed to be estimated on the basis of the model’s flight testing. Main results which could be helpful in the comparison with different aircraft’s are given in the table below (Tab.1).

Table 1 Main data for general characteristics and performance

General characteristics: Wing span 10.50 m Length 7.72 m Height 2.67 m Wing area 8.745 m2 Maximum take-off weight 998 kg Type of engine MISTRAL G-200 Maximum take-off power 149.1 kW Type of propeller 3-blades Propeller’s diameter 2.06 m Performance (for maximum take-off weight) Maximum speed (at 3km. m.a.s.l) 230 km/h Maximum speed (at 0km. m.a.s.l) 241 km/h Cruise speed 180 km/h Rate of climb 7.5 m/s

 

Figure 1 MOSUPS aircraft – ducted fan version. Project: Ł. Stefanek

3    

II. General concept The first part of the project was an overview of the solutions available on the market. A fixed, nose-wheel

landing gear with composite solid spring as the main landing gear leg was chosen in the course of the market analysis.

Fixed landing gear is lighter, cheaper and simpler to design, use and service than the retractable one. For this aircraft, the cost criterion was more important than performance, so a greater aerodynamic drag was considered to be less significant. [6]

The nose-wheel landing gear has many advantages as well. First of all, it provides safe landing both at high and low speeds. What is more, it allows more intensive braking without apprehension of overturn. It is more stable during taxying and landing with the crosswind than tail-wheel type too. The visibility from the cockpit is better as well as the comfort of the passengers due to a horizontal deck. [1]

Main landing gear leg is a composite solid spring. In this type of solution, the element which absorbs kinetic energy is a solid spring which is connecting the main wheel with the fuselage. The spring is installed at an angle which allows vertical movement of the aircraft and the absorption of energy. A deflection of the spring causes shift and rotation of the wheel. [7] The important fact is that damping is provided by the friction of the tires on the ground in the side move. The efficiency of that solution is not very high in comparison to other possibilities (like oleo-pneumatic absorber) [2], but it is characterized by low weight, simple design and low cost of production and conservation.

III. Location and basic dimension of the gear landing  

The location of the landing gear depends on the location of the aft center of gravity, propeller’s diameter, and the maximum angle of attack. The landing gear has to be high enough to prevent any part of the airplane from hitting the runway during the landing. Given that, the location of the landing gear is showed below (Fig. 2). With dimensions from the figure and with the requirement that the wheel track should be 3 meters wide, the basic dimensions of the landing gear are defined as follows:

• Angle of the leg pitch α! = 45   ̊ • Overall length l! = 2.03m • Minimum safe distance from the ground l!"#$ = 0.77m

Figure 2 The location of main landing gear

 

propeller 10  ��  15�  

4    

There are two types of dependences which are used to determine the vertical deflection of the leg:

Geometric– the vertical deflection is associated with the horizontal one, angle of attack, and geometry of the landing gear.

Energetic – the comparison of work done by the strut and the tire to total energy of landing.

Model assumes that the leg deflects and rotates to the point Z through the angle α. On the basis of the schematic below (Fig. 3) the relationship between deflection of the leg and the angle of rotation is:

2l! ∗ 1 − cosα −h!

sin α2 + α!

= 0

The energy of descending is the sum of kinetic energy (sink rate equals 2,95 m/s) and work done by the gravity W and the lift L in the way equal to travel of the center of gravity (which is the sum of deflection of the tire and the leg (hg + ho) [3]:

E =  M ∗ w!

2+ h! + h! ∗ (W + L)

Work done by the tire and the leg is known as [3]:

P = n ∗W ∗ (h! ∗ η! + h! ∗ η! ∗ η + h! ∗ η!) Where: n – load factor, ηg/ ηb/ ηo – efficiency of vertical deflection/horizontal deflection/absorption of the

energy by the tire and η – friction factor. The size of the load factor is bigger than the minimum in CS 23 regulations because of the experimental character of the aircraft and its value is ±4.4.

IV. Load cases Before commencing the design of any part of the landing gear, load cases have to be established. CS 23

regulations [4] contain explicit instructions on the definitions of load cases (CS 23 483,485,493 + appendices C). There are six load cases whose values for one wheel are given in the table below:

No. Case Fx [N] Fy [N] Fz [N] 1. level landing with inclined reactions 3834 - 13012 2. level landing with nose wheel just clear of ground 5385 - 18275 3. tail-down landing - - 18275 4. one-wheel landing 5385 - 18275 5. side-load conditions - 4895* 6511 6. braked roll conditions 5208* - 6511

Table 2 Load cases *Reactions applied to the point of contact with the ground

The designation of the forces are referred to the global co-ordinate system, in which Z-axis is the vertical

one, X-axis is the main axis of the aircraft and Y-axis is perpendicular to both of them, towards to the right wing. In spite of variety of loads and placements of their application, the dimensional case is quite clear – level landing with nose wheel just clear off ground because of the largest Z-force and high value of X-Force. In addition to that, strength calculations are concerned to the 6th case also (maximum arm of the X-force).

Figure 3 Schematic of deflection and rotation of the main landing gear leg.

α/2  +  αp  

Z  

u

αp  

5    

V. Main wheel assembly Nowadays, most of the parts of the main wheel assembly are produced by some specialized companies. With

that knowledge, tire, rim with disk brake, and ball bearings were chosen from companies’ catalogs, which were available on the Internet.

The choice of the tire depends on the maximum static load which is equal to the half of weight of the aircraft and maximum bottoming load (6th load case). The chosen tire is Michelin Bias Type III, whose rim diameter is 5 inches, static load radius equal 5.7 inches and bottoming load radius is 4 inches [8] Assuming that the radius of the tire is linearly dependent on the deflection of the tire, the radius for real maximum load and deflection of the tire were calculated.

Having chosen the tire, it was possible to choose the rim with disk brake and ball bearings (TOST company

[9]). Next step in the main wheel assembly project was the design of the axle of the main wheel. The model was designed in NX 8.0. The axle is loaded with bending moment MX and MZ from FZ and FX forces and in 6th it is loaded with torque MY. In order to ensure the safety factor 1.5, the axle is made of 40HM steel (yield strength 880Mpa).

Figure 4 The design of the main wheel assembly with the holder connecting the assembly to the leg

VI. Main landing gear leg design

The most important and the most extensive part of work on the project was the design of the leg. It is loaded by two bending moments MX’ and MZ’, two shear forces TZ’ and TX’, torque MY’ and compression force FY’. Apostrophe mark is related to the local co-ordinate system in which Y-axis is along the length of the leg, X-axis is parallel to global X-axis and Z-axis are perpendicular to both of them. To calculate the strength of the leg, we need to know the distribution of all the mentioned loads along the leg’s length. The calculations are based on the schematic from Fig. 5.

Maximum value of both bending Figure 5 Schematic needed for loads calculation

A   B  

K  

G  P  

6    

moments is at the place of restraint B. Compression force is constant and it concerns only the inclined part of the leg. Shearing forces are the greatest between restraints A and B. It is assumed that the freedom of rotation is taken-off at the place of restraint B, so torque is relevant for segment BK. Maximum of torque is on the segment BG.

The design of the leg section is shown in the figure 6. Because of light weight and high mechanical

properties it was decided to make the leg out of composite materials. In order to make calculations easier, the distribution of roles between the caps and the shear webs is assumed.

The caps are made of carbon fibre composite with fibres arranged along the leg (uni-directional, Young’s Modulus 135GPa, Ult. compression strength 1200MPa [10]). Thanks to that, it carries the load of bending moments MX’ and MZ’ and compression force FY’ effectively.

The shear webs and cover are made of glass fibre composite with fibres arranged on angles 45/-45 degrees in relative to main axis of the leg (Young’s Modulus 12,2GPa, in-plane shear strength 150Mpa [10]). Thanks to this, it carries the load of shearing forces TX’ and TZ’ and torque MY’ effectively. Glass-fibre fabric is simpler to bend under the large angles and it is more scratch-resistant than carbon-fibre composite too.

The core is made of polymer foam to assure the better work of the section, lower weight and higher stiffness.

Figure 6 Section of the leg

Strength calculations is the comparison of maximum stress in the caps (the sum of normal stresses from bending moments and compression force) and maximum stress in the shear web (the sum of shearing stress from shearing forces and torque) to permissible stresses for carbon fibre and glass fibre composites [5]. What is more, the stiffness of the leg has to be high enough to make deflection equal to calculations from energetic equations earlier. The results of strength calculations provided section’s geometry, which is shown in the table below (marks are the same like in Fig. 5).

Table 3 Sections' geometry

Section Height of module H

[mm]

Height of core h[mm]

Width of module b[mm]

Thickness of the shear

webs δ[mm]

P 54 30 35 1.5 A 54 30 36 1.5 B 54 30 56 1.9 G 54 30 60 1.9 K 23 12 36 1.5

Figure below presents the whole design of the main gear. There is an independent leg for each wheel, whose

connection to the fuselage is similar to the bayonet wing connection to the fuselage. The part of the leg is tapered to fit the opposite leg. In the case of using one integrated leg, load on the one side of the main gear causes uplifting the main gear on the other side which is unfavorable.

7    

Figure 7 Main landing gear

Figure 8 Exploded view of main landing gear

The important part of work was designing connections between the leg and the main wheel assembly and

between the leg and fuselage. Because holes in composite lowers its strength significantly and there are not many reliable and available studies about metal-composite adhesive joints, thus the design avoids such solutions.

8    

Figure 9 The leg – main wheel assembly connection

The connection of the leg with the main wheel assembly is presented in

Fig. 9. There are additional layers of fabric in the ending of the leg (1) caps. The leg is bonded with both halves of the holder (16a,16b) with glue. The holder is shaped to fit the nominal leg and the glue compensates inaccuracies of the production of the leg. To prevent compression of the leg during the attempts of slipping out of the holder, the space between the caps is filled with aluminum alloy dice instead of polymer foam. In the holder, there is the ending of the main wheel axle (17) (close fit). The halves are bonded with themselves with bolts (18a) (they are not going through the leg – no holes in composite). Two bolts (18b) join the axle with the halves. Every bolt is secured from slipping out by nut (19a,b).

The connection of the leg to the fuselage can be divided into two

subassemblies. 1) Clasps with bolts – their function is to prevent the possibility of

movement along vertical axis and along X-axis (parallel to main axis of the aircraft)

2) Pin joint with a ball hinge – its function is to prevent the possibility of movement along Y-axis (slipping out or in)

Ad. 1. The clasps (4,5) are slipped on the leg. They have matched hole for the bolt (6). The clasp is bonded

to the leg with glue during the first assembly. Inaccuracies in the dimensions such as the distance between holes in the frames are compensated because the glue, before it sets, has the capability of small adjustments. The bolt (6) goes through one of the main frames of the aircraft (2a), hole in the clasp of the first leg (1a), middle frame (3), clasp of the second one (1b) and the ending frame (2b).

Figure 10 Lock of the bolt

9    

The important questions is: how the bolt can be prevented from slipping out? It is secured by the ledge on the frame and the metal plate (8). During the assembly, the plate deflects under pressure of the bolt, and after that it comes back to the neutral position. This blocks the backward movement of the bolt. This kind of connection is easier in assembly because it requires the access to the frame only from the one side (in comparison to the nut). The middle frame and clasps effect is that the bolt is loaded only by the shear force (not bending moment) significantly lowering its diameter.  

 

Figure 11 The leg - fuselage connection

Ad.2. There is another insert made of aluminum alloy on the ending of the leg with a forged ball hinge. The hinge allows small movements of the leg during bending. The bracket (9) is mounted to the frame and the bolt (12) goes through the ball hinge and the bracket. The bolt is prevented from up-down movements and rotation by a profiled plate (13). The centering plate (15), which is glued to the frame, makes hitting into the hole easier and secures the bolt against vibration. Additional layers of fabric on the ending of the leg prevents it from slipping out. Slipping in is blocked by the geometry of the insert.

Figure 12 The lock of the second bolt

10    

Figure 13 The leg – the fuselage connection

VII. Conclusion The final main landing gear is, actually, heavier in the propeller version than the one with a ducted fan. Mass

analysis shows that weight of the leg is 9.15kg for first version and 7.34kg for the second one. It gives 1.81kg difference which is not enough to decide surely that design of the aircraft with the propeller is not possible. What is more, designed landing gear could be lighter. It would be helpful to use finite elements method to improve accuracy of calculations which should produce more specific results. An analytic method could generate errors due to chosen method of integration, distribution of roles between the caps and the shear webs of the leg and omission of the polymer foam in carrying loads. Moreover, it should be remembered that there were few verified data about the four-seater version of the aircraft and obtaining most of them was estimated or scaled. Designed connections: leg-main wheel assembly and leg-fuselage as composite-metal joints need to be laboratory tested.

Finally, the goal which is the design of the experimental aircraft main gear landing has been achieved and the construction fulfills the requirements and assumptions.

References [1] Cymerkiewicz, R., Budowa samolotów. Warsaw, Wydawnictwo Komunikacji I Łączności 1982 [2] Jenkins 1989 [3] Błażewicz, W., Budowa samolotów. Obciążenia. Warsaw, Wydawnictwo Politechniki Warszawskiej 1979 [4] CS 23 regulations [5] Bijak-Żochowski, M., Mechanika materiałów i konstrukcji, Vol. 1, Warsaw, Oficyna Wydawnicza Politechnika Warszawskiej 2006 [6] Sadreay M., Landing gear design. URL: http://faculty.dwc.edu/sadraey [sited: 18 February 2014] [7] Chartier B. and others: Landing gear shock absorber. URL: ftp://ftp.uniduisburg.de/FlightGear/Docs/Landing_Gear_Shock_Absorber.pdf [sited: 18 February 2014] [8] http://www.airmichelin.com [sited: 17 February 2014] [9] http://www.tost.de/PDF/Wheel%20Catalog%202012.pdf [sited: 17 February 2014] [10] http://www.performance-composites.com [sited: 17 February 2014]