2017 International Conference on Applied Mechanics and Mechanical Automation (AMMA 2017) ISBN: 978-1-60595-471-4

Analysis and Research on Wing-Fuselage Connection Design of Small Combined UAV

Pei-yuan LI*, Jing-wu HE and Yue-xi XIONG

School of Aeronautical Science and Engineering, Beihang University, Beijing, China

*Corresponding author

Keywords: Unmanned aerial vehicle (UAV), Combined type, Wing connecting.

Abstract. In this paper, the analysis and research is conducted on a wing-fuselage connection part

using a small combined unmanned aerial vehicle (UAV). The work is based on the characteristics of

flexible disassembly in order to be easy to carry and transport for the combined type unmanned aerial

vehicle (UAV). The combined wing-fuselage structure is designed with the structure design theory.

Analyzing each project from the internal stress and strain situation, mass attributes and other aspects

based on result from FEM sofware. With the comprehensive comparisons of the characteristics of

each project in the consideration of satisfying the strength and stiffness criteria it provides a reference

for the design of the combined type UAV wing-fuselage connecting structure.

Introduction

The unmanned aerial vehicle (UAV) has been widely used in today's society. For the common

UAV, its size is generally smaller than the volume of traditional aircraft, the cost is low, the service

life is short, and it often requires bulk transportation. In transit, its own geometric shape restrictions

often bring a lot of inconvenience, especially for some UAVs with large aspect ratio which need to

carry out high-altitude long-endurance operation. However, the combined UAV can be dismantled

into a number of independent standard modules according to the fuselage, wing, tail and landing gear,

etc. It is flexible and easy to apply, and also convenient for daily maintenance and packing

transportation. It can be put into use after a simple assembly work in the workplace when needed. The

combined UAV overcomes the above drawbacks. In this paper, a small-sized hollow long-endurance

solar UAV with large aspect ratio is taken as an example. Proceeding from the engineering reality, for

the connection problem between the wing and the fuselage, several reasonable design scheme are put

forward, and each scheme is analyzed through combined with finite element simulation results.

Figure 1. The outside view of whole machine. Figure 2. The sectional view of SG6043 airfoil.

The solar UAV is lighter in weight, the whole machine structure weighs about 18kg, and the

airborne equipment weighs about 20kg, which is a whole composite structure. The fuselage adopts

single body structure, the structure of the body is a hollow variable cross-section tube, and an electric

motor is installed in the head of the fuselage to provide power. About 400mm position after fuselage

head is equipped with a battery bracket. The wing has a wingspan of about 10m, and adopts the

straight wing of SG6043 airfoil, whose aspect ratio is about 11. The middle front of the fuselage is

connected with the wing, fuselage at the wing trailing edge about 150mm is divided into detachable

pluggable two segments, and the tail of the fuselage winds out the cylindrical interface in the

89

processing to connect with the horizontal tail and vertical tail. In the finite element analysis, in

addition to the parts connected to the wing body, the other parts are made to a certain degree of

simplification.

In a non-removable scheme of the real engineering practice, the wing is glued with the fuselage

through the foam block, the outer side of the foam is coated with carbon fiber fabric, and bonded

together with wing skin. The carbon fiber fabric participates in most of the load transfer, the foam

transfers a small part of the load and plays the role of shape. The scheme is robust and reliable, but in

the assembly and transportation, fuselage and wings need to be overall packed, resulting in a huge

waste of resources and space; and not removable, the destruction of a structure in the parts will cause

the whole machine scrapped. And because the surface of the foam block contains a large number of

small holes, due to the limitations of the level of processing technology, the glue will often penetrate

into the hole when applying glue, indirectly resulting in an increase in the weight of the fuselage

structure. According to the existing engineering experience, the increase in the structure weight

caused by the glue penetration can even be up to about 20% of the body itself. Therefore, it is very

important to find a kind of detachable and light-weight connection scheme. Investigating the world's

many UAVs' wing-body connection form [1], and combining relevant theoretical knowledge of the

aircraft structure design [2], this paper puts forward three kinds of wing-body connection scheme

according to the actual situation.

Introduction of Several Wing-Body Connection Schemes

"Ω" Type Bracket Connection Scheme

The "Ω" type bracket connecting scheme puts two pieces of inverted "Ω" connecting components

under the two wing beams, the fuselage passing through it, and the components are connected and

fixed with the lower edge of the front and the back beams and the lower abdomen of the body through

bolts. The structure of this scheme is relatively simple, and the process is less difficult. The edge strip

near the bolt hole of the wing-body connection needs to be widened and thickened, and the position

connected with the web should be reinforced. And because the fuselage may squeeze the lower

surface of the wing, in order to improve the local stiffness, a reinforced rib is also arranged above the

fuselage. The "Ω" connection component is simple in shape and has a composite structure, the middle

of which is 2mm thick foam material, and then the two sides of which is laid alternately with five

layers of T300-3k fabric with 0 ° and 90 ° on each side respectively.

Figure 3. The diagram of original non-removable

scheme.

Figure 4. The diagram of "Ω" type bracket connecting

component.

Wing-Body Fusion Half-Buried Connection Scheme

Wing body fusion half-buried connection scheme fuses the wing and the fuselage structure. Two

reinforced ribs are laid close to the two sides of the fuselage in the wing structure, and reinforced ribs'

web extend about a quarter of circular arc downward along the outer surface of the fuselage. The

connecting component wraps the fuselage and connects with three pairs of bolts. A semicircular

groove is made in the position that wing beam is fitted with the fuselage, the depth of which is about

one third of the web' height, and the lower edge of the groove and the skin are fitted with the body

tightly. In order to make up for the loss of strength caused by slotting, the wing beam is provided with

90

double side edges and stiffeners at the notch. A wedge foam block is used to maintain the shape

between the two reinforced ribs at the trailing edge of the wing. The advantage of this scheme is that

the fuselage can be closely attached to the wing, the weight of the structure is reduced, and the

transmission effect is better, but the manufacturing process is slightly more difficult than other

schemes.

Figure 5. The diagram of wing-body fusion half-buried connection scheme.

Rotary Bolt Connection Scheme

The rotary bolt connection scheme adopts two front and rear bolts with a butterfly-shaped lug boss as

connecting components. When installed, the bolt penetrates through the wing and the fuselage from

top to bottom, rotates 90 degrees around the symmetry axis of itself after through the fuselage, and

then is locked with the matching nut. The purpose of using butterfly bolt is to remove the work of

lifting the fuselage. The purpose of adopting a butterfly-shaped lug boss is to solve the problem that

the bolt can swing around the through hole of the wing flange after locked. The wing flange should be

reinforced at the corresponding position.

Figure 6. The diagram of rotary bolt connection scheme.

Finite Element Analysis

Then the finite element simulation analysis of the three connection schemes is carried out. Finite

element simulation work mainly focuses on calculating the strength and rigidity of the joint between

91

wing and fuselage, the wing structure and the fuselage structure of each scheme, so some details of

other parts are simplified, and the material of the structure of the body is mainly carbon fiber fabric,

foam and resin, etc. The bolts used for connection are made of aluminum alloy. The properties of the

material mainly used are in the following table:

Table 1. Parameters of T300 carbon fiber material [3,4].

ρ [kg·m-3

] E1 [MPa] E2 [MPa] Poisson's ratio G12 [MPa] G13 [MPa] G23 [MPa]

T300-1k 1200 126000 8000 0.33 3700 3700 3700

T300-3k 1550

Table 2. Parameters of the main materials in several other structures.

Material ρ[kg·m-3] E[MPa] Poisson's ratio

Aluminium alloy 1170 1000 0.38

Epoxy resin 1170 1000 0.38

Foamed material 35 686 0.32

And then according to the layup program, we create a composite layer or give the cross-section

properties for the different structure of the UAV, of which the layup of the skin is the simplest. The

two layered T300-1k fabric is laid by ± 45 degrees. The bolt hole of the wing and fuselage structure

needs strengthening to varying degrees. Reinforcement program can be adjusted according to the

specific finite element simulation results and theories about composite structure design [5]. Here no

longer say. The layup program of the other parts of the main parts is as shown in the following table:

Table 3. Fuselage layup scheme.

Material Thickness[mm] Angle[° ]

T300-1k 0.11 0

T300-1k 0.11 90

Foam 2 -

T300-1k 0.11 0

Table 4. Layer of wing beam.

Web Edge

Material Thickness[mm] Angle[° ] Material Thickness[mm] Angle[° ]

T300-3k 0.25 -45 T300-3k 0.25 -45

T300-3k 0.25 45 T300-3k 0.25 45

Resin 0.1 - T300-3k 0.25 -45

Foam 2.8 - T300-3k 0.25 -45

Resin 0.1 - T300-3k 0.25 45

T300-3k 0.25 45 T300-3k 0.25 -45

T300-3k 0.25 -45

Table 5. Layer of wing rib.

Web Edge strip and lightening hole flanging

Material Thickness [mm] Angle [°] Material Thickness [mm] Angle [°]

T300-3k 0.25 0 T300-3k 0.25 90

Resin 0.1 - T300-3k 0.25 0

Foam 1.8 （reinforced rib 2.8） - T300-3k 0.25 0

Resin 0.1 - T300-3k 0.25 9

T300-3k 0.25 0

The total lift force of about half a wing is gained of 245.2N by the calculation of wing 3D modeling

of CFD software. The total wing provides a total force of about 490 N. According to the arrangement

of the ribs, the whole wing is divided into several parts from symmetry plane. The lift force on each

section is integrated. Assuming that the resultant force point of each aerodynamic force is located in

the middle of the section, as shown in the figure below, because the force in the other directions is less

than the force in the Z-direction, we assume that the resultant force of aerodynamic force is along the

92

Z direction of the body coordinate system. The resultant force of each section is as shown in the

following table.

Table 6. Several schemes of loading arrangement of aerodynamic load.

Scheme

Direction

Original

non-removable

scheme

“Ω” shape Bracket

connection

scheme

Wing body fusion

half-buried

connection scheme

Rotary bolt

connection

scheme

F[N] F[N] F[N] F[N]

Wing root 10.762 10.762 3.223 10.762

16.186 22.66 7.539 22.66

26.952 23.168 19.963 23.168

26.833 15.843 19.945 15.843

26.62 15.78 18.545 15.78

26.286 22.315 6.969 22.315

25.776 24.674 17.893 24.674

24.98 24.721 20.149 24.721

23.67 24.471 22.538 24.471

21.285 23.67 23.668 23.67

15.855 21.285 23.474 21.285

- 15.855 24.2585 15.855

wingtip - - 21.285 -

- - 15.855 -

Besides the aerodynamic load above, the load kinds of full-aircraft are as follows. The load is

applied by overload ratio of 1.8.

Table 7. Other main loads.

Item F[N] Item F[N]

Motor peak tension 180 Tail gravity 22

Battery gravity 80 Vertical tail motor inertia force 20

The gravity of motor and blade 12 Horizontal tail anti lift 17

At the symmetry plane of the wing structure, the boundary condition is set to be completely fixed

U1=U2=U3=UR1=UR2=UR3=0, draw the mass and set the definition of the contact between the

components, afterwards, submit the calculation.

In the original non-removable scheme, the maximum stress appears in the joints of carbon cloth

and covering skin which coat and connect the foam blocks outside, and the maximum stress is

51.03MPa. In the new designed three kinds of removable schemes, the maximum stress of the "Ω"

type bracket type connection scheme and the blended wing body half buried connection scheme,

appeared on the bolt hole surrounding of the connecting members, and the maximum stress is

147.4MPa and 95.15MPa respectively. The maximum stress of the rotary bolt type scheme appears

inside the aluminum alloy bolt, and the value is 180.6MPa.

93

Figure 7. The maximum stress of several connection schemes.

In the original non-removable scheme, the maximum strain occurs at the junction between

the foam block and the fuselage near the trailing edge of the wing, its value is 1617µε, which

is due to the load on the tail and relatively larger distance from the tail and other reasons. The

other three removable schemes, the maximum strain all appear on opening edge of the bolt

hole, the value are 2480µε, 2617µε and 2776µε, which are all in line with the expected

requirements.

Figure 8. The maximum strain of several connection schemes.

Summary

The statistics of the stress, strain, and global assembly mass for several solutions are shown in the

following table:

Table 8. Statistics of the stress, strain and global assembly mass of several schemes.

Connection scheme Global maximum

stress [MPa]

Global maximum

strain [µε]

Connection

component stress

[MPa]

Connection

component strain

[µε]

Global

assembly

weight [kg]

Non-removable

scheme 51.03 1617 11.1 819.1 15.9

"Ω" type bracket

type connection

scheme

147.4 2480 79.59 1856 14.3

Wing body fusion

half buried

connection scheme

95.15 2617 95.1 1798 14.4

Rotary latch type

connection scheme 180.6 2776 180.6 - 14.2

94

It is worth noting that the strain of the bolt hole edge in the three scheme is calculated after different

reinforcement. In the reinforcement scheme, the degree of reinforcement of the fuselage in the semi

submerged connection scheme of the wing body fusion is the smallest, and only four layers of

T300-3k fabric are added in the corresponding adjacent area of the hole edge. Especially in the

rotating bolt connection reinforcement scheme, the degree of reinforcement is biggest. And because

in this scheme, the connecting member is only connected through front and rear bolt, in the

corresponding opening position, the load of this position is relatively large. So 13 layers of fabric are

laid near the opening of the wing beam flange; in the opening hole close position of the fuselage

again, there are more than ten layers of T300-3k fabric, even so, the strain of the whole fuselage is up

to nearly 2800µε.

As can be seen in the above many schemes, the stress and strain of the original non-detachable

wing body connection program are both the lowest, and relatively safe and reliable, but the price is

the highest mass. Three new proposed schemes, the blended wing body half buried connection

scheme performs more prominently, whose stress and strain values are more advantageous compared

to the other two schemes. It shows that the design of the structure is reasonable, the force is even, and

the mass is reduced by 9.4% compared with the original non-detachable one. The global assembly

mass of the "Ω" bracket type connection scheme is similar to the blended wing body half buried

connection scheme, but the biggest strain and connected component, namely the "Ω" connector, of

which internal stress is less than the blended wing body half buried connection scheme. In all

schemes, the mass of rotary bolt type scheme is the lowest, but because of the stress concentration

around the fuselage openings, the value of stress and strain are both larger than the other schemes. It

has the advantages of simple structure, quick connection, but the body may lead to swing during the

flight, which has impact on the stability.

References

[1] “A Complete Collection of World Unmanned System” writing group. “A Complete Collection of

World Unmanned System” [M](Chinese). Beijing: Aviation Industry Press, 2015.

[2] Li Zhengneng. Aircraft structure [M] (Chinese). Beijing: Beijing University of Aeronautics and

Astronautics Press, 2005.

[3] Wu Cunli et al. Dynamic analysis and optimization design of rib antenna for a composite material

[A]. MSC.Software Chinese user papers in 2004 [C] (Chinese). MSC. Software company, 2004.

[4] He Wei, Li Tingqiu, Li Ziru. Study on influence factors of hydrodynamic performance of marine

composite propeller [J] (Chinese). China shipbuilding, 2015, (04): 119-128.

[5] Wang Yaoxian. Structural design of composite materials [M] (Chinese). Beijing: Chemical

Industry Press, 2001.

95