ARTICLES RESEARCH & TECHNOLOGY

Advanced Remote Controlled (RC) Aircraft Design and Fabrication: Preliminary And Detail Design

x29-grunman-nasa

CHAPTER 4


Preliminary Design

Wing Structure

The wing structure is designed referring to various models of forward swept wing aircrafts manufactured in the past. The main structure is designed to consist of three spars, a front, central and rear spar. Front spar is made angular running through the quarter chord of the structure.

Central and rear spars are arranged so that they would meet the structural requirement. But later carbon rod is planned to be used in place of central spar which would provide support, integrity, stiffness and transfer the loads across through the structure.

The whole wing structure is designed as a single part so that fuselage is to be placed on it which would make assembly easier making a low wing configuration. From the aerodynamic analysis, wing is configured to have twist angle of 3 degree.

This feature is to be given proper attention during design process. No special feature is to be used in ribs except for areas for reinforcement to maintain structural integrity and the aileron control surface. The whole structure is to be composed of 12 ribs. But later additional two ribs are required for aileron structure and skin attachment.

priliminary-wing-structure

priliminary-design-of-wing-rib

Fuselage Structure

The fuselage structure is designed with ten frames and 4 longerons, enough to hold the fuselage together. The thickness of frames is also decided at this stage. Also, four stringers are required for the structural integrity. Two types of frames are to be designed: one simple rectangular with circular top and bottom and other similar frame but with cut off on the top part for the cockpit assembly.

frames-of-fuselage

In above figures, the four holes are for the longerons and the rectangular holes are for stringers to be attached. It is decided that the frames and longerons of fuselage is to be made out of ply wood. The frames are to be of 3 mm thickness in most of the parts and the longerons are to be of 8mm diameter.

Empennage Structure

The empennage of our aircraft only composes of a vertical tail. From the parameters calculated during conceptual design outline of vertical tail is drawn and based on it all the other structures are created. Similar to other structures the ribs are decided to be 3mm thick and a spar is used with a leading edge stringer to hold the structure together. Below is the rib structure of vertical stabilizer.

vertical-stablizer-rib

Canard

Variable canard structure is chosen to act as the vertical stabilizer in our aircraft. Since forward swept wing is our basic design configuration, canard is an important part of the whole aircraft structure. But unlike wing, canard is designed to be symmetrical. Fully movable canard structure is used which is rather easier to design for assembly.

canard-rib-structure

Control Surface Structure

Basically, two control surface structure is to be designed: ailerons and rudder. They are simply made out of cut outs from the respective ribs and then later to be assembled using some attachment features.

Detail Design

Fuselage Detail Design

The total length of fuselage is 1.5m with a pointed nose. A total of 10 frames are designed placed 14.725cm apart. While the frames in which wing is to be mounted are kept 16.1cm apart to meet the precision. Each frame is designed with previously calculated area. Since fuselage is to be structurally strong enough to carry load of servo, batteries, motor and engines it is decided to be made out of plywood for strength and stiffness.

frames-assembly-in-catia

The longerons extending from frame 5 to frame 8 are of 10mm diameter while on the other frames they are of 6mm diameter. The frames are designed to be 3mm or 4mm thick. The design is made in such a way that the frames and longerons locks in to themselves.

Another important structure in the fuselage is its base plate to control the torsion in the fuselage and ensure that the fuselage is straight during fabrication. This plate would be carrying the payload and in this case batteries and motor. Figure 6.2.2 shows the base plate structure. The frame on which landing gear is mounted is designed to be 4mm thick since it will be experiencing heavy loads during landing. Below is the figure of frames, longerons, and stringers assembly.

The frame on which landing gear is mounted is designed to be 4mm thick since it will be experiencing heavy loads during landing. Below is the figure of frames, longerons, and stringers assembly.

fuselage-assembly-of-frames-longerons-stringers

fuselage-detail-design-with-skin

The 3mm vertical ply plate is added between two frames over the base plate for the wing mounting. The vertical plate locks itself between the frames. These plates have another locking system which act as a passage for the wing spar so that the wing structure’s load can be easily transferred to the fuselage and better attachment of these two structures is assured.

wing-mounting-plate

In order to assure the fuselage structure, a base plate is added from fourth till ninth frame from the nose. The front part i.e. up to the fourth frame from the nose was planned not to use any base plate, but on assembly designing and observing the structure closely another base plate is added till the fifth frame. Below are the figures of the frames used on the front part and rear part respectively.

front-base-plate

rear-base-plate

A special plate is added in the tail section of the fuselage for the vertical stabilizer mounting. To ensure better attachment and structural strength by keeping in mind about the weight penalty, it is chosen to be made out of 2mm ply and 5mm balsa wood.

vertical-tail-mounting-plate

Another important component to be attached to the fuselage is the propulsion system. Since we are using two ducted fan engines on either side of the rear section of the fuselage i.e. between frame 9 and frame 10, frame 9 is designed with a special extension to hold the feature.

A figure showing the particular frame with engine attachment section is shown below. An additional frame exactly same to the 9th frame is the place at a distance of 25mm rear to it for the engine attachment. Engines produce thrust which drives the whole aircraft structure so such assembly should be designed with care.

For this reason, this section is designed to be of 6mm thick. Frame 9 is 3mm thick and the additional 3mm thick ply is glued to the frame to make it structurally strong to hold the engine. Also, it looks like that the frame alone cannot form the perfect assembly to bear such a high stress, so a mounting plate is designed to extend from frame 9 to the added frame as shown in figure 6.1.

This plate has two locks that lock the structure and prevents any displacement of the propulsion system. The fuselage already has a hollow central part so no additional holes are made for weight reduction purpose.

propulsion-system-mounting

Propulsion System Locks

As mentioned above, there are two locks that hold the engine between two frames and prevent the vertical displacement of the engine. These locks are inserted in the mounting plate in a previously made hole and glued.

propulsion-system-locks

Wing Detail Design

The first step in the design of wing is the selection of proper airfoil. Going through previously built forward swept modeled winged aircraft MEG 62-63137 is chosen to be idle airfoil. Creating an airfoil in CATIA is a very time consuming and difficult task.

But thanks to the software developers we came to know two software namely Profili and Airfoil. From Profili .txt file of selected airfoil is exported and inserting this .txt file and chord length in Airfoil software, airfoil spline curves are created in CATIA. In this manner, all the 6 major ribs of half wing structure are created in CATIA.

Ribs are to be designed such that they have features for the other components to be mounted and assembled. Below is figure showing rib structure. Each rib has a vertical slot through which the spar runs through the whole structure. Similarly as per to fulfill the twist angle requirement the airfoil structure is rotated to the respective twist angle in CATIA then holes are made for an arrangement of main spar such that during fabrication each airfoil will be twisted.

wing-rib-structure

Another important feature is the aileron connection. As shown In figure 6.2.1, the second figure is the aileron connection design. The ailerons are to be connected to the main wing structure via the plate extending along the vertical cut-outs of the rear section of the respective rib.

A similar plate lies on the upper surface of the aileron perpendicular to the one in the ribs; these two plates are then connected with the hinge that is screwed down. Besides, the plate on the aileron also allowed a position for the control horn.

wing-structure-for-aileron-attachment

Twisted-wing is a challenge to assemble during fabrication. As mentioned before spar passages were made to be angular, but this individually wouldn’t make the task any easier. Keeping this in mind, small 3mm circular holes are made at the tip of each rib about which each rib can freely rotate when a circular rod is inserted between them and on attaching the spar, wing twist is easily formed without any problem.

But using a circular rod at the tip can be a major problem after the aircraft is fully assembled and goes through flight test, so it is recommended to use the proper adhesive between ribs and spars to make the wing structure tight and firm.

As mentioned in the preliminary design stage, other than 6 main ribs four other ribs of 2mm balsa wood are added for the ease in the design of aileron respectively in the place aileron structure starts and ends. A carbon rod would be passing through the structure can also be seen in the above figure.

For each rib, Plywood is considered idle for the design of ribs in the wing structure firstly because it was easily available and for its stiffness and bending strength. Elliptical holes are made inside the structure for weight reduction.

quarter-chord-spar-of-wing-structure

wing-rear-spar

Above shown is the quarter chord or main spar for our wing structure. Since it is the main structure in sustaining the load during flight it cannot be broken and attached. For that, a single wooden block is used. Holes are made to reduce the weight and also for reinforcing ribs. A leading edge stringer is chosen to be the base for the wing assembly.

carbon-rod-passing-through-the-wing-structure

Figure4.18 Carbon rod passing through the wing structure

wing-bottom-view-with-skin

A carbon rod of 8mm diameter would extend from the third rib of one side to the other. The main purpose of using it is to support the structure during flight. Lift produced during flight give high loading in the structure creating the bending moment which tends to fail the structure.

To avoid this problem designers have been using carbon rod in RC aircraft design and so we followed the same trend in our design. In a forward swept wing, there is high load distribution in the wing root, unlike any other wing’s load distribution. Keeping this in mind, we decided to add one more spar in the rear part. The spar would run throughout the last structure in each side. Holes are made similar to the main spar.

Servo Plate (Horizontal)

A servo plate is attached to the third and fourth rib (from wing tip), by the middle section of the aileron for the servo attachment on both the wings. The reason to use two servo plates on each side is for the use of two servos, since our ailerons should also act as flaps during takeoff and landing or in other word flaperons. The plate is rectangular plywood with a hole for servo attachment inserted and glued between the two ribs.

servo-plate

Empennage Detail Design

The vertical tail is of relatively simple to design since it is neither swept nor twisted. Just like the wing, an airfoil is selected from Profili. NACA 0012 airfoil is selected for our design, which is a symmetrical airfoil for an aerodynamic reason. Also choosing this over a flat plate is for stability purpose, a flat plate would have been unable to hold up the horizontal tail. While importing file to CATIA, this airfoil didn’t have joined tail spline curves, so it is extrapolated to 10mm length and then joined.

The vertical tail is of 0.206m height and the chord length of 0.197m, bottom of tail and 0.0787m top of the tail. The vertical stabilizer is composed of four ribs each of 3mm thickness and the main spar about the quarter chord as shown in the figure 6.3.1. The ribs are separated 0.0687m apart from one another vertically.

Also, a 6mm diameter carbon rod is required to pass through the structure for the strength of structure and guide to mount the vertical tail to the fuselage. Spar is designed similar to the one used in the wing. A 3 x 3 stringer passes through the leading edge like in the below-shown figure to ease the task during skinning and structural integrity.

vertical-stablizer-top-view

verticle-tail-detail-design

The Rudder is also designed to have two spars:  Main spar about the quarter chord of the structure and rear spar at the place from where rudder starts. The main spar is to be made out of 3mm ply while the rear spar is to be made from 2mm ply. Holes are made in the ribs and spars for weight reduction as shown in the figure below. as shown in the figure below.

verticle-stablizer-main-spar-showing-elliptical-holes

Canard

Another important component of our aircraft is to be designed next. Canard has to act as horizontal stabilizer in this case but the lift is out of our concern so we decided to use a symmetric airfoil. After doing some research via internet RUTAN CANARD airfoil is our choice. It has the maximum thickness of 11.62% at 39.0% of the chord and maximum camber at 2.02% at 42.0% of the chord.

Just like the vertical stabilizer, canard also has four ribs separated at a distance of 6.52cm. There is a main spar running about the quarter chord of each rib and a carbon rod of 8mm diameter from second rib from root and inward. A 10mm hole for carbon rod insertion is made in the two ribs as shown in the figure below keeping 1mm clearance between the rod and the hole.

The reason for this specification is because the canard surface rotates about the carbon rod acting as a control surface during the flight. The ribs and spar of canard are also all made out of 3mm plywood. A 3×3 stringer passes through the leading edge keeping the structure straight and firm. Both the spar and ribs have holes in them for the weight reduction purpose.

canard-top-view

Figure4.24 Canard top view

canard_detail_design

Control Surface Detail Design

Rudder

The rudder is constructed from the cutoff parts of the rear section of the respective ribs in vertical stabilizer. It consists of 4 ribs spaced at 6.86cm from each other. On the top front part of the ribs of the rudder, a slot is cut off. Through this slot, a vertical plate runs.

This plate holds the structure together, provides guide for skinning and provides the location for servo horn attachment. Similarly, another plate just perpendicular to this vertical plate attaches the front open part of the rudder integrating it into a single structure. In order for the rudder to deflect, an angle of 30 degrees was cut out from its leading edge which can be clearly seen in the figure.

rudder-structure-displaying-30-degree-cutoff

rudder-top-view-with-vertical-plate

Aileron

The ailerons are the main component used for controlling the aircraft roll and have to bear high load during flight. So it is better to make safe design than to fail during the flight. Keeping this in mind, similar to the rudder structure two plates are attached to the front part of the aileron. The vertical plate serves as the guide for the construction of the aileron. Somewhere

The vertical plate serves as the guide for the construction of the aileron. Somewhere by the central part of the aileron, a spar runs through the length of the aileron keeping the whole structure together. Similarly, the connection plate is made for attachment with wing via hinges and also to keep the structure straight.

Aileron

After finishing all the basic to detail designs individually, the whole structure is assembled to represent the final structure of our aircraft to be built. Below figure shows the final assembly of the whole aircraft with each part defined at their exact position.

During designing for the features like holes and parts to be inserted in them we thought of keeping a little clearance of 1 to 2 mm in each part to ease the fabrication process. These features were of great necessity in the places where parts were assembled angularly.

Material Insertion and Weight Analysis

Another important feature found in CATIA is that after creating parts, actual materials to be used in products can be assigned to it and the weight analysis can be done during the design phase. In our design, we have created most of the parts from plywood and a few from balsa.

So to perform the weight analysis these material’s property is defined in CATIA like in the figure below. The density of plywood was set to be 500 kg/m3 and that of carbon rod was set to be 1400 kg/m3.Then the material is assigned to each of the parts using insert material command. Then after assembling the whole feature, their weight analysis is easily calculated via the command shown in the following figure.

material-property-defination-in-catia

weight-analysis-of-the_parts

Aerodynamic Analysis

Another most important task during a design process is aerodynamic analysis to estimate and analyze how the aircraft perform on air. Through this analysis, the exact position of aircraft components are analyzed and thus structures are designed in detail by adding features in exact positions. During our design phase too after creating each structure individually, their aerodynamic analysis was done by varying wing, canard and vertical stabilizer structures at different positions until a favorable result was obtained. Below is a short description of our aircraft’s performance.

During our design phase too after creating each structure individually, their aerodynamic analysis was done by varying wing, canard and vertical stabilizer structures at different positions until a favorable result was obtained. Below is a short description of our aircraft’s performance.

c.g-location-in-XFLR5

From XFLR5 CG of our aircraft by placing canard 40cm ahead from the leading edge of the wing is found to be located at 85.616mm behind the leading edge of root chord of the wing. External loads like batteries and control system will be installed in the fuselage later relating to this CG.

lift-vs-angle-of-attack

Above graph gives us the value of lift coefficient with respect to the different angle of attack. Here can see that the lift is continuously increasing with the angle of attack. Since the lift will keep increasing until it reaches the stall condition, for our aircraft stall is reached after 12 degrees angle of attack at 100 meters height from the ground with a constant velocity of 20 m/s.

lift-distribution

Similarly, the graph above shows the lift distribution in our aircraft. We can see a rough elliptical distribution with higher lift near the fuselage which highlights that the fuselage behaves as aerodynamic fences and high lift is produced in the wing root. This is also one of the main advantages of forward sweep wing design.

Structural Analysis

Before confirming the design for fabrication it was necessary to analyze the structure of the design in NASTRAN PATRAN to ensure the design and material selection satisfies our requirement.

Drafting

After all the structures are created in CATIA, next step is to create their .dxf file (file compatible to AutoCAD). For this another workbench in CATIA comes to use. Drafting is the process of creating 2D view of 3D models created in CATIA. In this phase all the possible views can be created in sheet.

For our design, we usually need front view of the respective parts. While for the parts created during assembly design even their isometric view can be selected for 2D drawing. The sheet used to import these views is of certain standard.

In our design after confirming the dimension of the wooden planks in which the final cutouts are made, the sheet of drawing is selected to be f D ANSI format with 863.8 x 558.6 dimension because the planks dimension was found to be about 900 x 700 in mm.

Since the parts to created are not of the same material so, different drafts are made for different material. For example; ribs of the wing, canard, and vertical stabilizer were to be made out of 2mm plywood. So their 2D view was created on a single sheet.

While the spars of them were to be made out of 3mm plywood, so in this case. all the spars were drawn in another sheet. During drafting, each part are first imported in the sheet and arranged inside the pre drawn sheet boundary so that all of them fall inside the boundary.

Similarly, care is taken to arrange in such a way that minimum material use is assured. Below is a figure displaying 2D drafting of ribs of the canard, vertical stabilizer and the wing all to be made out of 2mm plywood.

drafting-of-ribs-in-D-ANSI-sheet

Similarly, ribs of the wing, canard and vertical stabilizer are made in a different sheet but with the same dimension. Then these files were saved as .dxf file to be accessed in AutoCAD. Further editing can be done in AutoCAD, like the excess lines if any can be deleted. Or any last minute features can be added via AutoCAD interface.

One of the problems I encountered during importing these files in AutoCAD was that some of the features like elliptical holes inside ribs and spars were not displayed, since their dimension was not of effect in our dimension so I arbitrarily created this definition later in AutoCAD.

spars-drafting-in-CATIA

Next: CHAPTER 5