Fachadas y medianeras descubiertas 1. Grado de impermeabilidad

wire cut from polystyrene blocks, using the same technique as for the wings. However blue foam, of higher density was used, as it was found to be more stable under hotwire cutting. The two horizontal surfaces were attached using epoxy resin to create the horizontal tail section (Figure 103a). The addition of control surfaces and glassing of the remaining three sections was completed in the same way as the wings. The glassed horizontal tail is shown in Figure 103b.

A half aerofoil shape was cut into the ends of the vertical tail sections to allow the horizontal tail to allow a smooth transition from vertical to horizontal sections. The two verticals surfaces were first attached directly to the fuselage (Figure 103c) using an epoxy/micro-balloon mix and the horizontal tail was then attached to the verticals. As the empennage section was permanently fixed to the fuselage, much care was taken to ensure all surfaces were aligned correctly before attachment.

Figure 103 - a) Horizontal tail joined as a single piece, b) horizontatal tail after glassing, c) installation of vertical tail onto fuselage

5.4 Fuselage Construction

The final shape of the fuselage was based around the design of the pulsejet engine. It allowed for streamlining of the installed engine, sufficient fuel storage for 10 minutes of flight, as well as room for all other auxiliary systems. The final design was carefully planned to ensure that manufacturing could be completed with minimal complications. Wing flanges were important for both aerodynamic performance and for ease of manufacturing. The smooth transition from fuselage to wings was made to ensure that mould release was able to be achieved without damaging the final

a b c

product. A recess was also designed in the under-belly of the fuselage, which allowed the rear landing gear to be mounted flush with the body.

The information in the CAD package was imported into the CNC milling machine, allowing the aircrafts plugs to be shaped accurately. Two plugs were made, one for the port and starboard side of the aircraft fuselage. The plugs were made from Jelutong, which is a timber commonly used for modelling due to its consistent texture, allowing a smooth surface finish to be achieved in minimal time. After machining, each plug was painted and sanded multiple times, until the surface was determined to be sufficiently smooth for producing the moulds. Figure 104 shows the port plug straight after machining.

Figure 104 - Fuselage plug

The first step in creating the moulds from the plugs is to ensure that the moulds will be able to be removed from the plugs after the glass and resin has been applied. This was achieved by applying multiple coats of wax to the plug surface, with a final coat of PVA release agent. Following this, a layer of gel coat (Figure 105) was added over the surface of the plugs, allowing the same smooth surface finish of the plugs to be achieved in the moulds. This coat formed the inside surface of the moulds, which inturn forms the outer surface of the fuselage. Once the gel coat had set, 6 layers of 300gsm chop strand fibreglass mat were added, thereby forming the structure of the mould. The combination of the chop strand mat and vinyl ester resin softened the fibreglass, making it significantly easier to mould to the desired shape. The vinyl ester resin was also used to reduce setting times, compared to those required for most epoxy resins. Once set, the fibreglass moulds were released from the plugs and heated

Section 5.4 Fuselage Construction

to allow the resin to cure, thereby significantly increasing its strength. Wet and dry sandpaper, as well as surface polish were again used on the mould to obtain the smoothest surface finish possible. Further layers of wax and PVA release agent were then added to the mould surface in preparation for laying up the final fuselage.

Figure 105 - Gel coat being applied to plugs in preparation for creating the moulds

A thin coat of an epoxy resin/micro-balloon mix was chosen for the outer surface of the fuselage for a number of reasons. It provided the smoothest surface finish, helped to fill tight corners to allow the correct shape to be produced and also made the sanding and cleaning of the final surface easier. Onto this coat, one layer of 0/90°

85gsm and then three layers of 0/90° 320gsm aircraft grade fibreglass cloth were added. The middle layer of 320gsm cloth was added at 45° to increase the structures torsional stiffness. A high temperature vinyl ester resin, with glass transition temperature of 177 ⁰C was used in the fuselage to help protect it from the heat produced by the pulsejet engine. Fibreglass rovings were added to the corners of the flanges in the engine cut-away section for increased stiffness.

Figure 106 - Fuselage

After being released from the moulds, the two halves of the fuselage were joined using a layer of 0/90° 85gsm fibreglass on the outside, and a layer of 0/90° 320gsm Kevlar cloth on the inside. A layer of Kevlar was also applied to the nose of the fuselage for added impact resistance. The resultant fuselage is shown in Figure 106. Although it was expected that significant weight saving could have been achieved through the use of carbon fibre in the fuselage structure, this was avoided to help prevent radio interference between the R/C controller and receiver. Depending on the degree of interference, the radio signal can be lost mid flight. This was an unacceptable risk and hence was avoided.