Bo-Fighter - Airborne Construction Article

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One of the RAF's most successful and widely used twin engined fighter types, the Bristol Beaufighter borrowed a number of design features from its torpedo bomber forerunner, the Bristol Beaufort. Both aircraft were innovative for their time and leaders in their particular class. The Beaufort was the modern torpedo bomber and the Beaufighter was the first combat-worthy twin-engined fighter to enter RAF service and as such they found a rightful place in the Royal Air Force "hall of fame".

The Beaufighter has been re-badged by Airborne's R & D department as the "Bo-Fighter" in a profile, fun fly "stand way off, semi scale" version. The plans for this scratch build aircraft are designed for two .15 to .25 engines but at a scrape some rev-head could fit a brace of 32Õs. There's always someone who just has to go to that next level isn't there? The radio needs to be a 4 channel, 5 servo job.

Assembly

Fortunately for me payday coincided with the trip to the hobby store so I bought up big. With the shops balsa bin looking a little bare it was time to add glue to the growing pile of stuff on the counter. I always like to try new products and after an impressive demonstration of the range of Saturn Mega Bond adhesives, a selection of each was added to the pile. Covering and finishing was discussed while the bill was being tallied. A suggestion to use Oz Tuff combined with Flowgloss paints and a quick demonstration of this versatile film and paint combination definately looked simple. Just the way I like it.

The Bo-Fighter is a great quick build twin that's not going to break the bank.

Fuselage and tailplane

Survey the plan and become familiar with all component and balsa sizes needed and make a shopping list as you go. The fuselage is usually where I like to start and on the Bo-Fighter this is the easiest. The profile fuselage is constructed of 12.5mm solid nose and a built up tail area, all sandwiched between 1.5mm balsa sheets.

My method was to begin by photo-copying sections of the plan, nose, rudder and fin section along with the horizontal stabilizer. Place the photocopy of the nose face down onto some 12.5mm balsa sheets which have been joined together, now run a hot iron over the back of the sheet. When you remove the paper a reversed image is transferred onto the balsa sheeting. Use a sharp knife and cut out the nose section. The same technique is used for the horizontal and vertical stabs. Place the nose and fin pieces over the plan, which is covered with plastic wrap to protect the plan from glue. Using the 12.5-mm off cuts and a balsa stripper, cut 19mm strips to size and fit them over the plan until the fuselage begins to take shape. Make sure all the joins fit snugly then pin the pieces over the plan until the fuselage is laid out. Using cyano. adhesive, glue each join carefully and when dry remove the fuselage from the plan. Bend some piano wire for the tail wheel support and install it before sheeting the fuselage with 1.5mm sheets. Using PVA glue, smear it to both sides of the fuselage then fit the sheeting ensuring the grain runs horizontally along the fuselage. Place the fuselage on a flat surface with some weights on top and allow it to dry.

Fuselage prior to being sheeted for extra strength.

Finished open structure emperage prior to covering.

The tail-plane is constructed in two halves as it has a considerable amount of dihedral in it. The halves form an open frame construction and I fixed them together with Mega bond. Shape the tail feathers as per the plan and sand them smooth before you fit the hinges to the elevator.

When the fuselage is dry, sand and shape as per the plan. Mark the location of the 1.5mm ply reinforcing plates in the wing saddle and remove the balsa from under them. Now glue the ply plates into location ensuring they remain flush. Next cut out the slot for the tailplane as per the location on the plan. Slide the tailplane into the fuselage and make sure it is aligned and square before you glue it into position. Hinge and fit the rudder to the fin then place the fuselage aside.

Wing

The wing is constructed in three sections, the center panel, which houses all the radio equipment and the two engine nacelles, along with two outer wing panels, which incorporate the ailerons and their servo's.

The centre section is the most complicated and is started first. Wing ribs 1 through to 4 are split at their thickest section and are built on a long ply wood dihedral brace. Building over the protected plan glue the rib halves and spruce spars to the brace as per the plan, followed by the leading edge. Fit the rear rib halves to the brace along with the trailing edge. Build up the radio compartment and fit some pushrod outer tubing through the ribs for the throttle's. This then allows you to sheet the entire section with 1.5mm balsa sheet except for the radio compartments. They are covered with 1.5mm ply, which is a little more durable.

Wing is built in three sections the joined and sheeted.

The two outer wing panels, which include the ailerons, are simple in construction and are built directly over the plan using familiar build up techniques. Remember to include the servo mounting rails and drill a hole in each rib for the servo lead. Joining the wing is easy, lay the centre section on a flat surface over the plan and ensure it won't move. Next, dry fit the outer panel to the centre one with a 25mm packer under the wing tip. Sand whichever edge is necessary to ensure a perfect join, which conforms to the plan layout. I used Gatorbond Ca. and an activator to secure the two sections together, this ensured an instant, very strong, quick gap-filling join. Repeat the process for the other wing panel, this then allows you to final sand the leading and trailing edges.

Nacelles

The profile nacelles on the Bo-Fighter are built tough, they hold the motor, fuel tank and the main undercarriage. They are 12.5-mm thick and constructed of balsa and hardwood and sandwiched between ply sheet. Start by cutting out 4 of these ply sheets taking care when cutting the inside shape as it must be an exact copy of the wing profile. Lay the ply down on the plan and start cutting the hardwood mounting rails for the engine and undercarriage wires. Lay the engine mounting rails over the plan at a spacing to suit your size engines. I had two Magnum 25Õs brand new in their boxes. Both motors are set up with no down or sideways thrust offset, this makes building the nacelle very easy. Glue the hardwood rails into place and fill the remaining areas of the nacelle with balsa, remembering to fit ply supports for the cowls before adding the outer ply sheet. When both nacelles are all nicely sanded and shaped as per the plan, slide one over each wing tip until it gets to the join of the outer and inner panels. If all has gone well it should be a perfect fit with no free movement, if not you may need to do a little sanding or worst case, some packing out. Bend some piano wire as per the plan to form the main undercarriage legs. Using epoxy, glue the nacelles roughly into location, measure the distances for centerline to nacelle tip and the same for the nacelle rear. These measurements must be identical front and rear, repeat the same procedure for the other side ensuring that the distance from the centerline is exactly the same for each of the nacelles. Pin them into location checking that they are also sitting vertically and allow them to dry. Now to fit the fuselage, screw 4 pieces of aluminum angle to the fuse as per the plan. Make sure that the fuselage sits squarely and vertical in reference to the wing. Drill the angle to accommodate wing bolts which are then tapped into the ply supports built into the wing.

Engine fitted to the nacelle. Fit the plastic drink bottle cowl and see the difference.

Covering

Saturn Hobbies Oz Tuff was purchased for the covering job. This translucent, iron on film is applied using normal methods with the distinct advantage of being extremely heat tolerant. Sealing down the edges and blasting it with a heat gun, this shrinkable film quickly tighten up to form a tough, smooth surface ready for painting. Flow Gloss paints are available from Saturn Hobbies in a variety of primary colors but unfortunately Beaufighter's aren't, so some creative mixing was needed for the colors. After a lot of experimenting, I ended up with pale grey for the underside and a two-tone green camouflage scheme for the top. Vinyl cut roundelles were added to follow the RAAF's No 31 squadron scheme while some silver and black paint was used to create the cockpit window.

Fit-out

Begin by recessing the aileron servo's into each outer panel feeding the servo leads through the wing to the receiver area. Next, the throttle servo was laid flat and fitted with ball links to ensure the pushrods to the motor ran smoothly. The elevator and rudder servo's are installed so that the arms protruded above the wing surface. The pushrods for the tail feathers are made from piano wire and are attached to the side of the fuselage with the aide of small nylon undercarriage straps. Brass Du-bro threads are soldered to each end of the piano wire allowing adjustments of the clevises when they are connected to control horns.

Servos are recessed into the wings and covered with hatches.

Install the two motors onto the hardwood engine bearers on the side of the nacelles and connect the throttle linkages. Now fit the fuel tanks with the aid of some brass straps, followed by the main undercarriage. Once you have done all the plumbing, its time to individually run in the motors. Adjust the carburetor arm until they both open and close approximately the same amount. With a tachometer check the revs at idle and high speed then compare them, you may need to adjust the throttle clevises until they are running in synchronization and giving off that great harmonic sound we all love from twins. Finally fit the cowls, these are cut from the middle section of a 2 litre drink bottle. Using sharp scissors cut out access holes for our motor and muffler. Paint them up to match the color scheme before screwing them to the nacelles. The effect is rather good for a profile style model. The aircraft should be balanced so that the centre of gravity is 120mm to 130mm back from the wing root leading edge.

Flying

With one of the engines out of fuel asymmetric flight is no real problem for tose new to twins.

The deflection's were set up as per the plan but 300 grams of lead had to be added to the nose to achieve the correct centre of gravity. After it passed all the usual checks the motors were individually started and tachoed. Minor needle valve adjustments soon had both motors running as a pair. A refuel saw the Bo-Fighter making its way out to the field. Taxing proved a little difficult due to the fixed tail wheel but an increase in rudder deflection soon made this less of an issue. A final clear of the motors, and the throttle was pushed open to see the Bo-Fighter airborne. The climb out was smooth and assertive as the Magnums spun the 10Ó X 4Ó propellers with ease. The nerves eased as minor trimming was needed for hands off flight. Several circuits were needed to adjust to the different characteristics of flying a twin. The plain bearing Magnums provided ample power as the plane continued to climb as it was pointed to the heavens.

Aerobatics I hear you ask, not a problem. Spins, loops and of course snap rolls proved to be no challenge, knife edge flight and even loops are a breeze thanks to that flat fuselage. After a few minutes of fun the slow speed stall was tested to give us an idea what a landing would be like. This was quickly converted into a smooth and predictable landing as one of the motors began to surge. As the Bo-Fighter taxied back, the reasons for the surge was obvious, the tanks were nearly empty! Oops.

As flights progressed my confidence increased enabling low level inverted flight and aerobatics a lot closer to the ground. The response on the Bo-Fighter is great, enabling an abundance of manoeuvres, but I wouldn't call it a competition grade fun-fly.

The next time out the Bo-Fighter was modified with 6-oz tanks and a relocated tail wheel now connected to the rudder. Ground handling was greatly improved, as was hopefully the flight time. The plan has the tail-wheel in the approximate scale position but you may want to change it if you want a little better ground handling. Not a real problem but your choice. With both motors running well, they soon had the Bo-fighter back in the air. This time experimenting with the asymmetric characteristics (what happens if one engine cuts out in flight) was the aim. With one tank filled a third less than the other, it was a cautious wait as the Bo-Fighter circled high in the sky up wind. Five minutes went by before surge, cough, then the death of the port (left) engine. Immediately the plane began to yaw to the left so I feed in right rudder. Surprisingly the Bo-Fighter flew well on one engine as slow graceful circuits were still achievable at full power. Now for the landing, as the throttle is reduced and the plane slows down, less rudder is required. Apart from a little yawing, even with one engine the landing was still slow and simple.

Summary

The Bo-Fighter is an excellent introductory aircraft for those wishing they had a twin but canÕt afford one. Now you can! With friendly flight characteristics this profile model wonÕt be too daunting for those never game enough to venture past single engine aircraft and the profile desing doesnÕt cost the earth to build and construction time is way less than a conventional fuselage design. Fly it scale and slow or wring it out, this really is a twin with attitude.