Peter's 'Four Star'

THE FALL AND RISE OF THE SIG FOUR STAR 60

I loved flying my Sig Four Star Sixty.  That was, until one day, my beloved airplane rolled into a terminal dive.  The crash was not pilot error as it happened from straight and level flight and the radio did not go into failsafe as the throttle did not close as it ought to have done (I had lost all controls).  I sent the receiver, a Spektrum AR6200, to Horizon Hobby UK with an explanatory note.  They replaced it, free of charge, with a new AR 6210 unit “for peace of mind”.  I am full of praise for the speed and quality of service Horizon provided but I became increasingly suspicious of the reliability of the AR 6200 receiver as I had suffered an earlier crash of an Astro Hog in very similar circumstances to the Four Star.  Again there was absolutely no control of any of the functions as the plane descended.

 

 

 

Anyway, enough of the moaning.  I was determined to rebuild the Four Star.  The previous one was scratch built from the plan (don’t tell Sig) so I didn’t anticipate too many problems replicating the process.

As the photo shows, there is still a considerable amount of the original structure remaining and it is tempting to reuse this and graft on new parts.  However, my instinct tells me to start anew and then everything will be right.

 The first task, and perhaps the least interesting, is to make a kit of balsa and liteply parts.  First of all, I trace the outline of the components from the plan.  I use good quality tracing paper obtained from Windsor Photoprints, 130 Eglantine Avenue, Belfast.  An alternative approach is to cut up a second copy of the plan, but you need to look out for photocopying distortions if you use this method.  The outlines are then glued (using a Pritt stick) to the correct type and thickness of wood.

 

 

Whenever I purchase my balsawood stock, I always weigh each sheet and record the weight on the sheet.  This way I can easily identify the most suitable grade of balsa to use for the different parts.  Generally speaking, the lighter the sheet, the softer and weaker the wood will be.  I never use hard (heavy) wood unless I need its extra structural strength.  Weight is the enemy of a model aircraft because a heavy plane will have a higher stalling speed and will be less responsive to fly.

 

  

Using the tracing paper outlines as a guide, I then cut out the parts, leaving a small margin (about 1 mm) for final sanding.  As the wing ribs are identical, I make one from liteply and then use it as a template to cut out all the ribs.  I tend to cut out the spar slots slightly tight so that a final sanding will produce a nice snug fit for the spars.

 

 

Some people cut the ribs using the sandwich method but I find this is quite wasteful of expensive balsa wood and I enjoy the challenge of maximising the number of ribs from one sheet.  Incidentally, I find that 100 mm wide sheet is generally more versatile in use than 75 mm though in this case, 75 mm suited just fine.

Decisions, decisions.  Which to build first, the wing or the fuselage?  In this case I am going for the fuselage because it is more interesting to build and keeps the motivation going.

 

 

Having cut out the fuselage sides and formers, the first task is to glue the doublers to the front section of the fuselage, taking care to construct a left hand and a right hand version.  Contact glue or a thin coating of epoxy is better than PVA for this job.  The front formers are then attached to one fuselage side, using 30 minute epoxy for the firewall and PVA for everything else.  Despite my use of a set square to keep everything at right angles, I still managed to build some left thrust into the engine firewall which I only noticed when it was too late!  (Subsequently I was able to correct this by inserting a suitably angled shim behind the engine mount). 

The second fuselage side is then positioned and glued and when dry, the assembly is placed over the plan and the rear of the fuselage is pulled together, making absolutely certain that a banana shape is not built in.  The remaining formers are inserted and glued in place.

Once the glue has fully cured, the assembly is removed from the plan and the top and lower sheeting is added, together with the “T” nuts for the engine mount.

 

 

 

 

 

 

The 6 mm undercarriage plate and the wing bolt retaining plates are epoxied in place and the fuel tank supports are constructed.  Essentially at this stage, it is important to think ahead to ensure that internal structures such as the tank are completed and installed before they become inaccessible.  The tank is cushioned in its mount with foam to try to minimise fuel foaming caused by engine vibration.  Before finally mounting the tank, I pressure test it under water to make certain there are no leaks.  I always try to install the tank in such a way that it can be removed at a later date in case of damage or leaks in the fuel lines.

 The balsa strip shown in the photo is inserted to provide a joining surface for the two pieces of sheet used to construct the front top decking.

 

  

 

Work then starts on the rear decking by first installing the sub formers.  Note the use of two jigs to ensure that the rear panel of the cockpit is correctly aligned.  The photo also shows the considerable use of lightening holes in the liteply.  Nevertheless, the fuselage structure is exceedingly strong, possibly over-engineered, but not a bad fault.

 

 

 

 The stringers are then added so that they sit slightly proud of the formers.  This avoids the outline of the formers protruding when the covering is added, giving an unsightly “starved horse” appearance.

 

  

 

The remaining task to be completed before wing construction begins is the sheeting of the fuselage front decking.  As the decking is about 115mm wide, it is necessary to butt join two pieces of 2.4 mm (3/32”) balsa sheet.  I add the sheeting in two stages, gluing at one side first of all and then joining at the centre.  If the wood is too stiff to adopt the curvature of the top formers, damping it with water and the application of heat from a heat gun will soon have it in place.

Right!  Now is the time to make a start on the wing.  In the crash, the starboard wing remained structurally sound but the port one was badly mashed with most of the outer section totally destroyed.  After much thought (at least one minute’s worth) I resolved to build a new port wing and somehow graft it on to the starboard one.  At first I thought that I could join the two structures by bracing together the main spars, thereby leaving the centre section untouched.  However, I was worried that I might inadvertently build in a twist, so I decided to go for the nuclear option – saw the wing in two at the centre section!

 This drastic action left me with a very clean root section onto which I could join a newly constructed port wing.

 

 

 

Wing construction follows a fairly normal process starting by laying down the lower main spar, rear sub spar and the lower trailing edge sheeting.  Ribs are added, using a set square to ensure that they are glued at right angles to the building board.  The top main spar is added, followed by the upper sub spar, upper trailing edge, leading edge section and diagonal bracing.  The wing gains its strength from the “I” section beam formed by gluing 3/32” sheet between the upper and lower spars.  The diagonal bracing also helps to keep things rigid.  Wood selection is important here with hard balsa being used for the spars and soft/medium for the ribs whose only function is to hold the spars apart and give the wing its aerofoil shape.  The root rib was not glued at this stage because I wanted to make sure that it aligned with the root rib of the other wing half when the two halves were joined.

 

 

Once all the glue joints have dried thoroughly, the wing is taken off the building board and turned over so that the mounting box for the aileron servo can be added.  A cautionary tale follows …… Having spent a happy half hour or so building a very neat servo mount, I then realised that age had befuddled my brain (again!) as I had managed to construct the mount on the upper surface of the wing.  Patience, patience was needed at this point.  A final task was to install a paper tube to guide the aileron servo leads to the root rib.

 

 

 

The most crucial stage of joining the wing halves has now been reached.  A suitably shaped dihedral brace was cut from 1/8” plywood and the wing halves were carefully aligned and propped up to the correct dihedral.  Using plenty of 30 minute epoxy, the brace was glued and clamped to the main spars and left overnight to cure thoroughly.

 

 

The centre section sheeting was now added, together with a variety of patches to make good the sheeting damaged in the crash.  Following this, the upper and lower sub spars were added with the centre sheeting suitably trimmed to accommodate the ends of the spars.

 

 

 

Once the sheeting was sanded, two holes were cut near the root and string was passed through so that the aileron leads could be guided at a later stage.  You will notice from the photo that there is only one wing dowel so a second one was added at this stage by gluing it into solid wood inserted behind the leading edge.

 

 

 

The wing locating holes have already been drilled (undersized) in the fuselage former.  It is now necessary to locate the second wing dowel to accurately match the hole in the former.  To do this I sanded a sharp point on a piece of dowel which is a tight fit in the hole and located the dowel so that the point is just protruding from the former.  Locating the wing dowel in the other hole and firmly pushing the wing into its seat will produce an indentation in the soft leading edge of the wing.  A hole is drilled here and the second dowel glued in place.  I was much relieved when everything fitted perfectly and the wing retaining bolt holes can be drilled.  Incidentally, the piece of tape seen attached to the fuel tank will help to extract it, if that is ever needed. 

 

 

The centre section of the wing is given a good sanding and thin glass fibre cloth is applied to reinforce the join.  One layer is applied with a second, slightly narrower one added the next day.  Adding a generous amount of micro balloons to the resin mix for the final coat makes sanding the cloth to a smooth finish a much easier task.  I will only give the wing its final overall sanding immediately prior to it being covered, otherwise it will pick up the inevitable “hanger rash” as it is being fitted to the fuselage.

On the home straight now.  The next task will be to repair/renew the tail surfaces and generally fettle the fuselage prior to covering.

To be continued...

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