The plan build article by Peter Miller - 28/1/11
Many years ago I designed a model for 1.5cc diesels called Tequila Sunrise. This was based on the big Bendix Trophy racers of the ‘30s and came complete with a long and fairly ludicrous spoof history. This history must have been nonetheless convincing because I had requests for scale documentation from overseas modellers who wanted to build examples for scale racing. Two such enthusiasts from New Zealand scaled her up to .40 and .60 sizes and reported that she was a dream to fly - music to a designer’s ears! I recalled the Tequila Sunrise whilst desperately seeking inspiration for a new project, and the outcome is Miss Lizzy.
I like a bit of a challenge with my models and in this case it was the silencer system. Hiding a standard muffler would require the cowl to be huge, whilst cutting holes in a smaller cowl would look horrible. The best solution was to fit an after-market dumpy silencer and manifold, and after a fairly extensive search I ordered a seemingly suitable combination from Mick Reeves. Alas, typically, on arrival I found that the combination still wouldn’t fit in the cowl. Pondering this dilemma for a while I realised that I could fit the silencer in the fuselage. A seemingly straightforward solution, providing one remembers to insulate the tank from the silencer by incorporating a thick balsa floor between the two. Why? Well, I once had a control line stunter with a rear exhaust engine and couldn’t understand why the flight times were so short. When I subsequently gave the engine a long ground run I noticed that fuel was being forced out of the vents, due to heat from the exhaust causing the air in the tank to expand.
Anyway, with the silencing issue overcome, the rest of Miss Lizzy’s design was pretty simple.
The fuselage is built in traditional style, employing two doubler-faced sides that are joined by formers. Spend time getting F1 right; drill the required holes for both silencer and tank, then fit the blind nuts in place prior to assembly. Also prepare (but do not glue) the 1/16” ply front face of F1 - make sure that all the holes line up but leave it oversize.
Bind the undercarriage to F2 with thin wire and solder the joints. With the u/c in place one leg will have to hang over the edge of the bench during assembly. Join the fuselage sides with F1 - F5 and when the glue has dried pull the rear of the sides in, join them with scrap balsa infill and add the remaining formers.
Fit the 1/4” square spine and the cockpit floor, then add the 1/4” sheet balsa tank bay floor, ensuring there’s enough room for the silencer in the fuselage below it and that the tank is at the right height above. At this stage it’s a good idea to use the top of the fuselage as a pattern to form the silencer bay hatch (see later text).
The top of the fuselage is then sheeted with 3/32 (4” wide stock will just cover it). Next, fit the snake outers and glue in all the sub formers F1a - F5a. Fit the sheet between F1a and F2a and add the stringers, then add the sheet between the stringers and sand smooth. Plane the stringers such that they taper to nothing at the rear of the fuselage and take on a slightly rounded shape.
The sheet that runs along the wing root goes right back to F5 and then tapers to nothing at F6. When this is trimmed to match the fuselage sides there’s a large gap between them - leave this until the wing is fitted. Once there’s a perfect fit between wing and fuselage this gap can be sealed with 1/64” ply. Cover the bottom of the rear fuselage using 1/16” sheet with the grain crosswise; the last portion is ply, to take the tail skid. Fit the belly former to the bottom of F5 (there’s no need to shape it at this stage), add the bottom stringers, then plane to shape as shown in the side view. Fit the ply wing bolt plate with its blind nuts. The bolt location is marked on the wing later by fitting sharpened bits of nylon bolt in the nuts and pressing the wing down onto them to mark the sheet.
Moving forward to the silencer bay, you may wish to follow my lead and make a small access plate / hatch. You can do this by laminating a piece of 1/8” balsa between two pieces of 1/64” ply. The curvature can be formed by gluing the laminations together whilst in situ over formers F1 and F2. For ease, it’s best to form the curve on the top surface of the fuselage where there’s no interference from the undercarriage legs. There are no supporting formers at either end of the hatch, it’s simply screwed into some 1/4” hardwood at the front of F2 and into F1.
The silencer is held at the front by the hole in F1, and by a pair of 1/2” sheet cradles at the rear - one glued to the tank bay floor and the other to the hatch. I also made provision for a good flow of air through the tank bay by using a dummy oil cooler. The face of F1 is then laminated with the 1/16” ply that was prepared earlier, and a rim of 1/2” sheet segments glued around the front. Adding the headrest wraps up this part of the build, though do note that it’s best to cover this before gluing in place.
COWL AND SILENCER
The cowl can be built up, as shown on the plan, by rolling it around a suitable object. Incidentally, if you find such an object, please let me know! A more sensible idea is to buy the ready-made cowl from Vortex Vacform - their part no. CWR8 is the correct diameter but a bit too long as supplied. You’ll also need to make a small hole in the cowl to clear the manifold, though a small blister can be glued over this if you so desire.
Whilst wooden blocks are shown on the plan for mounting the cowl, I came up with a better idea in that I cut down some old engine mounts to make ‘L’-shaped brackets that are screwed to F1b. It’s important to fit the baffle shown, as this ensures a good flow of air past the engine. Do make the effort, for if you leave the cowl open there’s a chance that the engine may overheat.
The manifold is connected to the silencer with some bendy pipe, also available from Mick Reeves. I used Hermetite liquid gasket to seal all the joints and small self-tappers to hold the pipe to the manifold and silencer. The manifold is supplied with a ‘U’-shaped threaded rod to clamp it to the engine; you can use this, but I chose to drill and tap the mounting plate to accept the original engine silencer bolts, subsequently trimming off the excess plate. I also fitted a 4mm pressure tap.
A remote needle valve is a useful thing and I made the most of it’s mounting flexibility by attaching it to F1. The filler and pressure pipes were then fed out through a hole in the 1/2” sheet rim. I also added a connection to the pressure line so that it can be undone whilst fuelling up; this prevents the silencer getting filled with juice (in practice you can watch the fuel come through this pipe, then stop pumping at the appropriate moment).
The wing is completely conventional in construction; if your building board is at least 48” long then you can save time by building both panels at once. I’ll describe one panel here, just follow the same sequence for the second.
Shape the bottom of the lower spars as shown at the section of the tip. Pin down the lower l.e. sheeting and pin the lower spar to this. Fit all R2 and R3 ribs, gluing them only where they touch the sheet. When the glue has dried, fit the top spar. Add the tip pieces between the spars and shape as shown.
Lay down the 3/4 x 1/16” aileron spar cap strip and glue down the aileron spar. Now rock the wing back until the rib trailing edges fit into the assembly tightly. When dry, shape the top of the spar to match the ribs. Glue down the top cap strip, then fit the scrap blocks that take the hinges. Allow to dry, then rock the wing back to its first position and support the t.e. Take a minute to make sure there are no twists in the wing, then fit the 1/8” sheet l.e. strip; chamfer the bottom of this to match the ribs before fitting it. Apply glue to the underside of the l.e. and the ribs where they’ll touch the l.e. sheet. Bring the l.e. sheeting up to fit the ribs and l.e. (I used a lot of short pieces of t.e. stock as wedges for this job). Next, fit the webs in front of the spars.
Laminate R1 and R1a; the slot in R1 forms the hole for the wing dowel when the two wing panels are glued together. Add the dihedral brace and R1 parts to one wing panel using the rib angle template, then sheet the top of the l.e. on that panel (use clothes pegs or small bulldog clips to clamp the sheet to the spar and short map pins to hold it down to the l.e.).
With both wing panels constructed, pin the second wing firmly to your building surface and join the first wing to it with the dihedral braces, propping it up 11/2” at the outermost R2. Add the parts of R1 to the assembly, thus ensuring a perfect job. When dry, sheet the top of the second wing in the same way.
Time to fabricate the laminated tips. I had a 10” diameter saucepan that was perfect as a template, but you could cut a pattern from thick card. I used four laminations of 1/16 x 5/16” balsa and one of 1/64” ply. This, incidentally, gave such a strong tip that I didn’t need to fit the diagonal braces shown. Shape the tip to match the ribs etc. before gluing in place. There are two ways of bringing the sheet down to fit the tip: you can slice it into planks, gluing each down, then pulling successive planks to meet it, trimming to fit as you go (this is the safe option). Alternatively, you can trim the sheet slightly oversize, apply glue and then clamp the sheet down with big bulldog clips; this worked for me and I even got the slight double curvature!
Add the centre-section sheet, all the cap strips, the l.e. cap strip, the centre-section t.e. pieces and the ply plate under the wing to take the wing bolts. The servo mount is a piece of 1/8” liteply, to which are screwed Radio Active servo brackets, whilst the bellcranks etc. are obvious from the plan. Note that the underside of the servo bay is sheeted with 1/64” ply, which makes covering around the pushrod slot much easier and neater.
The ailerons are made from either 1/2” sheet or 1/2 x 11/2” t.e. stock; they’re a fraction under 1/2” so some trimming is needed. I used Great Planes ‘pivot point’ hinges but, since they’re tricky to locate in this country, Robart ‘hinge points’ will do fine, though they are slightly larger. The under-wing fairing is made with the wing bolted in place; a former at each end and two 1/8 sheet stringers with a little sheet at the front, aids covering.
As can be seen from the plan, the tail is from medium-grade 1/4” sheet. Two fairing blocks are positioned on either side of the fin and whilst they may look tricky to profile, the reality is totally different. First, tack-glue some scrap 1/4” sheet to the fuselage to take the place of the fin and tailplane, then tack-glue the blocks in place and sand the whole thing to match the correct profile. The blocks can then be separated and fitted to the finished tail assembly at a later date.
COVERING AND INSTALLATION
I used Solarfilm Supershrink Polyester covering for Miss Lizzy, for this is simply the best covering film I’ve ever used. The scallops on the wing were applied using a cooler iron, a hotter ditto being used in the centre of the panel. Fuselage embellishment was courtesy of Solartrim, whilst the cowl was painted with Humbrol enamel, which is an almost perfect match.
There’s really very little to say about the remaining installation. The servos go where shown and the receiver and battery fit just behind F2, secured with Velcro straps. Finally, the tank is fitted as per the plan (you’ll have to cut F3 away to allow it to slide in and out)... and that’s about it!
The usual story, a completed model and lousy weather at weekends. Fortunately, a flyable Sunday did eventually present itself and, with all the pre-flight safety and range checks complete, Miss Lizzy was fired up and took to the skies without any fuss. I didn’t even have to add a single click of trim - now that’s a first! She was quite fast without being ‘hairy’ - very steady and smooth, yet responding promptly to any control input. Just the way I like it. Rolls proved very axial, loops, flick-rolls and inverted were all performed very smoothly. Inverted needed the merest hint of down trim, and she flew inverted figure eights as easily as if she’d been upright.
Spins? No trouble; and centralising the controls stopped the manoeuvre at once. With a dead engine the glide was long and flat, followed by a smooth landing. Maybe I should have called her Miss Smoothie. Stuart Pickett, who pilots for me when I’m taking the photos, enjoyed flying her enormously and was making very low level passes within a few moments of taking over the controls.
Here’s a model that’s easy to build, has a really great performance without being nerve-wracking, is very aerobatic-capable and inspires confidence in the pilot. The silencer system works well and makes the model very quiet in flight. Go on, treat yourself, build a ‘real’ model and make those ARTF types sick with envy!
Name: Miss Lizzy
Model type: Semi-scale sports aerobatic
Designed by: Peter Miller
Wingspan: 51'' (1372mm)
Length: 37'' (940mm)
Wing area: 3.4 sq. ft. (0.3sq. m)
All-up weight: 4 lb 1oz (1.8kg)
Wing loading: 19oz / sq. ft. (5.8kg / sq. m)
Rec’d engine: .25 to .32 cu. in. two-strokes
C of G: 25% chord (back from l.e.)
Control functions: Aileron, elevator, rudder, throttle
Control deflections: Aileron ±1⁄2'' (12.7mm)
Elevator ±5⁄8'' (16.0mm)
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