I’ve always had an interest in American racing aircraft, and though I’ve built a few over the years, I’d never tried my hand at a Shoestring. Ironically, given that this classic ‘40s racer was popular with full-size home-builders for its simplicity, there are a number of problems in translating the aircraft’s design into a satisfactory scale model. Eventually, however, I was able to distil the shapes or styling of the original into this sports aerobatic model, the Bootlace.Proportionally, the fuselage is slimmer than the original and the cowl cheeks have gone, but she’s still got that distinctive shoulder-mounted wing, which keeps the centre of drag very close to the thrust line. The wing section itself has been thickened for better aerobatic performance, while also giving the model predictable handling at slower speeds.With the design finally down on paper, the construction of the model is fairly straightforward, as you’ll see. We’ll start with the fuselage, though before we begin, you might find it helpful to make yourself a kit by cutting out all the parts.
Okay, begin by preparing F2, i.e. drill the engine mount bolts and fit the blind nuts, if you’re going to use them. Note the hole in F2 for the front of the fuel tank. Glue the 1/16” ply doublers to the fuselage sides with contact adhesive. You can just see from the photo on page 56 that I made lightening holes in these parts, but you needn’t bother – the 10g saved isn’t worth the effort.
Join the sides at the tail and pin the assembly down over the plan with F5, then add F6 to F9. Once these have dried, and with the fuselage still pinned down, you can add F4 – F2, pulling in the sides to fit as you go. Make sure that the fuselage is straight and that all the formers are at 90° to the centreline. Incidentally, now is a good time to fit the 1/8” balsa treblers at the wing seat; if they’d been made in one piece and fitted before this point, you wouldn’t have been able to bend the fuselage sides to shape.
Fit the 1/4” ply undercarriage plate and all the triangular stock in the corners – most ARTFs omit these, which is probably why their undercarriages tend to be torn out when they land on anything other than smooth ground!
Next, sheet the bottom with 3/32” balsa, remembering to run the grain across the fuselage. Attach, but don’t shape, the 1/2” sheet between F2 and F3, the 1/8” ply tail wheel and strut mount, and the 1/8” balsa tailplane platform at the rear. With that little lot done, you can install the snake outers, if you like. For preference, I use heavy-duty Sullivan items.
With F6b installed at the correct angle, you’re ready for the rolled sheet sides. I find the best way to tackle these is to glue the sheet down to the sides and leave them to set, before wetting the outer surface and applying some heat as you pull them into the formers, where they’re glued in place. Once dry, trim and sand the balsa to match the top of the formers, and glue on the 1/4” sheet spine, which you’ll need to carve and sand to shape.
To finish off the rolled sheet work, glue a 1/4”-square longeron between F2 and F3 roughly in the middle and flush with the top of the formers. Then, using the same method as before, fit the rolled sheet over F2 and F3 so that it’s supported by the aforementioned longeron. The next job is to glue in the 1/8” ply plate at the rear of the wing cut-out, and add a piece of scrap balsa sheet (shaped to fit the wing) on top to increase the gluing area at the sides and back.
Note the triangular stock in all corners of the nose
When it came to fitting the motor mount and engine, I used a spinner with the backplate behind the prop’, which means that the propeller is at a constant distance from F1 even if you change to another size. Using scraps of 1/16” balsa as spacers, I glued the spinner backplate to F1, and then mounted this on the engine. An old prop’ hub is handy for getting the spacing right.
Now build up the cowling with 1/2” balsa and triangular stock, around the engine. When it’s dry, remove the engine and spinner backplate, and carve the cowling to shape, taking in the 1/2” sheet under the nose. So far so good, however, before you go any further, you’ll need to build the wing.
If you use the following sequence and build the wings together on one board, you should find their construction fairly simple. Make sure, though, that you make the spars from the hardest 1/4”-square stock that you can find; the rest of the wood can be medium grade.
To speed up production of the ribs, I photocopied the profiles and ironed them onto the balsa sheet. When you’ve cut them out, laminate R1a and R1b, pin down the lower spar, then glue ribs R2 to R11 in place, followed by the top spar.
When you come to add R1a/b, note that it must be at 90 degrees to the top spar (in plan view), and that it leans 1° at the top to accommodate dihedral. Finish off by fitting the 1/8” sheet leading edge and the 1/16” webs to the spar.
Cut the 1/16” sheet trailing edge to size, pin it down, then release the wing assembly from your board and rock it back onto the trailing edge. Support it in this position and make sure that it isn’t twisted. You may find that some ribs don’t rest evenly on the trailing edge sheet, in which case you’re likely to be suffering imperfect rib slots. Don’t force them into position, though. Instead, cut part way through the rib, just behind the spar, to provide enough movement for it to make contact with the trailing edge. Although this may weaken the wing in places it won’t be significant, especially if the cut you’ve made is filled with aliphatic resin glue. The aileron spar and the aileron leading edge are next to be fitted, followed by the aileron ribs; laminate a rib from several pieces, where shown, so as to create a slot for the paxolin horn.
Fit all the aileron hinge blocks, and the balsa filler pieces at the wing-bolt location, then sand everything smooth. Next, drill one or two small holes in the lower trailing edge sheet, at the inboard (root) end of the aileron, and through the horn slot. These telltale marks will be used in a later stage as a guide for cutting the ailerons free from the trailing edge.
Now add R1d, fit the upper trailing edge sheet, and glue the 3/16 x 1/2” trailing edge in place. You can now fit the top sheet to the leading edge and, when this is dry, remove the wing from the board in order to add the lower leading edge sheet.
The assembly is finished off with a 1/8” leading edge cap strip, which you’ll need to carve and sand to the correct shape. With the wing upside-down, carefully sand the root of each panel smooth and at 90° to the spar. Then, when you’ve got them to fit together perfectly (you’ll know, as both top spars will lie flat on the board), you can join the two halves (while they’re still inverted) using the 1/8” ply dihedral brace and 1/8” balsa piece that forms the rear of the servo bay and spans the R2 ribs. Once the glue has set, you’ll be able to lift the wing and add R1c, as shown, to complete the servo bay.
You can now install the aileron bellcranks and feed the pushrods through the wing, before adding the centre-section sheet and the cap strips. This is as good a time as any to build the tips, and to fit the balsa in-fill and ply nut-plate. Don’t forget to attach the nut before gluing the ply plate down, as once the plate is on the wing you won’t be able to exert enough pressure to seat it.
Now comes the clever bit. Open up the slots for the aileron horns (using those previously drilled holes as a guide) and dry-fit the horns. Then, referring to R6 as a means of checking the geometry, make up the pushrod – the latter having been designed so that it can be installed after the model has been covered.
When you’re sure of the fit, remove the ‘rod, and the horn, and cover the underside of that rib bay with 1/64” ply. Make a slot in the cover, as shown, and double-check that you can still fit the pushrod.
You can now separate the ailerons and drill them for the hinges. I used Great Planes’ medium size Pivot Points which are smaller than Robart’s Hinge Points. You could, of course, use the covering as the hinge, but I’m really not keen on this particular approach with a model of this size and power.
Open up the hole in the centre of the leading edge for the wing dowel and push the dowel into the slot in R1a. Don’t glue it at this stage, though – leave that until the model has been covered. You’ll find that the dowel hole in F3 is set a touch low in the former. This is so that you can relieve its top edge, a little at a time, until the wing just seats and with the dowel a tight sliding fit. Then, with the wing located, drill down through the blind nut and the plate in the fuselage.
Remove the wing and open out this hole to take the wing bolt; you’ll also need to make a hole in the bottom of the fuselage, which you should reinforce with a 1/16” ply doubler. Feeding the bolt through from the bottom, secure the wing.
FINISHING THE FUSELAGE
The top of the fuselage is a planking job, which may seem tedious but doesn’t really take very long. Besides, it’s a heck of a lot easier than trying to roll a sheet over the formers and shape it to fit the wings. Trust me on this. Incidentally, note that the wing-mounted half formers F3a and F6a should be spaced 1/16” from their facing formers on the fuselage to allow for the thickness of covering materials between the two.
When you’ve shaped the planking to blend with the rest of the fuselage, you can make the block that fairs the canopy into the rear fuselage. Don’t glue it on, however, until you’ve covered the model.
The final step here is to remove the wing from the model and face the ends of the cut-out (F3, F6b and the fairing block) with 1/64” ply. Note that the fairing block is faced separately from F6b as this not only prevents any damage to the edges, but looks much neater too.
The tail is made from 1/4” sheet. I used soft stuff and found I needed lead at the rear end, so maybe you should use medium sheet. Don’t forget the 1/4” dowel inserts for the screws that hold the struts. The latter are to prevent the tail from being knocked off in a rough landing – something I’ve experienced and seen happen many times, but not since I started fitting struts!
You can glue the tail and fairing blocks in place now if you choose, but I prefer to wait until the model has been covered, as it’s so much easier to do a neat job that way.
When it came to the undercarriage, I opted for a proprietary carbon fibre unit made by ATS Mayneline (part no. ATS 232/E). Nexus Modelling Supplies stock this gear, and as you’ll also need Nexus’ longer axle bolts (40mm caphead screws), you might consider buying the whole lot there.
The spats are by Micro Mold (cat. no. EV9), and each one comes in two halves which have to be joined. To glue and reinforce the spat joints, I made a paste from finely chopped scrap plastic and cellulose thinners, which you can spread inside the spat to bridge the joint. These units are big enough to take 3” wheels, though you’ll need the type with a good recess in the spokes on one side to accommodate the internal nut and leave room for the undercarriage leg inside the spat.
Why is the leg inside the spat? Well, it not only looks neater, but it’s also the simplest way of preventing the spat from rotating!
The whole main wheel assembly is held to the ply plate with four 1/2” long No.8 self-tappers. The tail wheel, meanwhile, uses a strut made from K&S 1/4” streamline brass tube, though you can get the same effect yourself by flattening some round-section brass tube.
Whilst the tail wheel set-up shown on the plan is, of course, non-steerable, you’ll be pleased to note that it works well. So, if you must have a steerable tail wheel, you’re on your own!
As the plans stand, Bootlace will accept an 8oz Radio Active tank, though for a more frugal four-stroke set-up you could get away with a 6oz job. Before you fit it, squeeze a thick bead of silicone rubber gasket (I use Solvol Instant Gasket) around the neck of the tank. This will seal the tank in the former (F2), but won’t set so hard that you can’t pull it out easily.
I located the servos as shown, with the throttle ditto in the centre and the rudder and elevator servos on the outside with their snakes running along the fuselage side. I made the exposed and unsupported portions of the snakes from thread rods, by screwing the threaded portion of each rod into the snake, and soldering a SLEC threaded adapter on the other end to take the quicklinks.
A simpler method is to push 16swg wire into the snake and then fit the normal short-threaded adapters to take the quicklinks. Last but not least, the battery pack was Velcro-fastened to the floor, just behind F5, and the bubble-wrapped receiver strapped on top of it, again using Velcro.
Working on the forward-most position for the C of G, set the control throws so that the ailerons have 5/8” movement each way and the elevator 1/2” each way. Give the rudder as much travel as you can. Your low rate setting should give 75% of these deflections. And that’s it. Done.
COVERING & FINISHING
I covered the model with Solarfilm’s Supershrink polyester, which I also used for the red trim; the yellow and black detail was made from Solartrim, while the lettering, meanwhile, is Sigma Signwriting’s handiwork.
The canopy is the result of a cunning piece of moulding, which involved carving a pattern from balsa, sanding it very smooth, and then wedging it into the top end of a large plastic lemonade bottle.
With the careful application of a heat gun, you can shrink the bottle over the pattern. Give it a coat of fuel proofer inside and out, and there you have it – a perfect canopy. Note that the pilot’s backrest is set forward from the rear of the canopy, so he doesn’t have to sit over the wing bolt!
She looks every part the racer!
INTO THE AIR
Fortunately, the summer outlasted the build, and provided perfect test flying conditions for the completed Bootlace. Taking off from what remains of the W.W.II concrete strip on our site, she flew straight and smooth, and climbed-out very rapidly – so rapidly, in fact, that on one particularly enthusiastic climb-out I pulled up into the vertical and the model almost disappeared out of sight!
Throttling right back to try out the slow-speed handling, I steadily fed in full up-elevator; she simply mushed and displayed no desire to drop a wing. Moving on to aerobatics the aircraft made loops, outside loops, vertical eights, rolls, flick rolls, hesitation rolls and inverted flight easy – something due, in part, to that alignment of the wing and thrust line.
I also tried Lomcevaks (a tumbling, autorotative affair better seen than described!), and she came nearer to completing these than most models I have flown. With the C of G moved back, I know that it would be possible to perform this manoeuvre consistently every time.
The real eye-opener, however, was knife-edge flight. I’ve never flown a model that was easier to knife-edge, and that includes Toot Sweet with a gyro fitted on the ailerons. I was able to fly round and back and forth for minutes at a time without losing height or rolling out.
Then came the inevitable bit – landing. The model’s long flat approach ended in a smooth touch-down that couldn’t help but make me look good. I like it! To really make the point, though, the model later pulled off a perfect forced landing. While Stuart Pickett was flying her for the camera, the engine stopped well out over fields that had been ploughed and rolled – not the best surface for an aerobatic model to land on. However, Stuart pulled off such a smooth landing in the rough that she didn’t even tip on her nose.
WORTH THE WAIT?
Without a shadow of a doubt. Those who watched the first few flights were impressed, Stuart Pickett really liked flying her, and I just love her!
In spite of all the things that I put her through during the first session, she never once tried to bite.
This model is a joy to fly, looks good, and gives you something that an ARTF never can – total involvement. Building, rather than assembling, your model affords you complete control over the materials and construction and you know it’s not going to fall apart in the air. The Bootlace also gives you something worthwhile to do on unflyable days, rather than spend hours on end in front of the TV. Best of all though, the end product is a singular aeroplane that no-one else can simply go out and buy.
Designed by: Peter Miller
Aircraft type: Sports aerobatic
Fuselage length: 45”
Wing area: 510 sq. in.
All-up weight: 5 lbs.
Wing loading: 23 oz / sq. ft.
Rec’d engine range: .40 – .46 two-stroke,
.52 – .58 four-stroke
Rec’d no. channels: Four
Don’t forget that you can talk to other Peter Miller plan builders and Peter himself in our dedicated forum section.
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