Please note that this article was first published over two issues in 2006.
The Basic 3D plan, laser/cnc kit parts and a wood completion pack can be purchased at www.myhobbystore.com
Designed to satisfy the needs of the up-and-coming 3D flyer, the appearance and construction of this model are completely different to my previously published efforts, which in the main have been semi scale, open-cockpit, golden era type sports aircraft. What I’ve tried to do with this one is to provide the builder with an aerobatic trainer that can also be an out-and-out 3D model, dependent on the wing type and size of engine fitted. With a .40 ‘cooking’ engine it’s a real pussycat, with a .46 it’s a lively workhorse, and with a .56 or above - wow!The model had to fulfil certain criteria. To be light in weight but strong enough to survive heavy arrivals (and I’ve had some!), easy to construct, cheap to build and easy to repair. It’s offered in two versions: a 58” span rounded-tip smooth flyer and a 52” span clipped-wing alternative to satisfy the hooligan element amongst us. The Basic 3D has been well proven over a two-year period, and provided you build it according to the plan you’ll have a model to be proud of that will excite you with its flying ability. Not pretty, maybe, but effective!
The design characteristics are typical 3D with a large area, low aspect ratio wing, large tail feathers and a long-ish tail moment to give stability and smoothness. All the control surfaces are large, giving good authority when needed, and slow speed handling is outstanding, which enables ‘kite flying’ in almost no wind. Okay, enough about how it performs for the time being, let’s get building.
WINGS & THINGS
The wings are conventional in construction, with a deep semi-symmetrical section and a blunt leading edge. Hardwood main spars give the wing strength, whilst cap strips and top / bottom l.e. sheeting look after some of the torsional loads. There’s some reduction in rib thickness towards the tip, which improves the look of the wing and emphasises the roundness of the bigger tips - “quite hawk-like,” commented one observer.Using the sandwich method, start by making a set of ribs from the templates shown on the plan, noting the five that are cut from liteply. Two of the liteply ribs are to be used as servo supports for the ailerons and require liteply doublers to hold the fixing screws. It’s worth fitting the servos at this stage, as screwing them in when the wing is assembled is more difficult. Lightening holes, if required, can be carefully drilled through the individual ribs using a Forstner bit of the appropriate size. Ribs inboard of the servos have additional holes drilled to accommodate servo wiring tubes, which are made by rolling some glued A4 copy paper around a suitable-size dowel a few times. Note that the three centre-section ply ribs have additional square sections let into their top sides, whilst all are notched into the l.e. sheet and false t.e. As you’ll detect, the spars don’t extend all the way to the tips, this particular job being performed by sheet inserts that bridge the gap and add strength.
Lay up and pin the bottom main spar and false t.e. over the plan and slot the ribs in place. Now fit the l.e., making sure all the ribs bed nicely in their cut-outs. Support the l.e. using scrap material before gluing it all together - I used medium cyano’ here to tack the parts, reinforcing them later with white glue. You did remember to fit the servo wiring tubes, didn’t you? Add the two square centre-section l.e. braces, which absorb the pressure of the wing fixing bands, and when dry, fit the top spar and the spar webs. The next job is to sheet the l.e. and centre-section, pulling the balsa down towards the tips. Anchor it well around the centre-section and braces to avoid any ‘creep’ that might occur under rubber band pressure. Cut a small hole in the lower sheeting and reinforce with thin cyano’ so the servo leads will exit tidily.
All dry? Okay, out with the razor plane and sanding block to shape the l.e. and sheeting to section. Since the l.e. is quite thick it’s a good idea to mark it along its length to show the widest point, as a guide for sanding. If you’re building the extended (58”) wing, add the wing tips by slotting them into the 3/8” ‘clipped’ tips. Small slotted blocks are needed to take the servo pushrods through the covering, so fit these next, and when that’s done, add all the rib capping pieces top and bottom and sand them to fit.
Whilst the ailerons are very conventional, good workmanship in this area will make for a strong section. Let’s be honest, it’s not uncommon to see such simple structures failing or warping at the club field because the individual joints aren’t well done. So, try to avoid joining this not-so merry band! Also, bear in mind that you may need to make adjustments to the position of the wider rib to accommodate the alignment of the aileron horn and your servo output arm position. Sand the surfaces carefully and make sure you provide a nicely rounded profile to the leading and trailing edges, then finish by soaking the horn mounts both sides with thin cyano’, to add strength. Incidentally, talking of horns, I made mine from glass fibre sheet and, having epoxy-glued them into position, they show no signs of moving. Of course, there’s nothing to stop you using commercial horns if you prefer. Anyway, that’s the wing pretty well finished; what you’ll need to do now is make an accurate template of its root underside cross section, for creating a well-matched joint with the fuselage.
THE LONG BIT
Study the plan and you’ll see that the fuselage has a slightly unconventional method of construction. Many undercarriages fail because insufficient thought has been given to the loads that occur in the fuselage, especially on landing; a little design application allows such loads to be harmlessly dissipated through the model. My Basic 3D has had more than its fair share of hard arrivals, and the undercarriage-to- fuselage joint has stood up to them very well. The undercarriage on the prototype was a commercial unit, but if you prefer to fabricate your own, the dimensions are shown on the plan. Anyway, back to the build.
Start the fuselage construction by cutting out all of the parts. If you need to splice the fuselage to get the right length then do so at an angle, different on both sides, and back the joints with 1/32” ply. Now fit the 1/32” ply doublers to the front section using contact adhesive. Having ensured that the starboard side is 1/8” shorter at the front to give the required right engine thrust, mark the locations for all the formers across the sides (this is best done with the two pieces laid together in mirror image).
Now’s the time to use the template that was taken from the underside of the wing. Mark the datum line between the leading and trailing edges, and align the template between them to set the wing at zero degrees. Mark this and cut it out. Fit the top and bottom longerons aft of and below the wing, and when set cut them to form grooves to accept the individual formers.
Referring to the plan details of the sub-assembly, you’ll see that the 1/4” ply undercarriage mount is supported by two 1/8” vertical curved plates. Make these bits first, epoxy gluing and pinning or screwing them together before adding formers 2, 3 and the servo mount plate F4 together with F5. This sub-assembly gives the fuselage considerable strength and is virtually indestructible.
Now to the main construction. Epoxy-glue the sub-assembly to both sides, ensuring alignment, and leave to dry under pressure. Pull the rear ends together with a temporary 1/4” strip where the stern post will go later, then fit formers 6 and 7 between the fuselage sides where previously marked. Fit the 1/4” tail skid / wheel mounting plate at the rear end.
Decide the position of the engine mount on F1, then drill holes for its retaining T-nuts (I mounted the engine so its silencer was on the centreline below the fuselage) and make provision for the three tank tubes. Use epoxy to glue F1 in place between the fuselage sides, noting the 3° right-thrust and 3° down-thrust. Epoxy the hard balsa triangular section supports behind F1, then fit the 1/2” infill to the tank bay bottom. Add the 1/4” wing bed pieces and the dowel exit strengtheners. By the way, now’s a very good time to fuel proof all parts between F1 and F5. With this complete we can fit the underside sheeting (between the fuselage sides) extending from the undercarriage to tail skid mounts and resting on the longerons. We’ll leave the top sheeting until later, i.e. after the servo pushrods are installed.
Trial fit the wings using 8” rubber bands to retain them, and check that the fuselage cut-outs are of the correct shape, making adjustments as necessary. The tank cover is the next job, made from laminated or block balsa and hollowed to save weight. This can be fixed to the fuselage or made removable if you prefer and, finally, for now, shape the rear edge to blend in with the wing l.e.
I think it’s fair to say that over the years I’ve made a bit of a name for myself as a lover of golden age designs, indeed many of my previous creations have been sport versions of open cockpit, ‘30s style aircraft. I love them all but, as you can see from this everyday sport aerobat, I’m not adverse to towing the conventional line now and again. As I mentioned last month, Basic 3D is designed to be an aerobatic trainer that, with the right engine, can also dip a toe (okay, wheel) in 3D water. It’s offered in two versions: a 58” span rounded-tip smooth flyer and a 52” span clipped-wing, hooligan alternative. Build it according to the plan and you’ll have a model to be proud of, and one that will excite you with its flying ability. Not pretty, maybe, but it is effective. So, let’s get on, finish the build, and see how she flies.
The tailplane is of straightforward, conventional construction and as with the ailerons, tidy workmanship will add strength. Note that the stabiliser t.e. and elevator l.e. are made from hardwood, which should ideally be degreased with meths just before gluing in place with PVA (it’s best not to use cyano’ for these parts). Either cover the horn mount with 1/32” ply or soak the balsa with thin cyano’ to strengthen it.
The fin is constructed in similar fashion, and you’ll notice that its t.e. runs right down through the rear of the fuselage acting as the stern post, so remove that temporary bit we added earlier; similarly the l.e. extends through the stabiliser to abut with F7. The rudder has small pieces of 1/32” ply inserted into either side of the balsa infill to give a good base for the horn - you can cut lightening holes in the same area if you wish. Sand the sides of the fin and rudder to get rid of any building imperfections then sand the top, leading and trailing edges to a smooth shape. Temporarily offer the tail parts to the fuselage to check for fit, making adjustments as necessary, but leave permanent attachment until after they’re covered.
Now’s the time to fit the servos and check for alignment with the horns - the decision as to how the pushrods exit the fuselage is yours. I used SLEC pushrod transfer linkages (Part No. SLO 46) and carbon fibre rods to drive them, but snakes are okay too. Do remember to fit fuel tube pieces to all the clevises to act as keepers, which prevent accidental release during high ‘g’ manoeuvres. Once the pushrods have been positioned, the top fuselage sheeting can be added and the whole fuselage sanded down. Due to the triangular section longerons within, the top can be well rounded, saving weight and giving a more pleasant finish. Finally - as far as construction is concerned - use lengths of 1/2” triangular strip to form reinforcement pieces for the tailplane mounting, noting that it’s easier to cover these before fitting!
COVERING & FINISHING
I wrapped the prototype in natural and red Solartex, but before doing so raised the grain of all the wood surfaces by wiping down with a solution of water plus a little detergent. When dry I gave the airframe a final, very light sanding just to smooth it over. After dusting thoroughly and covering, I gave the airframe two coats of fuel proofer (Ronseal quick drying satin varnish thinned 50:50 with water), applied with a wedge-shaped foam sponge. The varnish ‘does exactly what it says on the tin’ and gives a smooth and attractive finish that doesn’t colour with age. It’s also much lighter than polyurethane type finishes. I opted for fabric-sewn hinges to all surfaces, which give good support over the whole area and resist flutter in fast manoeuvres. If you decide to use an alternative method for hinging, then tape the undersides of the surfaces to seal from air leakage.
There’s ample space inside the fuselage cavities to position the Rx, and the battery can be moved around to adjust for C of G positioning. On the prototype, standard servos were used throughout - all bar a mini job for throttle - and these have all given very good service.
UP, UP AND AWAY!
A 46-size engine was used to power the prototype, and this was more than adequate for me. I felt comfortable with the performance right from the start, and it did everything I was capable of. I’ve enjoyed the aerobatic capabilities so much that I haven’t yet got around to trying prop-hanging or extreme aerobatics, but there’s no doubt that, given the right pilot, anything is possible. It did, however, fulfill the design concept in that it’ll take the budding pilot from elementary to advanced aerobatics. Basic 3D is very stable in flight, and outstanding in slow speed manoeuvres. It just doesn’t stall as such, only a nod and a mush; this means that low level, slow speed aeros are a doddle, but with the necessary extreme movements you do need to dial in lots of exponential.
The first flight was uneventful with a clean, well controlled take-off and transition to circuit height. There was a slight tendency to gain altitude by itself and subsequent adjustments were made to the engine thrust line (shown on the plans) to compensate. Thereafter, all the basic aerobatics were attempted: loops; bunts; rolls; tumbles; spins, and all were performed with ease. Truth to tell, with regard to the throws, only minor corrections were programmed into the Tx. The first landing was a gentle three-pointer into a stiff-ish breeze All in all, very satisfactory.
I hope you have as much enjoyment with Basic 3D as I have over the last couple of years. If you can, please do send me a photo or two and let me know how you get on with the more extreme stunts. May I wish you many soft landings.
Name: Basic 3D
Model type: Advanced aerobatic trainer
Designed by: Mike Keay
Wingspan: 52'' (1321mm) or 58'' (1473mm)
Fuselage length: 50'' (1270mm)
Wing area: 4.8 sq. ft.
All-up weight: 41/2 - 5 lb
Wing loading: 15 - 161/2oz / sq. ft.
Rec’d engine: .40 - .53cu. in. two-stroke or equivalent