the plan build article by Loris Goring - 22/10/12
With the dark nights only just beginning to recede and with the less than clement weather, many of us will be agonisingly short of stick time. But there is a solution - indoor flying. This discipline is great fun, but something that far too many enthusiasts have never experienced. And so it was that ed. Graham suggested I design an easily built, inexpensive indoor electric model to tempt the hitherto outdoor-only modeller from his fireside armchair. Enter Temptress - a simple slab-sider with a deep belly that easily holds GWS sub-miniature power and radio gear. The constant chord wing is easily built, with added riblets to make it look a little more posh and help give the aerofoil a better shape.
Many old hands will already have a suitable Tx and battery charger, all that’s needed in this case is to ensure that the GWS Rx crystal matches that in your Tx. Incidentally, if you’re new to this indoor flying lark, why not visit a forthcoming event and seek advice from the assembled crowd? Rest assured the feedback will be friendly and plentiful!
I must admit to being an ardent skinflint, and fellow clan members will be delighted to know that the airframe cost a grand total of £1.50 to build. Admittedly this doesn’t include the cost of the scrap balsa used, but then cutting the entire model from new wood isn’t going to break the bank!
To ease the building experience there’s a materials list towards the end of this article. Use light to medium grade balsa throughout, as the little motor is no jet turbine and it’s vital that you keep the weight down. Oh, and don’t forget to be sparing with the glue!
Temptress is essentially the shape of a 1940s rubber powered model, with slightly more structure inside to hold the radio gear and battery. If you have some 1/8” polystyrene foam handy this is ideal for the tail feathers, and it’ll save the nose ballast that I needed when experimenting with Corecell, a light foam product (available from SP Systems, tel. 01983 828000) that’s used in the aerospace and boat building industries. Advanced modellers might well be interested to explore this material, which is light and very strong when used as a core material between Kevlar, glass or carbon reinforcements. It’s not as light as polystyrene foam sheet but much stronger, and being thermoplastic it can be moulded. It glues with PVA and epoxy resin adhesives to all the woods we use.
Anyway, let’s crack on with the build. Protect the plan with either a covering of polythene or by rubbing a candle over the areas to be glued over. Done? Then let’s begin.
The two slab sides are built over the plan. Fashion the four formers, noting that the only tricky part will be cutting the holes in F1 and F2 so that the motor has the correct down- and side-thrust (thus ensuring the model flies straight whilst under power). While you’re at it, it’s also worth pencil-marking the formers with the position of the undercarriage, the top of the battery tray and the bottom ends of the servo / Rx tray. When the aforementioned components are installed this simple guide will ensure they’re properly aligned, either horizontally or otherwise. Right then, glue the formers to one fuselage side only, ensuring that they’re at 90°. When the glue has dried add the second side, and join the two at the tail end with the assembly sat over the plan view. To save space you’ll note that only half the plan view has been drawn, so you’ll need to project a mirror image in order to complete the detail. When you’ve done this, make sure everything’s symmetrical about the centreline and not bowed to one side or the other.
The Rx and two servos are mounted on a horizontal platform, with the top of the battery box below and space at both sides of the platform to pass the wiring through. The sides of the battery box are fitted inside the stringers to ensure the weight of the battery does no damage in the event of an arrival.
Okay, cut the Rx tray to size and open out the apertures for the servos. Servo screws will hold perfectly adequately in the 1/16” reinforcement, especially if a little cyano’ is run into the areas where the screws are to go. You might consider using Velcro for holding the electronic components in place, indeed I also used it to secure the battery. That said, be sparing with the stuff as removing the pack with a strong tug can cause damage.
Forward of the battery hatch the fuselage uses 1/16” light sheeting, whilst from the aft end to the tail 1/32” sheet is adopted. The battery compartment has an opening hatch made from 1/32” sheet, this fitted with a tape hinge at the front and, at the back, a small piece of sticky tape to fasten.
Note that to accommodate the dihedral small side pieces are fitted where the wing t.e. sits on the fuselage. Likewise, a light, shaped, balsa block (stuck onto the fuselage after covering, using contact adhesive) provides support for the leading edge. Cocktail sticks are used as dowels for retaining the rubber bands that secure the wing.
Both the top and bottom of the fuselage is left unsheeted until the radio and motor installation is complete (see below for r/c installation notes). Again, place the fuselage directly over the top view centreline to check that there’s no symmetrical distortion when doing this, and do be sure to provide air-cooling vents in the nose area to give the motor a longer life.
The 22swg piano wire undercarriage is bound with cotton then ‘fixed’ using balsa cement. Finally, check that the fuselage sides are perfectly vertical and the axle lines of the wire are ‘on track’.
Temptress’ aerofoil section is the easily built, flat-bottom Clark Y and shouldn’t cause you any grief.
Cut out 20 ribs from medium 1/16” sheet, then cut the secondary spar slots on the first four ribs for each side. Use the dihedral template on rib no.1 to get it angled correctly.
Lay the main spar, short secondary spar and t.e. on the plan, glue all the ribs down, then add the l.e. 1/8” square stick. As mentioned earlier, the 1/32” riblets between the main ribs do smarten up the look of the wing and preserve a better aerofoil. Complete the wing tip and then taper the main spar where it’s notched at the end. Finally, sand and round off the balsa at the tip.
I originally designed the model with a two-part wing for easy transport, but I admit that it would be less complicated (and lighter) to build it in one piece. The centre of my wing uses aluminium tubes to take a 1/8” wood dowel, but with a light model like this it will be perfectly okay to use rolled paper tubes. For this, roll a sheet of paper five times around the dowel, gluing it as you roll - it makes a perfectly strong tube when dry and couldn’t be cheaper - right up my street!
Before assembly, no.1 and no.2 ribs must have the holes cut for the tubes, noting that the positions in the no.2 ribs should take the dihedral angle into account. On the prototype, I simply filed the aluminium tube at the end to sharpen it, and just pushed it through the balsa of R1. If using a paper tube, then twiddle an ordinary drill bit in your fingers. We’ll cut the holes through R2 in a minute, however, in the meantime, to complete the tubes put a tiny amount of glue in the outboard end so that the dowels can’t slip into the wing. Okay so far? Good. With R1 sitting flat on the building board, prop up one tip by 40mm, insert the tube and, checking that it’s horizontal, push it towards R2. A dab of nail varnish on its end will serve to mark the position of the second hole which can then be drilled out. After repeating this on the other wing panel, check that the dihedral is correct and maintained by the joiners. I used 5-minute epoxy on my aluminium tubes but balsa cement is fine the paper dittos.
The wing l.e. sits on the aforementioned shaped balsa block, which ensures the wing has the correct angle of attack. Incidentally, the tailplane is at zero angle of attack when the mainplane is at 41/2 degrees. As the wing rests only on the block, the mainplane angle of attack can be easily adjusted with a sliver of paper or balsa under it.
Use a pencil to mark the tailplane outline onto either 1/8” Corecell or polystyrene, and cut out using a balsa knife (any warps in the Corecell can be corrected by heating with a covering iron and then placing it on a perfectly flat surface - glass is ideal). Lightening holes are then cut and a balsa spar inserted - note that using a lighter polystyrene foam sheet for the tailplane will save several grams in weight.
I also used Corecell for the prototype’s fin and rudder. Unlike polystyrene sheet, Corecell has a pitted surface that can be filled, and in this respect I used that excellent lightweight filler Delux Model Lite.
A piece of plastic card was used to draw the filler over the surface, and when dry was sanded down with very fine wet and dry paper, used dry. This surface can be painted with a water-based acrylic but I chose to cover both sides with Solarfilm Lite.
R/C AND CONTROL RUNS
Prior to completing the upper fuselage sheeting, I installed the Rx in an almost upright position so that plugs could be easily pushed home. Double-sided tape secured the Rx, and a length of garden twist-tie held the controller. My scrap box was raided for some old Sullivan inner snake pieces and the garden shed for some soft 1mm wire to make pushrods. Garden wire is easier to bend than piano wire and doesn’t distort on the short run in this model - it’s also cheap to replace if you get the bend in the wrong position! The wire was then passed through the horns and secured with some scrap plastic tube glued over the ends.
Before committing Temptress to the air I double-checked all the flying surfaces to ensure that they were true and at the correct angle of attack. The C of G was spot on the main spar, but I paid a small penalty by using Corecell instead of polystyrene foam sheet and had to add a touch of ballast inside the motor compartment. However, the model was still on its target weight of 7.5oz.
Joe Spiers accepted the job of test pilot whilst I got behind a camera at the Teignbridge Indoor Bowling Centre - many thanks to the staff there for opening up at dawn specifically for our test flights! First checks were for ground handling, and my generosity with the balanced rudder immediately demanded rates to be set on the Tx. The model now rolled straight without being twitchy on the sticks. Joe held down elevator until flying speed built up and then, with a little up, she took beautifully to the air and proved a delight to fly. There’s nothing more satisfying for a designer than watching his new charge do exactly what it was designed for!
My second test pilot was Geoff Bell, who flies models for the BBC in the making of wildlife films. This time we were flying in a more confined sports hall that made demands for very tight turning. Temptress still managed very well and proved she was tough when (...ahem!) flown into a wall. Both model and wall survived.
A GOOD NIGHT IN
They say that ‘staying in’ is the new ‘going out’, so there you have it - there’s no excuse for moaning when the winds blow and the rain’s horizontal. There’s bound to be some indoor flying activity not far from where you live - equip yourself with a Temptress and have some fun. She’s cheap, fun and comes back for more!
● 4 x 3/32 x 3/32” balsa sticks
● 1 x 1/32 x 36” balsa sheet
● 1 x 1/16 x 36” balsa sheet
● 1 x 3/4” wide trailing edge section balsa
● 1 x 3/32” balsa sheet
● 2 x 1/4 x 1/8” balsa sticks
● 1 x 1/8 x 1/8” balsa stick
● 1 small tube UHU balsa cement
● Piece of 1/8” dowel, plus paper or alloy tube (for wing joiner)
● 2 x 22swg piano wire (undercart)
● Soft 1mm diameter garden wire pushrods
● 1 roll Solarfilm Lite (1.27m x 0.7m)
Model type: Electric indoor
Designed by: Loris Goring
Wingspan: 36'' (914mm)
Fuselage length: 24'' (611mm)
Wing area: 207 sq. in.
All-up weight: 7.5oz (212g)
Wing loading: 5.3oz / sq. ft.
Motor: IPS DX2BB-B motor and g/box
Battery: 7.2V 250mAh NiCad
Control functions: Rudder, elevator, throttle
Control throws: Rudder ±1⁄2'' (12.7mm), elevator ±1⁄4'' (6.35mm)
Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!