Little Arrow (oz9023)
About this Plan
Little Arrow. Indoor RC model for the first time flier, using GWS hardware. Wing area 1.85 sq ft, weight target 5.25oz (150 grams).
Quote: "Daunted by the seeming complexities of radio-control indoors? Then try this ultra simple introductory design by Mike Roach.
One of the more enjoyable things in life is flying a model that is your own creation. You've done the thinking, the designing and the construction - and it flies! Success like this satisfies us on so many levels: saving money, the pleasure of seeing theories proved in practice, basking in the admiration of fellow modellers and it's all so simple and accessible for Indoor fliers.
To produce a first indoor model that flies successfully just needs imagination and the ability to meet one or two important targets.
Your design must match your flying ability. You might like to try ducted-fans, or a highly aerobatic IFO clone. And you certainly don't need me to tell you how to fly it. But if, like me, you are comfortable with two channels and just about okay with the throttle, then a naturally stable, slow-flying model is the one for you. In this article I'll show you how to design and make one that will fly superbly with the minimum of effort.
It must be 'legal'. Some indoor sites have a size, weight or power restriction, but you will almost certainly be okay if you stick to the 150 motor size.
You must want to make it. Don't waste your valuable time building if you are an ARTF modeller who prefers flying. Buy a Pico Stick and increase the dihedral (using thread tied from tip to tip) or a Pro Tech Butterfly: get up early one calm, sunny morning to get the feel of throttle as well as rudder and elevator, then go indoors and have fun.
General layout: The simplest way to start is with a 'wing at the front, tail at the back' design. All the technical stuff about thrust angles, point of balance and incidence is well known and you are pretty well guaranteed success. Just bear in mind that the best flyers have a wing loading of about 3 oz/sq ft. Assuming you use the 'industry standard' GWS geared 150 motor and associated servos, SC, Rx and 6 x 250 cells, the hardware will weigh about 100 grams or 3.5 oz. You can easily build a model that weighs 2.5 oz, giving a target flying weight of 6 oz and thus needing about 2 sq ft of wing. My scale Howard-Wright Monoplane (oz9893) uses this gear and has a wing loading of 3.7 oz/sq ft. It can fly very slowly and is agile enough for small hall flying, so the target loading is not only easy to meet, but has some leeway if you can't get light balsa.
An 'old-fashioned' curved plate wing section has the best performance. They knew this 100 years ago, so don't think you have to have an Eppler section for indoor flying. Most indoor pilots have a comfort zone of between 4 to 10 mph, which is just right for this simple airfoil. You don't need to worry about drag, you just need lots of lift. This is what the curved plate gives and the ribs are very easy to cut. I usually laminate mine from two layers of 1/16 square, but used 3/32 sheet sliced over a ply blank this time, and it has proved just as strong and durable. 100 metres of thread is lighter than an inch of ply.
Many quite hi-tech indoor models use rigging to brace the flying surfaces, and even work the controls on scale models. Ordinary thread is very strong, weighs next to nothing, comes in all sorts of colours and is stored free for you in the sewing box! Admittedly, monoplanes need slightly more structure to fix the bracing to, but everything else can be thinner and lighter and you won't need plywood reinforcement.
The motor can point straight ahead. What I mean is, you don't need to bother with down-thrust or side-thrust, because, unlike more ambitious or powerful models, trainers only fly gently and can easily be trimmed to fly hands off. You can also discover the delights of: power on, climb; power off, descend; just like the full-size.
Other layouts Canards, pushers, flying wings, twin motors, low wings, scale models and so on can all fly indoors, but might need a little more experimenting to find the right combination of balance and incidence to fly well.
How big? The GWS gear will let you fly a biplane of up to 34 in span or a monoplane of about 38 in span with a ready-to-fly weight of no more than 8 oz. A Speed 300 motor (or one of the new GWS twin 150s) will allow a 40 in monoplane and 10 to 12 oz, and with a Speed 400 you could easily go up to 48 in span and 16 oz, but might find it hard to fly in a small hall because it would not turn as tightly as a smaller model.
Little Arrow: This is my design for a cheap, light and forgiving indoor model, ideal for flying in the street if you dare, in the park on a calm evening, or in a four-court hall with your flying club. It's really only a set of suggestions, and I used spare wood from my scrap box for most of the parts and completed it in less than two days of building. You could cover the wings in tissue with a couple of coats of thinned dope or use any of the really light heat-shrink materials - Litespan, Airspan or similar. Just don't be too hard with the shrinking - it isn't necessary for flight and too much heat may well ruin the structure! I've used carbon rod for the leading and trailing-edges, but you could use balsa or hardwood dowel and incur very little weight penalty. I've tried to hide the cables and plugs away - I do hate to see these hanging down untidily - tuck them away, please!
Fuselage: This is merely 8 x 5 mm hard balsa, cut from a sheet of heavy timber I bought 27 years ago (I know, because my son doodled on it), which supports the little wing pylon and a banded-on motor stick which will break or fall off if you hit the walls, ceiling or floor.
The wings are held on with short brass tube dihedral braces and can be removed for transport if necessary. I find the most difficult thing about indoor flying is getting the model into the hall through a Force 6 blasting round the entrance. If you can fit it all into a box, it's much safer!
The undercarriage is made from 1.0 mm diameter wire and bands on to a dowel, as do the battery and speed controller. All these heavier components can be removed in an instant and fall off if you hit the walls.
Tailplane, fin and rudder: Make these over the plan from 2.0 x 5.0 mm strip, sand the edges (and the corners if you want it to look a bit better) and cover both sides. I can't see the point of only covering one side to save a quarter of a gram. And while we're on the subject, don't stint on the glue because it's 'heavy'. The glue in a tube of Bacia's solvent-free all-purpose in the green tube weighs 30 grams. You'll use perhaps a gram of it in this model plus half a gram of superglue, so use it well and forget weight gain because, with this size of model and the abuse you.are going to give it, a strong joint is more important than the weight it contributes.
Sewn hinges: My favourite method, this! Tape the surfaces together, leaving enough room to get a needle between them, then use coloured thread to match, and simply sew a figure-of-eight between them, using the same holes each time. Repeat four times for the elevator and three times for the rudder, and then remove the tape for a very free hinge. Put a drop of cyano on a pin and carefully touch it to the holes to secure the ends of the thread.
Wings: I had intended to use 3 mm balsa dowel for the leading- and trailing-edges and fill the bit in between with sliced ribs. None of the local shops had dowel, so I splashed out on two 2 mm carbon fibre rods instead. CF is very strong, but it does get expensive. Dowel is almost as light and easier to repair and the wing is not going to be stressed very much at 10 mph, is it?
The ribs are sliced from 2 mm medium sheet. It can be tricky, when using dowel leading and trailing edges, getting the rib length correct each time, so I've shown a neat solution which I think is original (there's no telling with this hobby, when every technique seems to have been first thought of in the 1930s). The photos show it better than words, but on your balsa sheet, mark two vertical lines the width of the wing chord and, for 4 mm deep ribs, mark the lines every 4 mm, then drill a 2 mm hole at each mark. Cut down the vertical lines and you have a set of ribs with a semi-circle out of each end, ready for cutting.
First though, using the drill, pull each hole back slightly, so that you have ? of a circle cut out, rather than a semicircle. Finally, using the rib template, cut the ribs so that the drilled holes are centrally located. You can gently sand the top of each rib just before cutting to get a nice surface finish.
Cutting carbon fibre rod: I'm told by people who work with carbon fibre every day that the dust is very dangerous if inhaled or if the fibres if they get under your skin, so I wear a mask when working with it. My suggestion for cutting the rod is to do so by rolling it under a knife blade and snapping it after five or six passes. I then sand the cut end so that there are no loose fibres left. I don't really like cutting it with a saw. I then immediately Vacuum up up the dust.
Building the wing: Pin the carbon fibre rods down over the plan and cyano each rib in position, leaving 10 mm of rod sticking out at the root. When dry, turn the wing upside-down and cyano the wing spars (made from 1.5 x 5 mm balsa) in place. They're only there to stop the ribs from being pulled in by the covering, so they can be taken from light to medium sheet. Cut a sheet of Litespan or similar, leaving at least 10 mm excess round the edges. Paint the inside edges with Balsaloc where they will touch the framework and right out to the edge. Wipe a glue stick around the wing structure and position the covering, using the adhesion of the glue to hold it while you get in exactly the right place. Then iron down the edges and, when it's looking neat and tidy, take the covering round the edges, using a pulling motion with the iron to tease out any wrinkles. Finally, fold over the LE and TE and stick it to itself for security in flight (see the 'how not to' photo). You can try some gentle shrink-ing, but too much will only 'banana' the wings.
Attaching wings and hardware Build a little platform for the wing from the same section balsa as the fuselage stick; two layers high for the LE and one layer for the TE. Reinforce with 0.4 ply as shown and drill for the wing joiners. These are 2 mm inter-nal diameter brass tube, 'bent to fit'. I glued the front one in place but left the rear one to float until I was happy with the set-up, then secured it with cyano. The servos, SC and Rx are taped onto balsa blocks in the usual places.
Flying: This is the second prototype. The first had a proper pylon and a softer fuselage and the banter from the crowd after one of the wings came off, breaking the fuselage behind the motor, prompted me to go the 'banded on' route. The version presented here is a very stable flier but is also manoeuverable and will fly very slowly on a touch of throttle, even within the confines of one badminton court, let alone the usual sports hall size of four.
The control surface movements I used were 45 degrees on the rudder and 15 degrees on the elevator (25 aim and 10 mm movement either way), but if you are new to Indoor flying, or indeed new to motors and throttles, fly it out of doors with the rates out at first until you get the hang of how it reacts to your piloting style. Experiment with the balance point: it can go at least 10 mm further back for a more responsive flight when you are used to the forward position. And go and have fun!"
Scan from DBHL, cleanup by theshadow.
Direct submission to Outerzone.
Update 19/03/2019: Added article, thanks to RFJ.
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