The bungee is essentially a super-size catapult that comes into its own when a conventional hand-launch just can’t cut it, providing that extra ‘va-va-voom’ just when you need it most. Blindingly simple in looks and operation, careful preparation and a few basic ground rules can make all the difference between a flat-out failure and a picture-perfect take-off. Let’s see what it’s all about.
FANTASTIC ELASTIC
The most critical part of a bungee launcher is the elastic itself. No matter how big the ramp or run-up, without the correct elastic you’re fighting a losing battle from the start. The traditional bungee cord – from which the launcher derives its name – consists of an elasticated rubber core encased in a woven polypropylene or cotton compression sheath and is available in a multitude of sizes, strengths and (usually quite garish) colours, the rubber composition inside being of most interest.
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A natural (latex) rubber bungee makes ideal catapult elastic. Offering excellent stretch and good rebound it’s just the job for smooth, controlled power delivery. On the downside its relatively poor resistance to outside elements, such as sunlight or extreme temperatures, can lead to the rubber hardening, this creating surface cracking or weak spots. Synthetic rubbers such as neoprene or silicone are more durable but lack the inherent stretch, strength and natural elasticity of latex. Whilst both material types are more than up to the job, the trade-off between outright performance of the natural compound and durability of the man-made synthetic is a point worth considering when deciding between the two. Whilst the cord’s outer sheath does a good job of protecting the elastic within, it can trap dirt and moisture beneath its surface. This ingress of grime not only damages the rubber core but also prevents the outer sheath from fully compressing, effectively reducing the overall stretch of the cord. All is not lost, however, and a regular dunking in a bucket of water will soon rinse away any foreign matter leaving the cord sparking clean and restoring its stretch as good as new.
Although not a bungee cord in the traditional sense, rubber tubing also makes for an excellent elastic. Again, available in natural or synthetic forms, tubing is lighter and more flexible than a fully sheathed cord and is ideal for smaller, lightweight models where the weight of a bungee should to be kept to an absolute minimum. For those blessed with a sunny climate, tubing treated with a UV inhibitor is well worth considering. Like sunscreen, the inhibitor reduces the damaging effects of UV rays on the surface of the rubber, keeping it supple and free from cracks and splits. Although slightly more expensive, UV-resistant rubber should easily outlast the untreated compound, usually paying for itself many times over.
Regular maintenance and cleaning is key in keeping any elastic in tip-top form. Avoid chemical cleaners and lubricants, as these can leave a sticky residue – a proverbial magnet for small stones and foreign matter. A wash-down in warm water after every outing should do the trick, adding a dash of washing up liquid or very mild detergent to remove any baked-in muck and grime.
WEIGHTS AND MEASURES
Like a conventional motor, the power of bungee should be tailored to suit to the model. Insufficient power and you’ll fail to break ground, too much and the model may prove a handful to control. Worse still, the excessive pull may stress and damage the airframe or simply rip the hook clean from the fuselage.
Ideally a bungee should be capable of pulling around five times the flying weight of the model. So, for a 6.6 lb (3kg) aircraft, aim for at least 33 lb (15kg) of available pull. Remember this is the pull strength of the elastic, not the breaking strain!
Whilst we’re on the subject, never overstretch the elastic in an attempt to gain more power. Stretching the rubber beyond its limit will seldom increase its pull, but can cause permanent damage to the cord, possibly leading to premature failure later in life.
Just as important as pull strength is the length and elasticity of the bungee. A short, strong elastic will initially produce bags of power, however its energy will quickly subside as the elastic loses tension. On the other hand a weaker elastic may not provide as much acceleration, but if stretched further will keep the aircraft under tow for longer. Striking a balance between outright power and tow duration is the secret to any good bungee launch – controlled delivery is the key. A lightweight park flyer, for example, will accelerate in an instant and require only a short, low-power elastic to hit take-off speed. Stressing the airframe or tethering for any longer than required is asking for trouble. Try a short 10ft (3m) length of elastic, tensioned to three times the weight of the model for starters. If initial acceleration is slow and the model fails to reach flying speed, increase the tension. If acceleration is fine but the model quickly runs out of steam, then use a longer length of elastic or stretch it further.
Bigger, heavier models require more power to get moving and spend more time under tow to gain full flying speed. 30ft (10m) of quality rubber, tensioned to pull four or five times the model’s weight should generally do the trick. Regardless of the model’s size, be sure to increase the bungee tension gradually, carefully monitoring the hook and surrounding airframe for signs of stress as you go.
Whilst the aim of a conventional bungee is to propel the model to flying speed, a glider or sailplane launch is all about altitude. A typical glider hi-start bungee launch uses a much longer length of elastic, coupled with an even longer flying line. A 90ft (30m) length of high-stretch rubber with an additional 300ft (100m) of lightweight static line is not uncommon, producing a strong, steady tow until the model hits release altitude, often many hundreds of feet above terra firma.
LOOPING THE LOOP
A bungee cord is capable of storing a lot of energy, so a heavy-duty metal tent peg or, better still, a spiral stake, is a must to secure it safely to the ground. Like an oversize corkscrew, the spiral stake screws deep into the ground to leave a short tethering post proud of the surface. Once in place it’s virtually impossible to pull out by accident and is invaluable for high-power bungees or when ground conditions are less than ideal.
Tie the bungee cord to the ground stake with a length of strong kite line or braided polyester. Alternatively, loop it and secure with strong cable ties. While a sheathed bungee is fairly resistant to wear and tear, exposed rubber tubing can chafe against the metal stake, so protect things here with a layer or two of gaffa tape.
The flying end of the bungee cord attaches to the model and pedal release via a series of metal split-rings, each tied to the elastic with strong line. Available from fishing tackle shops and adventure sports outlets, a heavy-duty split-ring looks similar to a keying loop but is much stronger and won’t pull apart or deform under tension of the line. Available in a multitude of strengths and sizes, choose a diameter that slips freely over the tow hook and has a pull strength in excess of the bungee elastic.
RAMP IT UP
A bungee can be used on its own (simply hand-launching the model or letting it drag along the ground) but, more often than not, it’s used in conjunction with a take-off ramp. In days of yore these were usually constructed of wood and held together with an assortment of bespoke bracketry, often proving a nightmare to construct and a right royal pain to transport. All this changed with the DIY boom and the advent of plastic plumbing pipe. Not only could your average enthusiast have a go at plumbing, it opened up a whole new world in ramp construction!
Unlike a conventional wooden ramp, a pipe ramp is not only cheap and simple to build, its push-fit plug-and-play construction makes it easy to disassemble and, thus, ideal for transportation and storage.
For a simple ramp you’ll need two 6ft (2m) lengths of pipe, four 90° bends and four T-sections. Almost any plastic pipe and matching couplers will do the job, but I find 1.25” (32mm) diameter polypropylene domestic waste water (overflow) piping to be the best bet. Unlike pressurised inlet pipes, waste outlets are gravity fed, using much cheaper piping and couplings – just the job!
First, cut a 13.8” (350mm) length from each pipe for the horizontal cross members, followed by a pair of 5.9” (150mm) front uprights and 2.75” (70mm) rears. This leaves two long sections for the main runners. Push-fit the T-sections into the ends of the horizontal tubes, and the 90° bends to the ends of the main runners. Next, fit one long and one short upright to each of the runners and then finish off by slotting the runners into the base sections. To prevent damaging the model on take-off, trim the protruding coupling edges flush or, better still, fit a length of foam lagging pipe over the runners, secured in place with sticky tape or cable ties. Job done!
Whilst such a pipe ramp is ideal for small-to medium-size models, scaling up for anything larger would prove expensive and wholly impractical. Bigger models usually do without, and either skid along the ground or use a bungee dolly. Unlike a conventional free-running dolly, a bungee dolly is attached to the elastic, therefore keeping pace as the model accelerates. Providing the ground isn’t too bumpy or uneven, gravity alone is usually enough to keep the model in place until the point of release.
ROCK ‘N’ ROLL!
An often-overlooked part of the bungee launcher is the foot pedal. Not only does the pedal give the pilot sole control of the release, it removes the tension of the elastic from the model until take-off, minimising airframe stress.
To construct a basic pedal, start with a 12” (300mm) length of wood for the pedal upper, and a slightly longer length for the base. No need to spend a fortune on the finest exotic hardwood here; I used 0.4” (10mm) thick x 3.5” (90mm) wide beech offcuts, courtesy of B&Q’s cast-off bin – a snip at a £1 for the two!
With both parts cut to size, chamfer the corners and sand with medium grit paper to remove rough edges and surface blemishes. Brush on a base coat of exterior varnish, working deep into the grain, then leave overnight to dry.
The rocker assembly uses a short length of 0.85” (21.5mm) diameter plastic plumbing pipe for the pivot, and four pipe clips – two securing the pipe to the base and two hinging the pedal. Lay the base flat on the workbench and place the pipe just forward of centre. Fit the clips over the pipe, close to the edge of the base, and secure with small self-tapping screws. Slide the pipe from under the clips then repeat the process for the pedal, this time insetting the clips around 1” (25mm) from each edge. Cut the pipe flush and remove any rough edges with fine sandpaper or emery cloth.
Fit the pedal and base together, slide the pivot pipe through all four clips and then rock the pedal back and forth, checking for ease of movement. If it binds or feels overly tight, back off the clips or trim the inside flanges with a sharp knife. The pedal should rock with gentle foot pressure but not feel loose so it can be triggered accidentally.
Next, drill and fit the retaining peg to the base plate. The peg has to withstand the full tension of the bungee, so use something substantial – I used an M6 roofing bolt secured in place with a nut and a couple of large washers. Round off the base with holes for the securing stakes, again sanding rough edges to prevent splintering.
Drill the peg clearance hole in the pedal upper and then, with the unit re-assembled, press the rear of the pedal down flat to the base (bungee release position) and cut the peg flush with the top of the pedal.
Apply a second coat of varnish, paying close attention inside the holes and edges of the wood where the grain is exposed. Cover the top of the pedal with grip tape or sandpaper, then loop a strong elastic band around the pedal preventing it from being accidentally triggered whilst armed. Easy!
PEDAL TO THE METAL
If this DIY approach doesn’t tickle your fancy, there are plenty of off-the-shelf alternatives available. For an all-in-one ramp and pedal, the aptly named Jet-O-Pult comes complete with bungee cord and ground stake, ready to get you started with the minimum of fuss and bother. Available from Fan Jets USA (www.fanjetsusa.com), this one-stop solution is ideal for small to medium-size park flyers and EDFs. Costing around £40 it offers great value, although you do need to factor-in additional overseas postage costs of around £10 – £20 if you live this side of the pond.
Our DIY foot pedal is just the ticket to get started, however a full-blown metal pedal is well worth considering if you wish to progress to bigger things. The WeMoTec BG 001 pedal (www.wemotec.com or www.rcwarbirds.com) costs around £25 and is as strong as they come. Fitted with a threaded, removable ground spike, the all-metal construction should ensure peace of mind for even the most powerful bungee.
HOOK, LINE AND SINKER
Now to the model. The tow hook position can have a huge effect on how the aircraft behaves during launch. In a similar fashion to setting the C of G, finding the sweet spot may take one or two attempts, however it’s well worth persevering as it can truly transform your take-offs.
For a conventional airframe configuration, position the hook 2/3 of the way back from the tip of the nose to the C of G location. This should give the model a slight nose-up angle of attack on leaving the ramp, which is perfect to encourage sufficient airflow under the wings without ballooning. We’re aiming for a controlled, shallow climb here, rather than a rocket-like trajectory!
If the model struggles to gain height or tucks under while under tow, then the hook is probably too far forward. If, on the other hand, it adopts a high (nose up) angle of attack, either climbing steeply, fishtailing or snap rolling as it leaves the ramp, the hook is too far aft.
In contrast, a glider (hi-start) bungee launch uses a much higher angle of attack, with the aircraft climbing rapidly, closely following the natural arc of the line. This is achieved by placing the hook much further back, usually just slightly forward of the C of G position.
Whilst there’s room for a little leeway, positioning the tow hook too far from the norm can result in a very brief (and usually expensive) first flight. If in any doubt, always set the hook slightly forward of the calculated sweet spot and work back, never the other way around.
BATTEN DOWN THE HATCHES
The acceleration of a bungee launch is way beyond that of a conventional take-off, and enough to break loose anything that isn’t firmly fixed in place. Pay close attention to the radio installation, power pack and flight battery. A loose Li-Po can easily shift mid-flight, damaging the airframe or upsetting the C of G, usually ending in catastrophe.
With the internals sorted, place the model on the launch ramp and check that the runners don’t foul servo arms, control horns or fittings beneath the airframe. If need be, adjust the distance between the ramp runners to suit – this is generally easier (and much quicker) than moving servos or modifying the model.
Last up, double-check that any battery hatches and access doors are secure and won’t flap open during take-off. If in doubt, run a strip of Blenderm tape around the edges to secure and seal them. Okay, then… let’s bungee!
LOCK ‘N’ LOAD
Before setting up, check with your clubmates and agree on a suitable ramp location. Disagreements often occur when others aren’t informed or included. In this respect, getting club members (especially the chairman!) on your side from the start usually works wonders.
Ideally the ramp should face directly into wind, positioned such that the bungee cord remains clear of the main runway at all times. No matter how well prepared you are, bungled approaches, dead-sticks and overshoots can happen at the most inopportune moment; the last thing a fellow flyer needs is a potential trip-wire across their landing path.
Ensure the ground is firm, (avoid puddles or overly dry soil, a sure sign of problems beneath!) and then hammer or screw the bungee stake securely in place. Mark the stake with a brightly coloured flag or ribbon; should a model stray off course, it’s easier to take avoiding action if potential hazards are clearly visible to the pilot or spotter.
With the stake firmly rooted, we’re set to tension the bungee, using a portable luggage or fishing scale to measure the pull of the cord. Quick and easy to use, a scale is much more accurate than pacing out. Most will measure up to 88 lb (40kg) of pull and provide a clear readout in grams, kilos, pounds or ounces. Get one, it’s a great addition to any field box and a bargain at less than a tenner! Okay, clip the scale to the bungee line and walk downwind, away from the stake. Never face directly down the line of the elastic whilst it’s under tension for should it suddenly let go you’ll be directly in the firing line. So, always avert your gaze. As the bungee cord tightens, keep an eye on the readout on the scales and mark the ground when the bungee tension reaches first three, and then five times the weight of the model. These are the provisional minimum and maximum tension limits. The optimum ramp position should ideally sit somewhere between the two.
So, with the ramp facing into wind, pointing directly towards the ground stake, secure some wire tent pegs in position. The wind can (and usually does) change direction throughout the day, so be prepared to ‘up sticks’ and move ground position to compensate. All set, then? Prepare for take-off!
FLY BY WIRE
Give the control surfaces a final check and then, with fingers well out of harm’s way, give the motor a blip and we’re set to arm the bungee. (Depending on the model you may find it safer to momentarily disconnect the battery at this point).
Tension the bungee cord and then, very carefully, hook the rear split-ring over the locating peg on the foot pedal. Place the model on the ramp and then slip the remaining split-ring onto the tow hook.
To reduce the risk of fouling the propeller on the bungee line it’s often best to take-off without power. Whilst EDF (Electric Ducted Fan) models don’t have such a problem, keeping the revs to a minimum will prevent the model from creeping forward and prematurely dropping the line off the tow hook.
For starters, concentrate solely on a clean getaway and test hop. Place your foot above the rear of the launch pedal, with fingers poised ready on the sticks. Once under way the model will accelerate at a rapid rate of knots, so be prepared. Keep your head up and face forward, not down at the floor! One last check that all’s clear then, with a firm push on the pedal, we’re off.
The model should shoot forward in a very shallow climb, so barring disaster there should be no need for any gung-ho stick banging. Wait for the line to release from the hook and the model to settle into a steady glide. Keep the nose ever so slightly up, then gently flare to land… Easy, eh? If the controls felt a tad twitchy, switch to mid or low rates next time and maybe feed in some exponential to dampen things mid stick. Practice the take-off a few more times, adjusting the trims and throws as you go.
Once you’ve mastered clearing the ramp and dropping the line, prepare to throttle up and fly a circuit. Be especially careful with prop-driven models as hitting the power too soon may hook the line on the prop, bringing the whole proceedings to a grinding (quite literally) halt. Once the bungee has dropped clear and the model’s settled into its glide, then, and only then, throttle up and power away. Should the aircraft kick or roll, be less aggressive with the revs, roll the power on smoothly then gradually climb away.
Things can still go very wrong, very quickly, so keep your wits about you. If the line fails to release or catches on the propeller, rather than fly out, immediately kill the power to scrub off forward speed and keep the wings level. Should the line still fail to release as the bungee tensions, its better to ditch slow, low and level than from altitude.
Ducted fan models don’t pose the same problem, as the fast-moving parts are safely tucked out of harm’s way inside the fuselage. They do take longer to get the power on tap, though, so be ready with the throttle quick-sharp, spooling up the revs as soon as the pedal’s released.
With the basics under your belt, experiment with different elastics, ramp angles and hook positions. Models exhibit different characteristics under tow so don’t presume that what’s good for one design is the same across the board. Be patient and make notes as you go – that way it’s easy to revert back should things not quite go to plan.
PINGED OFF
So, there we have it. Why not give the bungee system a go? With a little patience and practice you’ll be rewarded with picture-perfect take-offs time after time and, best of all, you’ll not be relying on the strong and steady arm of a press-ganged clubmate.
