Like it or not, aeromodelling in the 21st century is not the gentle introduction to handicraft skills that it once was. Newcomers to the hobby (sport if you must….) will almost certainly be guided towards the omnipresent ARTF trainer rather than a build it yourself kit, these days and some would argue why not? ARTF’s are a bit of a double edged sword because they eliminate a need to understand how an airframe is stressed for flight at the design stage but it cannot be argued that for the most part they represent a value and practicality that cannot be matched by a conventional build. I cannot put together a 46 sized trainer for the sort of money that a beautifully finished far eastern ARF version sells for, hence their popularity. ARTF most certainly does not mean Almost Ready To Fly however! It means in reality, Almost Ready To FIT, meaning that whilst you do indeed get a completely built and covered airframe in your glossy box of tricks, the really clever stuff is left up to you. Those cunning Chinese kit factory’s are very good at inducing eye appeal, hence the fabulous colour schemes that the ARTF’s carry, but aeromodellers with a couple of years or more under their belts will have learned to their cost that a finished airframe is by no means the whole story. I can knock out a wing a couple of evenings from stock balsa but perversely, fitting an awkward servo or two and a few linkages will easily absorb a similar number of hours. Any ARTF will take a newcomer at least a week of evenings to put together properly, which is unfortunately why a lot of them arrive at the field sadly lacking in preparation and attention to detail. If your model is to survive even its test flight then read on!
I have been asked to test fly many ARTF’s for experienced and not so experienced owners and without exception, none of them have been completely ready for the job yet. Faults identified and rectified on the field on the day in question range from totally unchecked C/G positions (disastrous) to linkages that either bind or have so much slop that the surfaces they are attempting to shift barely move. Moving swiftly on through a list of failings and potential crash making horrors we have hinge gaps that a half capable pilot could knife edge a Pitt’s Special through, clevis’ without keepers and fuel tanks that are barely affixed to the airframe. We then progress to the really nasty stuff such as wayward battery packs that produce interesting trim changes once the model is airborne, receivers that are just tossed into the fuselage and leads that dangle tantalisingly over servo output arms, ready and willing of course to unplug themselves with the very next control input. Believe me, even this lot merely heads up a very long list of regular failings, all of which threaten to end your day in misery. If you want to fly a radio controlled aeroplane then best you accept that, ARTF or not, there is a lot of preparation to do before you get anywhere near the flying field. These machines are miniature real aeroplanes and real aeroplanes crash if something goes awry during an aerial sojourn. All crashes, model or full size range from expensive and disappointing to injurious and trauma inducing. People get killed in full size aeroplanes when they crash and models similarly have killed more than one innocent bystander when they have lost control. You can minimise the risk from the outset by making sure that your engineering work has been carried out and quadruple checked to the utmost so that the machinery is at least up to the job.
Let’s start with the engine, which needs to be firmly affixed to the firewall. Engines are big heavy lumps of vibration inducing metal and they need to be firmly bolted to the structure. Good quality cap screws and locking nuts with washers should be employed and these will undoubtedly need a further nip on the spanners and drivers once the engine has been run for the first time. Wood crushes and vibration loosens – so double check before flying.
The fuel tank is the next area of concern and here I see so many avoidable problems it’s no longer funny. Make no mistake, if you have a tank problem then as a learner you will probably crash if your engine quits at an inopportune moment. That fuel tank, a very light and inconsequential bit of plastic, or so it would seem as you pop it into the fuselage, once full of liquid has a rather more impressive mass. Consider the sort of violent emergency recovery that a trainer may be put through where perhaps 30G is placed on the airframe. That plastic bottle that you timidly wedged twixt a couple of bits of foam as you hurriedly assembled your ARTF kit now seem horribly inadequate when you do the maths. Thirty times 10 ounces of fuel is three hundred ounces which is getting on for twenty pounds, meaning that your fuel tank now weighs twenty pounds as you pull out of that vertical dive in a blind panic, so best you fix it in place half decently. With the engine and fuel tank taken care of, we now come to the really serious business of fitting radio equipment. Radio equipment is very reliable and fairly durable these days, which means that in general it gets thrown into the model in short order with little care or attention to detail and this is where the problems manifest themselves. Think 30G as you plan each and every component and fit appropriately, because if you don’t then the moment it needs to withstand such force a failure will almost certainly put your model into the ground. The most serious failures are total rather than intermittent and the most frequent of them is usually induced by a lack of wiggly amps to essential components (like elevator servo’s etc) as leads come adrift in flight. DO NOT just rely on the plugs to hold leads together because they won’t, or not for long anyway. Heat shrink tubing, cable ties or self amalgamating tape will mechanically secure flying leads and secured in this way they will stay put, which is a major step in the right direction.
With all extension leads securely fixed together, the next job is to firmly mount all the servo’s and this means four screws, with grommets and ferrules correctly fitted. The washer part of the ferrule goes beneath the servo grommet so that the screw affixing it to the mount forms a complete isolation mounting point with the servo held only by the rubber grommet. Servo’s need to be isolated from engine vibration primarily, gliders are less fussy and so for that matter is electric power but for the moment let’s just agree that servo’s need to be properly mounted as per the manufacturers instructions. With your servo’s in place, now you can decide where the battery and receiver are to be placed. The battery is your on board ballast and as such it can be used to accurately achieve that essential centre of gravity adjustment so you need to position this item when all the other stuff has been done. Today’s receivers are light in the extreme and their mass can be considered inconsequential, so put them in a convenient and safe place. The receiver must be mounted firmly, not as I more usually see, just tossed into the fuselage on a bit of foam packing. Cable tie your receiver to a bulkhead or former in a foam wrap and most importantly, cable tie or put an elastic band around the crystal because if the crystal vibrates out of it’s socket (a regular occurrence) then it’s goodbye model!
Weigh your battery pack, multiply it’s mass it by 30 times and fix it in place accordingly. End of lecture – I am sure by now you are “on message” with the 30G scenario. Plug in all the gear and test your linkages and surfaces travel moments. If you have a COMPUTER RADIO, then adjust all your linkages MECHANICALLY with the clevis’s and horns - NOT electronically with the transmitter. Everything should be at right angles to begin with and all trims should be centred. Why single out the computer radios? Well if you have a non computer set you have to do it the correct way to begin with don’t you? The problem with computer transmitters is that newcomers (and some old hands for that matter) attempt to overcome diabolical radio installations with an electronic fix, which is nothing of the sort, it’s a very poor bodge. Here is a scenario for you. Take one thoroughly expensive modern high power servo – say a ten KG pull top of the range digital jobby costing eighty quid and compare it with a five pound rally special bargain servo. Both of these will move a control surface equally effectively so why the difference in price. It’s all about power and resolution and both of these can be ruined by a poor mechanical linkage and a lousy computer radio electronic “fix” as described. If your surface is moving twice as far as you want it to and more one way than another, the last thing in the world you want to do is reduce it’s travel on the transmitter program. You actually want to start with 150% travel on the transmitter and then sort things out mechanically. If you reduce travel by 50% then you have wasted 50% of your expensive servo’s resolution, and with it 50% of the mechanical advantage to boot. All you have achieved is to waste a load of money on an expensive servo that you didn’t need because with correct mechanical advantage a servo of half the power moving for twice the travel will be inducing exactly the same mechanical force to your surface and probably with equal resolution too. Of course if you start with a standard cheap servo then your control authority will be completely lousy in that situation.
Take the model jet, a fast ship that by its very nature needs tiny control deflections. Ten digital servo’s on an aeroplane like this, poorly set up with wasted resolution and travel means a thousand quid of wasted money. How the radio manufacturers must love the inept and uninformed aeromodellers out there who continue to fudge their installations with computer transmitters! Don’t get me wrong, a computer transmitter is a wonderful tool once you understand the mechanics of a model. Exponential control and end point adjustments together with servo reversing and mixing facilities all combine to give us fine tuning, but that’s all, just fine tuning. Remember, start with 150% on your travel adjustment and work mechanically as far as you are able, then fine tune with the transmitter if need be (unnecessary on 99% of trainers I would suggest). So what else can go wrong with my ARTF trainer then? I guess the truth is that anything that can go wrong will go wrong from time to time in this game but you can certainly minimise the odds with a bit of effort and common sense. Check everything twice and then check it twice again and you are halfway there! Happy landings. |
