Fail Deadly
The trenches are dug, barbed wire in place, the crossfire from the machine guns is murderous and the place is a minefield. Propaganda blares out from both sides. The searchlights probe for anyone in no-man's land. R.I.P.
It's not unfair to say that one of the major challenges for aeromodelling in the next year and beyond is how we choose to improve our safety record in the light of last year's fatal accident. The focus has, not unexpectedly, focussed on PCM Failsafe. If you've not already read the excellent PCM Failsafe bulletin from the B.M.F.A. on the subject, you really should do.
This webpage makes no attempt to persuade you to join either of the current camps on what to do, rather the intention is to get you to think about what you are doing, not just at the flying field but through the whole process of building your model. Our concern is that the amount of "noise" generated on the PCM subject will swamp other equally important considerations.
One thing has been established over the last year and it is written in letters of no rubbing out! You and you alone are entirely responsible for the structural integrity and the safe execution of each and every flight that you make, whether you are the builder of the model or not. Put very simply, if your hands are on the sticks, you are responsible. May it never happen to you, but you may be required to justify your every action, in a court of law, from the construction to the flying of your model. Since this is what is likely to happen, it makes sense that you should understand as much as possible about how and why your equipment performs in a given way. For example, if your radio does not have a failsafe fitted as standard to the throttle, any defence you have will be based on this fact. If, however you had fitted a throttle failsafe, you can argue that you recognised a problem, blew some cash on it and took this additional safety measure. Think how much better your defence would sound. This whole webpage is intended to help you to improve your own safety margin. It should not be taken as a set of instructions to follow - it is up to you whether you think any of the measures advised here are an improvement on any of your current practices. As always, if you've got a better idea on something, please let us know. A smart person learns from their own mistakes, a genius learns from others!
Building For Success
Switch harness failure is a depressingly frequent occurrence. If you have any doubts about the reliability of the switch - if you have to fiddle with it to get the aircraft to come alive or you seem to get intermittent power - then bin it. It's the same story with a battery pack - any doubts about it's reliability - bin it.
There are a number of things you can do to avoid unintentional disconnection of the battery pack - or indeed any of the other components plugged together in your aircraft. Despite the ready availability of "mate and lock" connectors throughout the entire electronics industry, the manufacturers of radio equipment have chosen to use proprietry friction lock connectors. Just take a look inside your PC. An equivalent connector to an R/C equipment one is that running from your CD-ROM to your Sound Card. It's small but it positively locks into position - try pulling it out without releasing the snap catch - impossible. Strange that equipment that sits on your desk has a better connection system than something subjected to a very harsh environment. It's not that the R/C connectors are cheap and nasty - they're just expensive and nasty.
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What to do? Where connections are to be permanent or at least semi-permanent, tie the connectors together. This can be usually be done for the battery to harness connector and, if you have separate servos in the wings, any servo to extension lead connectors. Use strong domestic thread or fishing line for this job. When you want to separate the connectors, simply cut the thread before splitting the connectors. One word of warning though - ensure you don't short-circuit the contacts when you are cutting any leads connected to the battery pack!
There's not an awful lot you can do for the receiver connections but if the receiver is well wrapped in foam, chances are that the cables won't disconnect.
That generally leaves the servo connections from the wings - aileron, flaps, gear etc. - to think about. If you plan for a neat and tidy installation of your radio gear, all these connections should be in a free space in the fuselage where any other servos cannot accidently trap or worse separate the joints. Self disconecting servos are not meant to exist! Even better is if the connections can be tucked away (or "trapped") in the receiver or battery foam once you have made the connection but prior to final fitting of the wing.
Cut off all the un-used servo arms in your installation - something that's not there can't cause any problems!
Install all your radio gear in accordance with the manufacturer's recommendations. Servos should be screwed into place using the (supplied) rubber grommets and stand-offs. These must be fitted to help isolate the servos from vibration. The receiver antenna should run pretty much in a straight line to the tail of your model and should not be "doubled back" on itself as this reduces the effective length of the antenna and really axes radio reception range. On a small model, leave the antenna trailing. No cables, including the antenna, should be tight or trapped - the wing to wing seat join should be inspected before tightening down the wing.
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To avoid putting unneccessary strain on the receiver antenna and, in particular, the solder joint on the receiver printed circuit board, make a small strain relief. Drill two holes in a piece of scrap balsa or dowel. Drill and cyano a small piece of pushrod inner into the fuselage top at a suitable position. When installing the receiver, feed the antenna through the drilled dowel, ensuring that there is some "slack" between the receiver and the strain relief, then out the fuselage through the pushrod centre.
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At the tail end of your model, feed the antenna through a small piece of fuel pipe and an elastic band. Make a small brass hook from brass and cyano into the tail fin. When dry, hook the elastic on the antenna over the hook, ensuring that the antenna is not pulled too tightly. Allow any excess antenna length to hang free.
Where you decide to position your battery pack and receiver are highly dependent on the model layout but two things are certain. Neither must be capable of movement and both should be well packed with foam. Best option is to either trap them in place using the fuselage structure using a ply plate - remember the wing can also be used to achieve this.
On the mechanical side, all control rods should be free moving with little or no slop. It should also be impossible for one control function to interfere with another - it's not obvious when you're building a model but the aileron servo which is generally in the centre of the wing may catch on any of the servos in the fuselage so check for "clicks, bumps and whirrs" by moving all the controls all over the place in all possible combinations - remembering of course to include trim controls.
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Although many kits and their instructions make no mention of it, all hinges on the moving surfaces should be glued and pinned in position. A good way of pinning hinges is to use domestic sewing pins. Once the hinges are glued into position, snap off the tip of the pin, sufficiently long enough to go through one side of the balsa, through the hinge and partially through the other side of the balsa. Ensure that the pin doesn't go all the way through the surface otherwise you'll finish up stabbing yourself! Push the pins in from the underside of the surfaces and they won't be visible when your model is sitting on the ground. Remember to pin both the fixed and moving sides of the hinge.
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Many models come supplied with plastic clevises for connecting push rods to control horns - their first (and only) flight should be from the workbench to the bin! Treat your model to metal clevises - it's worth the extra expense. Use a small piece of fuel pipe to ensure that the clevises cannot open unexpectedly.
Radio Gear
Many incidences of radio interference are "own goals" and can be avoided by pilot and club discipline. Whatever the frequency control system at your club flying site, be sure that you fully understand the system and be equally sure that both visiting pilots and newcomers understand the system as well.
There are a number of very basic practices that require to be followed:
- There must be a clearly defined transmitter pound where all transmitters not in use are kept and are switched off.
- There must be a system for identifying which frequencies and which transmitters are in use at any one time.
- You must know which frequency/frequency band your transmitter is using.
- Always carry out a full range check each flying day, preferrably with the engine running.
- Before each flight, check that all controls are working properly and in the correct direction.
Rigorous adherence to the above points will eliminate the vast majority of radio interference incidences.
Frequency bands (in the U.K.) are:
| Band | Mhz | Band | Mhz | Band | Mhz | ||
60 |
35.000 35.010 35.020 35.030 35.040 35.050 35.060 35.070 35.080 35.090 |
70 |
35.100 35.110 35.120 35.130 35.140 35.150 35.160 35.170 35.180 35.190 |
80 81 82 83 84 85 |
35.200 35.210 35.220 35.230 35.240 35.250 |
Even better, get a frequency scanner to monitor the airwaves and check that your transmitter is transmitting on the frequency you think it is. If the crystals are dodgy, bin them. D.M.A.C. unanimously decided at our last Annual General Meeting to buy a scanner for club use - a major investment for us but one we consider essential for the safe running of the club.
It's stating the obvious but always make sure that your transmitter and receiver battery packs are fully charged prior to flying.
Many modellers, particularly newcomers, do not have the more sophisticated radio gear which includes a PCM Failsafe while many may or may not have a PCM receiver. Many computerised radio sets come with a PPM receiver as a means to reducing the cost of the set. The first thing to establish is whether or not you have a PCM receiver which a quick check through your radio documentation should establish.
Let's say you do not have a PCM receiver. Given that the C.A.A. are very likely (within about a year) to introduce the requirement that your model must not leave the area if it gets radio interference, what can you do to meet this requirement? You cannot be sure that your engine will cut. The bottom line is that if your transmitter is PPM, you really should consider buying an inline failsafe for the throttle (at the least). These are not expensive at around £10-£15. Set the failsafe up so that the throttle closes when you switch off your transmitter - needless to say, do this at home and not with the engine running! If your transmitter is capable of using PCM, your options are to get a failsafe for the throttle or better yet, splash out on a PCM receiver. The latter is not however a very cheap option given that they cost in excess of £100. If you're a dedicated model flier, you can always tell yourself how much you deserve it and treat yourself to a full PCM set!
If you do have failsafe capability, how should you set it up? Unlike some of our famous peers, we're not going to tell you! All we're going to do is give you something to think about.
Firstly, failsafe mode is not intended to save the model - it is intended to bring an out of control model to the ground as quickly and as safely as is realistically practicable. Just accept that once you lose radio control, the model is wreckage - your only concern is the safety of others.
All PCM sets allow you to program either individually or collectively the servo response after loss of radio signal. The most obvious thing you should know is how to activate the failsafe. This means that you must read and understand the instruction manual for your transmitter. As with all things to do with programming, the instructions are very good at telling you how to do things but are appalling bad at explaining why you want to do them!
As the B.M.F.A. bulletin points out, the current default is for PCM failsafe to do nothing. This is probably worse than not having any failsafe at all since the receiver locks out the interfering signal and keeps the servos in their current position - not too smart considering you could be at full throttle and flying a very stable model! A PPM model will behave erractically and is very unlikely to go any appreciable distance. Since you (presumably) want to keep the model in the immediate area, logic dictates that you want to slow the aircraft down as quickly as possible. There is general agreement that the throttle should be closed. Some fliers, particularly those flying larger or petrol-engined models, consider that the engine should go to a minimum tick-over or idle rate just in case they regain control. They then have the option of manouevering away from any spectators or fliers. Should there be no recovery of radio signal then the model will be going faster than it would have been otherwise. A tough call and very easy to argue over so do what you feel comfortable with - stopped or idling. You should also consider having any flaps, spoilers and undercarriage go to their extended positions - they all increase drag and help slow the model.
The major area of controversy is what to make the aircraft do on loss of radio signal and opinion is sharply divided between a faction advocating neutralising aileron, elevator and rudder while a second faction believes the model should be put into a "flick roll" - full aileron, elevator and rudder - which, with a cut throttle, will be followed by an inevitable spin. Another thing to consider is whether the "jamming" is long term or intermittent.
Taking the second option first, the flick roll followed by a spin has the indisputable advantage that, irrespective of model attitude on loss of signal, the model will almost immediately lose all forward speed and come down at the lowest possible rate. However, some models have a very high and flat spin rate - they don't come down all that quickly and so the effect of wind can have a marked effect on the course of the aircraft. Should the radio signal be recovered once the model goes into a spin, the model could easily recover flying in both an unsafe and unpredictable direction and attitude - an unenviable position to be in, particularly with a lot of spectators around or an engine that was idling returning to full power. If the radio signal is intermittent and the model goes into a flick at irregular intervals, the flightpath is going to be very erratic. Best advice here is to forget the model, pick a safe spot and plant it.
If flying controls are neutralised, then the model will not lose speed so quickly. It's flightpath is however a bit more predictable - it'll go ballistic and pretty much in an arc from where the loss of signal occurred. Unfortunately, this could be towards spectators. It is however easier for someone to avoid the possible crash area if they know roughly the point of impact. If the signal is recovered then it will almost certainly be easier to re-direct the model to a safer area.
Two widely differing viewpoints and both equally valid. You have to make up your own mind on this. In the absence of any recommendations (which are unlikely until decided in a court of law), think about what and how you fly and choose the failsafe method you consider best.
If you suspect that you have been "jammed", immediately close the throttle on your transmitter - you don't want any surprise bursts of power if you regain control.
Another thing worth considering as a discipline at your flying field is the control of the increasingly common mobile 'phone. There is some discussion and reported incidents of mobile 'phones causing problems with the memory on computerised radio gear. Some reports allege that the 'phones "wiped" the memory set-up for their model. The consequences of this can be imagined. The 'phones need to be within an inch or two for this to (possibly) occur so the sensible thing to do is to keep mobile 'phones well away from any transmitters, preferably in the vehicle you arrived in. Don't hang them on your belt otherwise you could interfere with your own transmitter! If this is confirmed, it makes it absolutely essential that you always check radio operation for correct operation and direction of servo travel every flight - something you should be doing anyway! Just remember, it could be the failsafe that gets screwed up. This problem is not unique to model aircraft - car manufacturers, starting with Vauxhall, are warning of the dangers that mobiles may cause in motor vehicles by interfering with the engine management systems
Finally, it's up to you what efforts and expenses you are prepared to put into aeromodelling and the safety levels that you hope to achieve. Like everything in life, you have to try and achieve a balance and assess the risks involved. Guaranteed 100% air safety comes when nothing flies. Asssuming that this is as unacceptable to you as it is to us, then we all have to have confidence in the decisions that we collectively take about what the acceptable risks are and how we will manage them. Safety is a constantly moving target and how well we constructively respond to the changing nature of our hobby and the technologies involved is how we will be judged fit to run our hobby. "That's the way it's always been done!" is a complete non-starter.