(a) The aerial on your transmitter is an integral part of the set that is certified/tested by the manufacturer in order to qualify for the CE mark. If you are replacing a telescopic aerial on a 35 MHz set you should try to obtain the manufacturer’s spare part. If you can’t do this then the aerial you fit should be of the same specification (length, screw fixing etc) as the original.
If you wish to fit a base loaded or ‘rubber duck’ aerial, you should be aware that you may only use one of these aerials if the manufacturer has cleared your particular transmitter for such an aerial. If this is the case, then you should be able to buy the manufacturer’s authorised spare.
If your particular 35 MHz transmitter is cleared to use such an aerial by the manufacturer but you can’t get the original manufacturer’s spare then any replacement aerial you buy must meet the same specification as the manufacturer’s item. Note that, with this type of aerial, the specifications are more complex than simply matching the length.
You should be aware that fitting an aerial that does not meet the transmitter manufacturer’s specifications will result in you being considered to have introduced into use a new variant of the transmitter which will not be covered by the manufacturer’s testing/certification and CE mark.
If you wish to fit an aftermarket aerial you should first contact the manufacturer/importer of your transmitter for further information. You might also find information on the Ofcom website at www.ofcom.org.uk
(b) A dirty or oily telescopic transmitter aerial will degrade the range of your transmitter, sometimes quite severely, and may even affect the output frequency. Clean it every two or three months with methylated spirit or similar and never lubricate it.
(c) Take care to route your receiver aerial well away from any carbon fibre in your aircraft. Carbon fibre is electrically conductive and is a good aerial itself. Large quantities of it can blanket your receiver aerial completely and even a few strands used for strength can cause glitching in flight if they are close to the aerial and can affect the signal reaching it.
It has also been reported that some metallic covering films and certainly some metal clad airframes have also been seen to suffer from degraded range and glitching and the siting of receiver aerials in these types of model can be quite critical.
(d) A point that is often overlooked, even by experienced flyers, is that the placement of 2.4 GHz receiver aerials is much more critical than for 35 MHz equipment. You must read the manufacturer’s installation instructions very carefully and take note of the information they give you. If you don’t do this you may find yourself flying with equipment that is low on airborne range simply because the aerial configuration you have set up is inefficient.
(a) Dry cell batteries do have their uses in some transmitters but care should be taken to monitor pack voltage at all times.
(b) The use of dry cell batteries in airborne battery packs is strongly discouraged and they must never be used in the airborne pack if you have four or more servos operating.
(c) Subject to the advice given below, It is recommended that you only use rechargeable batteries in your radio control equipment. However, when fitting Nickel Cadmium (Ni-Cd) or Nickel Metal Hydride (Ni-Mh) rechargeable batteries to equipment designed and sold to take dry batteries, always ensure that the cells are soldered or welded into packs and that the packs are either hard wired or wired through a plug and socket into your transmitter and receiver systems. Do not rely on the spring type battery contacts in battery boxes.
(d) There are, however, exceptions to this advice. Some modern transmitters have very low current drain and are supplied as dry battery sets with battery boxes that are not removable. In these cases dry cells give an acceptably long operational life and may be used safely.
If you do use individual re-chargeable cells in these transmitters, make sure that the cells are removed at least monthly. While the cells are out of the transmitter, carefully clean the spring battery contacts and the ends of the cells before replacing them. You should also carry out this procedure if the transmitter has been standing idle for any length of time.
If you don’t take these precautions, your transmitter might suffer from the same symptom as many TV remotes when they stop working until you have disturbed the batteries.
(e) Lithium Polymer batteries (Li-Po) are being used increasingly in radio control transmitters and many flyers are retro-fitting Li-Pos in place of Ni-Cd or Ni-Mh battery packs. If you are considering this, it is essential that you contact your Tx manufacturer / importer for information on whether this is allowable in your transmitter. This is because there are significant issues with voltage differences between the different types of battery pack and the ability of any specific transmitter to cope with them. It is safer to consider using Lithium-Fe cells as their voltage is lower than Li-Po cells and equate more closely to standard transmitter battery packs.
If you are not given clearance to make this change but you still go ahead then you will run the risk of damaging your Tx and, in addition, any warranty you have will be invalid, you may not be able to have the equipment serviced and the CE mark on the transmitter will also be invalid. The legal responsibility that you then take on yourself is considerable and must not be underrated.
(f) The regular use of a battery checker for receiver batteries is essential and there are many cheap reliable units available, either hand held or on-board, to cover most battery types so no matter what type of cells you are using you can buy a checker to suit. Ensuring that the receiver battery has sufficient capacity to support the flight is essential if the pilot is to comply with the ANO and having the peace of mind in knowing that the last flight of the day will not be the last flight of the model is well worth having.
15.4 Nickel Cadmium (Ni-Cd) Batteries
(a) Ni-Cd cells will self discharge at a rate of around 20% of their capacity each month and if a stored pack discharges below approximately 1 volt per cell, there is a danger that one of the cells in the pack may be irreversibly damaged. The lower the voltage reached the more risk there is that this will happen. It is therefore recommended that all Ni-Cd packs be charged regularly, at least every few months, and that any pack not in regular use be initially stored fully charged.
(b) Ni-Cd cells are very resilient when trickle charged at around 1/10C (i.e. 50mA for a 500mA h battery). Most chargers supplied with radio equipment are designed to work in this range and there is little risk involved if packs are inadvertently left on charge when using them. Even if you regularly fast charge your cells, it is good practice to trickle charge them occasionally.
(c) Overcharging Ni-Cds at high currents (fast charging) can ruin your cells and has been known to cause battery packs to explode violently. Most fast chargers have a ‘delta peak’ voltage controlled cut-off and are generally very reliable. If you don’t have such a charger and wish to fast charge your cells then, as a minimum, you should use a charger with a timer or temperature controlled cut-off.
(d) If you have a charger capable of both discharging and charging your battery packs then you should fairly regularly cycle the packs as this will help to keep them in optimum condition. However, it is also good practice to occasionally trickle charge any packs that are regularly fast charged whether they have been cycled or not. Just make sure that the pack has been well used or discharged before you start (no lower than 1 volt per cell though).
15.5 Nickel Metal Hydride (Ni-Mh) Batteries
(a) Ni-Mh cells can self discharge at a rate of up to 40% of their capacity each month and the danger of a stored pack discharging below 1 volt per cell and possibly causing irreversible cell damage is therefore considerably greater than with Ni-Cd cells simply because it will occur sooner. It is therefore recommended that all Ni-Mh packs be charged more regularly than Ni-Cds, at least every two or three months, and that any pack not in regular use be initially stored fully charged.
(b) Ni-Mh cells may be trickle charged at around 1/10C (i.e. 50mA for a 500mA h battery) and most chargers supplied with radio equipment are designed to work in this range.
However, Ni-Mh cells are more fragile than Ni-Cds and are susceptible to damage by overcharging even at normal trickle charge rates and should never be left connected to the charger longer than is necessary. The ‘safe’ constant trickle charge rate is very much less that that provided by the standard type of charger supplied with most radio equipment so the possibility of overcharge damage when using these trickle chargers must always be borne in mind.
(c) Ni-Mh packs can be charged at high currents (fast charging) but overcharging can quickly ruin the cells. Most fast chargers have a ‘delta peak’ voltage controlled cut-off and are generally very reliable but you must ensure that the one you are using is specifically designed for Ni-Mh batteries.
(d) Ni-Mh packs may be cycled, as with Ni-Cds, and you should consider doing this fairly regularly. However, it is also good practice to occasionally trickle charge any packs that are regularly fast charged whether they have been cycled or nor. Just make sure that the pack has been well used or discharged before you start (no lower than 1 volt per cell though) and note the advice in (b) above.
(e) A noticeable feature of Ni-Mh technology has been the increasing capacity of the cells for any given cell size. For instance, the early AA pencells were rated at around 700 mA h but you now see capacities of around 2000 mA h for the same cell size.
The only way this extra capacity can be achieved is by increasing the surface area of the active components within the cell and, for a given size of casing, this can only be done by making these components thinner. The problems that this will give you are increased internal resistance (the cell won’t give it’s energy up as easily and may get hot) and increased fragility of the cell. Thinner materials can be damaged more easily, both electrically when charging or discharging and mechanically, for instance, due to overheating when soldering or being over-stressed in a crash.
These problems may not be apparent in your transmitter pack but you should think carefully about using very high capacity Ni-Mh cells in airborne packs where the demand on the batteries will fluctuate and can be much higher than in a transmitter. You can easily get into a situation where a high capacity pack is unable to supply the voltage required by some hard working servos simply because the internal resistance of the cells will not let the energy stored in them be released quickly enough.
15.6 Low Self Discharge (LSD) Ni-Mh Batteries
Originally developed by Sanyo under the trade name ‘Eneloop’. This type of cell is now produced by several other manufacturers.
These cells have such a low self discharge rate that you can treat them very much as you would a Li-Po and charge them when you come in from flying rather than the day before you go out.
They are robust and can be charged with a standard Ni-Mh battery charger. They are a little more expensive than standard Ni-Mh cells but their very slightly higher operating voltage gives good energy storage levels and the claimed number of possible charge cycles is greater than the standard cells. The technology is certainly worth considering as an alternative and very useable battery, especially in Transmitter applications or in airborne packs that cannot be readily removed from the airframe for charging.
15.7 Lithium-Polymer (Li-Po) Batteries
Li-Po batteries are now used by a very significant number of model flyers and they must be treated differently to the more conventional rechargeable batteries.
For full details on safety and use of Li-Po batteries please see the BMFA booklet – Battery Safety Booklet 2014 which is available from the Leicester office or for download from the BMFA website.
15.8 Battery Isolator Switches
One of the most dangerous points in the flight preparation of electric models is when the flight battery is plugged into the model. A freshly charged battery has a lot of power locked up in it and many models are very awkward when it comes to connecting the battery pack, especially as you usually need both hands to do the job.
Consequently, if the pilot fails to set the throttle to the correct setting or the onboard electronics in the ESC fail, it’s very easy to have a propeller or rotor come to life when you least expect it to, with possible serious consequences.
Under no circumstances should an isolating switch be placed between the ESC and the Battery unless it has been designed specifically for that usage. Current flow from even a 3S LiPo pack can reach 60 amps. With some models, it is difficult to connect the battery to the ESC while keeping your arms outside the propeller arc. In such cases, an external arming plug is recommended typically of the type and rating that is used to connect the battery to the ESC. At least one manufacturer is offering a battery isolator switches covering 100 A and 200 A but these are in excess of £100 currently. The use of a spark arrestor to eliminate the crack when you first connect a battery to the ESC is good practice. Spark arrestors can be made by the modeller or else bought commercially.
15.9 BECs / UBECs / Receiver Batteries
A large majority of Electronic Speed Controllers (ESC) have a built in battery eliminator circuit (BEC) and the use of the BEC to run the airborne radio package of electric models is very popular.
However, there are factors that you should bear in mind when using or considering the use of the BEC.
All BECs are limited in the amount of current they can supply. The cheaper BECs can usually supply current that is adequate for most sport models with three or four servos but if you are using more servos than this or are using digital, large or special servos, you should check the specifications of the BEC you are using to see if the current it can supply is adequate.
Remember that digital servos may require more current supply than you might expect and, no matter what type of servo you use, any binding or stalled servos or high aerodynamic loads will also pull significant current. Helicopters can be particularly demanding.
If you have any concerns, there are two ways to improve the situation and give your airborne system the ability to supply the current that the receiver and servos require.
(a) Fit a Universal BEC (UBEC). This is a stand-alone BEC unit that is not reliant on the ESC circuitry. These units are usually quite cheap and you can check the current capabilities of the units before you buy.
(b) Fit a separate receiver battery of an appropriate capacity.
Both of these solutions are valid but you should think carefully about the model and flight requirements before making your choice.
For instance, if you have a model that requires nose weight, it would make sense to fit a separate receiver battery and use this as part of the weight required. An electric powered glider might also be a good candidate for a separate battery as you may reach a situation where you have exhausted the propulsion battery but may still have significant flight time to come, especially if you are thermalling.
There is one other point that you must bear in mind and that is the ESC will have limits to the voltage (number of cells) and to the out-put current in amps. The BEC output will be specified in amps at the standard voltage (4.8 to 5.2 volts) but the BEC has to handle the total voltage of the supply pack (e.g. 12 volts for a 3S Li-Po). The higher this voltage the greater is the power dissipated in heat which might require a reduction in the output current demanded of the BEC to avoid overheating and possible damage and failure. It may be that, in these circumstances, the BEC will not be able to safely supply the current needed by the airborne RC pack. If this is the case then a separate receiver battery will be essential. The ESC manufacturer’s documentation should indicate the BEC current limits at given main pack voltages.
15.10 Black Wire Corrosion
(a) Systems fitted with rechargeable batteries, particularly the older Ni-Cd batteries, can suffer from black wire corrosion. When this happens the surface of the copper strands in the core of the negative (black) wire in a circuit receive a coating of black material which works inwards until all of the copper in the wire has corroded. This corrosion has a high electrical resistance so as it gets deeper into the wire it lets less current through until eventually your radio stops working.
(b) The wires which are most affected by this corrosion are the negative wires from the battery to the switch in both transmitter and receiver wiring but in severe cases the corrosion can go much further than this and in extreme cases has even been seen in servo leads.
(c) The causes of the corrosion are very complex but it seems worse on batteries in storage, particularly in a damp atmosphere, or which have been allowed to go flat. Well used and maintained batteries certainly suffer much less but they are not immune to the problem.
(d) Unfortunately, there is only one practical way to find out if your wiring is suffering from black wire corrosion and that is a visual inspection of the core of the wire. If you are competent to do this, inspect the wire leading from the negative terminal of the battery. Stripping back a very short length of outer will show if you have the problem.
(e) There is no cure for black wire corrosion other than removing the affected wire and replacing it with new.
(f) If you are unsure of any of this advice, it will be well worth sending your rechargeable batteries and switch harness back with your radio equipment when you have it serviced with a specific request for black wire corrosion checking. Several companies specialise in supplying batteries and they might also be able to help. Another source of advice could be your local model shop but failing all this you should ask an experienced modeller for assistance.
(a) It is essential that you use the correct specification crystals in any non-2.4 GHz transmitter or receiver you are using. Not all crystals are the same and you should NEVER use one manufacturer’s crystal in another’s Tx or Rx. The only exceptions are many of the aftermarket receivers and their manufacturers actually specify which crystals are compatible.
(b) When buying crystals, always take care to specify in which individual piece of equipment they are to be used. Original manufacturer’s crystals are always the best choice.
(c) Receiver crystals are a fragile point in any airborne R/C system and they are susceptible to crash damage. If you have any concerns about your Rx crystal after an incident then you should replace it with a new one. This could be a very good investment if you consider the implications of crystal failing in the air a few flights later.
(a) CAP 658 says:
For All Model Aircraft
Any powered model aircraft fitted with a receiver capable of operating in failsafe mode (i.e. PCM receivers, Digital Signal Processing (DSP) receivers or 2.4 GHz equipment) must have the failsafe set, as a minimum, to reduce the engine(s) speed to idle on loss or corruption of signal.
This means that you will have to carefully consider what type of receiver you are using in ANY i/c or electric powered model, even the smallest.
For Models Weighing 7 to 20 kg
A serviceable ‘fail-safe’ mechanism should be incorporated to operate on loss of signal or detection of an interfering signal. For example on a power driven model this should operate, as a minimum, to reduce the engine(s) speed to idle.
For All Gas Turbines
All gas turbine models should be fitted with a failsafe. This must bring the engine to idle in the event of radio interference or failure. The fuel system must be capable of manual shut off via a fuel valve or fuel pump switch.
(b) All PCM sets, most DSP 35 MHz receivers and all 2.4 GHz equipment have settable failsafe modes and if you are using any one of these then you must take care to set the failsafe to at least engine idle.
For over 7 kg, you must ONLY use failsafe settable equipment and, again, set to engine idle as a minimum.
(c) As a reminder, nearly all PCM and DSP receivers and all 2.4 GHz equipment defaults to ‘hold last position’ out of the box so if you don’t set the failsafe, then that’s what it will do. This means that, for even the smallest model, interference or loss of signal will mean throttle and control lock-on and a potential flyaway or high throttle, high energy impact. If ever you re-bind a model please remember to recheck the failsafe as some sets may revert to default settings under these circustances.
(d) Users of any failsafe capable radio equipment (PCM, DSP or 2.4 GHz) should check fail-safe operation before each flying session. With the model restrained, switch off the transmitter and ensure that all relevant controls on the model move to their pre-set fail-safe positions. Switch the transmitter on again and make sure that normal control operation returns within a few seconds. If the fail-safe does not re-set quickly there may be a fault so DO NOT FLY. Also remember that if the failsafe is set to retract the undercarriage the model will need supporting off the ground.
To be safe, You must take the positive step of specifying what your failsafe should do instead of leaving it set at default. Read your radio manual carefully for details of settings.
If you don’t initially understand the instructions for setting the failsafe on your equipment then you MUST take steps to find out how to do it. This is one thing you cannot ignore and ignorance of the procedure is not an excuse that can be accepted.
Note: If you have PPM equipment and don’t have a DSP receiver but are using an add-on failsafe, it too should be set as a minimum to low throttle.
Glider Failsafes for Models Weighing 7 to 20 kg
The requirement in CAP 658 to use and set failsafes applies to these silent flight models too, although obviously the ‘setting of throttle’ does not apply. You should remember that the reason that the CAA requires failsafes is to prevent flyaways, not to deliberately crash the model, and you should set the controls of your model with this in mind. Application of spoilers, ‘crow’ brakes or even rudder and elevator to spin the model might be appropriate.
Electric Model Failsafes
The setting of the failsafe to, as a minimum, reduce the engine(s) speed to idle, obviously applies to all electric models too. However, given the ability to re-start the motor(s) at will, it makes sense to have the failsafe cut the motor(s) completely. This will give you the desired ‘minimum power’ situation and will avoid you having to decide on what idle speed you might need to set.
The development of modern electronics means that it is now possible to fit model aircraft with what are known as ‘Intelligent Failsafes’. These are particularly applicable to multi-rotor aircraft and full details are given in the Multi-rotor section of General Model Safety later in this Handbook.
15.13 Frequency Identification
Users of 2.4 GHz will not have or need any method of frequency identification but for users of 35 MHz there will be many occasions when others might need to quickly identify the frequency you are operating on and your transmitter should carry an easily visible channel identification pennant;
15.14 Mix-and-Match Tx and Rx
Using different makes of transmitter and receiver is common practice when using 35 MHz equipment, especially with the large range of aftermarket receivers available. There is a point you must be aware of, however, concerning manufacturers guarantees. A matched Tx and Rx will be warranted by their manufacturer both as individual items and to work together as a pair. If you ‘mix and match’, the individual warranties still apply but you have no guarantee that the pair will work together. In this case you take upon yourself the legal responsibility of making sure that your equipment operates correctly.
15.15 Mobile Phones
Although mobile telephones operate on frequencies far removed from our model control frequency bands they are a major addition to the increasing background radio ‘noise’ that our equipment has to filter out. In addition, there is some evidence that there may sometimes be an interaction between mobile ‘phones and microprocessor controlled transmitters.
Many mobile ‘phones transmit powerful signals regularly even when on standby and BMFA recommends that they are not taken into the pits area and especially not on to the flying area. Many ‘phones also emit a powerful signal pulse when switching off, which is also something you may have to consider. Your radio equipment has a hard job to do filtering out background RF radiation and you could make it much worse with your mobile ‘phone.
15.16 Module Equipped Transmitters
Plug-in transmitter modules sometimes suffer from corrosion of the connecting pins, especially if the transmitter has been operated in a damp or humid atmosphere. Unplug it regularly and check for dirty connections. Carefully clean the pins with methylated spirit or similar (check that the solvent doesn’t affect the plastic before you use it).
Broken fixing lugs on the plug-in module is another problem that may affect a module equipped transmitter. Never rely only on the connector pins to hold the module in. Modules in this state have been known to fall out of the transmitter without warning, sometimes with a model in flight.
There have been several cases of transmitter neckstrap users accidentally knocking the throttle stick open when getting ready to fly. This can have very serious consequences so take great care with your pre-flight preparations if you use a neckstrap.
The use of radio control equipment by heart pacemaker users has been investigated but no direct interaction problem has been identified. If you are a pacemaker user, however, and you require more information you are strongly recommended to speak to the Consultant who fitted your pacemaker. He should have all the technical specifications of the particular unit you use and should be able to identify any problems you may have.
Do not use standard inexpensive servos in any situation where flight loads are likely to be very high, i.e. virtually any flight control on a large or fast model. Standard servos have many uses and are usually very reliable and good value but they simply do not have the torque, precision and power of a servo designed to cope with very high loads. There is an enormous range of servos available so think about what you expect of the servo and choose carefully. If your model is large or likely to be fast then don’t automatically fit the cheapest you can get or those that simply come to hand in your workshop.
Many modern models feature long servo and battery leads and the trend towards separate aileron servos in each wing means that even quite small models might have extended servo leads fitted.
If you are using 35 MHz equipment, these long leads can make excellent aerials, feeding signals back into the receiver and possibly causing interference. Any extended lead should be de-coupled either by a using a commercial opto-electronic de-coupler or by looping the lead several times through a small ferrite ring which may be obtained from your local model shop.
It should be noted that this should not be a problem with 2.4 GHz radios.
High power, high torque and digital servos may have a high power drain and you should carefully consider the capabilities of the batteries you use with them. Multiple battery systems may be required in some cases. This is especially so if you expect your servos to work hard in your model. The more work you expect them to do, the more current they will take to do it.
15.20 Switches and Wiring
The standard airborne wiring harness and switch sets supplied with most new radio equipment, and also many of those available as aftermarket spares, are usually rated at approximately 3 amps. You can recognise this quite easily as the three core flat cable and plugs used are similar or identical to normal servo connector leads.
When multiple, digital or high torque, servo installations are used, the 3 amp limit can very easily be exceeded, sometimes by a large margin. So if you are using a demanding servo setup (and, for instance, most 3D capable fixed wing and Heli models or larger or faster models will be) then you should think very carefully about using a higher rated switch and wiring harness.
Many modern radios have the ability to downlink telemetry data from your model to suitable ground receivers such as laptops, tablets or smart-phones as well as your transmitter. If you are using this facility it is essential that you have a helper to monitor the data and not to do it yourself unless it is auditory only. You are obliged by the ANO to remain in visual contact with your model at all times.
If you use a smart-phone as the receiver you should ensure that it is switched to ‘flight mode’ as this will enable you to take it on the flightline with no risk that it will operate as a mobile phone whilst you are flying.
15.22 Transmitter and Receiver Issues
With new or repaired radio control equipment, a ground range check is essential, preferably in a model and with the model’s engine running if possible. Check the manufacturer’s literature or website for guidance on your transmitter or, if this is not possible, look for a minimum range of between 30 and 50 metres with the transmitter aerial down and no servo jittering.
2.4 GHz equipment usually has a ‘range check’ button that enables a ground range check to be done, even though the aerial cannot be altered. It is recommended that you make use of this facility regularly so that you can monitor the performance of your radio.
It is good practice to carry out a routine range check on your equipment at regular intervals, at least every month or so, and a check is advisable if you have not flown for a few weeks. You should also be prepared to do a range check if you feel that you have a problem with your radio equipment or if you have removed and replaced crystals or a transmitter module. If the model has a spark ignition or electric motor then the range check should always be carried out with the engine running.
If you use aftermarket 35 MHz receivers be aware that many are designed for indoor use, especially the very lightweight models. The range and ability to filter out interference of such receivers may not be suitable for outdoor use and you should take care that you are aware of the limitations of the equipment you are using.
When selecting which receiver to buy and use you should consider carefully where you will be flying and remember that to a great extent you get what you pay for. Single conversion receivers are usually the cheapest and work well in most circumstances but the more expensive high specification or dual conversion receivers are generally more capable, especially with outside interference rejection. Small 2.4 GHz receivers often have a limited range and this is stated in the documentation.
If you are operating in a busy radio environment (such as at a busy club site or on a site known to be subject to outside interference) then you should seriously consider only using higher specification or dual conversion receivers or moving to 2.4 GHz equipment.
The radio spectrum gets busier by the day and your transmitter signal has to be filtered out by your receiver from every other signal out there. This situation will only get worse and there are already some sites where only high specification, dual conversion or 2.4 GHz equipment is safe to use.
With certain types of transmitter, when setting up mixers and servo interconnections on a model it has been shown that, in some circumstances, the trim button will work in the opposite direction to that expected. Take a few seconds in the workshop to ensure that every control and trim works exactly as it should.
Problems have been reported with the binding of 2.4 transmitters and receivers when the Tx is surrounded by metal, such as in an open car boot or an open transmitter case. Make sure that the Tx is in ‘clear air’, close to and in line of sight to the Rx when binding. But note that some 2.4 systems require a minimum separation (usually a metre or so) for binding to properly take place.
Next Section 16. RADIO CONTROL AND YOUR CLUB