Radio Installation


4th October

Note regarding the use of non-metal geared servos in aircraft powered by petrol engines
Taken from HITEC RCD

Note: Be aware that high vibration gas (not glow) engines combined with large control surfaces and large throws (3D models) can put undue strain on the gears. Metal geared servos are recommended for these applications even if under 12lbs.

28th April 2014

What is Blackwire Corrosion and how can it be avoided?

Black Wire Disease - or the dreaded Black Rot!

The problem relates as much to NiMH batteries as it does to Ni-Cads since both systems employ the same seals and electrolyte (KOH).

It's certainly true that Black Wire Corrosion (popularly referred to as Black Rot, or Black Wire Disease), was a topical subject that cropped up a great deal just a few years ago but, like exhortations to always carry out a range check before flying your model, hardly ever gets a mention these days. With the advent and rapid growth of new battery technologies, coupled with the resulting popularity of electric flight, Black Rot has taken a back seat to the more dramatic concerns of exploding lithium polymer batteries and the challenge of careful and controlled discharging and balancing them.

What is it?
So, for those of you who may not have come across this phenomenon and - like plenty of other model flyers - are still depending on Ni-MH or Ni-Cad batteries for your model power, just what is Black Wire Corrosion (BWC)?

BWC is the name given to corrosion of the copper negative lead of battery pack wiring which eventually corrodes the copper to a nasty black sludge. It is first evidenced by signs of green verdigris appearing around connectors but, like many a human disease, is not always immediately evident. It therefore needs to be checked for and corrective action taken before serious damage is done - the outcome of which is for you to eventually lose electrical power leading to loss of control and probably loss of your model!

What does it do?
BWD produces an electrical effect as if a resistance has been introduced into the circuit which both reduces the charge accepted by the battery and limits the current that the receiver can draw. This means that your radio may seem to be working satisfactorily when first switched on, but the battery will drain very quickly and if the current draw is increased to a fairly high level, such as would be the case when several servos are all demanding power at the same time, the resistance of the wire causes the battery voltage to drop to a level below that at which the receiver will work. After the resultant disaster, you are likely to attribute it to radio interference since the radio will appear to be working perfectly, no servos are jammed and nothing appears to be wrong with anything at all!

What causes it?
It is generally agreed that in the first place, BWD stems from storage of your radio gear with the battery installed in damp conditions - so it can apply to a transmitter just as much as a receiver. If a wired-up charged battery is stored in these conditions this gives rise to an electrolytic effect that gives rise to an acidic condition at the negative terminal of the battery. The negative lead then starts to corrode, running back from the battery pack towards the switch harness to the receiver. The copper wire loses its 'coppery' look and turns almost black as well as becoming brittle and inflexible. (Interesting question - is it referred to as black wire corrosion because the corrosion turns the wire blue/black or because it occurs in the negative wire which is normally the black one?) Those of us whose climate means we are inclined to 'lay up' our models for a season may store them in an unheated garage or garden shed and this, unfortunately, is a perfect environment for BWC to occur!

The black wire syndrome is an occurrance in battery packs (Ni-Cds) and (Ni-Mhs) where the negative wire becomes corroded (turns from shiny copper to blue-black). This is the result of either a shorted cell in the pack, the normal wearout failure mode of Ni-Cds, or cell reversal when a pack is left under load for an extended period. The sealing mechanism of a Ni-Cd cell depends to some degree on maintaining a potential across the seal interface. Once this potential goes to zero the cell undergoes what is called creep leakage. With other cells in a pack at some potential above zero the leakage (electrolyte) is "driven" along the negative lead. It can travel for some distance making the wire impossible to solder and at the same time greatly reducing its ability to carry current and even worse, makes the wire somewhat brittle. A switch left on in a plane or transmitter for several months can cause this creepage to go all the way to the switch itself, destroying the battery lead as well as the switch harness. There is no cure. The effected lead, connector, switch harness must be replaced.

This leakage creep takes time so periodic inspection of the packs, making sure that there are no shorted cells insures against the problem. The cells should also be inspected for any evidence of white powder (electrolyte mixed with carbondioxide in the air to form potassium carbonate). In humid conditions this can revert back to mobile electrolyte free to creep along the negative lead. Some "salting" as this white powder is referred to, does not necessarily mean that the cell has leaked. There may have been some slight amount of residual electrolyte left on the cell during the manufacturing process. This can be removed with simple household vinegar and then washed with water after which it is dried by applying a little warmth from your heat gun.

What can be done about it?
To avoid it happening in the first place the ideal action to take if your model is going to be laid up for any length of time is to remove the battery pack and store it in a warm and dry environment as you probably already do for your transmitter. (Some pundits suggest you should store the battery in a cool place, like the refrigerator - that's model flying for you!). Unfortunately, this solution is not a very practical one when you have your battery pack tucked away deep inside the model but if you can remove it, it would certainly be for the best. Bear this in mind at the building stage and see if you can make your battery more accessible for removal if you are likely to have to lay the model up for any period of time.

If you cannot take this preventative measure, then you need to check the wiring from time to time, looking for signs of the corrosion at the battery terminal. If it's there, then your only option is to strip out the wiring and replace it, including the switch harness. Again, not particularly easy if the battery is difficult to access, but it may save you a lot of grief, and maybe money, later.

Key Points:

  1. Check the switch and if possible the battery leads for a green coloured powder type substance on the negative leads. If either show any signs of green, remove and dispose.
  2. BWC can, if left unoticed, spread to the reciever and servos rendering the entire system useless.
  3. BWC can also appear in transmitter batteries and can spread to the circuit board.
  4. Applies to Nickel Cadmium (Nicad) and Nickel Metal Hydryde (Nimh) batteries.
20th March 2014

Setting and operating a failsafe

If a radio system is fitted with the option of a failsafe, it must be used.

Failsafes must be used on all models weighing over 7kg.

All 2.4 radio systems are fitted with a failsafe.

Depending on the type of receiver, 35mhz radios use failsafes.

You should consult your manufacturers manual.

Click on the link below to download the article

/public/Setting and operating a failsafe.doc

/public/Setting and operating a failsafe.pdf


Aerial positioning with 2.4 GHz

Recently there have been two or three instances of glitches on 2.4 GHz at the flying field. Perhaps we've got so used to being protected from interference by modern technology that we've forgotten, or tend to neglect, the basic rules that still apply. 

1.  Rx Aerial alignment 

2.  Keep the Rx aerials clear of other conductors

3.  What about the model itself ?

4.  Positive Rx aerial location

5.  Range testing

 Click on the link below to download the article

 /public/Aerial positioning.doc

/public/Aerial positioning.pdf