PPM/PCM & MORE

Two questions on the forum prompted asking MacGregor JR for information. 

These are edited copies of the original questions and the reply we received.

Question 1

After much hunting and head scratching.  I have got myself a cracker of a plane to fly.
Question is I have always used dual conversion PPM receivers and up to now have been more than happy with them.
But now I have got quite a chunk of money invested in the new plane I 'was' going to fit a PCM receiver until one my our more experienced club mates asked me WHY??
As he quite rightly put it, I havn't had any problems with the PPM, at a competition it would probably be only me flying and if I did get any interference would I rather have a glitch or PCM lockout??
At my stage of competing (the very first rung of the ladder) my head is now telling me to stick with what I know is good for me.

Right or wrong?? ...or indifferent Very Happy

Question 2

The second question was about the need for dual batteries and switches.

 

22 11 2005

The following is the reply from MacGregor to the original questions, however I think the most important thing here is the telephone number in the last paragraph, if you have a problem or need some advice talk to the people who supply the equipment.  Although I have had no further contact from Ripmax (Futaba), I am sure they offer the same service and basically the same advice. Ashley.



Dear Mr Hoyland

Thank you for your letter of 9th November.

It is often the case that modellers see PCM receivers as some form of magical safety device for their expensive model. This is not the case. The responsible reason for using PCM should be to enable and to use the Failsafe feature to cut the engine and to set the controls so that the model descends as quickly as possible to prevent it flying away and causing possible unseen damage to persons, property, vehicles, vessels or other aircraft. In blunt terms, in a situation where extreme interference has overwhelmed the receiver, it is better for a model to crash with its engine cut and within sight of the pilot and any spectators rather than flying away and possibly causing an accident

In the case of a PPM receiver, during periods of extreme interference the model may crash or flyaway, with the engine still running. For this reason alone we would strongly recommend that all GBRCAA pilots use PCM receivers, with FAILSAFE enabled for the throttle servo and set so that the engine cuts.

The question of what to do with the remaining flight controls during Failsafe is always a tricky one. Some people say it is better for the controls to be set to bring the model down in a gentle spiral descent, while others may say it is better to just return the controls to neutral and let the model assume a long glide. This is all very well if the model spends most of its time flying straight and level, but an F3A model will rarely be in this position.

However, since most interference is of a very transient nature (severe interference is almost always caused by another transmitter on the same frequency), it may be best to set the flight controls to HOLD their last good position so that the model does not veer off track too much. Chances are that any transient interference will be over before you (or the judges!) even know it! But what about the engine? The temptation here may be for an F3A pilot to set his throttle to HOLD, or to go to idle under FAILSAFE, so that they can carry on flying their round without the engine cutting. Well the HOLD option is not acceptable due to the very small, but possible risk of a flyaway. Setting the engine to idle may be tolerable during competitions, but for sport flying or practising (where frequency control may not be so good as at a competition) then the best setting has to be a full engine cut. After all, modern prop blades, even at tick over, are quite capable of inflicting severe injury or damage if they come into contact with any thing or any person. No doubt the GBRCAA will have its own recommendations on this matter.

Other benefits from using PCM receivers include slightly longer range and the ability to extract very high levels of resolution from high quality servos, resulting in very fine controls, if needed. F3A pilots will no doubt be using digital servos in their models and it should be remembered that these were developed expressly for use with PCM receivers In some cases, use of a digital servo with a PPM receiver may even cause internally generated glitching! So if you intend to use a PPM receiver you should stick to using non-digital servos with standard or coreless motors.

JR PCM receivers are also a lot more affordable now. You can buy a seven-channel unit from JR for just £70.00, while the top level 10-channel dual conversion PLL synthesised unit can be had for a reasonable £125.00.

However the overwhelming case for using a PCM in an F3A model has to be one of safety, with the emphasis on safety of persons and property rather than being a safety device for the model itself.

No doubt you would forgive me if I saw this as an opportunity to start a long sales spiel about all the different PCM receivers that JR have to offer, and how wonderful they are compared to anything else offer. But I will refrain from this as it is the safety message that is most important here. All I will say is that as most F3A pilots have already discovered, the use of any JR or Futaba PCM receiver, when used with a matching computer transmitter from the same company, should ensure almost trouble free flying – assuming you use good quality batteries and wiring. Which neatly leads me on to your second point…

Our attitude to batteries is quite simple. If JR don't make it, we don't recommend it! At this time all genuine JR batteries are Ni-Cads. In our experience JR packs are very reliable and can be fast charged at the field if necessary (a lC charge will top up a battery between rounds). So there really is no need to use any other system. Nearly all JR servos are rated at 4.8V for use with four cell packs. Use of 6V packs is not recommended There are a wide range of servos available, so choose ones that give the correct specifications you require at 4.8V, rather than trying to boost a lower rated type by applying excess voltage to it.

We would recommend that you fit the largest capacity JR Ni-Cad pack you can pack into your model (currently 1900 mAh). Buy yourself a fast charger (ideally one where you can see the capacity that has been replaced) and top up the battery after every three or four flights (less if lots of digital servos are used!). Use a decent battery monitor and check it between all flights to ensure that your battery is sufficient for another flight. Use only genuine JR heavy duty switch harnesses and genuine JR extension leads. Poor wiring is a common cause of radio problems. Decent chargers have a discharge function - use it two or three times a year to cycle your batteries. This will help keep them in tip-top condition.

If you wish to build in some form of redundancy into your radio wiring then use a JR Multibox. This small unit allows up to four servos to be run from one receiver channel (ideal for multiple servo installations operating one control surface). The Multibox allows each servo to be adjusted so that they all have identical performance. What's more, it allows these servos to be run from an independent battery if required.

Finally, do remember to replace batteries and switches from time to time as these items have a finite life and even the best quality ones will fail eventually if you don't do some preventative maintenance.

I hope that this is of some use to your members. If any GBRCAA member needs further advice on the use of JR radio equipment, please don't hesitate to contact our Customer Service Department on 01753549111.
Best regards,
Kevin Crozier
Director, R/C Division

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