RADIO INSTALLATION

By Stephen Carr

RADIO MAKES
The types of radio equipment used by the team are split between Futaba and JR. The systems in use are the Futaba Field Force 7 & 8, the JR MC16/20, and the Futaba FC-18 & FC-28. The MC 16/20 and FC-18 & 28 are ideally suited to scale flying, as many of the auxiliary functions can be positioned at a comfortable position.

Everyone in the team uses Pulse Code Modulation (PCM). There have been many discussions amongst modellers and in model magazines about the benefits and drawbacks of PCM verses PPM. Many people believe that PPM ( an un-coded FM signal ) is better as it allows you to fly through interference. I think that this view comes from a lack of proper understanding of the PCM system. On the Futaba PCM system, the receiver can lose up to 50% of its signal and still operate correctly. It uses its internal memory to replace the missing data. This means that you can fly through interference without it affecting the model at all. It is only when you exceed 50% signal loss, that the radio goes failsafe. PPM on the other hand will react to any amount of interference. The model may well have buried itself long before you get to the 50% signal loss situation ( been there ). 

METAL TO METAL NOISE
One source of interference that both systems suffer from is metal to metal 'noise'. A PPM model may frequently glitch or glitch when at a certain angle to the transmitter.  PCM may briefly go failsafe at certain angles if the noise is serious enough. If it is only mild interference, the PCM is now at a handicap due to doing its job. Because the PCM set will make up the missing signal, you don't realize you have a problem. Now you are flying around with let's say 25% on board interference due to metal noise. The PCM is now only giving you 25% insurance against outside interference, so it doesn't take much to push it into failsafe. Try to eliminate as much metal to metal contact as possible. Use nylon ball links on throttle arms and electrically bond adjacent metal items with a wire soldered between the two.

OPTO-ISOLATORS
Extension leads are another source of model interference. Long leads act like aerials and can feed interference back into the receiver. S.M. Services ( Tel: +44 (0)1234 751095, or www.smservices.net ) produce a little black box known as an Opto-isolator which can handle 8 channels. The Opto-isolator goes between the receiver and the servos. It works by converting the signal pulse to the servo into a light source and then back again. This prevents interference from passing back to the receiver as there is no direct link. It requires the use of two Ni-Cads; one for the receiver and Opto-isolator (4.8v), and one for the servos ( possibly 6v ). 

The radio installation in Stephen's P-47N "Big Stud". The cockpit floor takes up much room which is why the rest of the installation is spread along the wing seat area.

Using dual Ni-Cads has two advantages. Firstly, any interference is not passed into the receiver via the power leads, and secondly, the Rx Ni-Cad can maintain a constant voltage to the receiver. Some makes of receiver are sensitive to voltage changes and can go failsafe if the voltage drops too low. Some brands have been known to enter failsafe due to a brief voltage drop and remain in that state indefinitely, until the power switch is cycled. If you have a large model with several large servos, during some situations you will be pulling high loads from the Ni-Cads and this may cause the receiver to reach its failsafe point. With an Opto-isolator, the servos have their own power source.

As mentioned earlier, a 6 volt supply can be used on most systems. On long extension leads as there can be a voltage drop due to resistance causing low power and incorrect operation of the servo. When using long leads, use a heavier duty cable than normal.

Rx BUDDY
What is an Rx Buddy? Another little black box from S.M. Services. By C.A.A. law, any model over 20 Kg in weight must be fitted with a failsafe, but also must be fitted with TWO receivers. The most common way to wire the receivers to the controls is to have one receiver operating one elevator, aileron, one engine ( for multi-engine ), and rudder, with the other receiver operating the remaining controls. In the event of a receiver failure, some control is maintained in order to land or crash the model in a safe location.

The Rx Buddy connects the two receivers to all of the controls. It has a built in opto-isolator and battery backer. It constantly monitors the main receiver and Ni-Cads, and should either fail, will automatically switch to the secondary receiver or Ni-Cads. This system maintains full control of the aircraft. The two receivers need not be on the same frequency. Should the main receiver go into failsafe due to interference, the secondary receiver is selected and an additional pilot with a reserve transmitter can take over flying the model. This system is used on the USAAF Team's B-17 and will be used on the C-47, A-26 & B-26.

SWITCHES & NI-CADS
So what else can be done to improve radio reliability? The majority of radio failures are due to switch and Ni-Cad failures. You've built your model costing hundreds or thousands of pounds, and it all relies on a switch worth pennies? All electro mechanical items have an expected mechanical life of operations before failure occurs. The switch on your model also has to contend with vibration, temperature extremes, moisture, oil and fuel. All of these factors will reduce its working life. Are you also still using the standard radio switch on your mega-sized model? How much current is it designed for and how much are you pulling through it? By using two switches, you reduce the chance of failure dramatically. If the chance of failure is 1:1000, then the chance of both failing together is 1:1000000. 

By using two switches, you now have two power leads to plug into your receiver, so reducing the chance of a faulty plug contact. If you have two switches, why not two Ni-Cads? When using two Ni-Cads, it is safer if they separated by diodes ( one way valve ). If one Ni-Cad should fail or short ( and it does happen ), you don't want the other to try to charge it.  The diodes can drop the voltage by 0.7v so a 5 cell 6 volt pack is used, but since it is sharing the load, it need only be half of the capacity of the single ( i.e. use 2 x 700mAH and not 1 x 1400mAH ).

AERIAL EXTENSIONS
There is one last step to improve security. Most receivers come with a standard aerial length, which is a fraction of the wavelength of the radio. It is a compromise due to the impracticality of long aerials on normal models. Once we get into large and giant models, the 1 metre aerial barely reaches outside of the fuselage. We now have an airframe large enough to extend it. By soldering a new longer wire on to the receiver, it becomes out of tune as it is designed to work with the 1 metre length. Instead we solder one leg of a 27pf capacitor to the end of the original aerial, and to the other leg we solder our extension of whatever length is suitable for the model.

Remember aerials tend to work better with a 90-degree bend somewhere, so I tend to run it along the bottom of the fuselage inside, then out of the rear and up the fin post, then forwards again to the scale aerial mast. The capacitor now deceives the receiver into thinking it still has the 1 meter aerial, while allowing the ac signal to pass through from the extension. This can significantly increase range, particularly useful on large models when the circuits flown are often much greater than with an ordinary model.

Since I started using the above information in my large models in 1998, I have not had one radio problem, despite often flying low down and far away during some manoeuvres.