As model sizes have increased, so to have the number and size of servos resulting in additional loads on the battery and wiring systems. From a standard analog servo providing in the order of 2.5Kgcm torque, we are now using digital servos well in excess of 8Kgcm and also ganging them together to drive the control surfaces for very large sailplanes.

The seemingly never ending growth in model sizes raises some significant additional issues:

• As the power requirements of the servos increase, the batteries are required to provide much higher bursts of current whilst maintaining sufficient voltage to ensure continuous and reliable operation of the receiver/s. All types of digital receivers (PCM, 2.4Ghz, IPD etc) have a much greater dependency on the minimum recommended supply voltage that was generally the case with analog receivers therefore this issue must be appropriately managed. 

• If all servos and receiver/s are connected to a common power supply the failure of one device on the system has the potential to drag down the battery voltage to a point where the total system fails. This could simply be a case of a large combined load causing a significant voltage drop across an under rated switch or connector or in the worse case, a short circuit causing total loss of power to the receiver. 

When considering how to manage the above issues scale glider pilots should look to the following power supply practices often employed by their colleagues operating turbine and giant scale power models.

Dual Batteries.

This is not a new idea however the use of dual batteries has been made easier for the average R/C installation through the development of dual power capable receivers. The internal circuitry used to manage two battery packs may range from simple diode isolation to a more sophisticated approach using dedicated DC power management techniques. All this means that the user needs to understand the individual receiver capability before making a judgment on its suitability for a particular installation.

A few of the latest twin battery receivers have had their internal capabilities increased to support large servo power requirements.  Whilst  advertised current ratings of 3-5amps may sound impressive this may still be inadequate when many high power servos are installed.

In larger sailplanes we agree that twin battery redundancy is good practice however appropriate isolation is also required to prevent single component failure pulling down the total power supply. If this is not done all you are really doing is making one large battery with only a marginal improvement in system reliability.

Servo isolation

OK, so you use a dual battery system with a battery isolation circuit. What happens when one of your servos shorts out while you are flying… well you crash of course!  Without the ability to isolate each servo and the receiver/s from the power source, a short circuit  (yes – it has happened to us, and fortunately the tug was on the ground when we saw the smoke) will draw very high current from the power bus. This high current can potentially melt the power wires, connectors and switch or cause an on board fire – in any event the likely outcome will be disastrous.

So, what is the solution? Fortunately there are a range of commercially available products designed to manage dual batteries and servo/receiver power isolation. Options include the PowerBox range of products which advertise many features, but at a significant price, and a more cost effective solution provided by the Smart Fly power distribution systems. The PowerBox range of products seems to be targeting the “Jet Set” whereas the SmartFly range has solutions suitable for the large scale and sport flyer.

The SmartFly units that we have tested over the past six months have proven to be simple to install, robust in construction, and very reliable. Even the sophisticated PowerExpander EQ10 unit, which provides extra features for adjusting and matching servos, is relatively easy to set up using the provided instructions.  I used the servo matching feature to ensure perfect matched travel alignment of the twin digital servos on the DG303 elevator.

Overkill or under resourced.

There have been a few individuals who view our installations as overkill! However, we have seen a few models that have been destroyed due to a loss of power to the receiver or servos. These expensive models may well have been flying today had they been fitted with a quality dual power distribution system.

Some fliers are tempted to go the “home made” solution and there is certainly a lot of often conflicting advice available on the net. I have previously been guilty of this approach however the problem with these solutions is that each and every installation becomes a prototype with all of the problems associated with that first flight becomes not just a test of the airframe, but also the home brew system. Using a well tested commercial solution means that there should be one less item that can go wrong through an unintentional design or construction error.

At approximately 10% of the cost of the average large scale airframe, the SmartFly solution is a cost effective way of ensuring that battery or power system failure is minimized as much as is possible.

DG303, Cirrus and a Tug as examples:

Over the past months, we have tested three different SmartFly solutions in three different Setups.

Our Hots Tug

The photos below show the PowerExpander Sport Plus in use on our modified Hots tow plane (2.5meter). The sport plus comes with the standard SmartFly features, but includes a power regulator for the receiver and an optical isolated ignition switch. This means we can use large capacity 2s lipos for power and should for whatever reason a cronic power failure occur, the optical isolated ingiton switch will cut the motor, rather than leaving it in the last known state. Optical isolation also reduces the chance of electrical "noise" from the motor interfereing with the radio gear.

The Paritech DG303

The requirement for the DG303 (5 meter) was a little different where the requirement was to provide matched y leaded servo to some surfaces and also to allow fully redundant and isolated power to the receiver and separate power stream to the servos. It was understood that with an 8 servo wing, the power loads would be quite significant. The us of the PowerExpander EQ10 also offered an elegant solution to the issues of only having 9 channels available out of the JR/Spektrum solutions at the time. The filtered and regulated power to the receiver also overcomes any low voltage issues of the JR/Spektrum receivers. All of the SmartFly units are also totally compatible with the latest Futaba 2.4 offerings.

The Basic Cirrus

Now, this sailplane is not complex nor demanding. At 5 meters and with its "old-style" design, it is best described as the "Cadillac" of my fleet. But the use of the PowerExpander Pro allows for peace of mind with regard to all of the standard features provided by these units. One of the optional items we have used on all of the installations is the Failsafe - switch. This utilises the on-board electronic switching that if the mechanical switch was to fail at any point, will always fail  "closed" (switched on). Normal mechanical switches (in the case of twin batteries you have to use 2), when they fail, will fail "open" (switched off). Just another point of failure eliminated.

Good Products still mean good practice.

One of the problems we see in the field appears to be the replacement of good maintenance practices with new technology. A typical example of this is the apparent reduction in charging and cycling of batteries due to the large increase in capacity now available.  Regardless of the technology used, there is still no replacement for good practice. No matter what systems you are using, the maintenance of your airframe, batteries and radio gear are as important today as they were in the days of home made gear. 

Actually, the larger the models we fly, the more important is our duty as the pilot/builder to ensure these models as safe and reliable as possible.