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Curing Stalling Servos
Smart-Fly power management to the rescue
Text, photos and video by Tom Hintz
Flight video by Clark Ponthier
Posted – 12-15-2015
From the beginning I had a nagging feeling that something was not right within my Aeroworks 30cc Freestyle Extra 260 QB-L. It always did tight, quick snap rolls on take-off and its mid speed range roll rate was very quick. However, at full throttle the roll rate slowed noticeably. Full throttle into a stiff wind results in an even slower roll rate. It was becoming obvious that the servos were stalling.
My Original System
I am using HiTec HS-5565MH servos with one on each aileron, one on each elevator half and another for the rudder. These servos feature coreless motors, dual ball bearings and can generate 153 oz./in. of torque on 6V and 194 oz./in. on 7.4V. That high voltage rate is actually a bit more than AeroWorks recommends for this plane. The most logical cause for these servos to be stalling is because they are not getting enough voltage to generate all of the torque they are capable of.
The original system used a pair of 2700mAh, 7.4V Lipo packs to supply power to the receiver then on to the servos and ignition. I had a battery isolation unit that monitored the batteries to isolate one if something goes wrong without taking the other pack with it. That was located between the individual switches controlling those packs and the receiver. A battery eliminator unit sent power from the receiver to the ignition (no ignition battery) and functioned as an ignition cutoff which is the main reason I installed it.
While this is a fairly common setup and it did work under low to moderate aerodynamic pressures it had several points where voltage drops occurred. That includes each switch, the battery monitoring system and the ignition battery eliminator. Add those voltage drops to the limited capacity of the receiver to pass a sufficient volume of power to the servos and they could be seeing well under the 7.4V originally entering the system from the batteries. As the voltage being fed to the servos drops so does the torque that they can deliver. This is almost certainly why my servos are stalling at speed under the highest aerodynamic pressures.
The Smart-Fly Cure
The new power system is based on the Smart-Fly PowerExpander Sport Plus with built in Ignition Cutoff. To eliminate as many of the voltage drop points as possible I am using Smart-Fly’s Pin and Flag Failsafe System Access Panel and the Ignition Access Panel. The Failsafe System Access Panel eliminates the voltage drops across the old switch contacts (two switches) and the plugs going into the switches. The Smart-Fly PowerExpander Sport Plus system allows me to use two plugs from each battery to double the current paths which roughly halves the voltage drop of one connection. The entire power system is armed by removing the one pin and disabled by inserting the pin again.
The Smart-Fly PowerExpander Sport Plus has the battery isolation circuitry built into it so that if one cell goes bad that pack is isolated so that failure has no impact on the other pack. This system draws from both packs equally when they are within about 0.050V so they are drawn down equally.
While most of the signal wires are just that, no power wires, the first and last wires deliver a filtered 5.0V power to the receiver so you have the dual battery protection. Power from the batteries comes in from both sides of the Smart-Fly PowerExpander Sport Plus and through the end signal wires to the receiver so if one battery goes bad the system can isolate it and you fly on power from the other side. Both of the receiver leads on the Smart-Fly PowerExpander Sport Plus board draw power from both batteries for physical redundancy.
There are also two plugs available on the Smart-Fly PowerExpander Sport Plus for each receiver channel. Think of these as a built-in Y-cord that provides additional versatility when programmable servos are used. More and more people are using HiTec servos (or similar) that can be programmed/matched with tools such as the Hitec HFP-25 Digital Servo Programmer & Tester That lets you run two servos off of one channel and do the matching (including reversing) with the programmer rather than in the radio.
This can be a huge benefit when you start running out of radio channels on more complex models. Many larger planes use two servos for the elevator halves and one servo for each aileron. Normally we have to use two of the Auxiliary channels for the second elevator and aileron servos. The Smart-Fly PowerExpander Sport Plus frees up those channels for other uses like retracts, flaps or a choke which can save you a bunch of money when you don’t have to buy a radio with more channels.
This board also has the fiber optic ignition kill system built in. You get the fiber optic receiver and a length of fiber optic cable for connecting that to the Smart-Fly PowerExpander Sport Plus board. This way there is no direct electrical connection between the ignition and the main Smart-Fly PowerExpander Sport Plus with the receiver to eliminate that potential source of interference. The fiber optic receiver is essentially an On/Off switch that controls power from the ignition battery to the ignition system on the motor. We set this receiver up on a radio channel with a toggle switch so we can shut the motor down from the radio.
There is a small amount of current draw from this system even when the pin is inserted. However you really have to neglect your batteries for this to matter. The Smart-Fly PowerExpander Sport Plus unit using the Fail Safe switch (Pin & Flag or Slider) will draw less than 10mAH per day per battery. That means that given a fully charged 2000mAH battery on each side of the unit it would take a minimum of 200 days to reach the safe fully discharged state. Even if you fly the battery packs down to a remaining 25% of capacity it will still take about 50 days to reach the safe fully discharged state. If the Ignition Cutoff Access Panel is used without a switch the draw is less than 5mAH per day from the ignition battery.
All this means that if you charge your flight packs before each trip to the field this bit of draw means nothing. The only time the draw of the Smart-Fly PowerExpander Sport Plus and related Access Panels comes into play is if you seriously neglect your batteries. If you stop flying for extended periods like over the winter months you can unplug the packs.
I began by installing the Ignition Panel, the ignition battery (in the motor box) and the Ignition Cutoff Receiver that accepts the Smart-Fly PowerExpander Sport Plus fiber optic cable. The Ignition Panel has a fuel dot built in so I located the Access panel where the old duel dot was. The fiber optic receiver was also mounted to the top of the motor box where it meets the fuselage to make accessing it with the fiber optic cable easier while keeping it out of the way.
During the installation of the fiber optic receiver I thought there was a short or a bad plug somewhere because the LED on the Ignition Panel was flashing on and off now and then as I worked with the receiver itself. After about 30 minutes of checking everything I realized that the port on the fiber optic receiver where the fiber optic cable plugs in was aimed up toward the ceiling and as I moved it around it was “seeing” an incandescent bulb on the ceiling and that turned the ignition on, lighting the LED. Duh! I show this happening in the video so you can skip my bit of confusion.
When mounting the Smart-Fly PowerExpander Sport Plus board remember that the end with the fiber optic port should aim in the general direction of the fiber optic receiver. You can flex the fiber optic cable quite a bit to weave it through structure and components as needed but I try to keep the “turns” to a minimum when stringing this cable.
The Smart-Fly PowerExpander Sport Plus board was mounted to the floor of the fuselage using a pair of hardwood rails I cut to just keep the board suspended. I added a piece of foam padding to the floor at the center of the Smart-Fly PowerExpander Sport Plus board to dampen the vibrations it will experience in operation. The hardwood rails are glued to the wooden fuselage floor, plus I used longer screws than needed to be sure they went through the floor below the rails for a little extra security.
As I removed the servo plugs from the receiver I noted the letter by the signal plug on the board that replaced it. That told me where to plug that servo in along the outboard rails of the Smart-Fly PowerExpander Sport Plus board. This really is a simple task if you just read the letter indicators at the base of the white signal wires.
My receiver is attached to the Smart-Fly PowerExpander Sport Plus board using strips of the heavy duty hook and loop material supplied with this system. This is some tough stuff so only a pair of ¼”-wide strips are needed to secure the receiver.
All that remained was plugging in the batteries and the charge plugs from the access panel into the board. The switch wire goes to the port on the Smart-Fly PowerExpander Sport Plus board marked “Switch” and the panel LED plugs into any unused servo port as it simply draws power to light the LED when the system is activated. There is also a jumper wire supplied that goes from the Ignition port on the Smart-Fly PowerExpander Sport Plus rails to the receiver channel being used to control the Ignition Cutoff. For this plane I used the Gear channel with On being when the Gear switch is pulled towards the front of the radio. I chose that position because it is way easier to accidentally flip this switch towards the operator than to push it away and I do not want the motor to shut off by mistake.
I did some bundling of the wires within the bay to be sure nothing gets hooked on the pull-pull rudder system. This is also a good time to be sure that none of the leads has tension on them and to just clean up the installation by securing loose wires. The last step here was to add my piece of Gorilla tape across the signal wires and onto the receiver body as another level of security. See my story “The Cheapest Safety Device” for more on this piece of tape and why you need it also.
At the Field
Any doubts I was having about the effectiveness of the Smart-Fly PowerExpander Sport Plus system went away seconds into the first flight with it installed. With the Aeroworks 30cc Freestyle Extra 260 QB-L up to speed I hit the ailerons and the roll rate was much quicker. I kept trying full aileron throw into and with the wind, at maximum speed and partial throttle and the roll rate was much quicker in every situation.
I also found that giving the servos more power perked up everything I did with the Aeroworks 30cc Freestyle Extra 260 QB-L. Anything I tried, especially maneuvers entered at speed benefited from the surfaces overcoming air pressures and going to full throw quickly. Tumbles, snaps and things like knife edge spins all became more violent and tighter. The Aeroworks 30cc Freestyle Extra 260 QB-L had become far more “active” at speed but all of the great low speed handling remained intact.
The electrical efficiency of the Smart-Fly PowerExpander Sport Plus System with the access panels did wonders for my Aeroworks 30cc Freestyle Extra 260 QB-L. Eliminating the multiple add-on systems and switches made a huge difference for me and I suspect it will for you also. Keep in mind that in addition to reclaiming lost voltage to perk up the servos we also get the safety of redundant batteries, ignition kill and the dual servo inputs that let us do more with moderately priced receivers. The Smart-Fly PowerExpander Sport Plus System is a win-win deal.
The Smart-Fly PowerExpander Sport Plus (with ignition cutoff) sells for sells for $179.95 (12-15-2014), the System Access Panel and the Ignition Access Panel each sell for $34.95. I think that is very short money for the performance and safety this system brings to our planes. Smart-Fly has many other systems designed to protect larger planes as well.
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