Current Draw and What it Does to Your Radio System

(Pre Lipo Info but still good to know)

April 18, 2004
Ron Lund

If you are using a single 4.8 volt battery pack to power your model's radio system, it is very important that you understand this. And if you're using digital servos, it's vital.

For you electrical engineers out there, we're going to keep this very simple. If you would like to submit a technical article, I would be glad to post a link to any article you would care to write. If you disagree with anything in this article, please tell me why. Feedback is welcome.

The Basic Facts

When you put a constant or temporary load on a battery, the voltage of the battery drops. The more load, the more the drop.

Digital servos put a higher load on a battery than standard servos.

Any servo that is operated with undo load (binding) puts a very large load on the battery.

What can I do About It?(see the side note)

There are several things you can do to prevent this, now that your are aware of the problem.

First and foremost is to make sure none of your servos are binding. This is very simple. Connect a voltmeter to your receiver, I like the I4c onboard voltmeter, but you can use anything that reads voltage. With all the servos neutral, observe the voltage. A freshly charged 4 cell pack should read between 5.4 and 5.1 volts. Now move the collective all the way to one extreme. Check the voltage. After you've completed the stick movement, the voltage should return to it's original value. Now move the cyclic stick to one corner. Again, the voltage should return to it's original value. Repeat this at all 8 compass points around the cyclic stick and at the other end of the collective range and in all modes. If you see the voltage hold at a lower reading, do something to eliminate that binding, whether it's reducing the travel of a servo or modifying your mechanical setup.

Charge your model more frequently. The more voltage you start off with, the less likely it is that you will dip into the voltage area that is dangerous. I don't necessarily think a larger capacity pack is the answer. The larger capacity packs do extend the time you would be operating at 4.8 volts, but they seem to drop into that area in about the same time as smaller packs. I would much rather see someone using a freshly charged 1400 mah pack, rather than an 1800 or larger pack that has a couple flights on it and is offering the pilot a false sense of security.

Change your power system. Go to a 6volt battery system with a regulator. I've never been a big fan of 6 volt systems because of the extra expense and weight, however with the increasing popularity of digital servos, it's almost become a necessity. Regulators are a lot better now than in the past. New technology in batteries, such as Lithium Ion and Lithium Polymer batteries has progressed to the point where they are very reliable and give extended flight times. Be aware that using a 6 volt pack is not a substitute for a vigilant watch on your voltage.

Ron Cosby EE, says a heavy duty switch helps reduce voltage drop to the receiver. I agree. Some of the switches that come with the less expensive radio sets aren't very good. Futaba makes a heavy duty switch for digital servos and we have the rocker switch. Both have good heavy wire and good contacts. The gauge of the wire and the length of the wire is the key element.

More on this subject. Submitted by Electrical Engineers in plain English. If it's not in plain English, it's not here.

Copyright © 2010 Ron Lund, RONLUND.COM and Ron's Heliproz South All rights reserved

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What can I do if my tail goes hard over in the air?

More than likely, you're going to crash because you're going to panic, which makes things even worse. If you can think fast enough try this:

First, warn the people around you! Yell and scream. It could save someone's life.

Immediately switch your gyro to normal mode and relax on the cyclic. This might allow the battery's voltage to recover. If you are inverted, roll out or, if you're too low, land inverted, using what time you have to get the model level. If there's time and altitude, rolling out puts less strain on the servos than pushing over. Gently maneuver to a landing. If possible, get in Throttle Hold mode. That's one less servo drawing current from the battery. By relaxing on the sticks, the servos will reduce the load on the battery and the voltage should come back up, hopefully to a point where the radio will operate normally. By switching to normal mode, once the voltage recovers, you should regain control of the tail. If you leave it in Heading Hold mode. It might operate, but it will be trying to go off to one side because when the gyro initializes in HH mode and the model isn't still, as in this case, the gyro will be biased toward one side or the other and the tail will be uncontrollable, unless you get very lucky.

It is very important that you have your normal mode set up properly. This is a basic concept in a good gyro setup anyway.. A lazy person can get somewhat satisfactory results with a poor mechanical setup as long as he isn't pushing the model in backwards flight. Take the time to adjust your servo horn position and your tail rotor control rod to the right length. It could save you a heartbreak.

Remember that while the voltage is below the receiver's threshold, PCM Failsafe doesn't work. Bear this in mind when doing the post crash autopsy.

Did You Know?

A larger battery pack requires a charger with more current output. The wall charger that came with your radio will never fully charge a pack larger than the one that came with your radio, no matter how long you charge it. The initial voltage might read a full charge, but the battery is NOT going to be fully charged and you are NOT going to get the time out of the battery you expect.

It's a good idea, when you get a new switch, to cycle it about 50 times before you put it in your helicopter. This "breaks-in" the contacts.