The idea to test servos at rated torque-load and measure the current drain has been in the back of my mind for some time, filed in the "nice to know" file. With model airplanes becoming bigger every year along with more functions, this means more servos added to a flight system. A larger battery pack is self evident but what size do I use? A Sears Diehard is definitely out of the question!
That is the whole reason for the research and this article ... to give some idea of the current load a servo uses. First, let's review batteries. Capacity of any given Nickel Cadmium battery is the amount of electrical energy that can be delivered over a period of time. Energy is measured in amperes or milliamperes (ma) and time is measured in hours. So, capacity (C) is measured in ampere hours or more common for our use, milliampere-hours (mah).
We can compute the actual operational capacity of a fully charged battery pack by converting the time it took to discharge the pack to hours [divide by 600 and multiply the result by the discharge current (300 ma).] For example, pack A took 105 minutes to discharge 105/60 = 1.75 hrs x 300 ma = 525 mah.
Every nicad battery pack has a rated capacity (225 mah,450 mah, 550 mah, 1200 mah, etc.). If the operational capacity of the pack is determined by a battery cycler, such as the ACE DIGIPACE, and meets or exceeds the rating of the battery pack, then everything is O.K. If it does not, then perhaps the pack should be checked.
How does one choose the right size battery pack? Most R/C systems include a 500 mah pack with the flight system and in most cases this is adequate for the flight system using the standard servos that come with the radio. Let's add up the total idle current drain of each servo plus the current that the receiver uses.
A typical receiver uses around 20 ma's of current. From the servo test chart using the IDLE MA column, we find that an Airtronics 94510 uses 8.5 ma current at idle, servo centered, no 1oad. We are using 4 servos in our imaginary plane so 4 times 8.5 = 34 ma. The total would be 20 ma + 34 ma = 54 ma.
If we are using the 500 mah battery furnished with the flight system, we can see that we can leave the system on for 9.25 hours, based on a discharge rate of 54 mah. The discharge time is equal to 500 ma divided by 54 ma equals 9.25 hours. But that wouldn't be very exciting just watching your airplane set there on the runway ... now would it?
At the other end of the chart notice that the stalled current is 720 ma., times 4, equals 2770 ma. At that rate, a battery pack wouldn't last very long. For this reason make sure that servo end travel is not jammed. Servos draw current whenever your thumb tells it to move to a new position, in relation to centered, or in the case of a retract servo, from one end to the other end of travel. If no load is applied, the current drain will be at idle. So if the push rod is not working freely, the friction may add excessive load other than the load of the control surfaces.
Let's use another example. We have the following set up; throttle - S128, ailerons 2 - S128's, rudder - S134, elevator - S134, smoke - S148, flaps - S134, retracts - S134G, brakes - S148, and bomb drop - S 148. Neat model! From the following chart the total idle current is 124.6 ma. At idle, using a 500 mah pack, you would have about 4 hours of idle time. If we totalled up each servo's average current drain, we find that to be 2940 ma.
Since we are using only flight controls in the air, the nets total only those averages and add to that the idle current of the other servos. Now we have 1357 ma. (assuming that the receiver is using a separate battery pack, a must with big birds). Using this formula we can see that the 500 mah battery pack is simply not large enough.
In this case we need to use a 1500 mah battery pack, giving you a flight time of about 60 minutes. Let's say that you fly 3, 13 minute flights, starting when you turn on, till you turn off the radio. You would have used up 39 minutes of flying time, leaving some 21 minutes to spare. Times may vary accordingly to your particular installation.
By using the chart, one can determine what size of battery pack you need for your particular installation for safe flight operations. Knowing your battery capacity is a real good idea. I use an ACE DIGIPACE, but any other similar instruments would be fine.
The chart was developed by using a servo driver furnished by Custom Electronics and a HP clip-on Milliamp meter. Each servo was firmly mounted, then using a calibrated and balanced yard stick, a calibrated weight was applied at the set distances ... using the formula Torque oz/in = Wt. x Arm. Each servo was tested several times to obtain an accurate reading.
I want to thank each manufacturer for sending me their servo for testing and chart development.
SERVO TEST SHEET TORQUE OZ/IN = ARM X WT
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MANUFACTURER | |
AND RATING | MA. CURRENT DRAIN AT % LOAD | MA.
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| TORK | IDLE | 25 % | 50 % | 75 % | 100 % | | AVERAGE
SERVO | OZ/IN | MA | LOAD | LOAD | LOAD | LOAD | STALLED | LOAD
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ACE 14G26A | 45.0 | 7.5 | 25 | 54 | 94 | 215 | 665 | 97
ACE 14G26AB | 45.0 | 6.1 | 27 | 75 | 130 | 310 | 640 | 135
AIRT. 94102 | 50.0 | 4.5 | 20 | 190 | 310 | 410 | 520 | 232
FUTUBA 94510 | 110.0 | 8.5 | 55 | 290 | 385 | 500 | 720 | 307
FUTUBA 94732 | 67.0 | 6.6 | 65 | 235 | 380 | 415 | 580 | 274
FUTABA S114 | 167.0 | 9.3 | 75 | 330 | 530 | 740 | 910 | 419
FUTABA S128 | 48.7 | 9.5 | 35 | 125 | 245 | 340 | 590 | 186
FUABA S134 | 112.6 | 9.7 | 145 | 365 | 565 | 780 | 920 | 464
FUTABA S134G | 173.8 | 20 | 85 | 390 | 505 | 630 | 910 | 402
FUTABA S148 | 42.0 | 9.0 | 45 | 170 | 245 | 325 | 600 | 196
FUTABA S9101 | 41.7 | 9.2 | 50 | 165 | 245 | 335 | 500 | 199
FUTABA S-34 | 112.6 | 9.6 | 135 | 360 | 570 | 765 | 910 | 457
WORLD S-16 | 180.0 | 7.5 | 95 | 340 | 520 | 620 | 860 | 394
WORLD S-29 | 34.0 | 6.2 | 20 | 40 | 75 | 170 | 505 | 76