I didn't know where to put up this warning, but since the failure of this brand of servo has made many of the observers believe we had a compromised RF system, I thought I would put it up under "Radios".

For 7 months now I have had a Bird of Time that was getting "hijaaked", that is, I would loose control for 6 to 20 seconds and then control would come back to me. This cycle could repeat 3 or 4 times before I was able to get the large bird back on the ground (or crash). One of the ground observers noticed that all of my problems were with elevator control, never with rudder, aileron, or motor. When your 3 meter glider is doing inside and outside loops on a 20 foot radius, you do not notice the "small things".

I have replaced everything in the plane several times except for the motor. Several ESCs, several BECs, standard 72 MHz Rx's and two different AR6000 spread spectrum receivers. Everything works for a while, and then the problem comes back. I replaced the elevator servo with a new one (of the exact same type) just as a safety precaution and the problem still kept happening.

After the first crash of the BOT, I bench tested all the parts for several hours to look for a failure. Nothing happened. Then I noticed that my old elevator servo was running very slowly.....like taking 10 seconds to move to the limit although my Tx stick did it in 1/10 of a second. Then slowly the elevator servo would only move in one direction...and slowly....and take almost a minute to return to center.

After my 10th flight with the new elevator servo, I noticed this strange slow response. I assumed it was coming from the Rx battery that I had put into the system to remove any dependence on a BEC. I removed the battery pack, put back the BEC, and now the servo worked just fine. So I chaulked it up to battery problems.

Two flights later I was flying and the same problem on the elevator.....I would loose control for 10 seconds and then it would come back to me. I did an emergency crash dive to get it to the deck and land it. This time the elevator servo was toast. It would not respond to any signal....in the plane or on the bench. I opened it up and found the miniture circuit board was fried. I tested the system with the original elevator servo. I worked for an hour on the bench and then that elevator servo also fried its circuit board....with the wonderful smell of cooked G-10 going all over the garage.

I put in a different brand of elevator servo because my supplier didn't have any more of the Blue Bird BMS-380 MAX competition units. I used the BMS-380MG unit which has almost the same torque but is quicker. Ten flights later nothing had failed. Then it happened.

I was 10 minutes into thermal soaring and the rudder servo died. I could smell the G-10 PCB board from way down the field. Luckily I had redesigned my ARF BOT to have ailerons/flaps/spoilers. As my other posts have shown, the ailerons are basically worthless. It takes almost 15 seconds of jambed ailerons to make a 90 degree turn. But with no rudder.....that was all I had. It was a windy day so I could only make turns in one direction. I was high up and it took two circles of the field to get in an upwind approach where I could bring it down. Thank goodness the field was empty. On weekends you could have 100 soccer players occupying their end of the Great Meadow. Anyway, the flaps and ailerons worked. It landed with a few bounces, but everything was OK.

As the picture shows, the BMS-380 MAX burned a hole in the servo box right through the body. This was similar to the two other failures I had seen.

So the bottom line is: if you have BMS-380 MAX servos, consider a change.....immediately. My five 380 MAX units came from two different production runs separated by 3 months. I still have two more in the wings and they will be replaced this week. The BMS-380 MG is basically the same servo in a different package with a different motor and different electronics. This far they have not shown any problems. The BMS-380 MAX all worked fine for about 15-20 flight hours, and then the problems started. I had bench tested them for three battery packs before I installed them. I used a watt meter to measure their torque at stall and how much current they took out. They matched the printed spec perfectly. For a 15.6 gram servo that puts out 57 in-oz of torque, they are perfect for a glider.

The BMS-380 MG weighs the same but only puts out 50 in-oz of torque.

So my conclusion was that I did not have any RF issues (or if I did, they were minimal). What was happening was that the servos were getting hot (or shorted internally) and they would drive to an extreme position and hold there. After 10-20 seconds they would cool down, and then they would operate as normal. Twice I have seen this phenomenon happen on the bench using all the same batteries, ESC, BEC that are in the plane. After the servos cool down, operation returns to normal. Remember, in a glider you are making very small servo motions and you try to fly "hands off" as much as you can to conserve kinetic energy. You put an absolutely minimal load on your servos.....to do otherwise costs you air speed. So these servos never saw any fast motions or high surface loads. They had long teflon push-rods inside plastic tubes that were firmly mounted to the fuselage. Everything moved very smoothly.

So I just want to warn the flying community. There may be a serious safety problem with the Blue Bird BMS-380 MAX servos. I fly all of their other types and have not had any problems. If you have this specific type, consider putting in something different. Servos are cheap compared to loosing control and having the plane come through the flight line.

If anyone else has seen this phenomenon, please comment here so we can form a consensus that BMS could use to re-design this specific unit. Thanks.

Crazy Ted -- glider pilot because thermal flight really lifts your spirits.

P.S. In the pictures I show the BMS-380 MAX box and the BMS-380MG. Very different packaging. You can see the black hole in the side of the servo. The white smugges are hot glue that I used to hold the servo in position in its frame.

Originally Posted by tjcooper

I didn't know where to put up this warning, but since the failure of this brand of servo has made many of the observers believe we had a compromised RF system, I thought I would put it up under "Radios". <snip?
I have replaced everything in the plane several times except for the motor. Several ESCs, several BECs, standard 72 MHz Rx's and two different AR6000 spread spectrum receivers. Everything works for a while, and then the problem comes back. So the bottom line is: if you have BMS-380 MAX servos, consider a change.....immediately. My five 380 MAX units came from two different production runs separated by 3 months. The BMS-380 MAX all worked fine for about 15-20 flight hours, and then the problems started. I had bench tested them for three battery packs before I installed them. I used a watt meter to measure their torque at stall and how much current they took out. They matched the printed spec perfectly. For a 15.6 gram servo that puts out 57 in-oz of torque, they are perfect for a glider.What was happening was that the servos were getting hot (or shorted internally) and they would drive to an extreme position and hold there. After 10-20 seconds they would cool down, and then they would operate as normal. Twice I have seen this phenomenon happen on the bench using all the same batteries, ESC, BEC that are in the plane. After the servos cool down, operation returns to normal. If anyone else has seen this phenomenon, please comment here so we can form a consensus that BMS could use to re-design this specific unit. <several senteces snipped out> Thanks. Crazy Ted .

Hi Ted, appreciate the in depth explanation, and whilst I have not used the same brand, I wonder if Electronic Speed Control - Why BEC and LVC cause most EP RX problems. may be explain some of the cause. BOT was designed as a 2 channel model with 2 X standard 3kg servos. A 3m glider loaded up with motor, battery pack and that many servos could be to much for the ESC / BEC employed leading to failure of the servos. A separate RX pack is recommended for that many servos or at least a 5A BEC. These details are covered under several sections on my web page such as "Electronic Speed Control (ESC) and DIY ESC Circuits " and "Glitches & Jitter in RX & Servo - causes and cures " amongst others.
Regards
Alan T.
Alan's Hobby, Model & RC FAQ Web Links

Alan,
thanks to the pointers to threads on LVC and BEC problems. I have seen these discussions before and I really worried about this prior to making my first flight with the BOT. I had a 2 amp BEC in the first ESC I used. On the bench I had the plane run through 3 battery packs while I tested out the current drain through the BEC with a specialized watt meter. Even when I stalled two servo surfaces, I could only get to 2 amps worst case. After my first "hijaaking" events, I replaced the BEC with a stand alone switcher which was rated for 3 amps continuous and 5 amps for 60 seconds. I again bench tested the whole system and measured individual currents from each servo under moderate load and under full stall. My results showed that it would take all 4 servos to stall to exceed 3 amps (imagine how hard it is to have 4 surfaces in your hands and try to stall them all at the same time!!!!).


I got so paranoid about the current loading and possible "boot up timing problems" with the DX6 that I put a separate battery pack in the plane which was wired in parallel to the BEC. Testing showed that since the NiMh voltage was always slightly higher that 5 volts, that the BEC in the ESC (I had another change of BEC and ESC for the 3rd time) never turned on. That is rather neat (but not all BEC units work that way), since I have parallel voltage sources that don't fight each other. I measured my 4 NiMd cells after 1 hour of flight and I had not drained 25% of the cells.

So you see, I have really tested the theory that all these strange effects were related to Rx problems (replace receivers 4 times), BEC (replace that 4 times), or ESC (replaced that 3 times). I had also tested the failure of the elevator servo but I replaced it with exactly the same model and type.

It was only when the rudder servo failed that it dawned on me I had a failed brand of servo.....nothing else. That BMS 380 MAX have had 3 of the 5 units failed all the same way within 20 hours of service. That really points a very strong finger of bad design for that unit. That is why I want to warn other pilots who use them to beware of this poor design. I don't want anybody else to spend the 100 hours of diagnostic work that I have spent tracking down this problem (and the replacement of the center section of the BOT wing when it went in nose first into the field).
Crazy Ted -- glider pilot for peace and quiet

I too have had two BMS-380MG failures. Mine were running on 6v Rx pack, were yours at any point tjcooper ? I notice the plastic geared version is only spec'd at 4.8v but the metal geared version is spec'd at 4.8v and 6v. I can't imagine the two having different electronics so I wonder if this maybe something to do with problem. I assume the BEC's you used were outputting 5volts ?

It has been over two years since the servo problem started. I got new units and put them into the ailerons of the BOT. Just out of "safety" worries, I replaced the last one this week. It had the start of the same PCB burn out. This means it is a design problem of the worst type.

Please see this warning to the R/C community:
https://www.rcgroups.com/forums/show...8#post10048039
tjcooper

UPDATE: forgot to mention, I have run the battery in parallel with two different ESC's now. No problem in over a year of testing. When I put the battery on a special in-line tester, all that happens is that the current drops with one of my ESCs when I turn it on. For the other ESC, there is no change of current. So it appears that most modern ESCs have BEC units that just wait for the voltage to approach 5.0 volts DC before the BEC starts putting its current into the circuit. This is a great thing. It makes the two circuilts act as a fail-safe to each other.