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Hitec 7955 caused jet crash

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Hitec 7955 caused jet crash

Old 05-14-2017, 11:23 PM
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wichet
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Default Hitec 7955 caused jet crash

Hi everyone, wondering if someone could shine a little light on the subject. Was flying my Hotspot yesterday but unfortunately 2nd flight of the day lost some control of the plane which resulted in a damaged jet. The jet is setup for the typical delta setup of elevons, appon instepction the RX is fine, RX batteries sitting at 90% charge, BUT the left wing elevon has gone from a + 5mm reflex on the wing to a - 5mm. The servos are Hitec Titanium 7955 so would of thought bullet proof, it has seemed the servo has lost it centre or something.
Any ideas?
Thanks
Old 05-14-2017, 11:45 PM
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Henke Torphammar
 
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Hitech HV servos cant do HV was the reply... this was at only 2S life

Old 05-15-2017, 02:43 AM
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How do you know the RX is fine?
Old 05-15-2017, 04:11 AM
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I have hundreds of flights on 7955s. In 170cc aerobatics with bad engine vibrations to many jets over 20kg with no problem .
I own more than 50 of 7955 and i had no problem all this years.
Mechanically its impossible for this servos to jump a gear IMHO.
I remember years ago i made a mistake on programming of four 7955 gang rudder setup ,and i ended burning the one was not tuned.
But the other three didnt manage to damage that ones gears.Only fight against it and burn it.

I also have a hotspot. With twin 7955 ganged up per elevon.

Do you have two elevons on each wing ? or single surface?
If you have one elevon on each wing how many servos you run on it?
What is your receiver setup? Batteries? Powerbox?

Have you opened the servo to check the gears?

my2c
Old 05-15-2017, 04:14 AM
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DelGatoGrande
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Originally Posted by Henke Torphammar View Post
Hitech HV servos cant do HV was the reply... this was at only 2S life

7955 is not a HV servo


.
Old 05-15-2017, 04:20 AM
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wichet
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I am using a PowerBox Spektrum receiver, had no flashing lights on sat Rx to say interference or such, have unplugged the left wing and tried a number of different servos on that channel all seems fine on the channel. I will be sending it to Powerbox for a diagnosis just be sure thou.
Old 05-15-2017, 04:23 AM
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Single elevon on each wing with a single servo. Powerbox Spektrum Rx, 2 2s lipo for power, this is my 3rd Hotspot I've owned over the years and the first time this has happened. Not opened up the servo yet
Old 05-15-2017, 06:30 AM
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FalconWings
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I run 7955's on LiFe.....so far so good. Not sure I would run LiPo without a regulator.
Old 05-15-2017, 06:47 AM
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I guess OP is using a Powerbox with build in spectrum receivers, so I guess the Servo did not die due to over voltage but was operating at 5,5V or the like. An old 7955TG would not survive long with a fully charged 2s Lipos voltage.

Many years ago, I guess around 2007 and 2008 there was a batch of 7955's with problems. I can remember that well. However any servo can die at some point right?

Hope the model is no total loss.
Old 05-15-2017, 06:47 AM
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flyinfool1
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Another thing to check is to be sure you have the correct servo arm for the servo. If the splines do not match it could have jumped a tooth on the arm.
Old 05-15-2017, 07:18 AM
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Are you using plastic servo arms? Could the spline have jumped a couple of ridges?
Old 05-15-2017, 09:23 AM
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SWB servo arm
Old 05-15-2017, 09:27 AM
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SWB, then it's likely not that.
Old 05-15-2017, 10:00 AM
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Yep it's got me confused also. This is the only thing that seems to be wrong on the plane
Old 05-15-2017, 10:04 AM
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racer8297
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Ok. Is there any chance that something changed in your TX setup? Like maybe you inadvertently changed the sub-trim on that channel?

How are the control rods and horns?
Old 05-15-2017, 10:11 AM
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No nothing, had my first flight then put TX away in box, Taken it out again, Switch on made sure everything had bound, started turbine then half way through the flight it happened.
Old 05-15-2017, 10:13 AM
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racer8297
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Wow. Is it possible that the servo broke a tooth or two for whatever reason?

And, as said before, it could just be that the servo has reached the end of it's usefulness!
Old 05-15-2017, 10:47 AM
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Zeeb
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Originally Posted by DelGatoGrande View Post
7955 is not a HV servo


.
That is incorrect. Hitec told us the 7955 was an HV capable servo from basically the time it was introduced. The twitching shown in the video is a 7950 which has had twitching issues especially on long extensions that Hitec recommends using a booster which they manufacture.

That said; I do not run Hitecs on my turbines but use quite a few of them on my gassers.
Old 05-15-2017, 10:58 AM
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I have crashed years ago because of digital metal gear servo(not 7955) loosing center. Not jumping gears but loosing pot to output shaft contact due plastic coupler shrinking in low temp. It jumped out of center and eventually jammed into mechanical stop(over travel) and resulted slow roll with opposite full aileron.

/Jyri
Old 05-15-2017, 11:48 AM
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My first thought was a servo arm skipping a tooth. The second thought was the Trim Setting or Offset got changed between flights. I would also check the control surface for binding (e.g. w/o being attached to the servo and when attached) Are the servos endpoints set to high allowing the surface to reach its physical travel? You can see any binding and endpoint problems when you put an ammeter in the servo's circuit. Amps take a quick jump up and you got binding, or a physical endpoint, etc.. Put a Programmer on the Rxer's output and see if the resolution is steady. If it is not, the fault could go as far back as the transmitter's stick, etc.

I have spare gear sets and replace them myself. Open up the servo an inspect it. If one is not comfortable with that, I would suggest sending the servo back to Hitec. They service them for free. If the voltage applied to the system is correct the servo should have handled most any load we could throw at it.

I have dozens of hitec servos. I have physically worn out two. However, I also have had one fail on my very first Giant Extra. The servo locked with the Left Aile up about 30 degrees. I am pretty sure I burned it up during my setup on the bench. Since then, I bench test things a lot more before they go to the field. But, ...

For those who don't know, Powerbox has two voltage settings (i.e. 5.9 and 7.3 VDC) A servo rated for 6 VDC is not designed for peak load at the HV setting. If one does not believe that 7.3 VDC is NOT good for a LV servo, one might want to get the scope out and watch what is going on with the current and voltage as you load it and a few other servos at the same time. At the very least put an ammeter in the circuit and get an understanding for the current at load (e.g. probably 8+ amps peak with 3-5 showing on the ammeter.) You will probably see a current change as the servo heats up.

I do run all HV servos, but set the PBox at 5.9. If the regulators ever mess up the entire system will still handle the 2S. But as far as I am concerned putting in a 6 volt servo on a 7.3 circuit is asking for unnecessary issues. At the very least, it introduces unnecessary risk. They give us the specs on these things for a reason. And, we are frequently pushing these servos to their limits. Why start out with a known design flaw?
Old 05-15-2017, 12:10 PM
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Yeah, "regular" voltage servo's can't be expected to live long on 7.4V.

That's why god made regulators.

Sorry about your crash!
Old 05-15-2017, 12:20 PM
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Originally Posted by Zeeb View Post
That is incorrect. Hitec told us the 7955 was an HV capable servo from basically the time it was introduced. The twitching shown in the video is a 7950 which has had twitching issues especially on long extensions that Hitec recommends using a booster which they manufacture.

That said; I do not run Hitecs on my turbines but use quite a few of them on my gassers.

"Operating Voltage Range (Volts DC)4.8V ~ 6.0V"

HS-7955TG High Torque, Titanium Gear, Coreless Ultra Premium Servo | HITEC RCD USA
Old 05-15-2017, 12:25 PM
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DelGatoGrande
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... it sounds like a programming error. Do you use conditions ? Maybe when you flipped your retracts switch you changed to another trimming set up..
But you must take the servo apart.This will show you all


my 2c
Old 05-15-2017, 12:36 PM
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Shoot, I remember when we started using 6V nicads and being told that THOSE were HV! LOL
Old 05-15-2017, 04:06 PM
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Wow, I just got off the phone with a flying buddy getting his opinion on these very servos. I'm getting ready for a big Viperjet and since 8611s are not available I wanted feedback on the 7955s. My buddy has run a bunch of these for years in 40 and 42% 3D models and has never had one fail. He has no less than 11 in his latest bird and flies the snot out of it with no issues. He even said he thought had stripped a gear on one a while back, he called Hitec and they said no way, we don't see stripped titanium gears, your aluminum servo arm is stripped. He pulled it apart and the arm was in fact stripped.

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