Stevojet

My Feedback: (1)

We have had a series of flap servo failures on several turbine powered jets at our airfield of late. The latest victim was an Avanti that experienced 2 fried Hitec 7954 flap servos in flight causing catastophic failure. I experienced a similar failure in my Ultima-tun 2 meter turbine powered jet. While I did not lose my plane I do not want to fly again until I understand why these high voltage digital servos are failing. None have been observed to be binding or otherwise hitting the end of their control throws. No shorts in any wiring have been discovered either.
Tomorrow I am sending several of these off to Hitec to take a look at. In the meantime I wonder if anyone has had a similar experience?

Steve

wojtek

My Feedback: (73)

We have had a few high end Hitec servos burn out in the past ( HS5955 ) .. I have had 5945 and 5925 servos burn out also. Too many first hand as well as reported failure on Hitecs for me to bother with them, or to have to worry . I have never had any other make of servo fail on me.


Voy

Len Todd

Have you programmed the failsafes? The default for failsafe off. Are you setting them up with a ammeter? I would think so, especially if they are ganged. On the flap, what were the current readings when the flap was fully up? Mine uses .08 amps at up. That is a little much. But when flying the air pressure on it apparently unloads the servo cause the overall mah usage is just not there after several flights.

I had a relatively new 7955MH start to get overly noisy last week. I took it apart and the gears were not lubricated very well. Lubed it and it quieted right back down. Also, the HV servos are not made to run straight off 2S LiPo w/o some sort of regulation down to 7.4, etc.

If you are using HV servos, I assume you are running them off a power distribution system versus trying to run them off the Rxer buss. Running them off the Rxer buss is a good way to have voltage fluctuations. Digitals can all operate at the same time putting over an Amp a piece on the buss (e.g. 8 servo could easily cause 10+ amp peaks, etc.) The resulting voltage sag could burn up a servo.

gapellegrini

Steve,

Is people on your club slowing down the plane before flaps down? One good idea is to slow down the plane, gear down, flap approach and some seconds later full flap with flap in slow motion. I have seen 8711 burn down doing all this above except slowing down enough the plane.

Just my 2c

Gabriel

HarryC

My Feedback: (1)

It would be unusual to advertise a servo as being HV and not be able to run straight off a 2 cell lipo, after all that is the whole point of HV. Hitec's website says they run from 2 cell lipos and makes no mention of having to regulate them. The torque and speed are quoted at 7.4V but that is just standardising the specs at a battery's nominal voltage, all brands do this for all types of battery, for example quoting the specs at 4.8V but any 4 cell nickel battery will be well above 5V much of the time, it doesn't mean you have to regulate it down to a max of 4.8V. Other brands of HV servos also quote the specs at 7.4V but state their voltage range is up to 8.4V

I can understand a voltage spike burning up a servo but not a voltage sag, can you please explain?

Justflying1

Well over 50 Hitec Digital servos of all sizes in Jets and props. Only ever had a couple fail on the ground while setting up which was 100% my fault. Like already stated try slowing the plane down and put the flaps down slowly like full size does. Also try and not pull 30 G and maybe the plane want fall apart either. Sorry just had to throw that in.
Ok I'm now sitting down with my pop corn and ice cream so fire away LOL

The couple I had fail where on valves when I was setting my plane because they where binding. Again 100% my fault. Hitec all the way for me.

Ron Stahl

My Feedback: (12)

I worked for a hobby shop during the last 15 years. Only ever saw Hitec servos fail from the thousands we sold over those years. I would never use one in my planes.

paulhat

Interesting thread.
I have been using Hitec servos in all my jets for say 10 years now with no issue at all.


Paul.

Stevojet

My Feedback: (1)

Len,
I am running all servos off the rx bus with power fed to a 12 channel spectrum dual battery input with 2 each 2 cell lipos. I have not programmed the failsafe overload feature. That sounds like an excellent idea. I fly 6 flights daily and use about half the lipo capacity so do not think servos are drawing excessive current. Have not checked anything with ammeter yet.
Steve

The Great Squintini

+1 I use Hitec in my jets as well and never had any servo issues what so ever. Then again I also never try to over complicate things also. Hitec 7955TG off of a 6v regulator....Never had a servo fail. I just might go buy several more for 89.00 a pop ;o)

RCISFUN

My Feedback: (44)

I have used Hi-Tec with no problems also in my Jets, I don't like digital on the flaps as they seem to be the most susceptible for binding issues.


Here are a few threads on this subject you may want to look at.... the RCU search tool is your friend


http://www.rcuniverse.com/forum/rc-j...vo-choice.html

http://www.rcuniverse.com/forum/rc-j...ps-futura.html

http://www.rcuniverse.com/forum/rc-j...102-flaps.html

http://www.rcuniverse.com/forum/rc-j...ervo-fire.html

ravill

My Feedback: (11)

I've had TONS of those little JR servos fail.

I've never had a standard futaba servo fail, 9001 e.g.

I've had a bunch of Hi-Tec 5645 fail.

I've never had a failing JR 8411 or 8611a (the bulk of my servos).

I've never had a failing Hi-Tec 5995 or 7995 (Which I've abused PLENTY).

And I'm no pansy with my airplanes. I only trust the Hi-Tec 5955 and 7955's.

I won't use any micro or "small" servo in my jets. My bandits have 8611a's in the elevators. I love the size of the JR 3421, but when I can use something bigger, I do.

Since JR has stopped making the 8611a, I'm forced to use the 8711. I'm using them for the first time. I don't like using servo's I don't know.

I'm more likely to trust an unkown JR servo than an unknown Hi-Tec servo. I'm old fashioned like that.

MikeMayberry

My Feedback: (1)

Hey guys... sorry to hear about the issue. To have two servos fail simultaneously in the flaps to me would indicate that there was a slight load on the servos when they were up. It is always good to check the current in the position with an ammeter to make sure they are not drawing too much current. You can also set overload protection with any one of the Hitec servo programmers which will throttle the servo power back as needed to attempt to keep it from burning up. I usually set this to 20%These are extremely popular servos and there has been no indication that there is any issues with them. It seems these situations may be application/installation related. That's not to say that servos can't fail as they can, just as any electronic item can, but it would be very odd to lose two at once.

Hitec servos carry a two year warranty so if there is an issue, send them in and we will repair or replace them at no charge.http://hitecrcd.com/support/warranty-service

Mike.

airraptor

My Feedback: (66)

I dont fly jets but still fly some 150-190mph planes and use only Hitec servos. My favorits are HS65, HS 85/87, HS5245, 7975HB, 7940, 7945 and 7950's. For flaps on large plane I use a spektrum 600 although its a retract servo it has a speed of 1 second and 260 ounces of torque metal gears and fully programable.

Hinckley Bill

My Feedback: (569)

And that's why I continue to buy and use Hitec servos........great customer service is what makes the difference.

757Driver

My Feedback: (90)

Lots of jets and IMAC big birds. All 5955 and 7955. No issues. Love em!
thanks hitec!

edit, oh yeah, 7954 and 7950's.

Stevojet

My Feedback: (1)

Update...
I spoke with Hitec today and sent 6 servos from three different turbine jets in for them to take a look at. Got them on the phone easily and they were very responsive. I want to keep using these servos so I hope they can find the issue. I plan to buy a programmer and program in the overload protection per Mikes suggestion.

Steve

Stevojet

My Feedback: (1)

Just fyi to all these 6 servos came from 3 different turbine jets all flown by very capable pilots with first rate install capabilities and many years of experience flying turbine aircraft. NO binding over travel of the servos or shorts in the wiring were observed in any of the three installs. All servos except 1 were being used as flap servos on jets that weighed in around 26 pounds or so.

Steve

essyou35

My Feedback: (11)

I have had several hs-81 servos go bad.

I have used 7954SH a bit, no problems. When your servos fail, are they only after a few flights or after a whole bunch of flights? I have used the hell out of the cheaper 654MG and never had an issue. I did mess up the gears on one once but the servo worked, I only noticed after I manually moved an aileron and felt a crunch.

The general rule (or myth) is that electronics either fail in the first 15 minutes of use or last forever. I always turn my jets on when I build them and let them sit for 45 minutes, periodically moving the sticks around.

I am putting 4 used ones in my mig-29. I used to know be a fan of used, but if I find someone I think I can trust that didnt abuse them, but used them a bit, then I like to buy used for this very reason. They are then proven servos!

757Driver

My Feedback: (90)

I always burn in servos for an hour or so in the test function of the TX after the airplane is setup. That said, there isn't any load presented. I wonder if anyone sells a servo exercise rig? I envision something like spring either side of center to load the servo in both directions oe one direction such as flap.

I love over the overload idea and will also be programming mine.

RCISFUN

My Feedback: (44)

Here is a good thread by olnico

http://www.rcuniverse.com/forum/rc-j...vo-sizing.html

ira d

I wonder if the servo is under load you should have a much shorter burn in period being the typical rc flight is 10-15 min.

OhD

Steve, anything new from Hitec? It sure seems like the servos must go into the equivalent of a stalled condition. This can happen if they go into a high frequency oscillation so they are essentially starting and stopping at a high rate. This is when a servo motor draws the most current as there is no back emf when the motor is not moving.

I gave Gary some regulators to protect the rest of the system but I haven't heard if he tested them yet. It would be nice if they also protected the servo, but maybe the servo has built in protection from what I read in this thread.

Let me know if you learn anything.

Jim O

Stevojet

My Feedback: (1)

Jim,
Hitec received the servos Monday and said they have a 7 day backlog of work so I don't expect to hear back until next week. I will keep you posted. If this wind ever lets up it will be nice to get out and fly. Hope to see you out there.
Steve

Len Todd

To move a control surface with a given amount of air pressure on it, it takes a certain amount of watts to move the surface. If the voltage is lower, due to whatever reason, more current is required. (Watts = Voltage x Current.) To hold that surface in place on a flap, it takes that amount of current until the surface is returned to neutral position.

With analog servos, they each operate a different times. With digital servos, they can all operate at the same time. When they all operate at the same time (potentially), all the individual currents add up. When you constrain all that current on a typical Rxer buss, the buss is essentially a resistance and voltage on the output side of the Rxer will go down (voltage sag) resulting in an increased current draw to get the power needed to move all the surfaces. That is why there are power distribution systems out there. When Resistance goes up the Voltage goes down (Power = Current x Resistance.) You may not ever see a drop on the bench. But when the plane is moving, it is there. The faster it moves, the more prevalent it is. At some point, the Battery's capability may even begin to factor in. Long servo leads and maybe a little resistance in the connectors and now what is the voltage at the servo going to be?

When you throw these conditions together with several digital servos all servos operating off that tiny Rxer buss, in a high speed plane, you have a situation looming (i.e. Many or all the servos operating at the same time with high loads on the surfaces.) Now, you throw in a flap servo, at maybe high speed, and there is now a constant high current load placed on that Rxer buss and the battery, plus all the other control surface loads. The flap's constant load can be up around 5 Amps+. As the flap servo begins to heat, the resistance goes up and even more current is needed resulting in an even higher current draw. That's why the Ultra Premium servos have obvious heat sinks. They can handle the higher currents and resulting heat.

Bottom line: Slow down before dropping the flap. Make sure you have a servo that can handle the load and resulting heat for a long period of time. Set the servo's failsafes. Keep the battery near fully charged (as the Voltage goes down on it, the above negative effects are amplified.) When using several digital servos, consider a power distribution system. Remember, the flap servo, when down, is a constant load versus just momentary like most of the other servos. It has to be a pretty healthy servo capable of handling the load and the resulting heat for a long period of time.