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RC Radios, Transmitters, Receivers, Servos, gyros Discussion all about rc radios, transmitters, receivers, servos, etc.

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Old 09-27-2014, 07:05 AM   #1
solentlife
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Default Serious flutter of servos !

Having just bought 2nd hand the Extreme Flight Extra 300, 58" - I find I have a problem with the elevator and rudder servos.

The original owner chose to mount the servos at rear of fuselage instead of the closed loop rudder and split elevator as in kit manual.

When Rx is powered up, the servos center fine. But when operating sticks and return to center - the two servos start very fast fluttering - so fast in fact the surfaces blur !
The two servos do it with / without Orange 3axis stab connected.

The system is :

FrSky 8ch HV Rx
Blue-bird BMS 761DD 26gr Digital low profile high speed servos.

I am wondering whether to add soft iron rings to the leads ? Maybe there is some signal inductance or other causing this ?

If anyone has ideas or seen similar and a solution - please I am all ears !

I have ordered analogue replacements after substituting my standard analogue servos for test. But they are too deep case to fit. It needs low profile. The plain servos worked fine with no flutter.

Nigel

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Old 09-27-2014, 07:25 AM   #2
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( Quote )
When Rx is powered up, the servos center fine. But when operating sticks and return to center - the two servos start very fast fluttering - so fast in fact the surfaces blur !
The two servos do it with / without Orange 3axis stab connected.


IMHO the problem is with the sevos, they are unable to learn your Transmitter Pulse Signal and lock on to it, the Servos dont like your Transmitter. and Neither do I LOL, Ok Just kidding, I like your Transmitter

BMS-761DD Low Profile Digital Servo 4.4kg / .13sec / 26g
With a low electronic impedance and precision movement, these low profile servos are designed for high-performance F3A precision aerobatic use. They are also a great choice for moving mechanical retract gears in a realistic manner.
The servo will automatically learn the user's transmitter pulse timings and lock into the signal every time. ( Yea Right ) The servo also includes a signal buffer in the CPU with fail-safe functionality. The motor is wound with 2PEW thin coated wire for better electrical conductivity and is able to withstand high temperature without degradation.

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Old 09-27-2014, 07:36 AM   #3
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Give those Blue Bird servos to someone that you dont Like LOL, OK, Just Kidding again they might work with a different Transmitter.

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Old 09-27-2014, 07:48 AM   #4
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I thought all the same as Chellie said and more. BUT it turned out to be a little simpler than I thought ( pointed out to me by 30 year kit builder). He told me that the space at the hinge point of elevator was to wide (makes elevator flop). So rehinge and make hinge point tighter. If the elevator still flutters add hinge tape over hinge point all the way across elevator (both sides). The key here is to add weight to help servo center. This worked great and I have done this to three of my larger planes when needed.
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Old 09-27-2014, 08:22 AM   #5
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Originally Posted by suresharp View Post
I thought all the same as Chellie said and more. BUT it turned out to be a little simpler than I thought ( pointed out to me by 30 year kit builder). He told me that the space at the hinge point of elevator was to wide (makes elevator flop). So rehinge and make hinge point tighter. If the elevator still flutters add hinge tape over hinge point all the way across elevator (both sides). The key here is to add weight to help servo center. This worked great and I have done this to three of my larger planes when needed.
Thats interesting to Know, I do know that to heavy of a Control surface load like a stiff hinge will also cause flutter, not letting the servo center, we all run into all kinds of interesting problems with RC Aircraft That makes us pull our hair out or turns it Grey

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Old 09-27-2014, 08:39 AM   #6
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Default Servo flutter

I don't know which way to go now after hearing two plausible reasons for servo jitter. My thoughts are, if you uncouple the servo from the horn and it still jitters it could be the servo. Substitute it with another servo of another manufacturer and see what happens. I confess to being very old school about a lot of things and I was taught that if my control surfaces were not free to drop under their own weight then they weren't installed properly! The fear was that too much drain from the flight pack to move the servo because of the control surface stiffness could cause a dramatic reduction in usable battery life or, in the worse case, catastrophic failure because the battery ran out of oomph!
I don't discredit what has been told about stiffening up the hinges just intrigued. I know that since I seal my hinge points that there is always an associated increase in stiffness so there might be something in that.
This is the fascination of the hobby, many solutions to many problems. I am glad the jittering has been solved. For what it is worth however, and it supports Chellie's argument, there have been a number of servo failures and model losses because of the little blue babies!
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Old 09-27-2014, 09:03 AM   #7
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Originally Posted by Kvar View Post
I don't know which way to go now after hearing two plausible reasons for servo jitter. My thoughts are, if you uncouple the servo from the horn and it still jitters it could be the servo. Substitute it with another servo of another manufacturer and see what happens. I confess to being very old school about a lot of things and I was taught that if my control surfaces were not free to drop under their own weight then they weren't installed properly! The fear was that too much drain from the flight pack to move the servo because of the control surface stiffness could cause a dramatic reduction in usable battery life or, in the worse case, catastrophic failure because the battery ran out of oomph!
I don't discredit what has been told about stiffening up the hinges just intrigued. I know that since I seal my hinge points that there is always an associated increase in stiffness so there might be something in that.
This is the fascination of the hobby, many solutions to many problems. I am glad the jittering has been solved. For what it is worth however, and it supports Chellie's argument, there have been a number of servo failures and model losses because of the little blue babies!
Hi Maurice I know I have run into all kinds of Issues with RC Aircraft, Most of the time I am able to figure out the problem, but sometimes it Bites me in the Behind That is what keeps up Going when i first started with E Power, I am a Old Nitro Flyer, I had a heck of a time with Jitter on my Planes with 72 MHZ, found out that most 72 mhz receivers dont like E Power, to much RFI for them, Berg and Hitec receivers solved that problem, but i did crash a few planes before i figured out what was going on, the plane would get about 200 feet away from me and I would loose all control and crash, even after doing a range check too that showed no problems untill the plane got into the air, after 48 years with Model Aviation, I am still learning New things, Take care and have fun, Chellie

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Old 09-27-2014, 09:10 AM   #8
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This is very common on digital servos on 3D models with large control surfaces, elevators are usually the worst. It gets worse when the servos get a little older and develop some play in the gears. It's actually a bi-product of one of the main advantages of digitals, that is their faster and more powerful centering compared to analogue. The problem is that they come back to centre so fast that when connected to a large surface they can overshoot, then they come back the other way and overshoot again etc etc...
The idea about tightening up the hinges might work as it adds a little friction damping so slow the servo down a fraction, but the way i do it is to use a small piece of the 'loopy' side of sticky back velcro and stick this on the balance horn of the surface in question so that it rubs on the fixed part of the tail as the control surface passes through centre. This always works very effectively for me and has the advantage of only adding friction at the centre point, not through the entire movement.

See the Velcro in the yellow circle (taken of my Extreme Flight Edge):


FYI... the manual gives two options for the elevator, it recommends single servo on the elevator for electric and dual servos for i.c. power.

Sounds like the previous owner has used the second elevator servo position for the rudder instead of closed loop.
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Old 09-27-2014, 09:39 AM   #9
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Lots of great info on this thread so far, Thanks JPF for the info on the Digital servos and Velcro Idea. If I Read Nigels post Right, He Was Saying that the servos jitter with or with the control surfaces being hooked up, so that indicates to me that the digital servos are the problem, and it may be be that they are unable to read the transmitters signal, or that the pots in the servos may be bad, Myself, I stay away from Blue Bird and Corona servos, had nothing but issues with them, the turnigy and Hxt servos have been good to me, so i stick with them.

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Old 09-27-2014, 12:42 PM   #10
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Looks like a lot of people dont like this servo these servos sound like a POS to me


BMS-761DD Low Profile Digital Servo 4.4kg / .13sec / 26g
With a low electronic impedance and precision movement, these low profile servos are designed for high-performance F3A precision aerobatic use. They are also a great choice for moving mechanical retract gears in a realistic manner.
The servo will automatically learn the user's transmitter pulse timings and lock into the signal every time. The servo also includes a signal buffer in the CPU with fail-safe functionality. The motor is wound with 2PEW thin coated wire for better electrical conductivity and is able to withstand high temperature without degradation.
The servo comes with an accessory package and the universal JR/Hitec "S"
1. Nylon gear with smooth curvature
2. Double M9.6 ball bearing (si-oil)
3. 3 pole high-torque motor with low RF noise (CE certified)
4. Rubber seals for waterproof case
5. Futaba compatible 25 tooth output splines
6. Heavy duty servo case
7. Digital control IC - SOP package with 12bit data transfer
8. MOS-FET motor driver chips that can handle 7A peak loads
9. Digital motor drive with ultra-square PWM signal
10. Automatic determination of servo center pulse width
11. Metal gears of aluminum 6061 material combined with light and strong C95 material



Weight: 26g / 0.92oz
Dimensions: 42 x 21.5 x 22 mm / 1.65 x 0.85 x 0.87 inch
Torque At 4.8V: 4.4kg/cm , 62 oz/in
Speed At 4.8V: 0.13 sec / 60 at no load









PRODUCT ID: BMS-761DD



Weight (g) 26 Torque (kg) 4.4 Speed(Sec/60deg) 0.13 A(mm) 27 B(mm) 43 C(mm) 24 D(mm) 21 E(mm) 55 F(mm) 16 Update/Add my own data
Customer Data


Weight: 58g
Quantity:

International Warehouse stock

10+




Price $21.65







acc.


  • Futaba/JR Connector caps (5pcs/set)

    Combo Price: $1.00 IN STOCK











Further discounts available for registered & logged-in customers.
Customer rated 5
crowns


Total of 6 discussions.

Le_Biscuit69 71 points - 3/6/2014

Here is what the data from hte back of the box says : - Weight : 26g/0.92 oz - Size 42x21.5x22 (mm) / 1.65x0.85x0.87 (inch) - Torque At 4.8V : 4.4Kg/cm - 62oz/in - Torque at 6.0V : empty - Speed at 4.8V 0.13 sec/60* at no load - Speed at 6.0V : empty - Ball Bearing Dual/M9x7 - Motor type : 3 pole magnetic motor - Operating voltage Range 4.8V - Operating temperature Range -20*C to 60*C - Idle Current : 5mA at Stopped - Running Current : 400mA - Stall Current : 1100mA - Dead bend Width 2usec - Direction Re-Clock Wise / Pulse Traveling 1000 to 2000 usec - Horn Gear Spline : 25 Segments (Same as Futaba) - Connector Wire Length 3000mm (11.81 inch) - Connector Horsing Universal - Main control IC : Mitsubishi Digital IC - Power Push Chips : Mos-FET ships


Lazar 5 points - 4/15/2013

these servo are not a good choice. I took my two and defective. one trembles and the other motor very hot - it hot.


Janine 26 points - 3/3/2010

I have the same problem as bostjan, adjusted the servo's with servotester and now they don't work on my spectrum 7
Anyone with a solution?


bostjan 18 points - 12/14/2009

i have problems with these servos. They works only with servotester. When I connect them to RX. they are not working. Is it possibly that servo remembered signal from servotester and locking on that signal. If it is so, how can I learn servo to remember signal from Receiver?

Bostjan

Janine 26 points
Hello Bostjan, did you find a solution already? have the same problem

Janine 26 points
Found my servo's working again after they were at room temperature and about 3 minutes learning

Sumer 31 points
I don't think that this servo has a memory to remember a previous signal. But these servos can have problems with certain receivers.

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Old 09-27-2014, 12:47 PM   #11
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Nigel, Take a BFH to the Blue Bird Digital Servos, that will fix them for good LOL


http://www.rcgroups.com/forums/showt...18533&page=139

And here the problem I have:
Today when my bird was ready for long range test, i have found that my ailerons are not moving. (Last week maiden flight was on 2.4ghz, but for today I have switched to DL). After some checking on the field I gave up, returned to home and started to investigate. All wiring was correct. And I checked another servo on aileron channel and they worked fine. The conclusion I made is that DragonLink has problems with this servo and maybe others because i have found another post claiming the same problem and here is Mike's answer ).

My servos are as following:
Aileron - BlueBird BMS-761DD
Rudder - BlueBird BMS-705
Elevator - BlueBird BMS-706

The only servo that does not work are 761DD which are Digital Servos. And they does not work only with DrafonLink because I have tested them on my Hitec Optima 6 and Optima 9, on Spektrum AR8000 and OrangeRx R610 for Spektrum and they are working good.

So, question to Mike and Daniel: Why do we face such problems, me being at least second with problem when Digital Servo is not working on any DragonLink channel?

************************************************** ******************************************

http://www.rcgroups.com/forums/showp...&postcount=445



JettPilot
Long Range FPV !!!



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Joined Apr 2005
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Quote:
Originally Posted by gent96
Anyone, please? Mike, Where are you?

Hi,

Are you saying that your digital servo does not respond on any channel of the Dragon Link RX ? For some reason it is not liking the PPM being put out by your RC radio, since Dragon Link reproduces the RC transmitters PPM output. I would try using a buffer on this servo to see if it work. Do not by the cheapest 2 dollar buffer you can find, get a good one if you want it to work. Sometimes digital servos can be particular about its signal, given that this is a cheap chinese digital servo, it is no suprise that it might work with some RX'es and not others. The first cheap digital servos I ever bought would not work on my expensive JR receivers, never again will I buy cheap servos !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Mike

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 09-27-2014, 01:45 PM   #12
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I use Blue-bird servos in my big biplane and also in the big Cessna ... but I stick with analogue as I do not need digital precision.

These two POS servos were already installed and as JPF says and I indicated earlier - the guy has substituted one elevator servo for rudder. The size of servos used are bigger than manual gives for elevator - so at least he compensated there. I knew this when I bought the model and all tested ok with Servo-tester.

I have considered modifying to manuals pull-pull rudder and have a standard profile servo in one slot for elevator .... but then it means I have to plug up the empty slot as well as cut slots to feed pull-pull cables.
The servo location is narrow and needs low profile cases.

I tried coils on the leads to see if it was wayward signals ... no change.

The annoying bit is having to wait for replacement servos.

Nigel

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Old 09-27-2014, 01:50 PM   #13
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Default Servo installation

Love this thread. For what it is worth I never mix my servo type or manufacturer on my models. Not suggesting anything it is just what I do. Secondly, I always run my rudder on a pull pull system, light and accurate and I suppose if one wire ever broke you might still have some control; don't know, never happened. Because I generally build large aeroplanes I more often than not run two servos to the elevator. I use a Y lead. I don't know exactly when I started doing this but maybe in the back of my mind it might have been about redundancy, back up etc.
If I use digital servos all the servos on that model are digital. Same goes for some of my analog. I mainly use digital on my 3D.
I was wondering if a pull pull system provides some sort of dampening and actually there is a force balance across the control arm? Imagine there would be.
Guys and gals, these are just a few things that I do and I have been wrong many times but all these things work for me.
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Old 09-27-2014, 02:02 PM   #14
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I like Pull-Pull and have a box of fishing trace wire and crimps along with screw end terminals specifically for it.

But this is a model already built and installations made. So I do not want to modify and create repair patches.

I have no real reason to use Digital and for 45 odd years been happy with analogue. As said - these were already fitted to the model.

I always think it funny that people today go on about Digital and such ... it was not so long ago that top line JR / Futaba etc servos were less than half the speed of a cheap Tg9e or HXT servo. I have JR 501, 5001, 101, 1001 servos as well as Futaba's in a box ... all from 1980's .... all were used in Display and Competition models. When I fire them up today alongside a budget cheap chinese servo - WOW how slow they are !! The chinese job whips back and forth quicker than they reach max deflection !

On a slightly different subject - some may question my putting a 3 axis Stab in the model. There's actually good reason. It will help in taming her down for landings and take-off runs. For out and out fun - the stab can be switched off in flight ... giving me best of both worlds.

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Old 09-27-2014, 03:50 PM   #15
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Did you try the Velcro trick Nigel? Works every time for me. You might have to add velcro to both balance horns on the elevator but often just one is enough.

FWIW the worst servos i had for this were also Bluebirds, they did it from brand new whereas most servos have the wear a bit before they start. Funnily enough some of the best were Bluebirds too, but the latest coreless motor ones, they are great servos.

The best stabiliser i've come across is the Lemon Rx one.. and it's also the cheapest! The Lemon one gives the option not only of turning on and off but also adjusting gain in flight. It also seems to 'interfere' much less with aerobatics than some others, in fact I can leave the Lemon one an all the time and there are no adverse effects. I've just ordered three more to put in various models.
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Old 09-27-2014, 06:25 PM   #16
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Nigel,

"When Rx is powered up, the servos center fine. But when operating sticks and return to center - the two servos start very fast fluttering - so fast in fact the surfaces blur ! "

It sounds like you have a modern "Galloping Ghost System" they used to flap every thing so fast the surfaces were hard to see.

All kidding aside, I agree with the people talking about the mass of the surfaces, once moving in one direction with a high speed digital servo, will travel past the center point then the servo tries to correct, and being a digital servo it give full power to the motor which causes it to over shoot the other way.

Analog servos taper off the power when they come close to center. This would fix your problem but they have less holding power for the same reason.

I got into a discussion with a VERY good RC pilot, about what would happen if we setup a 3D plane with all servos being the super extremely fast helicopter tail servos, some of them are 0.04 seconds or faster. His explanation was that the surfaces would never stabilize because of the moving mass, and would whip themselves to destruction.

A tail rotor has very little mass, so the servos can handle that.

I also have the exact same plane as you, so this post really caught my eye. I haven't set my plane up to fly yet. I'm not sure I would of thought of the problem before I would have bought the servos.
Thank you for bring this up.

Dave R, Proud PGR rider.
When you have flying skills like mine,
You become a master at repair.
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Old 09-27-2014, 07:56 PM   #17
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I haven't tried Velcro yet. To be honest, like other guy - I've grown up trying to get surface hingeing as free as possible without slop.

I did note that if I knocked the surface - it would most times stop fluttering. But slight movement and away it went again.

I got finally to rudder being quiet, so only elevator playing up now. But as soon as I EPA'd the range to recc'd - the fluttering was constant. Not even weighting the elevator or knocking it would make it stop.

The only time all is steady - is when full deflection is given and held.

This is not good enough - so final solution is swap out the servos.

Bluebird servos have given me great service until these. So digital BB's are of my list now.

As to the Orange stab .... I have the Atmel connector and USBasp items .... so will update the firmware to give variable in-flight gain as well as dual aileron capability.

Nigel

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Old 09-27-2014, 09:26 PM   #18
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If you feel the need to swap servos fine, but a couple of patches of velcro in my experience totally solves the problem for all time and you are still left with the positive centering advantage that digitals give you...

It might be that the bluebirds are so 'fluttery' that the velcro trick doesnt work but I'd say that it is worth a try at least... no?
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Old 09-27-2014, 09:36 PM   #19
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FWIW.. the older type Bluebird digitals like the ones you have werent great to be honest, but the new coreless ones are really excellent, some of the best I've tried. They are vary fast and positive and no flutter at all. The Coreless ones are suffixed 'DMH'.
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Old 09-28-2014, 08:05 AM   #20
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Originally Posted by JetPlaneFlyer View Post
If you feel the need to swap servos fine, but a couple of patches of velcro in my experience totally solves the problem for all time and you are still left with the positive centering advantage that digitals give you...

It might be that the bluebirds are so 'fluttery' that the velcro trick doesnt work but I'd say that it is worth a try at least... no?
OK OK .....

My problem is 'traditional' thinking on this. If a servo doesn't work straight - then I don't trust it.
Hardening up center as suggested seems contrary to years of practice.

To be honest the limited number of digi servos I've had have been disasters. Corona's. So this has fallen into that arena.
Digital are supposed to improve our hobby not find ways to combat new problems related to them.

I will try the Velcro - but whether I trust to fly or not ....... being honest ...

Nigel

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Old 09-28-2014, 08:27 AM   #21
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Nigel swap out the servo. If it is not working its not worth risk and cost of crash.
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Old 09-28-2014, 08:42 AM   #22
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Don't right off all digitals because those Bluebirds flutter, first it can be easily fixed and second not all digitals do it by any stretch. Some develop it when they get warn but most never do it at all. Like most things, you to some extent get what you pay for and the cheaper ones are most prone.


Anyway, on servos that I've had the problem with the velcro works perfectly and permanently. The Extreme Flight Edge in the previous photo has had the velcro for two years.

FWIW they don't do it in normal flight anyway because the airflow is enough to give the necessary damping. I'd agree to change them if there was any possibility of the issue causing failure but once Velcro'd (and assuming it works) this really cant possibly cause any problems.
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Old 09-28-2014, 08:55 AM   #23
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Nigel, Dont write off Digital sevos as JPF Says, The Better Quality ones are great, I use them in my Helis, its the Cheaper, want to be Digital servos that will give you Problems, Quality servos are not cheap, but you get what you pay for

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 09-28-2014, 11:21 AM   #24
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+1 Cellie
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Old 09-28-2014, 11:23 AM   #25
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Sorry Chellie I fluttered ;<)
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