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HS-5475 Servo Gears Stripping in Normal Flight

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Old 08-08-2004, 08:44 AM
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hilleyja
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Default HS-5475 Servo Gears Stripping in Normal Flight

I'm posting this in this forum because I'm getting no response from Hitec in their manufacturer forum. Maybe someone here can tell me what's wrong.

I previously posted in the Hitec forum that all of my 4 HS-5475 servos stripped their gears during flight in my GP Christen Eagle II. I got no response from a Hitec rep.

I replaced the gears in all 4 servos, changed out the CA hinges for Robarts, and sealed the hinge lines. This should have solved any flutter concerns.

I've also yet to wring this plane out. My flying to-date has been very conservative, wide turns, simple loops, mostly 3-point feather landings.

Well, yesterday I flew her for the 3rd time since replacing the airlon servo gears. Upon taxiing her back from her last landing one of the servos was screaching like a banshee -- yep, the gears were stripped. Whats going on here? I can't blame it on unusual flutter anymore. Is the vibration fron a gas-ignition engine too much for this servo's gear train? I looked up the specifications for these servos and they are well within the limits recommended by Great Plains for this airplane.
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Old 08-08-2004, 09:06 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Jim, I think I saw a note where Mike was on vacation.

I have several of the 5475's, but on smaller planes. Only replaced one gearset, but that was from contact with ground. Fly every once and a while with a guy that uses them on a GP Patty Wagstaff on the ailerons and he has stripped out several gearsets. I don't think the servo was ment for his application.

Ed M.
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Old 08-08-2004, 09:17 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

It is next to impossible to give you accurate/valid advice without seeing and examining your plane. Are you absolutely sure that there is no slop in the system such as enlarged holes in the horns or servo arms, not stiff enough push rods, play in the hinges, loose servo mounts, etc.? Your problem sounds like it is caused by flutter or possibly jammed servos at the end of their travel. These servos are typically pretty good so there is most probably an external cause. Are you running excessive voltage on the servos? Have an experienced long time flyer look your set up over, he/she may be able to give you some good advice.
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Old 08-08-2004, 09:42 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Jim, I think I saw a note where Mike was on vacation.
My 1st post on this subject was before he went on vacation -- he responded to subsequent post from others.

It is next to impossible to give you accurate/valid advice without seeing and examining your plane. Are you absolutely sure that there is no slop in the system such as enlarged holes in the horns or servo arms, not stiff enough push rods, play in the hinges, loose servo mounts, etc.? Your problem sounds like it is caused by flutter or possibly jammed servos at the end of their travel. These servos are typically pretty good so there is most probably an external cause. Are you running excessive voltage on the servos? Have an experienced long time flyer look your set up over, he/she may be able to give you some good advice.
I understand what you are saying here. Please read my other posts on the subject. I have examined my application, made changes to the hinges and sealed the hinge line. The servo/airlon connection is a simple direct rod connection, common for in-wing airlon servos, there is NO SLOP. I took extra care in installing the servos -- they are tight, they are new, and they are spec'd well within Great Plains recommendations for the Christen Eagle II.

I have installed in-wing airlon servos is well over 30 airplanes over the last several years. I have never had a servo gear train strip out on me other than from a crash. I have even had airlon hinges fail in flight like this one and that didn't strip the servo gears except for this one.

Fly every once and a while with a guy that uses them on a GP Patty Wagstaff on the ailerons and he has stripped out several gearsets. I don't think the servo was ment for his application.
Tell me more here. I've included an excerpt from both the GP Christen Eagle II and Patty Wagstaff instruction manuals that define the minimum servo requirements. In both applications, the HS-5475 is within their limits for all flight modes.

Here are the Hitec Specs for this servo:

Torque 4.8/6.0v : 61 / 76 oz 4.3 / 5.5 kg.
Speed 4.8/6.0v : 0.23 / 0.18 second


From GPMA-1217 Instruction Manual

Technically, the Christen Eagle is a giant-scale model, but
it's not really THAT large! Expensive, high-torque servos
aren't necessary to fly this model, but “standard” servos
should not be used either. Servos with a torque rating of at
least 50 oz-in [3.9 kg-cm] should be used on the ailerons,
elevator and rudder.
If you plan on doing lots of aerobatic
flying where the rudder will be used heavily, you might
consider using an even higher torque servo for the rudder.
Of course, a standard servo may be used on the throttle.
From GPM1305 Instruction Manual

The Extra 300S ARF can use a simple 4 to 6-channel radio
with several “Y” reversing and non-reversing harnesses or it
can use a 6 to 10-channel computer radio. Even though the
airplane flies fine with a simple radio, we strongly
recommend that a computer radio be installed as this will
make it easier to obtain the best performance of all flying
styles possible with the Extra 300S ARF. Also, all servos
used on this airplane should be capable of delivering at
least 60 oz-in of torque with the exception of the throttle
servo. Digital servos are highly recommended for 3D flying
.
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Old 08-08-2004, 10:05 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Jim

I saw your original posts in this and the other forum and felt your concerns and or problems were addressed. I like others believe flutter was/is likely your problem. There is simply no way we can ascertain specifically what is causing your problems. You've made some changes that may or may not effect the flutter issue, IMO.

Let me suggest that flutter can be realized of the wing itself. Its pretty common knowledge that other ARF manufacturer's have had similar problems with their wings and flutter (metal geared servos and all). That said if you feel confident the the problem lies within the servos gear-train STOP using them.

There have been reports from others with regard to the 5475 gear-train failures. If I recall they were generally on larger gas powered models. The Karbonite gear-train is not as strong as metal geared servos. Some improvements have been implemented on newly released Karbonite servos which incorporate approximately 30% more gear-train mass than that of the 5475.
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Old 08-08-2004, 10:23 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

ORIGINAL: mglavin

Jim

1. I saw your original post in the other forum and felt your concerns and or problems were addressed. I like others believe flutter was/is likely your problem. There is simply no way we can ascertain specifically what is causing your problems. You've made some changes that may or may not effect the flutter issue, IMO.

2. Let me suggest that flutter can be realized of the wing itself. Its pretty common knowledge that other ARF manufacturer's have had similar problems with their wings and flutter (metal geared servos and all). 4. That said if you feel confident the the problem lies within the servos gear-train STOP using them.

3. There have been reports from others with regard to the 5475 gear-train failures. If I recall they were generally on larger gas powered models. The Karbonite gear-train is not as strong as metal geared servos. Some improvements have been implemented on newly released Karbonite servos which incorporate approximately 30% more gear-train mass than that of the 5475.
1.) I received no response from a Hitec rep. I did get a response from another RC Universe member and I did modify my application to reduce or elliminate flutter potential.


2.) OK, I see what your posting but don't understand. What do you mean that the flutter can be realized by the wing itself? It has always been my understanding flutter is caused by slop, slop in the servo gears, slop in the hinges, too much gap in the hinge line, and too much speed for the application. I took care of all that. There is no slop in the hinges. There is no slop in the push rods, and there is no gap in the hinge line. And I do not fly this airplane at an excessive speed. And, believe me, if my wing was fluttering I would notice it. So what you are telling me is the ARF manufacturer has spec'd the servo requirements incorrectly. I will bring this to Great Plains's attention. Meanwhile Hitec and other servo manufacturers should rethink their specifications. If this servo, HS-5475, is not capable of performing within its torque range then they should not be spec'd at that range. How can you not expect people to use them in applications that include their spec ranges.

3.) I wish I would have seen some warning from Hitec that this servo was not capable of being used in my application. I would have chosen a different servo and would not be out the $38.00 for new gears and now more money for another set of gears.

4.) What other choice do I have -- it is obvious now the servo does not stand up to my application. There is nothing more I can do with the airframe.

BTW, while were on the subject of flutter. Let me point something out about the Great Plains Christen Eagle II. This is not a 3D airplane. It does not have extra wide and long airlons. The airlons are tightly embedded into the wing geometry (they do not extend to the wing tips), they are ribbed constructed and not made strickly from balsa sheeting, and are by that nature somewhat insolated from normal conditions that cause flutter.
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Old 08-08-2004, 10:59 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

I am NOT an expert on flutter issues but am aware that a wing itself can and will induce flutter. Remember all the problems modelers were realizing with H9's original 1/3 scale Extra ARF or how about DPM Extra? These problems were attributed to the wing and or the aileron itself flexing, inducing flutter at low speeds in straight and level flight. I witnessed a failure of H9's Extra, 8411s, good hardware and all. The pilot was on the maiden flight straight and level at a reasonable speed and hell broke loose. Again the wing was the problem not the servo, installation or hardware thereof.

I am not suggesting the HS-5475's may not be your problem. These servos are not offered as a metal geared servo replacement. For the most part, modelers would not use a nylon geared servo in applications were gas powered engines are utilized. These are simply standard servos with Karbonite gears and digital amps. Yes they do make more power than standard servos, does that mean they should be exposed to the abusive nature of gas powered models? I think NOT!

Have you spoken with other modelers that are flying this model? What servos are they using? Is anyone using nylon geared servos?
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Old 08-08-2004, 02:10 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Another thing that can cause flutter is just surfaces themselves that are to soft or will flex. You want the entire aileron to be very stiff, i.e. if you grab one end and try to twist it, it should not twist under reasonable force. The same thing applies to the wing, if it easily twists, it will be more apt to flutter. Realize that EVERYTHING WILL FLUTTER given the right conditions. You just have to make sure that the natural period of resonance is outside the boundry you are operating in. Since our worry is usually in the low killocycles or less, stiff structure (resistance to bending) will increase our chances of not having a flutter problem under normal operation. Also, never place your aileron horn in the center (spanwise) of a flying surface. Try to hit a 1/3 span for best resistance to induced flutter. 1/5 or 1/7 of span is usually okay also.
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Old 08-08-2004, 04:00 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

To be fair, The 5475 is a very low-cost digital servo and is billed as a "standard" servo - hell there are many conventional servos that cost up to twice as much!

In fact, the 5475 is just a $17.99 475HB retrofitted with a digital amp!

Even the world of RC isn't immune to the old "you get what you pay for" adage and I would suggest that you consider moving to something a little more robust for your application.

For an extra $20 per servo you could go to the 5645MG and get 250% more torque and 20% more speed along with a far more robust metal geartrain (albeit at the cost of an extra 1/2 oz per servo).
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Old 08-08-2004, 09:52 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

The only sure cure for flutter is static balance of the control surfaces. Try that and I bet your problem will go away
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Old 08-09-2004, 07:44 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Please explain what I need to do to "static" balance my control surfaces.
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Old 08-09-2004, 09:36 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

While both static balance (equal weight on either side of the hinge line) or airodynamic balance do little or nothing to eliminate flutter but both have many other advantages. Flutter is just anything oscillating at its natural resonance and; when the external stimulus from any source excites the structure at that frequency it will flutter. The secret is to make sure that resonance point is outside the range of the stimulus. You do this by making the stucture stiff for those frequencies. In our case, the stimulus is usually air turbulance over the structure but it could also be the result of the engine or prop.
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Old 08-09-2004, 10:37 AM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

ORIGINAL: Rodney

While both static balance (equal weight on either side of the hinge line) or airodynamic balance do little or nothing to eliminate flutter but both have many other advantages. Flutter is just anything oscillating at its natural resonance and; when the external stimulus from any source excites the structure at that frequency it will flutter. The secret is to make sure that resonance point is outside the range of the stimulus. You do this by making the stucture stiff for those frequencies. In our case, the stimulus is usually air turbulance over the structure but it could also be the result of the engine or prop.
As you say flutter is a resonance condition. It can happen with the aircraft sitting still with the engine running. The vibration of the engine can excite the resonate frequency of the structure and cause flutter if conditions are correct. Stiffening the structure will shift the frequency of the structure and appear to help. Static balance will change the frequency drastically and is the best cure as it eliminates gravity as a power source. Flutter can still occur due to the inertia and buffeting of the air but usually at such high speed it is no longer a factor to consider.
All full scale aircraft employ static balancing of the control surfaces as a matter of design.
If you want more details I will refer you to a friend of mine, a retired design engineer for the Boeing Co., an expert in the field of flutter analysis.
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Old 08-09-2004, 12:29 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

I too have had problems with Hitec Karbonite gears have” broke several” and in all cases it have been in a plane that pushed the limits of the servo, all have been Larger heaver planes with larger Control surfaces and Powerful motors at the top of the MGF airframe limit.

I have several planes with these servos and have had no problems 4*60, Sig Rascal 40, Senior Telemaster 91fs, World 40 Cub; I think we can see the pattern hear.

Best advices is use Metal gear servos and chalk it up as a learning experience. I learned once again the old saying you get what you pay for and our variety of large Models with big motors and large control surfaces NEED LARGER, MORE COSTLY METAL GEAR SERVOS TO EQUAL THE LARGER MASS and MUCH LARGER AERODYNAMIC FORCES.
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Old 08-09-2004, 02:46 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

ORIGINAL: hilleyja

Please explain what I need to do to "static" balance my control surfaces.
The easiest way is to attach a wire to the end of the control surface that extends ahead of the hinge line outside of the wing or other surface. Place a weight on the end of this wire to almost make the control surface stay at any position when the servo is unattached. You do not have to be exact at this weight.
Calculations to determine the exact weight required are very complex. In full scale work the exact weight is usually determined in flight test. At least they were in my day. I guess with supercomputers you could calculate it now. Someone would still have to program the computer though
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Old 08-09-2004, 03:17 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Well, static balance, flutter, etc is good for discussion. And frankly I'm talking about an ARF here. All I did was assemble it per instructions; I won't be making modifications to the airframe to compensate for design problems. If it was designed improperly I wouldn't be the only one with these problems. What I have learned from my experience has been aluded too by many. $35.00 servos don't belong in $1300 airplanes. Take the servo specs with a grain of salt. The HS-5475s probably do live up to their torque ratings. They don't however, survive the kind of vibration put out be a gas-ignition engine. Every single servo failure has been around the neutral location -- my limited understanding of dynamics tells me the damage was probably caused by vibration in the airframe, not over-torguing the control surface beyond the servos rating. If I had done that then the gear breakage would have been well away from the neutral location. 2 of of the 5 failures I have had has been one tooth on the shaft gear. This tooth was adjacent to the very center, i.e., the servo was in neutral when the gear broke. The other 3 failures were several teeth adjacent to the center, one of which was on the gear adjacent to the shaft gear.
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Old 08-09-2004, 05:59 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Your failures are consistent with a resonate condition (flutter) You cannot attribute this to bad design as the manutacturer has no control over the installation of the servos. I don't think it is the fault of the servos either.
I think it is just an unfortunate coincidence.
If you don't want to counterbalance the ailerons try adding weight to the ailerons. That might shift the resonate frequency enough to solve the problem. Its more dangerous -though- as it could make the problem worse.
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Old 08-09-2004, 08:24 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

I've been trying to warn people on this forum not to use Karbonite servos on the big ARF's. My 25% CAP went in really hard with the HT 635's. The ail's stripped out and I lost them. I landed very hard using rudder and elevator. Luckily I only had to buy a new wing and the fuse was in good shape. I bought 4 of the 5645's maidened it last weekend and it flew beautifully. No problems at all (168 ounce inches vs 82 oz-in of the 635's)

People, please don't use Karbonite gear servos or any plastic servos on 90 and larger planes. Also be aware of the mechanical advantage of how you setup your control horn and servo arms. The servos side at 1.25 inches and the control horn side at .75 inches will give you monster throws, but it will strip out your servos on the first turn. I setup my CAP for 1 inch on both sides and no problems at all after 5 flights. I'm a happy camper.

hilleyja, those servos are way too small for that big pitts. Get the 5645 or 5945, setup your control horn and servo horn for a 1:1 mechanical advantage and say good bye to your servo problems.


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Old 08-09-2004, 08:30 PM
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Default RE: HS-5475 Servo Gears Stripping in Normal Flight

Hi Jim, I'm using 5475's all around on a GP Patty, no problems yet, but I'm also using a twin cyl. engine on a "Hyde Mount"---pretty smooth---overkill?, maybe, but I'm NOT having servo problems---can we make the argument that airframe vibration is the problem?---
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