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Old Jan 19, 2009, 02:34 PM
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Stamford, CT
Joined Feb 2002
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Gordy's right. Plastic gears are better for the elevator where very precise control is required. The plastic gears have an interference fit so there is no gear lash (i.e. slop). That is the teeth are literally forced together tightly. This is not possible with metal gears, as the gears would bind. F3A pilots only use plastic gears for this reason.

George
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Old Jan 19, 2009, 02:59 PM
RIP MC
fnnwizard's Avatar
United States, CA, Midway City
Joined Dec 2003
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I just removed the suspect servo out of the plane and noticed it is impossible to turn the horn by hand. I took it apart to see if any of the gears were broken but they are not, so it maybe that the brushes are welded on?

Also while having one apart, I got an idea...

At certain points of the travel the slop is less, so I plugged the servo into my rx to line up what neutral on the plane is (servo would be off neutral in this instace), marked it on the output gear, and move other gears around until I noticed the least amount of slop.

Is this too extreme. Maybe, but I would like to get as close as zero slop at neutral, all other points I can program radio to compensate. Tried it and it actually works.
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Old Jan 19, 2009, 03:41 PM
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USA, KS, Wichita
Joined Sep 2002
1,130 Posts
DS3421SA Servos on Elevator

Target,

The nylon gears are more precise but they are very suspect in strength. I took out a set on my Espada R on launch and was very lucky I did not end up stuffing the plane. I am pretty sure 99 percent of the pilots out there could not tell the difference between the two in operation regarding the slop. I usually see planes with a more slop in the linkage its self than you will ever find in a metal gear servo.

See Ya,


Pat


Quote:
Originally Posted by target
Here's a newbie question for you all:

Is it a no-no to use a 3421SA on the stabilator, due to a risk of stripping it?
As I understand, the "SA" variant will have less slop, something I would think is very desirable for a pitch servo. But not if it's risky.

Thanks for varied opinions.

Target
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Old Jan 19, 2009, 04:05 PM
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United States, CA, Midway City
Joined Dec 2003
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Quote:
Originally Posted by pmccleave
Target,

I usually see planes with a more slop in the linkage its self than you will ever find in a metal gear servo.

See Ya,

Pat
This is true Pat. I've bought a few used planes when I first started and have trace the slop to these areas.

If there is slop, the first place I look is the clevis pin to brass horns. Replace them horns and if there is still slop, it would tend be in the servo arm to clevis pin connection. Then after that it would be in the gear train.

Now at deflection there could be more "slop" introduce by the use of inequate rods or bent rods that bend even more under use. But where I have also found, and most miss, is in the clevis pins themselves as detailed in my quick and crude drawing below.

That's why I only use to use the multiplex clevis exclusively. The pins enter the other side of the clevis with a point so it stays "center".
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Old Jan 19, 2009, 04:38 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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I was mainly concerned with the abuse the elevator servo might see in a contest use, where there will be "dork" landings. Does the elevator servo, on say, the Xplorer see much force from a dork landing???
In slope landings, tail-slap is a similar concern, but most slopers are v-tailed these days, so the servos for the tail have an easy job, and you don't really need a metal geared servo there. But try to find a resin geared 761...

So, we have Pat chiming in on the side of caution, stating that the amount of gear slop is negligable, and there is safety in the gear train.
Gordy and others prefer the resing gears for slop-free operation.


I guess the REAL QUESTION should be:

Has anybody stripped a 3421SA on the elevator of a cross tail from a dork landing with no other damages being incurred??

Thanks, Target

PS, Tuan-
Ever try the MPJet clevises with the plastic body and seperate brass pins that can be replaced?? I love them.
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Old Jan 19, 2009, 05:05 PM
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Chicago IL
Joined Apr 2004
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Target, I am unaware what damage it had seen previously but Peter Goldsmith of JR stripped a 3421SA elevator servo on his Pike Perfect at the NATS. He stripped in on launch and lost at least one other servo in the wing. He still flew for several minutes using the trailing edge camber and his rudder for control.

Personally, I'd stick with the original 3421 over the SA.

Jeff
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Old Jan 19, 2009, 06:23 PM
Fly R/C writer
Redlands, Ca
Joined Dec 2004
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Hi Target,

I use the 3421 metal gear servos for flaps, and everything else gets 368's. I'm one of the ones who found out the hard way about mylon gears and huge flaps...they don't mix. (ya know the saying, friends don't let friends use nylon gears in the flaps!). The really good thing about the 3421 is that the metal gears will go right into the nylon gears' place, which is normally not the case.

Tuan, as for that flap being resistant, you do have to break in the hinge line to free it up, especially for an RDS set-up. I simply run the blade of a wide screwdriver over the hinge line from inside the flap while holding the flap wayyyy past 90-degrees. It sounds bad, it feels bad when you do it, but it works.

Mike
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Old Jan 19, 2009, 06:39 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Quote:
Originally Posted by mlee8249
Hi Target,

I use the 3421 metal gear servos for flaps, and everything else gets 368's. I'm one of the ones who found out the hard way about mylon gears and huge flaps...they don't mix. (ya know the saying, friends don't let friends use nylon gears in the flaps!). The really good thing about the 3421 is that the metal gears will go right into the nylon gears' place, which is normally not the case.


Mike
Hi, Mike-

I think you only picked up part of my post; my question was not if it is deemed acceptable practice to use 3421SA's on the flaps; I most certainly would not!

My question, is, what about using the 3421SA for the elevator?? How large is the risk, and how great is the gain??
It's starting to sound like the safe choice is the metal geared servo for contest use, and some may be using the SA in planes that see less spot landings.
What is your vote?

Thanks, Target

PS, Thanks, Jeff for the heads up on a near mishap; I'm leaning towards the mg version.
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Old Jan 19, 2009, 06:53 PM
Fly R/C writer
Redlands, Ca
Joined Dec 2004
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Oops, sorry about that. I wouldn't have any hesitation of using the SA at the elevator. It is strong enough, torquey enough and fast enough. I prefer a very fast elevator servo. As for surviving a dork landing, there are a couple of dynamics at work here. One is the shock itself, and that is simply taking anything electronic and seeing if it will survive G-forces. Then there is the force of a pushrod impacting on the control horn of the servo. There will be some, depanding on how hard you hit, what the control rod is made from, what's on the other end of that rod, and how much flex is in the tail boom when you impact. If you watch some slow motion films of sailplanes on dork landings, you would be surprised as to how much flex occurs! It's quite a bit. I have seen some where I thought it was pretty amazing that the boom did not snap. And all that stress is hitting the servo arm. But I have used the 3421SA at the elevator and never had an issue.
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Old Jan 19, 2009, 06:53 PM
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San Diego, CA
Joined Aug 2004
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From the guy who is not scared to give the plane a good dork, it is acceptable to use the SA servo on the stab. I use them on my vtails all day long.

I would also use a tight MG.

In order of my personal prefference

1- a very tight MG
2- a SA
3- a sloppy anything is unacceptable

Another choice is to use one of the newer servos based off the 3421. I still go with the 3421mg for flaps, but switch to the faster 3400 or XXX. there are a few new JR digis with the 3421 case that are even faster, and MG. they seem to have very tight geartrains.
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Old Jan 19, 2009, 06:56 PM
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Chicago IL
Joined Apr 2004
924 Posts
I just received two of the z3650 they're lightning fast at .09 sec but have less torque at 42 ounces. They're going on Supra stabs and I'll report back after I fly them.

They use the same case as the 3421 and are a drop in replacement.

Jeff
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Old Jan 19, 2009, 07:02 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Well, I have a grab bag of opinions, none too conclusive, though.....

I have time; I haven't picked up the plane yet.....


Thanks for all the feed back to this question.

T
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Old Jan 19, 2009, 10:20 PM
Registered User
Stamford, CT
Joined Feb 2002
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I'm not trying to beat the nylon gear drum too loudly. I would certainly not use them on flaps. But flaps don't require the precision of an elevator. I've never had a problem with nylon on an elevator even after severe dorks. My friends who fly helicopters gave up on metal because they start out nice and tight and then loosen up over time and need to replaced. The helis have a lot more vibration than a sailplane and I am sure this is an issue with wear. I'm equally sure, however, that over time metal gears will also wear in a sailplane and create some gear lash.

George
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Old Jan 19, 2009, 10:38 PM
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United States, TX, Weatherford
Joined Nov 2002
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I've been working on my hinge lines tonight. I did all the flexing and running the screwdriver down it and all... and it really works nicely. Again, thanks Mike... I just about have all the bugs and gremlins worked out of this thing and have toned down my throws, added some exponential, and added some more elevator compensation. I still need to replace one set of servo gears before permanently closing the doors on my servos. Right now I just have them taped on. What do you guys use for attaching the servo covers in a semi-permanent fashion. I was considering clear silicone... but whudda I know?

Jack

By the way... I did one high-speed run yesterday and this thing is really quiet... How can I startle my slimer buds with such a quiet airplane?
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Old Jan 19, 2009, 10:45 PM
Eggcellent...
tewatson's Avatar
United States, CA, Orange
Joined Oct 2006
2,468 Posts
Quote:
Originally Posted by schrederman
What do you guys use for attaching the servo covers in a semi-permanent fashion.
3M #600 tape, 1/2" wide, with pieces cut so the corners don't overlap. Properly applied, you have to be less than one foot away to see it and it's field serviceable.

Tom
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