IFlyEm35

My Feedback: (5)

So in the world of endless servo options I am curious what people think the best all around servo specs are. Coming from the world of 120cc+ 3D birds monster torque and speed are king. But are there any jets out there that really need 600 oz of torque and .08 sec transition? It wasnt that long ago that the JR 8411 was the high end of servos and it's only 144 oz.

So my question is for the run of the mill 120 - 210 size jet whats a good torque range? Using a 2.5M Viper with a 180 as a benchmark, would say metal gear 300 oz servos suffice? Would a JR 8411 be enough? What would you use?

Dave Wilshere

Reliable....
Centering...
300oz is about right, but all UK CARF Vipers fly with JR 8411's on the Horizontal tailplane. 8911 on everything else.
Mechanical setup is everything, some designs have terrible linkage and horn set ups!

IFlyEm35

My Feedback: (5)

Good info Dave.

Currently I am using MKS servos and absolutely love them. Wish they had a 400 oz option in the 1200 series but it tops out at 319 oz with the 1220. In my 1:4.75 Hawk I did MKS 599's all the way around but seems way overkill on everything but the tailplane.

Dave Wilshere

Holding power is important too. People say you don't need powerful servos etc. But You cannot overpower a control surface, if the servo is not working hard, it will last forever. My Classic Flash has had the same 8411's in it for 11 years, no thought of changing them even after 100's and 100's flights.
I'm very much a JR servo person, I've sold thousands and the non user problem rate is counted on one hand.
Although there is a batch of 8911's arriving in England any day the 8411 is almost certainly finished and that does worry me after 17 years flying jets and always having that option...

Springbok Flyer

Very valid comment. I cannot but wonder why my 20 year old planes are now suppose to have 10 times the torque they were built with? 20 years ago I installed 1.8 and 2.5 kg servo's in my 1/4 scale Bellanca Decathlon, and 1/5th scale Spitfire - both are still flying today. An Aviation Design Mirage in 1997 used 5kg servo's on the delta configuration running a 10kg AMT turbine. The large Franz Walti/BVM Rafale originally flew with 7.5kg MPX servo's running an AMT Olympus.

Regarding your Viper question - my SM 2.5 Viper with 180 has JR8411's allround, except on rudder and flaps I have JR8711's. So 300oz is more than enough.

Cheers,

Jan

David Gladwin

I recently bought a JR NX8931 servo for evaluation.

I was so impressed I bought another 8 for my Hawks. They are brilliant servos, powerful, wide voltage, tight neutral, ( flutter resistance) no buzz and fully programmable.

Love the slow start function.

Best servo I Have ever encountered.

For smaller applications, the NX 3421 is also an outstanding servo.

I am sometimes asked about servo life expectancy:

I have a couple of BobCats with JR 9411 servos on the rudders. These servos get a lot of stop /starts because of the gyro action, they have a hard life. They have been in service for 16 years and 600 flights.

Those servos still work as well as when they were new and they will stay in position until, and if, they fail, if ever.

David.

Len Todd

CARF UltraFlash specs call for 8411s. Put 8411HVs in the Horizontal Stab. Used HS5585MH (194-236 oz-in) elsewhere except in the rudder. Had a HS7950TH (400+ oz-in) for that simply because of the potential for heat build up in where it is mounted.

Use HS5585MHs almost exclusively in all my giant planes. I am not a 3D flier. But the Sprint and the Checkmate are subjected to sever acro workouts every time they fly. I do have a HS7950TH on the Sprint's flap.

DrScoles

My Feedback: (18)

I always thought we were over doing it with the 500oz torque stuff.... until I put a video camera on my 1/4 scale Yak. Buddy thought my flaps weren't deploying on landing, he had a little digital video camera... he was right. 500oz servo was not pulling the flap down. It came down after I landed! This is on a flap with the potential for lateral binding because of the single servo off-center and only pulling on one track, but still... you'd think 500oz on a fairly smooth flap mech would be sufficient.

Len Todd

A good way to see if your flaps are coming down in flight is to take out any down elevator mix, hit the flaps and watch. Most planes will go up.

My Sprint has one large flap. I put a large torque servo in that one because the HS5585MH (200+ oz-in) stopped working after a few flights. Hitec repaired the servo. The motor was apparently burned out. I was putting in the half flap at full speed to slow it down for eventually landing, and occasionally I would forget to take it off half flap for the whole flight. Did I mention that the Sprint had a P140 in it?

So I agree that a large Flap surface needs a high torque servo. But, on my Giant Extra and Checkmate, I have the 5585s and they handle the huge control surfaces just fine. Also, My Sprint has 5585s except for the flap. They have held up just fine to several hundred flights. We'll see about how they hold up in the Ultra Flash. They held thru the maiden and one subsequent flight. But, ... if the manufacture suggested 8411s, (144 oz-in) then the 5585s are even better.

LGM Graphix

My Feedback: (22)

I can agree with the sentiment that you can't have too much power, but at the same time I've said for years we overpower the crap out of surfaces. I've seen the math for surface loads and often I look at them and think so if that's what is claimed to be needed how did we ever fly anything years ago? I remember back in the day when Futaba standard servo's were 29oz and were used on 1/4 scale cubs, all surfaces. I remember 72" span Extra 300's that used a single servo in the wing with torque rods for ailerons and it was only a 40oz servo.
That's not to say we should put in the bare minimum, but we hardly need 120oz servo's in the same airplanes today.

In 1999 my first Fiberclassics Kangaroo with a RAM750 had JR DS8411 servo's on the Elevon's, just one servo per surface, that's what was called for, that's what I used. I used JR DS168 thin wing servo's for the rudders and a JR NES4131 servo for the nose gear steering. Incidentally, my first successful ducted fan jet was a DL Aeromodels Cyclone (little smaller than a Bandit ARF) and that jet used all analogue servo's, JR NES517's on every surface. Those servo's were only 60oz I believe. Never had any issues there either.
The Jet Model Products Firebird was originally flown using JR DS8231 servo's on all surfaces except the flying stab. My Firebird has 8611's but that's only because I already had them. The Stab servo is the robot Hitec servo (forget the number) but is only 220oz of torque, Tom uses it for it's massive gear train and how tight the servo is.

My CAI Raptor which is a 250+mph jet uses JR NES2721's on all surfaces.

As I say, there is nothing wrong with overpowering surfaces, and I'm not one to cheap out on equipment, but reality is, if I need to buy 7 servo's just for flight surfaces and a $140 servo is more than enough, I'm not going to spend $250 for a high voltage super power servo just to say I have it when it's not really necessary.

Just my opinion.

jofunk

Let me preface this with agreeing that you cannot have too much torque on a control surface. Looking back at what Ziroli and Byron drew and designed into their plans and airframes, giant scale airplanes with torque tubes and bell cranks everywhere and standard servos driving them........ This is not scientific but I was heading to a giant scale meet with a Ziroli Skyraider fuse. strapped to the top of the car. I have a standard servo an each elevator half. Out of curiosity I deflected both elevators up and headed out for the 3 hour trip. On the way I got the car up to a little over 80 MPH. When I got there I was expecting to see the unpowered elevator back to neutral. To my surprise they did not budge. With all that said, I look at 3 D helicopters getting put through the paces and I wonder how the servos survive.

FalconWings

My Feedback: (57)

Search Oli's servo sizing guide. One of the best documents out there. I think it's on his website too.

A little bit of math can go a long way!
Cheers,
David

Dr Honda

My Feedback: (4)

As some post above say... "We over power". And I totally agree.

Most of my jets are budget builds... but no one says they are unsafe. I don't have high end composite models, or strange configurations... so I use regular servos. For safety, and the reduction of slop... I like metal gears. But conventional servos work fine. (non digital) Besides... digitals suck more power. Yes... on a big gasser 3d model... you need large, quick throws... and the power to hold the thrust moving over the surface, so you don't have a choice. But jets (for the most part) have small surfaces, that don't move all that much. The FireBird I just posted about has all Hobbico servos. (rebranded HiTec) I bought them in the marketplace, new in boxes, enough for the entire jet for $65 !!!!!! (pics in my firebird thread) My Shock has all Power HD's in it. (220 oz, metal gear, BB) I think I got them for $12 each. And my "Trainer" Falcon 120 has servos I had sitting around.

It's up to you, and what your wallet will support. But take a look at the surface, and decide if you need a $100 digital... or if a $20 quality servo will do the job.

My 2 cents... take it for what it's worth.

Desertlakesflying

My Feedback: (28)

His fuel tubing advise it spot on so the servo sizing must be on point too.

Meanwhile I'm over here flying 277 oz-in servos that run 6.6v on all surfaces including the barn door flaps on my T-One without any issues at all. Even pushed the speed a little with the landing flaps down to be sure with no issues...

IFlyEm35

My Feedback: (5)

Great info! I am probably going to make the MKS 1220 my standard servo for everything but extreme applications. For those I'll use a 599 or 777. The Savox looks nice as well but I helped my LHS become an MKS distributor and want to give them as much support as possible.

MKS Servos USA :: High Voltage Coreless Servos :: HV1220 (0.115 sec/60° - 23 kg-cm (319 oz/in) @8.2V)

SECRET AGENT

My Feedback: (18)

Dr. Honda, you're a brave man using Hobicos in a jet. The only plane I ever lost to a servo had Hobicos in them. First flight one of the aileron servos went to full deflection and died right there. By saving a couple bucks it cost me in the end. I realize that can potentially happen with any brand, but in 20 years it hasn't. Just my observation.

Dr Honda

My Feedback: (4)

Thing is with Hobbico... you have to know what ones you are looking at. They re-branded a bunch of different companies servo's. Some were better than others. There are some hobbico's that I won't touch. But, with that said... and with what you said... any brand can fail, and even "High End" servos give up once in a while. (but yes, the QC is normally better with the high end stuff) Also... before I mount a new servo... I put them on a cycler and just let them burn-in for 30~40 min. First, it helps seat the motor brushes... but it will also weed out a manufacturing issue. On the FireBird build... I found a faulty Y-connector during the test.

I'm not trying to say to just buy a cheap servo, and "Go-for-it". What I'm trying to say is... our jets (for the most part) don't need supper fast, supper powerful, brushless/coreless digital servos.

Ad Clark III

My Feedback: (8)

I have never flown an airplane and afterward thought, "boy, these servos are just too darn powerful". I have however, almost crashed before due to marginal servo strength. I am also not smart enough to calculate inflight loads, so I just go by a simple equation, just double the servo size(oz)recomendation. If I am still not sure, then throw 50 more oz on top of that for aunt Sally and you should be just about right...lol I also like the fact that a powerfull servo that is not used very hard tends to last a long time/resist flutter and one of my jets turned 20 years old this season. My .02 cents.

David Gladwin

Right on!
Some years ago there were reports of soft elevator response on BVM BobCats and several were lost because of this problem.

When I built my first BC I didn't care for the servo spec. for the elevators or rudders and installed more powerful JR 9411s and never experienced the problem. I understand that many other BobCat owners took my advice and similarly upgraded and found the poor response was cured. Of course, I got a lot of criticism from some quarters for speaking up on this issue, but after 600 flights and 16 years of use, my two BobCats are still going strong on the mostly original servos, one of which is flown with JetsMunt Merlin 100 which makes it rather quick!

The rudder servos must take the biggest hammering, being continuously active with the gyros, they are still as good as new.

Of course, the issue of having enough power to deflect a control surface is not the whole story, as Oli agreed. I always use very powerful servos with very tight holding power as a primary defence again flutter on surfaces which have no mass balancing, it has worked well since my first fast jet, a JPX powered Sagittario.

I belive, too, it is a myth to say that using powerful servos is a waste of battery power. Years ago I did some basic testing and found that power consumption is almost directly related to torque generated but with modern digital servos rather more efficient that earlier analogue units.

You can never have too much power on a surface, but too little can, and has been, fatal.

David.

FalconWings

My Feedback: (57)

Not likely in the jet world. but in the IMAC world when you use multiple servos for a single control surface and don't synch them right, the more powerful they are the earlier they lead to binding and catastrophic power drain. Same goes for full scale. Do take into account if you ever plan on using duplex servos.
David

mauryr

So synch them right and provide an adequate power source that doesn't allow the power drain to become catastrophic...

Len Todd

There is a good article in Model Aviation this month on Digital Servos. Most folks underestimate the required power supply and circuitry needed for digital servos. When you look at the current consumption peaks of digital servos, especially when stopping and starting movement, and figure that the peaks from multiple servos can overlap each other, only then can one see that these tiny Rxer buses are barely adequate for our high torque Digital Servos, let alone high torque conditions. Take a couple simultaneous 9 amp peaks and then look at the voltage drops. You can have the most powerful servo in the world, and if you try to run them thru the typical Rxer buss, you may end up with big issues (i.e. a lot less torque than you think when the voltage drops across that Rxer Buss and long extensions.)

For matching ganged servos, one needs to get the in-line ammeter out and match the linkages and endpoints for minimum current use at max throw (e.g. ammeter in servo one's circuit while adjusting servo #2' linkage and endpoints, etc.) Ganged servos on long ailerons can have different throws because of the aileron narrowing toward the end of the wingtip. Kind of need programmable servos to match them up correctly (i.e. so that the total current stays low at max throw.) Either that or one could put each servo on an individual channel and match them up using individual travel adjusts, if you have the channels available. Or X-Buss or S-buss, etc. which affords individual servo programmability. Either way the end result is to look at the current used and minimize it.

An ammeter also is a good way to find the physical max throw of the surface. If the current goes up significantly you are at the end of the surfaces throw. Many times servo max travel can exceed the surfaces physical throw. Typically not a problem once you set the throws. But on initial setup (i.e. while building) you can stress things and kill servos or weaken hinges, etc. The ol' Hanger 9 in-line ammeter has proven quite handy over the years.

FalconWings

My Feedback: (57)

I don't disagree, but again, added complexity.

Kelly Rohrbach

My Feedback: (1)

I find building another plane more complex than just doing it right the first time.
Tim "the tool man" Taylor was not always wrong. MORE POWER !!!!

David Gladwin

I agree with much of what Len Todd, wrote but I go further to ensure perfect servo matching. I have two Airworld Hawks which have two hard connected servos on the stabs.

I put a hangar 9 meter in BOTH servo cables. I then adjust them at centres and end points so that current draw is equal.

I then slowly move the stab and monitor currents, and at any point at which there is a current increase, suggesting a mismatch, however slight, I then adjust one servo at those points until again currents are equal. Even tiny adjustments, and the Weatronic system is brilliant for this, can produce significant current reductions. My JR 28 x can also do this very well but does not have the resolution or adjustment points of the Weatronic system.

As a result the servos are moving in perfect unison, giving them an easy and long life. After 8 years and 200 flights, this system is obviously working well.

Not it sure why falcon wings would suggest that adding an adequate power source, ie two batteries, is adding complexity without significant safety benefits !

David