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Dec 29, 2013, 10:25 PM
Registered User
Avaldes's Avatar

New Futaba servos!!

After the thread last week about the new high voltage SVI servos, I had to get my hands on some to evalute. Full disclosure is that I am sponsored by Futaba and have flown their products since 1995. Having said that, I am truly excited by these new servos as they represent some very interesting new technology. I have repeatedly said that the old S3150 was a great servo 8 years ago. They have been very reliable/robust for me and they are very popular in Europe...much more so than here in the US. However they are not the fastest or strongest servos anymore and they are not rated for LiFe batteries by Futaba. Even though I have run them on LiFe batteries for years, many people gripe about the rating...understandable.

In the mean time, competitors like JR/MKS and Airtronics have introduced new servos that are VERY popular.

This weekend I received three new servos:

- The BLS173SVI for elevator use. It is a direct fit replacement for the JR DS3421. Below is a picture of this servo installed in my Maxa. So far I really like it...smoking fast (.11s) with 95 ounces of torque on LiFe. Even faster and more torque if you run LiPo's (I don't). This servo is fully programable, including the controller gains inside of the servo. I haven't tried that yet.

- The S3173SVI and S3174SV. The difference is that SVI models have a new connector on the servo case and the SV models use standard servo cables. The 3173 has the same mounting lug configuration and size as the DS398. They too are fully programmable.

All of these servos are compatible with standard PWM cables and receivers. They are also compatible with Sbus and Sbus2 radios. Since I don't have any Sbus receivers (yet) I am just running them in standard PWM mode. One neat feature is that the wing servos have two connectors on the case, enabling you to daisy chain the servos together in the wing, reducing the total amount of wires needed.

Further, if you run them in the Sbus configuration, they use serial communication and only need two wires (positive and negative) per servo. So it would be possible to substantially reduce the amount of wires and cable weight in an airplane.

One benefit of the Sbus2 receivers are that they include servo position in the telemetry, if you run the SV/SVI servos..very cool.

I hope these servos perform as well as they can. The new features are great and I hope the centering and ruggedness are up to my high standards. The BLS elevator servo is a real winner right out of the box. Time will tell if they meet my expectations.
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Dec 30, 2013, 03:48 AM
Registered User
Tuomo's Avatar
Nice to have new Futaba servos around. It was about time to get company to the excellent 3150 and 3156. Higher voltage is future and those of us who are using Futaba 2.4 system will appreciate the s-bus feature.

Just a note. It is funny that these servos are same size as similar JR servos. To me this speaks about the market position JR servos have in F3X.
Dec 30, 2013, 05:11 AM
F3B and F3K
RetoF3X's Avatar
Hi Aaron

Nice servos, what is the upper operational voltage (per specs).

I do not like your rudder servo installation, that servo horn looks way too long and you will probably only run a small fraction of the available servo throw.

Can´t you flip it and use a short servo horn for best mechanical advantage?

Dec 30, 2013, 09:13 AM
Registered User
Are these Futaba servos available in the US?
Dec 30, 2013, 09:16 AM
Registered User
Found them. Tower has them.
Dec 30, 2013, 09:50 AM
satinet's Avatar
They don't seem that new. High voltage is great but the problem with s3150s is the relatively sloppy gear train. Are the mechanical parts different?
Dec 30, 2013, 08:42 PM
Registered User
Avaldes's Avatar
Tom - I have no idea if the mechanics are the same. I have not had a reason to open one up yet. Just doing the "tick-tick" test seems to be very tight, equivalent to the 398 it is pictured with. I am installing this set in a windy Maxa4 and will see if/how much slop they develop.

Tuomo - Interestingly the 3150/3155/3172/3173/3174 and the JR 378/388/390/398 all have the same basic case dimensions. The JR servos appeared flatter than the Futabas, but when you compare the actual "box" they are the same. The mounting features on the new servos really accentuates that similarity.

JR makes great servos. It is hard to fault any of their models. In the past 15 years, the only "so-so" servo they made was the 378/388...and I think they addressed those centering/slop issues on the 398/390 servos. The 368 was a classic servo, in my mind it was truely a benchmark for small servos because it was so tough. I have repeatedly stressed to Futaba that JR/MKS/Airtrx have done a great job of listening to the requirements of F3X soaring pilots and developed servos specifically to meet those demands. I hope that these new servos will take Futaba a step closer to the competitors. I can't wait for brushless wing servos to come out. I think that will be a great day for all modelers. Serial (Sbus) servos with brushless motors will be a fabulous technology for our airplanes.

Reto - The box says "Rated Voltage = 6.0-7.4V" and "Operating Voltage = 4.0-8.4V", weight = 20.6g.

I know the rudder linkage isn't optimized, however the horn on the Maxa rudder is very long so you need a LOT of servo arm to get serious rudder travel. Right now I have it set to bang lock to lock at full rudder stick. So it seems to be good enough for now. I was circle towing today and it worked just great. It is easy to forget how nice and linear a standard tail is for rudder authority compared to the little V's we fly in the F3B models. If it bothers me in the future, it is easy enough to flip the servo like you suggest.

PS - The new elevator servo (BLS173) is KILLER! I can feel the difference in speed/torque on tow. It made controlling and adjusting the zoom trajectory easier. It might be mental but I'll take everything I can get These are backordered right now, but when they come back in stock I might even put these on the V's for F3B use. Certainly can't hurt anything.
Last edited by Avaldes; Dec 30, 2013 at 09:27 PM.
Jan 01, 2014, 11:58 PM
Registered User
Avaldes's Avatar
Upoon closer inspection, I noted that the gears on the new servos are silver in color as opposed to the brass color of the previous 3150 servos. I would guess that this is a chrome or nickel coating to add hardness to the gears in an attempt to extend the life and reduce slop. Again I have not opened any of the new servos, but the harder gears would certainly be a good start.
Jan 02, 2014, 01:42 AM
Registered User
Tuomo's Avatar
Any idea how the S3174SV compares in holding power to 3150 at low voltage, say 5 volts? (I am still stuck with the ancient 4xnimh power in my planes.)

3150 is really a very nice and dependable wing servo. It has been my primary aileron servo for the past 10 years but for flaps it was always a little marginal in power. When JR378 ( I use the Graupner version) came out it solved the problem, although at the expense of poor centering. Now JR has fortunately got better so that Futaba faces a really stiff competition. I will definitely check how they compare.

As a criticism I must say that Futaba provides us with a surprisingly crappy set of servo horns. The small horn is too flimsy to used in wing and the large one… …where I am supposed to use that??? JR/Graupner servo horns are elegant to Futaba offerings, and often give real benefits in installation (without hacking the plain round disk).

If S3174SV behaves as nicely as 3150, but is faster, and does not generate much slop in use, I see no problem using it in elevator. BLS173SVI type servo is nice and good, but light air F3J planes are ok with lesser equipments.
Jan 02, 2014, 01:42 PM
MSgt, USAF Ret.
Jim Frahm's Avatar
What's the speed and torque of the 3173 servo?

Jan 02, 2014, 03:20 PM
Stable genius
vespa's Avatar
The S3150 was replaced by the low/high voltage programmable S3171SB/S3172SV in 2009.
The S3172SV was replaced this month by the S3174SV.
None of the above servos are wing-mountable and all are extremely slow. Maybe useful if your flap servos are mounted in the fuselage? With grommets.

The S3173SVi is Futaba's first F3X sized wing servo and it really appears to have the potential to be the world's ultimate flap servo. It's very powerful, very configurable, very tight, and has the best possible mounting lug configuration. It's not as slow as the S3150 (nothing is) but it's not a normal-speed servo either, at 6.5V I'd say it's very similar to the JR DS368 on 5V. On 8V it's about the speed of a 398HV at 6.5V, IMO barely fast enough for a primary controls on a floater but fantastic for flaps. Hopefully they will release an aileron-speed version soon so we can fit the whole wing with these great looking servos. A wing-mountable version of the promising new S3270SV would round out the whole family! It's a great idea to have removable wire leads protruding from the bottom so that they don't interfere with the mounts and the plugs are even 100% Voltz-compatible in case you are as old as I am. Unfortunately, the leads are standard length instead of wing-servo length.

The 3173 geartrain is identical in style and materials to the 3155 and 3172, though each is a slightly different ratio. I don't have a brass 3150 to compare against but I'd expect they are also identical except for the brass output gear. I did, however, find one significant new feature in the 3173 geartrain housing: The intermediate gear axle shafts of the 3173 are very firmly press-fit into the top case and precisely slip-fit into machined bronze inserts in the mid-case. Older servos simply have these shafts slip-fit into the plastic at both ends. These shafts do not rotate so it's not an attempt to address wear but it likely reduces the tendency for the plastic support to deform under severe loads and it's conceivable that they might even post-machine the bronze inserts after molding for higher precision.

Programmability is awesome! Quantum leaps above any servo ever made! Travel can be increased all the way up to (and beyond) the mechanical stops in the geartrain for a total of 163 degrees of motion. Not only is the usual stuff programmable (deadband, center, reverse, etc.) but some really great advanced things also, such as acceleration and deceleration (separately) which can allow you to tune the servo for fantasic response without the risk of harmonic resonance. The 14SG manual explains some of the programming terms on p.65. Below is the list of programmable options and my best guess at translating the Futabese terms:
  • Travel: Caution: travel can be increased beyond mechanical limits of servo gears. Up to 163 deg total travel is possible between mechanical stops.
  • Neutral: +/-30 deg offset
  • Deadband (0.03-3.98deg): Positioning accuracy. Default 0.145deg. Lower values will center better and buzz more.
  • Speed (0.047s-12s/60deg): Slow motion.
  • Reverse: Norm/Rev
  • Type (NORM/RETR/OLP): Retract mode will temporarily increase deadband if stalled for >30 seconds with no change in input signal. Likely very wise for F3X flaps. OLP is some undocumented "Overload Protection" mode.
  • Smoother: High pass input filter. Ignores stick shaking yet still responds immediately to sudden inputs. Default On.
  • Softstart (0.8s-12s/60deg): Servo will slowly center upon power-on. Any movement of the stick will cancel the softstart and immediately revert to normal full-speed operation.
  • Boost (3%-45%): I couldn't see any effect thru testing. The manual seems to suggest it will increase torque for small deflections at the expense of "roughness". It's worth noting that it is always on, but if set to "on" it is more on.
  • Damper (0-255): Derivative term of sorts (deceleration only). Low values will allow overshoot/oscillation. High values will make the servo slow when nearing final position. Default=112.
  • Stretch (.125-8): Proportional term. Larger values improve acceleration and small deflection torque at the risk of oscillation. Default=2.
  • Buzzer: Faint beep on power-up and maybe upon lost signal?
  • Init: Reset to factory defaults.
  • Write: Servo will respond immediately to all parameter changes but will not retain any changes thru a power cycle unless "Write" is executed.
  • Recall: Connects to servo and reads current settings.

Below are some photos comparing the 3173 to the 3172 and 3155. The motor/pcb is pretty tightly press-fit into the case so I did not remove it to examine the potentiometer.
Last edited by vespa; Jan 02, 2014 at 06:59 PM.
Jan 02, 2014, 06:56 PM
dnbarrie11's Avatar

New Servos

I have used the 3150 for flaps on 6.6 volts for years. Same with the 3155's. I know they do not show voltage above 4.8 but the Robbe site did. The 3150 and 55 on 6.6 were quite fast.

The 3171 was the first SBus servo in this case. It was a great servo. I also ran that on 6.6 volts to make it fast and powerful. Disappointed it was discontinued.

The 3172 came out last year as a HV servo but was extremely slow. No way to juice it and make it faster. The nice thing about all of the above is that there are servo frames for mounting.

Now the 3173 comes out with an all new mounting system that is not retrofittable to current frames. Not sure why Futaba didn't provide both type of lugs. I received several of these and they are quick and powerful. The plugs on the bottom are nice and functional for SBus. I'm not sure how to mount these in a wing and still take advantage of the thinness. Had they provided both lugs we would have had a choice. And, there is no way to mount them in a fuselage.

The 3174 is out and is the same as 3150 series that is SBus and HV. Same stats as the 3173. It would have been nice to have the option of the SBus micro plugs on the bottom. Should have some of these tomorrow and I'm going to replace my 3171's. So it appears the goal was to have a wing version and fuselage version.

I'm really excited about the 3270's. They are slightly larger than the 3156 and will have two varieties. One with the micro plugs on the bottom and a standard plug type. Don't think there will be a "wing" type nor do I think it is needed if a frame becomes available. The 3270's will be very fast and have torque capable of handling flaps one Open class planes. These should be available in the next couple of weeks.

Contrary to what was mentioned by a couple of others, I never found any issues with gear train slop. With 6.6 volts, all of the 31xx series (except the 3172) were quite fast. Especially the 3155. I had several hundred flights on these with no slop noted.

In summary, Futaba has some great servos in the game with something for everyone. Good times!!

Darwin N. Barrie
Chandler, AZ
Team Futaba
Jan 02, 2014, 08:09 PM
Stable genius
vespa's Avatar
What do you mean by "both types of lugs"? Like a 94761? IMO that's a terrible compromise. The 761 has really lousy wing mounts that are too far from the "top" where the loads are centered and there's no way to make screws sit flush. Countersinking the holes inevitably breaks the brittle lugs. The 761 also has terrible fuselage mounts. Grommets? Really? And 4 of them? What year is this? The 761 mounts were so unbelievably stupid that companies actually sold "frames" for them since the servos don't mount well on their own.

Futaba did it right. They gave us "both types of lugs" -- the 3173 and the 3174. Unfortunately, they're still partying like it's 1979 with those crazy grommet things, but at least they genuinely offered both types of lugs. Seems they're hot on JR's trail. But then again, why would you want a tall thin servo like that in a fuselage? That's what 3270's are for. Or DS95's, or 761's, or BLS173's, or DS3517's, or...

What is your concern about mounting them to a wing and taking advantage of the thinness? Just get some lumber from the shed, a couple of old screws, and BAM!!! you gotcha some 11mm thick servos installed. I can't imagine a better servo mounting method than the 3173. Unlike the crappy 761, it mounts flush, and unlike the crappy 6125 or DS095, there's enough room for screw threads.

The 3174 doesn't have bottom plugs because it's a fuselage servo and would have clearance problems if a plug protruded from the bottom. Also since it's in the fuselage, it's likely close to the RX and thus does not need any daisy-chaining ability. Note that the 3270, also being a fuselage servo, has just one plug on the side, not two on the bottom.

P.S. I agree that the 3150 on 6.6V is fast enough for flaps.
Jan 02, 2014, 08:37 PM
Daryl Perkins's Avatar
Vespa, lmao! That's funny... I was on the phone with Darwin last week and told him exactly the same thing. The 3173 is pretty much perfect.

Darwin's personal gripe was that it wasn't a drop in replacement for the 3150. That doesn't mean the new lug placement isn't better, it just doesn't fit into his old airplanes. AND... since I never used a 3150, I couldn't care less.

Vespa, our only difference of opinion... The 3150 isn't fast enough for much of anything... even on 12 volts...
Jan 02, 2014, 09:48 PM
Stable genius
vespa's Avatar
Finally! Someone who shares my neurotic obsession with fast servos!

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