When I bought my Futaba 7C, it came with 3151 digital servos, so these are what went into my LT-40. I have just bought a Twist, and thought I'd get a flight pack with 3004 servos, put these in the LT-40 and swap the digitals into the Twist. Whats the benefit of digital servos? For an aircraft like the Twist would it be worth the time to swap the servos?

Thanks!

Peter

Hi Pete

from what i can understand, digital servos are alot faster and more accurate that anolouge servos but they suck much more battery. If the LT-40 is a trainer i would use the normal servos in it and put the digitals in the twist. Digital servos are really only needed for 3D and patten aerobatice where you need fast precise movements. for a sports model i wouldnt bother with them mainly because of the price.
I hope i helped

Dean

Actually, you can easily look up the specs for almost any servo.
And actually, speed isn't really a function of the type control used in the servo.

For example, if you go to Tower's online catalogue, the specs for those two servos are there.

The two are almost dead equal in specs.
They're both about the same speed, .21 versus .23
They both put out about the same torque, 43 versus 44
And they're both about the same weight, 1.5 versus 1.3

However, they differ SOMEWHAT in price, $35 versus $13.
You can actually buy 3 of the 3004s for $2 more than one of the 3151s.

I asked the same question of at my LHS; About the same servos. Here are the specs:

S3151($34.99 at Tower)
Speed: 0.21 sec/60 degrees at 4.8V
0.17 sec/60 degrees at 6.0V
Torque: 43 oz-in @ 4.8V
54 oz-in @ 6.0V
Weight: 1.5oz (42g)

S3004 ($12.99 at Tower)
Speed: 0.23 sec/60 degrees at 4.8V
0.19 sec/60 degrees at 6.0V
Torque: 44 oz-in (3.2 kg-cm) at 4.8V
57 oz-in (4.1 kg-cm) at 6.0V
Weight: 1.3 ounce (37.2 grams)

And then one you did not mention...

S3003 ($19.99 at Tower)
Speed: 0.23sec/60 deg @ 4.8V
0.16 sec/60 deg @ 6V.
Torque: 44 oz-in @ 4.8V
56.8 oz-in @ 6V
Weight: 1.3 oz (37.2g)

As you can see all these servos are nearly identical in performance. My LHS said that the S3151 servos would give a smother transition in movement but due to there frequent error checking They will use your RX battery up faster ( I have no idea how much faster.) For the money Go with the S3003's I imagine your flight pack came with 4 S3151 servos well Sell those on ebay to some tech junky and go buy 10 S3003 or S3004's so you are ready for the next plane after that.

Digital servos in an LT-40 is a bit of overkill. So, if you're going to get new, less precise servos, go ahead with the swap. I use Futaba S3004 or Hitec HS-425 servos for most applications. I've got a few S3003 servos but since they don't have ball bearings, I usually only use those for the throttle channel.

I've yet to find a deal on a complete flight pack that can beat careful shopping for the individual components. The three receivers I use are Futaba R127DF, Hitec Electron 6, and Tower System 3000. The Tower receiver has been a good deal because it comes with a crystal. It used to be 7 channels, and from reports was identical to the R127DF. According to Tower it's now an 8 channel. I haven't heard who makes it.

When I know I'll be putting together a new plane, I start watching for sales at places like Tower, Servo City, Hobby People, and a few others.

Edit: Almost forgot, another reason I buy components instead of flight packs is so I can buy a stronger NiMH receiver pack instead of the nicads that come with many of the prepackaged receiver packs.

Tower says the S3001, S3003, and S3004 have high current draw. They say nothing about the S3151 having high current draw. Then the LHS says the S3151 ( digital ) drains your battery quicker. Could some one explain this a little deeper? Thanks

Well now, here is something that doesn't happen too often...... I just don't have anything else to add to the discussion. Chuck and others have hit the nail on the head.

Ken

Bubba, the digital servos have an amplifier that updates the motor with the stick position 3 or 4 times as fast as an anaolog amp. Speed and torque is more a function of gearing than servo amp, there's cheap digitals and good analogs.

The fact that the update pulse occurs more often is why they use more current.

Here's a white paper from Futaba on the details.

http://www.futaba-rc.com/servos/digitalservos.pdf

Something that you will discover is that Tower's descriptions, or recommendations, are not always always correct. For example, their recommendation that trainer planes should have wooden props has been discussed widely here. The concensus was that it's a bad recommendation. However, no matter how many times people tell Tower about it, they cling stubbornly to their recommendation.

From what I have read elsewere, In an rc plane application, we couldn't tell the difference between an analog or a digital servo by flying the plane. I think I will go with a bb analog servo and spend the savings on a better battery.

Hmm... sell more props maybe.

I have never broken a plastic/nylon/whatever it is prop when there was not more broken things that I was more worried about.

Just for informational purposes I want to address this so you guys will understand what you get from a digital servo.


In a trainer for a new pilot this is probably true. HOWEVER, it is very different and it is performing differently than an analog servo. As your skills advance as as you fly more demanding planes you will most certaily be able to tell the difference.

Heres the basics of what the difference is in general laymans terms.

An analog servo receives a signal from your stick via the transmitter. This signal basically tells the receiver to tell the servo to move by sending a certain amount of power to the servo. BASICALLY, lets say it tells the servo to send x amount of current to the servo. Well, lets say the plane is flying fast enough that the air pushes back hard enough to push back against the surface, and the servo give a little and the surfaces loses some of that deflection. The servo doesnt care, it did what it was asked to do. If the surface force overcomes the servos force,,, oh well. Its about how the servo goes about moving. With an analog the stick tells it to provide a certain amount of current,,, thinking that X amount of current = so many degrees of deflection... Which was true with a static load on the control surface with the plane sitting still on the ground..

You as the pilot correct for this by adding more elevator without even realizing it, it causes mushy response. But with a small slow flying plane that is typically lumbering along you(I) really cant tell the difference.

Now, in a very general respect heres a digital. With a digital servo, the servo BASICALLY is told to deflect say, specifically 10 degrees (not actually, but for illustration). Simply put, a digital servo knows specifically what position its angle of deflection should be given a certail signal from the receiver. If the force of the air on the surface tries to push the digital servo back it adds more power to make sure that it holds its position. It will NOT get pushed back. If it needs to exert more force to hold its deflection angle at 10 degrees it will do so.

What you get with a digital servo is better response. Granted with a trainer and most sport planes they are not necessary. But if you are flying large planes or trying to fly precision aerobatics the digitals give you a more predictable response. It makes a difference, a big difference.

For instance. In my 37% Ulitmate Biplane (which I lost last weekend by the way, top wing sheared off in flight at an IMAC contest) anyway, when I tried to do a "wall" maneuver the plane was mushy and marginally responsive. It would stand straight up but because the servos didnt hold well it would climb in a sharp arc, it didnt just stand straight up vertically and slide forward belly 1st without gaining altitude. This was because the elevator servos were giving in to the extreme force.

( Wall- full up suddenly , the plane suddenly stands up on its tail without climbing and looks like it hit a wall belly 1st. [8D])

Then I put Hitec 5955 digital servos in and I get an instant change. The elevator response is so crisp that at about 30mph when I jam in high rate full up elevator the plane stands straight up and actaully leans back a little without climbing an inch. I go from 30-40mph forward flight to an instant hover in a split second without gaining any altitude. That is what digital servos are for.

Mike, sorry to hear about your loss. Ditto with your post. With time, or maybe I should say experience, you can easily feel the difference between a "good" servo and a not so good servo. How much can be felt depends a lot on the plane and how it is flown. I don't believe digitals benefit just pattern, IMAC and 3D, but any high end servo, digital or analog, would probably be a waste on a trainer. There are cheap digitals, and better digitals. On the same token, there are cheap analogs and better analogs. There really is a difference between servos other than torque, speed and cost. One of the most important things I've learned in my 8 years in this great hobby/sport is don't cut corners on engines and radio gear. I'd much rather run a great engine with good servos in a junky plane, than run a great plane with a junky engine and servos.

Early on in my second year I had a Masters pilot comment that my Ultrasport felt like it was hunting. I wasn't the wiser… it flew just like it always did. He had a spare coreless Futaba 9101 analog, which is a good analog servo, and he insisted we try it on my ailerons as opposed to what ever $7 servo I had in there (This was before digitals were out and coreless analogs were the best available.) I was pretty much convinced this was all a waste of time, but I figured what the heck. So we did the swap at the field, and flew again. I'll have to say it was a noticeable difference. What I always thought was "normal" I realized was dependent upon the servo. There wasn't anything broken or wrong with my $7 servo, it was just much less precise than the 9101, and it was clearly reflected in the air. The next day I bought 9101's all around for the plane and the plane flew much better. It stayed in perfect trim, seemed smoother on controls, more crisp and precise.

I know this subject often ruffles feathers, and there is nothing wrong with a $7 servo. But there IS a difference between a $7 servo and a coreless digital F3A servo besides the price. How much difference you will see/feel will depend upon the plane and your sensitivity. Cheers.

Wow. Thanks Mike and John. That really clears the air for me and a lot of other people too, i assume. I will do the analogs in the trainer and when I get a bigger, better, faster, cooler, kick butt plane, it will for sure get the digitals.

I' been flying for a long time [9 or 10 years] and never bought a servo other than jr 537's. I've had .60 size extras and .60 stingers,and so on. I just bought a little toni with a saito 100 on it and 5 DS811's. The plane flies great . It came with a JR 421 witch is an entry level computer radio and does not have dual rates. I bought it as a ceiling deal at our ONLY local hobby shop and was told it had been flown. It needed a fuel tank , battery and switch. Any way, the specs for the plane are 54oz so the 811's match well,but 2 flights bring a fully charged 1450 millie amp nickell metal down to 80% charge. The aileron servo's do chatter alittle bit but I don't like the power they use.

Mikeeast

Thats a very good explaination of the real time differences in digital vs analog servos. Noted Thanks.