Robert_Ellis

I am about to put together a 25% Sukhoi and was planning on using HS 5645MGs. Seem to be a good match for torque and speed. But they start to add up $$$ wise.

If there are 2 servos of the same torque/speed rating, metal gears etc., one is digital, the other is not. I have a Futaba 7CAP computer radio which allows me to set endpoints, sub trim etc.

1. What is the benefit of the digital servos?
2. Do you need to get the Hitec programmer to actually use the capabilites of the dig servo to set things like that actually in the servo?
3. If I just set end points etc in my radio does it matter then whether I have digital servos?

Thanks in advance.

- Rob

MinnFlyer

My Feedback: (4)

1 - With Digital Servos, you get to pay more money and they run down your batteries quicker.

2 - No

3 - No

Good deal, huh?

Jetdesign

My Feedback: (8)

Digital servos do a better job of holding a given position, most importantly the center position. They constantly are seeking to find the exact same center position, and use all their strength to find the position, and thus are using more energy. I compared digital to analog servos in my Venus 40, and noticed that with two different analog servos, the rudder didn't return to the same position when I input right ruder then left rudder. What does this mean? Turn with rudder and the plane feels like it's a click or two out of trim, so you adjust with trim. Then you turn the other way and you're like 4 clicks out of trim.

The digital servo returned to the same position, at least as I could tell by naked eye and use of a straight edge. I have converted all my planes to digital servos (they are all precision aircraft). For sport flying, or a trainer or something, maybe it's not worth the extra money. But, $30 each, and you can use them in your next plane, etc. So I only buy digitals now.

dhal22

so for sport flying we use analog and for competition we would use digital? sounds simple to me.

david

Gray Beard

My Feedback: (-1)

This may be A first, I'm going to agree with Joe!!!!!!!!!!!!!!!! I had to be talked into going digital, I couldn't force myself into spending the extra money. Then one day I flew two CG Extras, one with and one without digitals and was sold. Like Joe, my first set was the cheap HTs but they were good enough for what I needed. On some of my servos I have today you couldn't get the gear set for Joes 30 bucks. I still use analogs but on my lower end planes or planes that just don't care what you put in them. I just finished rebuilding my CG Extra and went with analogs {605s and 635s} on elevator halves and ailerons but because I do A lot of knife work I put in A 6985 digital on the rudder. A 1/4 scale Sukhoi without good servos is just wrong, the dark side is cheaper, not better!!

Robert_Ellis

OK. I was thinking that was the case - that they were better at holding and returning to zero. Thanks much.

- Rob

sir crashallot

a friend of mine had that problem. he created a little system using 2 little springs and they pulled the rudder back to the origional place. it worked quite well too, he used it on his piper cub as when he was taking off the rudder was conected to the tail wheel and it would slightly move and cause the plane to try take off sideways which i had the same problem too using a steerabl tail wheel on my old flair cub. But in my case i was learning to take off and didnt react quick enough and went home with a plane that did its own clipped wing conversion. my solution was i fitted a dragable tail wheel instead never had the problem again. but my friend persisted wit his and devised the spring method. I supose its a great little thing if you dont want the expense and waisted battery usage. il try dig up a dagram and scan it 4 on here

BalsaBob

My Feedback: (24)

Digital servos do not jump/twitch when you turn a 2.4 receiver system on. In my planes that have retracts (which still have all analog servos) I replaced my small analog retract servo with a small digital servo. The jump/twitch of analog servos on control surfaces (on power up) is not necessarily a problem ... but I did not want any movement at all on the retract servo on power up.

Lomcevak Duck

My Feedback: (14)

I used to think digitals were a waste of developement on an already saturated field of excellent servos. That is, until I bought some. I have found that by using digital servos in a setup with minimal linkage slop, my flying has become much more precise. It has become smoother and the aircraft seems to react imediately instead of waddling through high throw manuvers.

I have been buying Spektrum DS821 standard size digitals (these are rebadged JR digitals) from Ebay. If you look closely you can find them every so often for the same price as decent analog standard servos. I have not noticed any decrease in battery performance and these servos have held up quite well for about a year now on a .90 size profile. They are showing no signs of tooth loss or center point slop, and I have not been nice to them.

JohnW

My Feedback: (6)

Like others have stated, the benefit of digitals is their ability to deliver their torque much quicker when deflected, thus holding their commanded position better. With analogs, when the control surface is being deflected by airstream forces, the analog servo slowly starts feeding the motor power, but full power isn't delivered until the surface has been pushed quite far from its commanded position. Digitals can apply full power with much less deflection. The result is the digitals hold their position better. You can notice this in the air, mainly with trim, but under high loads, such as seen with some aerobatics, you can also tell the difference. Everything else is basically the same, i.e., Torque is torque and speed is speed.

Servos can make the difference between a so-so plane, and a really good flying plane. They won’t on every plane and some pilots don’t care. But for those of us that do care and have planes and flying styles that benefit from the extra holding power of precise servos, really like the higher end digitals. My “eye opener” was when a fellow pilot “made” me try a high end analog coreless servo many years ago (before digitals) on ailerons. Did a side by side comparison on the same day. The difference was very noticeable. Since then, servo precision has always been important to me. To me, they are not a waste of money and I could care less if they pull a bit more current from my battery if the plane fly’s better.

One note, just because a servo is digital doesn't automatically mean it is more precise or better than an analog. Deadband, pot quality, motor type, gear train and lash, etc. all still come into play. I built a testing rig that checked the ability of a servo to return to center after it was deflected with no load. I tested several new servos, JR8411s, HS645MG, and others. The winner, by a large margin, was a used Futaba 9101.... that's an analog coreless (discontinued I think.) Don't get me wrong, the other servos had more torque and I like and use JR84XX series servos, just making a point. Low torque operations, like throttle, I'll use a analog 9101, very precise.

Taurus Flyer

Rob,

I read this thread a little late, but do not see any post with the next information.

The analog servo’s do use the power for the positioning and holding their position synchronized on the modulation pulse train.
Result is that they all separate use electrical power from the battery on other moments.
Maximum moment current from the battery is the maximum current from the servo with maximum load. (and of course the receiver!)
There is one situation you can have the double load, that’s when using two servo's parallel with a Y harness or mixer in the plane (ailerons!).

Digitals use the power by way of an independent pulse train from the battery so there is no separation or synchronization.
More servos’ can use electrical power on the (fractional?)same moments.
Wiring from the battery to the servo dividing up has to be better , also the internal resistance of the battery has to be lower when using digitals for this reasons to prevent voltage drop when electrical power is used under heavy loads!

Cees