There seems to be quite a bit of discussion over the last few days whether the carb. return spring should be disconnected or not. It was a slow day here today with Obama in town (can't go fly until he leaves the area) so I thought I'd dig out a few servos and carbs, jig up a test fixture and actually measure how much current is required to hold a throttle at full on position when the spring is in place. Also, I tried to come up with a few comparisons as a measure of how much power is required. Keep in mind this is at full throttle and though I didn't make a column for half throttle, the current draw was considerably less at partial open positions.

This is a purely unscientific test but the numbers don't lie. The servo current required to overcome the spring tension is very low by any comparison. In fact so low that if you can hear your digital servos buzzing they might be drawing more current than if they were connected to a spring in place. I'm sure that just sitting on the ground, if you have large control surfaces, those servos are probably drawing more current than the throttle servo with a spring in place.

I don't expect much comment about this as charts and facts rarely produce much of a discussion in the forums and I'll leave it at that.

Edit: PDF upload finally successful!

.

You wanna e-mail it to me and I'll give it a try. Not saying I can, but two head's are better than one.
I think I know the result's already, but would be nice to see them in writing.

.

PM'd you.

I once heard that a servo when moved to a position has a brakeing effect. As long as the stick is not moving there is no more load than at neutral.
Don't know if that's true or not. Some test I ran seemed to confirm it for me, or close enough anyway.

Never mind. Gotta look up.

Truckracer do me a favor.Do the same test, on just one servo,without any thing hooked up to the servo. For comparison.

Which test is that?

Well the Hitec 7985 certainly exhibits that characteristic. It just seems to go to sleep while still holding the throttle arm in position against the spring. I did notice it sort of wake up from time to time though as you could hear it and see some current on the meter .... then it would go back to just the idle current again. I watched that cycle repeat itself several times during the test. I wonder if this is what causes the "kick" some people complain of on the 79xx series servos on some control surfaces? I thought it interesting and certainly not something I had ever noticed before with any digital servo.

Measure the current draw, neutral, half throw, full throw,of the servo without it being hooked to the throttle. Just curious as to what it draws empty.
I'd do it myself but I currently don't have a meter.

That would be the idle current shown in the first column ... nothing connected to the servo and that doesn't change with servo position. There is also no load, run current but I did not measure or record that. But as soon as the servo comes to a stop, that is idle current.

For the record, high torque servos can have a extremely high run current, even without a load. Many early (read that old) digital servos had a very high idle current but it should be noted how low the idle current is for the examples I tested. The oldest digital servo tested was the JR and it had the highest idle current.

I got ya. Excellent!
Those show just how minimal a throttle spring is for a servo. Maybe people will get it now. NOT

Thank's for posting those!</p>

Truckracer,
  Thanks for taking the time to conduct that test as well as posting that graph as we all know how hard that can be.
  Great info. I honestly would have thought the servos would have drawn a little more current than that, not a lot but a little more. i was thinking around 100ma range thereabouts.
  I have not subscribed for a long time now about disconnecting that spring but I remember back around 1990 when I got my first G38 I did disconnect that spring and removed it as it seemed very stiff. When you would push it  full throttle and let go it would SLAM back closed really hard. At that time I was the only one at my club who had a gas engine and didn't have anyone else to comment on it. Also at that time only running 4.8v packs as well. I sure don't run that voltage these days.
  Thanks again. I hope people will read you results but you know......   They will have to SEARCH.....  So maybe not.

Great info. I wonder why the Savox servo took more current. It's funny, here we are looking into how much power it takes to operate a throttle while a different thread has guys telling someone that Futaba 148 servos will work on his 1/3 scale Weeks Special. Only on RCU LOL

Wow! i had to look at thread thread myself to see that. Even All Day Dan didn't say anything about that. I would only use 148s on throttle 20 years ago on large planes. Also I think he said 128s which I think those were even low budget 148s. That Plane won't last long on either. Unless he runs maybe 3 per surface. But still.
</p>

Pamster, I disconnected my throttle springs for years. It just seemed the thing to do. Everybody I knew that ran gassers did that so I just followed along. Then one day I some great brainstorm in this old head and decided it wasn't really necessary and have left them in place ever since. Its such a simple thing but it can certainly stir up an interesting conversation whenever it is brought up.

Wow, regarding the 148s. OK in their day but certainly not for that application.

I'm not sure why the Savox servo drew more current than the others other then it just seems very active. Seems to be working all the time even without a load connected..... very tight deadband. Any small load resulted in quite a bit of current draw. Seems like a very good servo though so it must just be its nature. A bit more gear play than I would prefer though.

Truckracer. thanks for the info on the Savox 1258's I have a couple of them on the way. Will be using them on elevator and rudder on my new 50cc Laser. Wondering how the gear play compares to a new JR 8611a? Although those too were a bit sloppy and only got worse I never really noticed in flight. Would send them into Horizon when the pots started to go out and they would fix them.

I'm not sure how the 1258 gear play compares to the JR 8611a but it is on par with some other JR servos I've owned. I would be curious how your new 1258's are. I've heard they have improved and if so, I will send mine back for gear replacement. I'd call mine borderline excessive. I have every confidence they will resolve the problem though.

A servo at rest should draw only the current required to run its electronics. There is no voltage sent to the motor.
A spring on the throttle is probably light enough load that it will not overcome the friction in the servo gears so the servo does not send any voltage to its motor.
I just wonder why that spring is so important to argue about.

Very good information.
The throttle spring help you if you loose your linkage, in some rc cars it is used as mechanical fail save and it will move the servo to idle/ brake position if the radio looses the signal or the rx looses power.

Interesting to know how little the difference really is.

Do we have to thank Obama for that now?

No argument from me, just observation due to curiosity. The spring on the larger carbs is fairly large and strong so it certainly creates work for the servo. The readings show the servos easily overcome this load though.

Regarding no voltage being sent to the motor while the servo is at rest with a load in place, only the Hitec 7985 drops back to idle current from time to time, the others all had a fairly constant current draw under load.

Thanking Obama is optional. I'm sure he will do a good enough job thanking himself.

I disconnect. If the linkage falls off, you don't preflight well. If you have a loss of signal and fail-safe doesn't kick in, you have bigger problems than a wayward throttle. <div>
</div><div>I have a gas heli with a G-23, and the return spring was quite strong. I don't see the point in driving the servo unnecessarily, especially at full throttle. Interesting data for that combo of engine/servo. </div>