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Question? Hitec servo compare- mini vs micro

Discussion in 'RC Radio Systems' started by Alky6, Sep 12, 2018.

   
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  1. Snoopy1

    Snoopy1 640cc Uber Pimp

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    Eeddddd
    Very interested in what you have said about micro and mini servos. Does that also hold true if I would like to use a micro or mini servo on the throttle of larger 30cc gas plane.
     
  2. Xpress

    Xpress GSN Sponsor Tier 1

    Yes.

    Throttle is a unique application. The carb doesn't need more than 2 oz/in to open or close it, so literally any servo you have will be more than overly adequate to operate the carb. However, there are 2 important factors when choosing a throttle servo:

    1. Centering. Or rather, since the servo is rarely at 1500μs in throttle application, how accurate is its position sensing? Can the servo accurately move to the same position time and time again within +/-0.05º? Can it do this while under extreme stress?

    2. Durability. Things like metal gears also apply to this, but generally as the servo size increases, it becomes more and more durable. Micro and mini servos are designed for smaller applications where high vibration environments are not a factor. We can make these servos so small because of tiny surface mount components, which are very sensitive to vibration. Since glow/gas engines are constantly subjecting the airframe to high vibrations, these little components get stressed and can eventually fail. Standard sized servos are not as prone to this because the components used are generally of sufficient size and durability.

    It's not that the servo can't operate the carb, it's that they generally can't cope with the vibrations. Now, I have heard of some that have been using mini's on their throttles for years without an issue. Why they have been able to get away with it is beyond me, but in my experience, and seeing servo failures first hand, it's not an ideal choice for use with ICB engines. A mid grade standard size digital is a much better choice for throttle applications due to more accurate movement and more durable electronics and geartrains.
     
    stangflyer, Jetpainter and Snoopy1 like this.
  3. So,
    What’s your go-to throttle servo?
     
    Snoopy1 likes this.
  4. Snoopy1

    Snoopy1 640cc Uber Pimp

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    Thanks for taking the time to reply and thanks for the explanation.
     
    stangflyer likes this.
  5. Jetpainter

    Jetpainter 640cc Uber Pimp

    I always thought it was pretty rediculous to take off the throttle return spring like some do, I never saw the need, it takes so little torque to move it.
     
  6. Snoopy1

    Snoopy1 640cc Uber Pimp

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    I agree I myself have never removed the throttle return spring, and I do not understand why they do it.
     
  7. Jetpainter

    Jetpainter 640cc Uber Pimp

    I also look at it as another safety precaution. If you have a failure in the throttle linkage, let's say an arm falls off or the pushrod breaks, it kills the engine. Not great when flying, but way better than if God forbid it would happen on the ground without the spring. Who know what could happen.
    Speaking of pushrods, I think that is another thing that puts stress on the servo. The throttle pushrod is usually the longest on the plane, and mounted where it is it vibrates like crazy at times and transfers that right to the servo.
     
    Alky6, Snoopy1 and stangflyer like this.
  8. stangflyer

    stangflyer I like 'em "BIG"!

    So, this is very good information to have for those that don't know or just getting into the hobby. I must admit, I have considered using a micro or mini servo for throttle applications. Primarily because they do nothing but move a very (next to no friction) minimal drag mechanical device. However, there were reasons I did not. One obviously being the robustness of the servo. Just as Tanner pointed out, Yes, they will work...but for how long?

    Mine used to be the HS5665. I say used to because I had never had any problems. But then I was doing some set up on a new plane with a new 5665 and noticed it had a butt load of slop in the gear train. Did I get a bad one? I checked all my other planes as they are using the same servo. Yup, all of them are sloppy as hell. I am surprised that I did not notice it with my engines' idles. So what about it Tanner? Got a suggestion of a better servo for throttle usage?

    I would never disagree with you Doug, but I do have mixed feelings about the throttle spring thing and throttle pushrod lengths. Yes it is ridiculous to remove them because it takes so little energy to move the throttle. However, (just my thoughts) When you are applying direct and opposing force on an object, in time something will give. Experience has a way of smacking you in the forehead. I look at it this way. When you over amp an electric motor, it will burn itself up. If you continue to feed current into a stalled electric motor, again it will burn up. So, that throttle spring is not much pressure to our "Manly" hands and fingers right? Think of the size differentiation related to the servo motor. My 3w engines have had some of the strongest springs I've ever seen on the throttle arms. I absolutely do not keep them attached. On my very first VVRC Slick 540, I had an amazing running RCG 50cc engine yanking that plane around like it was a rag doll. I did not remove the spring on the throttle. After about 7 flights, the servo (HS5665) burnt clean up. I checked my linkage for binding. Impossible to have any as it was direct from the servo arm to the throttle arm. Could it have been a faulty brand new servo? Sure, but I have not had the problem again since I started removing the springs. Anyway, you have your spring attached, your throttle is held open for extended periods of time, putting a constant load on that little electric servo motor. I would expect that little electric motor is heating up. Especially if you are throwing gobs of 2s mah voltage at it. I watched a guys plane fall from the sky (literally) at huckfest a few years back. Smoking all the way to the ground. Cause? Stalled rudder servo. The electric motor overamped and caught the plane on fire. She was a smoking pile when it hit the dirt. Now I do understand that a throttle servo will never "EVER" have the loads slammed at it that most of our control surface servos experience. But still, a stalled servo is a warm motor looking for a flame. Because everyone knows I just love to slam my planes with huge mah batteries. LOL. Maybe I am just too cautious. The other reason I do not use them is kind of the opposite of what you outlined. If I do lose a throttle linkage in mid air...(which it did in fact happen on Baby when she was but 35 or so flights old), I don't want that throttle slamming shut on me. As was with Baby, when the linkage came off I was in a hover about 20 feet above my head. (The ball link melted because the header came off the cylinder due to inadequate muffler bolts, thus melting the ball link) Fortunately the throttle did not slam closed. Otherwise Baby would have been toothpicks from the rudder up when she hit the pavement backwards. As it was, I was able to limp her out of the hover, bring her around for final and kill the engine from the switch on my radio. Landed safely and was repaired with Nord Locks on the exhaust headers. Yes I understand your point of losing a throttle arm while on the ground and the imposing risks. But honestly, everything is a risk with these spinning props. Every precaution taken is a risk eliminated. Just like the throttle linkages. Keep them as short as possible and isolated from as much vibration as possible. I hope you understand I am not disputing your input. Just that I feel differently about throttle springs.
     
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  9. Snoopy1

    Snoopy1 640cc Uber Pimp

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    I agree with most of what you said, but if the servo is rated correctly then it should be able to hold the load indefinitely at that position or any position. That is why they are named as a servo. But that does not hold true if it is stalled out if you stall a servo them it is game over or not if it has an internal current limit. That just my understanding of a servo.
     
    stangflyer likes this.
  10. stangflyer

    stangflyer I like 'em "BIG"!

    I believe you are correct. And honestly, the HS5665 is rated far above any torsional loads "ANY" throttle spring could ever exert against it. I guess I just don't like those silly springs. LOL
     
    Snoopy1 likes this.
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