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Wednesday, July 23, 2014

Metal gear servos on park jets - "necessity or overkill?"

Hi everyone -

If you have watched my walk around videos for the last several planes that I have built, you probably noticed I am a big fan of lighter, less expensive nylon gear (NG) servos.  About a year ago after watching my first "overweight" RCP F18 V3 self destruct in front of my eyes (wing snapped in flight due to too much weight, too high a wing loading... :(), I really started to focus in on building light and started seriously looking at ways to reduce every gram of weight possible.

Previously to this time, I followed the RC Powers recommendation of using metal gear (MG) servos for everything, which not only added weight to my plane, it reduced a lot of weight from my wallet... :/  So I took the chance on using NG servos on my planes then and have not looked back since .

Here are some quick weight and cost comparison numbers to help explain some of what I mean.

Popular MG servos

  • Hextronik MG-14 metal gear servo currently recommended by RC Powers master parts list - weighs 14 gr/0.5 oz - cost is $7.95 http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8303&aff=46898  I have a couple of these servos and this is the actual weight on my inexpensive kitchen scale.
  • Hobby King 939MG Metal Gear Servo (used to be one of the recommended servos on the RC Powers master parts list). These are advertised as 12.5 gr servos, their actual weight on my scale 15 gr/0.5 oz, current cost $5.75 http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14458
  • RC Timer 9018MG servos, these are advertised as 9 gram MG servos, they actually weigh 15 gr/0.5 oz and cost $4.29.  These are my favorite metal gear servos, and not just for the cost which I will explain in a bit.  http://www.rctimer.com/product_525.html  Here is an interesting note to watch for.  RC Timer sells 6 packs of this same servo for $27.99.  You would normally think that bulk packs would save you money...well, $27.99 divided by 6 = $4.67... :/, so cheaper to buy them individually.
Popular NG servos
So I'm not saying throw away your MG servos or never buy another one, they certainly do serve a purpose for certain applications.

About a year and a half ago, I had built a Boxermad84 (before it was FRC Foamies) F35 600, it was a small, fun plane to fly which I built elevons only and initially had the Turnigy D2826/6 motor in it.  It was one of the first park jets where I took the chance with NG servos (HXT 900s to be exact).  So as I gained experience in flying, of course I wanted to see how fast I could get this plane to go.  I kept increasing motor size until I got to the NTM Prop Drive 2700.  I got this plane up to over 90 mph with the NTM...very exciting stuff for me at the time.  However, one thing I started to notice was at speed (lets say 70% throttle or higher), when I tried to turn, I got what I call "prop wash or servo blow back".  The elevon servos could not hold the elevons in place due to the excessive strength and turbulence of the prop wash caused by the 42 oz of thrust the NTM was generating.  So I would get kicked out of turns because the servos could not hold the strain.  Fortunately I never stripped a servo or lost the plane, I swapped the NG servos for MG servos and life continued along merrily.

So as a rule, with any plane where I am going to use the NTM Prop Drive 2700 motor, I use MG servos on the elevons.  It just buys me a little piece of mind that my servos can handle the stress and thus far even at 109 mph on a 4S with my Mig29 V3M3e3, the RC Timer MG servos I mention above have worked flawlessly... :)

On any other planes where I am going to use the Focal Price 2700 or lighter motor, I go with the RC Timer 9 gr NG servos for all the control surfaces and thus far, have not had a single problem in hundreds and hundreds of flights in temperature conditions from -5C/23F to 30C/86F.

So you might say, "well why worry, I am only saving 4 gr per servo".  Well even with my NTM planes, I use NG servos on my ailerons and rudders, so in fact I have saved 16 gr/0.56 oz.  But more than that, I save $9.24 which can buy a lot more servos, props and other goodies, or just stay in my pocket for other things in life... :)

By using all NG servos on my current plane the Mig29 V4M3e3 over using MG servos, I save 24 gr/0.85 oz and a total of $13.86, now I have saved enough to buy another Focal Price motor!

So why do I prefer the RC Timer servos both MG and NG?  Well, of course the price is right, much less expensive than the HK servos and shipping is pretty cheap.  Now, a word of caution in dealing with RC Timer (and Focal Price for that matter), what you save in shipping cost, you pay for in shipping time, so don't be surprised if it takes 4-6 weeks for your products to get to you from RC Timer/Focal Price.  This is why I normally make my orders worth while and buy enough servos for at least two airplanes at a time.  I also get my 6x3 EMP props from there http://www.rctimer.com/product_218.html , best deal I have found, so I don't make small orders from there, I get plenty of goodies and end up being my own supply source.

But cost aside, they just plain work better in my humble opinion than the HK choices.  They are quieter, smoother and come with longer servo arms giving me a bit more flexibility in push rod placement.  Here is a side by side picture comparing the black RC timer servo on the left to a blue HXT 900 servo on the right.


I don't work for RC timer nor do I get any special deals because I write about their stuff, I use their stuff because it is inexpensive and it just plain works time after time.  I have hundreds if not thousands of flights in total with planes using RC timer MG and NG servos and have not had a single problem to date.

So you might be apprehensive about using NG servos.  If RC Powers recommends MG, then why should I listen to this goofball writing this post?  Well, I'm certainly not trying to discredit RC Powers or anyone else, I am simply passing on my experiences from buying, building and flying with NG servos over hundreds if not thousands of park jet flights.  I push my equipment to the max on a regular basis and have not had an issue with any NG servos for park jet applications (other than in the story of the Boxermad84 F35 600 I told above).  As I already wrote, I would recommend MG servos for elevons on a plane where you might use the NTM 2700 motor or a motor that produces that much thrust or more, but for ailerons and rudders, NG servos work just fine flight after flight.  

And for the Focal Price and lighter motor planes, NG servos for all control surfaces have worked for me on over a dozen planes.

So after all these numbers and rambling, what is the answer to my question?  Well, for the one application with the NTM Prop Drive motor, I think that MG servos on the elevons are a necessity.  For all other applications, I believe and have proven time and again with my own flying experiences that MG servos are overkill for park jets.

So hand on heart I feel very, very confident in recommending NG servos for park jets within the criteria I mentioned.  Not only do they lighten the load on my plane while giving me excellent performance, they also significantly lighten the load on my wallet, which I think we all appreciate... :)

Up next in the theme of keeping things light, strong and inexpensive, the sequel article to this post, "making light, strong, inexpensive pushrods from scratch"  :)

Cheers,

Scott



4 comments:

  1. Scott,
    Another very thorough review. I see the beginnings of NAMC ParkJet Wikipedia.
    I too fell into the metal gear mantra. My first build was the F-35 with six Hitec 82MG servos and 6 sticks of hot glue. What a dog! I ripped all of the servos out except for the elevons. Flew, but was still a lead sled. Since, I have learned to build light and following your lead switched to nylon gear, well almost.
    I built the F-22 with all nylon gear servos and they couldn't handle the load on the elevons and switched back to the Hitec MG. Problem fixed.
    Having quite and investment in Hitec MG servos, I recycle them into the elevons on my new builds. Clearly, with our downsized elevons they are overkill.
    I have a stockpile of the Tower Pro 92 Carbon Gear servos and have been very happy with them.
    You were so kind to send me a set of the RC timer nylon and metal gear servos. As per your experience, the metal gears are fine on the NAMC downsized elevons. I have not had any problems.
    When it comes to control rods, I have noticed a difference between 3 and 2 mm rods on the elevons. I plan on videoing to see if the 2s are flexing.
    Moving forward through testing!
    Stephan

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    1. Thanks Stephan... :) Not sure about Wikipedia, but lots of stuff I have learned through hundreds of flights and trying many different combos of gear. Hard to believe that in the past I have built planes upwards of 2 lbs in weight... :/

      MG servos definitely do have their place in park jets, but as we both have come to realize, there are plenty of lighter, less expensive options available that work awesome. I had even experimented with 5 gr plastic gear servos for rudders/ailerons on a couple different planes, but the servo arms were pretty short and the performance was just not up to par.

      I know that you and I have tried a couple different options for scratch building our own light, strong, inexpensive pushrods for our planes, so it would be awesome to have your take on how you make your pushrods when I write the article on how I like to make my pushrods. That would give folks even more options to consider depending on what they like or what materials they have available. As we have both discovered, there are lots of options other than the heavy Dubro 2-56 threaded pushrods. I hope to get started on the push rods article later today or tomorrow.

      Cheers,

      Scott

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  2. Greetings Scott,

    I've been away from modeling for awhile and very happy to have found your spot here at the NAMC and hope this message finds you well. I'm finally preparing the build for the Tomhe F22 I think I mentioned to you once before on RCP. I would like to try the linked 2x4 option this time around, with the addition of a single servo for rudders. I was wondering your thoughts for that plane, in that configuration and wondering if a MG servo would be needed for that duty?

    I've always been obsessed with weight and would like to make some smart choices with this one and be on the lighter end of the spectrum, but still have a plane that tears up the sky and is fun to fly. I was considering the standard set up I've used twice before for the Mig29V2 and my Zagnutz 52" wing. I purchased all the components before from RCTimer and would probably do so again. I like ordering everything from one place and they didn't have the 1600mAh batteries back then so the choice was a 1500mah, or 1800mAh.

    *2826/6 2200KV
    *6x4 prop
    *9g MTG servos
    *40A ESC
    *1800mAh
    *Orange RX

    I was hoping for some recommendations from all of your time spent experience and knowledge on the subject of park jets.
    Thanks for your help,
    RT

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    Replies
    1. Hi RT -

      Glad that you found our blog. I don't participate in the RC Powers forum any longer, so if you ever want to discuss anything or ask questions, this is definitely the place to find me... :) As you can see by all the blog posts, I am still very active building, modifying, flying and testing planes, I just don't write about it on the RC Powers forum any longer. Long story, but life is better that I am here freely and honestly expressing my thoughts and findings... :)

      Welcome back to the hobby as well, hope things are well with you... :)

      The 2x4 configuration is simple and light, but does cut down a bit on the ability to adjust your aileron servos as the elevons and ailerons are slaved together. I would be careful in the first few turns and rolls that it doesn't over roll on you. The main thing I did not like about the 2x4 setup is the drag caused in the pitch by the ailerons also acting as elevators. I found loops and other pitch type maneuvers to be very "draggy" depending on how much up elevator you put in. You can still do decent loops, it just takes a little more finesse with power and elevator input to prevent a stall going over the top dragging all that deflected control surface.

      A single plastic/nylon gear servo for the rudders would work just fine, I still use the single HXT 900 servo on my old workhorse F18 V3 and it has held up for well over 300 flights.

      If you are going to go with the 2x4 setup, then perhaps you need to stay with the metal gear servos since each servo is moving two control surfaces, otherwise with the power setup you are using nylon gear servos in my experience would be just fine and save you about 4 grams per servo if you went with elevons only.

      If weight is really a concern, you could also go down to a 30A ESC for the motor you are using. I can't attach the file here, but if you go to post #127 of this link http://www.rcpowers.com/community/threads/parkjet-motor-bench-testing.15685/page-7 you can download the latest version of the spreadsheet I did on all the motors I tested. I think the Turnigy and RC Timer 2826/6 motors are pretty much identical (I never actually tested the RC Timer 2826/6, but from all I read they are basically identical motors), the highest amp draw I saw in testing was 29 amps on medium setting, I would probably just leave your ESC on the default setting of low. Based on the ESCs I use, there is a saving of about 7 grams between a 30A and a 40A ESC. I'm not sure if that is all that important, but it is another option for lightening things up.

      Yes, unfortunately RC Timer does not sell good quality batteries or a wide range of sizes, from research I have done, I would be very cautious of the B grade batteries that they sell or used to sell. I have been having amazing luck with the Zippy Flightmax line of batteries from HK as of late, they are inexpensive and thus far very durable. I have had great luck with this 2200 battery which works well for all the motors I run https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=9942

      I hope this has been helpful to you and I look forward to hearing how your Tomhe F22 flies for you. Good luck... :)

      Cheers,

      Scott

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