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

• Hobby King HXT900 9 gr servos.  These very popular blue servos have been around a long time and I strongly suspect are made by the same company and sold as Tower Pro and other brand names at almost every hobby shop or online vendor around.  Actual weight 11 gr/0.4 oz, cost at HK - $2.69 http://hobbyking.com/hobbyking/store/__662__HXT900_9g_1_6kg_12sec_Micro_Servo.html.  
• RC Timer TS-9018 9 gr Micro Servo.  Actual weight 11 gr/0.4 oz, cost - $1.98 http://www.rctimer.com/product_458.html These are my favorite nylon gear servos and not just for the price which I will explain in a bit.

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