As a sequel to my previous article about using lighter, less expensive nylon gear (NG) servos in place of metal gear (MG) servos in most circumstances to lighten my planes, I wanted to write about my method for scratch built pushrods. This is certainly not the only method for making your own pushrods, I know Stephan uses a different method, but for the same reason, the pushrods end up being lighter and less expensive in the long run.
Up until about a year ago, I used to build notoriously heavy, too much glue, components that were too heavy, etc. My planes still flew OK, but I was missing out on a lot of potential performance because my planes were so heavy and had such high wing loading. And it also took me several catastrophic crashes to realize that heavy planes are actually less durable than light planes. The more weight coming to a sudden stop, the more damage, often non-repairable. Now with my lighter planes not only do they respond quicker when I start to get in trouble, but if I do have the odd "gonk", the damage is far less serious.
So when I took the leap to start using NG servos to help lighten my planes, I also started making my own pushrods to save weight and in the long run a bit of money.
RC Powers recommends using Dubro 2-56 threaded pushrods on their master parts list http://www.rcpowers.com/community/threads/master-parts-list-official.8300/. This is fine for beginners as they are easy to bend and easy to adjust, all you really need is a "Z" bend at the servo end and the adjustable clevis at the control horn. Again I'm not trying to undermine or criticize RC Powers, just discuss some ideas for lighter, less expensive options to achieve the same goal.
The Dubro pushrods are heavy and in the long run more expensive than making my own. One pair of 12" Dubro push rods weighs 14 gr/0.5 oz. The cheapest cost that RC Powers lists them for is $3.49 for a package of 5, so essentially $0.70 each. So if you like to use 6 servos like I do, you have to buy two packages of 5 as I use 6 push rods. So that is fine, unit cost is still the same.
So here are the products I use to make my own scratch built pushrods
- Nylon clevis from HK - http://hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=8421 cost is $0.64 for 10, so lets say for arguments sake $0.07 each.
- 2mm carbon rod from RC Foam, sold in 1 meter or 39" lengths - http://www.rcfoam.com/carbon-fiber-sheets%2C-carbon-rods-and-tubes/carbon-rod/carbon-rod-p-737.html one length costs $1.60, so $.04 per inch.
- 2mm quick connectors from HK - http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idproduct=8519 I actually normally use ones from RC Timer, but the RCT website seems to be down as I write this. So the quick connects from HK are $0.12 each.
So lets look at my recent Mig29 V4M3e3 build to figure out what it would cost to use Dubro pushrods versus scratch built both in weight and cost.
I use 6 servos for all my planes, so 6 pushrods would be needed, cost would be $4.20 for 6 Dubro pushrods. Unfortunately, I would end up wasting some of the rod as once I cut it off, I don't have much use for the scrap rod. My elevon pushrods are each 10" long, aileron pushrods 5.75" long and rudder pushrods are 9" long.
So here is where the math gets a little anal, the Dubro pushrods weigh 0.02 oz per inch, so if I need 49.5", my pushrods will weigh 1 oz.
So now lets look at cost and weight for my scratchbuilt pushrods. So I need 6 clevis at $0.07 = $0.42, 49.5" of 2mm carbon rod costs me $1.98 and 6 quick connects at $0.12 = $0.72, total cost is $3.12.
For weight, the clevis are 0.5 gr each, so 3 gr total. The carbon rod weighs 1.5 gr per ft, so 6 gr plus lets round up 1 gram to include a bit of weight for the epoxy needed to make them, so 7 gr plus 6 quick connects at 1 gr each is 6 grams, total weight is 16 gr or 0.56 oz.
So I have saved myself $1.08 in cost and 12.5 gr/0.44 oz in weight. I know that this perhaps doesn't seem like a big saving in either category, but in the long run it is for me. Both types of pushrods can be recycled between airplanes, so once a set has been made they can continue life through several planes.
I waste less material with the carbon rod method as I can cut the 2mm carbon rod exactly to size and any little bits I have left over I keep to recycle if I need a little bit of extra reinforcement in a plane here or there.
Granted, not everything about the carbon rod pushrod setup is easy, they cannot be bent like the Dubro rods, but for my preference, I prefer pushrods with no bends. To accommodate for this shortcoming, I simply angle my servos like in these pictures.
The rods are quite simple to make. I cut the rod to the length that I need, then take a razor knife and gently shave/whittle the ends so that they will fit easily into the clevis and quick connect. Normally I have to drill out the hole in the clevis so that the rod will fit in. I still want it snug, but not so tight I might split the clevis putting the rod in. I then mix up a little 5 minute epoxy, put a dab in the clevis hole and a small bit on the end of the rod and slide it in so that the sleeve in the clevis is filled with the rod for a good secure bond.
For the quick connects I use from RC Timer, they have a little nut that goes on a threaded shaft to hold them to the servo arm, I always put a very tiny bit of blue loctite on the threads to keep it secure. It is important not to over tighten the little grub screw in the quick connect so that you don't crack or crush the carbon rod. The nice thing is though that with a little tightness, the grub screw bites into the carbon just a tiny bit and prevents slippage.
I do prefer the very fine adjustment available to me with the quick connects also. With the Dubro pushrods they are very adjustable, but sometimes I end up just that tiny bit off since they can only be adjusted a half turn at a time where the little nylon clevis screws on and off of the rod. I also find they are a bit more of a pain to adjust as I have to keep removing the clevis from the control horn, make a few turns, hope it is enough, then reattach. This can take a few tries whereas with the quick connects, one adjustment and I'm done.
So as I mentioned earlier, not a massive savings in cost or weight, but when you are really looking to save weight on your plane, a few grams here and there all add up and the more weight I can save without sacrificing strength and performance, the better. And if I save a buck or two here and there, that is an added bonus... :).
Anyway, another option to consider if you are looking for a simple, inexpensive way to lighten up your plane and save a few pennies.
Cheers,
Scott
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