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Jan 07, 2012, 02:20 AM
Three green

Wooden dowel torque rod?

I have a question for the old pros or whoever. I am building an Ultrasport 40 kit (conventional sport plane design), and to avoid using the steel torque rod to connect the elevator halves, I was thinking of building in a wooden dowel torque rod through the fuselage sideways below the horizontal stabilizer. It would just rotate in the plywood fuselage sides and stick out each side a little. It would have horns in the center and one on each end outside the fuselage. Then it would be rotated by pull pull with the servo in the normal location over the wing. The horns on each end of the torque rod would then be connected to each elevator individually alongside the fuselage. Will it work? Any thought welcome. Thanks!
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Jan 07, 2012, 03:21 AM
Registered User
Toysrme's Avatar
why not go old school and run a traditional dowel pushrod with bent wire ends. you secure the rods in the dowel but putting a little channel in the dowel , drilling a hole in the end of it for the rod to grab &then just wrapping thread around it. Epoxy that and you're done.
you can easily make a Y at the back end from securing a metal rod on each side. leaving extra rod & threading it. then cut them down to the size you need.

are you trying to make it... less stuff hanging out the plane? or what's the purpose?
Jan 07, 2012, 12:49 PM
An itch?. Scratch build.
eflightray's Avatar
CT, I'm not too sure why you don't want to use the conventional steel torque rod, but from what you are suggesting, I always try to look for potential problems.

You say, "a wooden dowel torque rod through the fuselage sideways below the horizontal stabilizer", but torque rods are normally on the hinge center line, otherwise they tend to move through an arc and need a larger hole in the fuselage.

Also I'm not quite sure on your "pull-pull" method. I assume you mean it would be inside the fuselage. I would suggest drawing it out first on a piece of paper to scale, as I would have thought it would foul the rudder.

One alternative to a torque rod joining the elevator halves, is to not have one, but use two servos, one for each side.
Or even a single servo, but then run pull-pull wires to each elevator half.
Jan 07, 2012, 02:06 PM
B for Bruce
BMatthews's Avatar
If you really want to go to that much trouble it'll work. But instead of wood dowel, which is never very round or accurate in size, I'd suggest using aluminium and brass tubing. The brass for main shaft as you can solder the arms in place and the next size up aluminium for the pivot bearings.

Frankly it sounds like a lot more work than using a regular pushrod. On a .40 size model there often isn't a lot of room back in the tail for all the stuff you're considering. If you really much use pull-pull then I'd just join the two sides with the U wire and put control horns on top and bottom of one side of the elevator and run the PP lines to those. But with all the funky angles making it hard to keep a tight system I still come back to just using a pushrod. They work and they are simpler to make and align for equal travel each side of center.
Jan 07, 2012, 11:33 PM
Three green
I thought one 3/8" dowel 3" long, and 2 push rods 4" long, and a bit of kevlar string would weigh less than a 12" and 24" push rod. As for the the wire torque rod connecting the elevator halves I find it hard to get it just right with no play and all the weight of that rod is right at the extreme aft were 1 gram that needs to be balanced out, adds about 5 grams.

I'm just kicking around ideas. This setup would be the same as the 2 servos on the sides of the tail only instead of heavy servos, it would be a torque rod sticking out each side driven by a servo up front. I know I could do it the conventional ways but I could also buy an ARF
This dowel would be built right into the fuse with plywood horns epoxied on and fuelproofed.

Anyway, thanks for responses. I do often get a bit carried away trying to re-invent the wheel, so maybe you guys have reined me in a little.
Last edited by CT-7; Jan 08, 2012 at 12:22 AM.
Jan 08, 2012, 01:45 AM
B for Bruce
BMatthews's Avatar
You're quite right that a lot of this stuff would be heavier. But a GOOD low slop accurate pull-pull that won't alter the tension in the loop is a tricky thing to make. And it would require very low slop bearings for the cross shaft. If you think you want to go that way then have at it.

On the other hand one option is to use a dowel alright. But use it as the joiner for the elevator halves instead of the music wire one. It WILL be lighter than the wire. Then make up a nice light but stiff pushrod from some carbon tubing. Keep in mind that the pushrod weight isn't all the way back at the tail. It's centered about half way back.

Finally I'd also suggest that there are better ways to save a few grams. Many of them start with building from scratch and picking wood that is as light as possible for the task of each part. Or in the case of a kit to examine each peice and if it's made from unreasonably heavy balsa to replace it with a new part cut from lighter density wood. From there the weight is further controlled by using adhesives that are light and only as much as needed.

Wood used in the tail surfaces is especially important since, as you say, a gram at the rear means 4 to 5 grams up front. So building the tail surfaces out of wood which is as light as possible, even to the point of replacing the kit wood, is a far better place to save than to try to build in a pull-pull system in a model where there isn't a lot of room back at that location to add in a cross shaft and arms.

There's nothing at all wrong with over thinking a situation. The key is to not let it blind you to where the rest of gains can be made and then to persue the options which matter the most.
Jan 08, 2012, 10:36 AM
Three green

Excellent stuff to ponder! Thankyou very much for your time. The tricky part is that untill its all up, I dont even know if the plane will be nose or tail heavy. Oh well, this is one of the things that makes the hobby so intresting.
Jan 08, 2012, 01:15 PM
B for Bruce
BMatthews's Avatar
I've been working with balsa long enough that I can simply look at and hold the wood for the tail surfaces in my hand and know if they are OK to use or will see duty as epoxy mixing sticks. I know that this sort of "feel" is something that only comes with time.

To help you get the same sort of feel here's an old trick. Push on the wood with a thumbnail. If it dents easily and deeply with light pressure which doesn't cause any discomfort at all then it's soft. On the other hand if you have to press hard enough that nail creates pretty hard pressure bordering on discomfort just to leave any mark at all then the wood is way too heavy and should be replaced with lighter stock. Or in the case of simple solid sheet surfaces you can measure the area and weigh the wood. Some simple calculations will find the volume and density of the pieces. From there if they are heavier than 6 to 7 lbs/cu foot then it's time to replace the wood with lighter stock.

Just building and THEN checking for stuff like this is just not being proactive enough. Light building starts with checking the wood for suitability as you remove the parts from the box. Once you have the glue bottle in hand it's far too late. Time spent evaluating the existing wood from the kit would be a much better alternative to trying to design a pull-pull system to save a few grams. If you NEED to save a gram or two it's only because you started worrying about the whole weight thing way too late.

Yeah, I know it's a pain to find that the kit wood is just not suitable and have to go and buy wood or order online and wait. But look at how much more fun the model will be to fly for many years if the wing loading comes out on the lighter side of the acceptable range for this design. Isn't that worth a bit more trouble?

So dive in and evaluate the wood first. If it is heavy then you can save enough weight by replacing it with light stock so that you don't need to go to the extraordinary efforts to save a gram or three in the control system.

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