In the midst of putting a Javelin together and looking at the push-rod for the elevator(s). It uses a split elevator with the standard wishbone push-rod system we've all seen many times. My question/problem: The whole wishbone (well, that's my term for it) push-rod system sucks, in my opinion. Most of the systems I've seen have the servo axis normal (perpendicular) to the elevator hinge line- as does the Javelin. Due to the angular displacement brought about by the rotation of the servo, the split elevators are- to some degree- not coincident in their travel throughout the full range of deflection.

If the servo were turned 90 degrees so its axis of rotation was parallel with the elevator, this problem would disappear. Granted, the elevator motion/rotation is not linear since the servo wheel/arm, etc, has differing amounts of angular displacement as it's rotated through the limits of its rotation but the push-rod would not be imparting differing amounts of input to each elevator half- thereby eliminating a built in roll component.

Now, all that being said, I'm well aware that plenty of very good planes exist with this system but I've got to wonder about it, all the same. Other than turning the servo on its side or installing one of those nifty bellcranks Central sells (http://www.centralhobbies.com/contro.../Bellcrnk.html), what are my choices? Connect the elevators and use a single push-rod? Two servos?

The question is basically an academic one; as I said, I know there are plenty planes with this set-up. It still gives me a big pain when I see it and wonder if anyone else has pondered this. Sorry for the long winded discussion.

DEPS works very well. Its lighter and way cheaper than the Bell Crank.

Many on this forum have used it and swear by it. I have used variations of it on everything from 40 sized models to 120-140 pattern models. Unless you are going to be using 3D throws greater than about 25degs this is the system. I would recommend it for sure. I had some questions and asked Troy via email and he responds with a ton of information its very helpful to me

Central Hobbies sells this system, and Troy has written several threads and has instructions on the Central Site on how its installed.

I had a bellcrank and hated it. I think it is a MKS from Central Hobbies. First, mucho overpriced. It's heavy. It limits travel with the same circular control arm motions as mentioned, only you get double the effect from the servo and bellcrank. I took mine out after much work thru one side of the fuselage with covering ripped off. I went to a single dowel rod and two rods to the split elevators. It is the best thing I've done to that plane. The clevis hitting the control arm on the bellcrank really limited throws and flexed the h@ll out of the single control rod from the servo.

Check out my post on dual pushrods in the Classic Pattern sub-forum. Works like a champ.
Rich
TeamVortex

SamD,

The wishbone pushrod system for the Javelin works just fine, even with the elevator servo positioned per the instructions. The servo axis can not/does not affect the movement of the elevator halves independently. For any increment the servo end of the pushrod travels backwards (forwards), each of the wishbone ends of the pushrod will travel backwards (forwards) by the same amount, and equally. There may be some flexing of the pushrod because the servo arm is moving laterally as well as front-to-back, but that doesn't cause the elevator halves to see different inputs from the pushrod. There may be lighter systems, stiffer ones, or ones that offer more fine-grained adjustment of the individual elevator halves, but that's a different issue.

I'm also not sure how easily you would be able to shoe-horn a DEPS into the Javelein given that the fuse is pre-built, pre-covered, and extremely narrow. You need to be able to adequately secure the DEPS at a couple of points along its length (inside the fuse). Even if you could do it, I'd still question the worth of putting $30 or $40 worth of CF rod systems or coupling bellcranks on a $130 plane. It is, however, worth replacing the supplied 3- or 4-hole plastic control horns with the bolt-type horn (as well as changing the landing gear). +

Vortex,

Checked out the pic of your dual pushrod system and I like it; clever method of eliminating the differential the two elevator halves will see with a typical split pushrod system. Probably won't be able to install that in the Javelin without a lot of butchering but I'll definitely keep this in mind.

Rbacons,

As you stated, trying to modify the Javelin is probably not in my best interests (other than landing gear and a few other minor points); the question was really more of an academic one- other than some quick and easy fix. I'll need to disagree about the lack of differential between the elevator halves, though. The pushrod is moving in two axis (well, actually three if you count the up and down movement the elevator imparts to the pushrod) as the servo wheel moves about its axis of rotation. The pushrod moves both fore and aft and side to side- the side to side motion imparting the differential in the elevator halves. Granted, it's a small amount, but it's still present. I suppose it's easier to visualize if you picture a split pushrod lying flat on the table with a vertical axis (perpendicular to the table) in the center of the rod. Moving the rod side to side about this axis shows that the two pushrods (split pair, that is) go through an axis of rotation- thus imparting differential to the elevator halves.

Keep it coming guys, more discussion is good!

Servo wheel/arm rotation in a horizontal plane (as with typical servo mount) definitely causes some side-to-side motion of the pushrod/wishbone configuration. When I wiggle the servo end of the pushrod (unattached to servo) from side to side, even a little, I can see differential movement in the elevator halves. Some guys say there is no visible effect to the plane in flight, but if it's visible on the bench, I don't want it. So I've been side-mounting elevator servos for the last half-dozen planes or more.

The Central DEPS system is pretty good, but I make something pretty similar with Darroll Cady pushrods, just as effective and a bit less expensive. These dual pushrods do need one or two bracing/damping points along the mid-tail area, and on the Javelin you'd have to cut into the balsa sheeted bottom to do it, then re-sheet and re-cover. I ususally strip all the covering off as my first step with an ARF. Sorta contrary to the general idea of ARFing, but not a big deal to anyone with kit-building experience.

The vertical arc in servo output for a side-mounted servo will introduce some non-linearity of control movement relative to TX stick movement, but that domain is already full of non-linearity; but at least the differential between elevator halves is gone. And besides, once you're you're flying, you move the sticks according to what your eye sees the plane doing, not according to the geometry of where the stick is.

Major,

Your thoughts coincide with mine; if I can see it on the bench I don't want it on the plane. I thought about side mounting servos on the Javelin but the fuse is really too narrow unless you make gross adjustments in servo location to avoid interference between the two servos. On the other hand, it would be nice if more models simply designed the servo installation such that the servo axis was parallel with the control surface axis- thereby eliminating all this playing around. The non-linearity of the control movement is a given in all our models until you start playing around with a pull-pull system with pulley/wheel attach points that maintain a constant radius for pay-out of the cable- but that's a whole other breed of cat.

At what point is bashing an ARF a diminishing return vs. just building kit? That's the question I always struggle with and always seem to come up with a different answer depending on what I'm working on. Still this whole issue of differential in the elevator halves is probably a second or third order problem (this being the opinion of wiser person than me and my feeling, too). Regardless, the imprecise nature of the split pushrod systems "offends" me...

I will say the Javelin is worth bashing, since there is hardly a kit to compare to it...maybe the CA Widebody.

Yes, the narrow fuse is a problem with the Javelin. As to gross adjustments in servo location, I hardly ever use the stock servo mounts. After I strip the covering off, my next step with an ARF is to cut the servo mounts out of the fuselage. Apart from having DEPS elevator and pull/pull rudder servos at different heights, I like to do a trial CG balance before I settle on exactly where on the fore/aft axis I will mount my servos.

When I do another Javelin (I have two NIB), I'd probably use a low-profile servo side-mounted for the elevator, and live with the plane of output rotation being not quite centered horizontally. If I didn't want to do that, I'd set up a vertical tiller bar to drive the pushrod, and drive the tiller bar with a standard mounted servo. All this sounds like worse than building a kit, and in some ways it is, since it's always easier to build it how you want it from the start.

But the Javelin does fly beautifully, and I consider time spent on these mods to be well rewarded once the plane is in the air.

One of these days Hitec (the great innovator in servos) will bring back the linear output. (Maybe. I hope).

As to the second/third order level of the problem: I think we've seen such great progress over the last couple decades in model aerodynamics that control systems/linkages are to me now first order problems (along with power systems).

But keep all this in perspective: do what you can with your Javelin, and you will most likely have a lot of fun flying it; then get yourself a good kit (Widebody or ?) and pull it all together on that. Working out problems in every new plane makes us better prepared for the next one after that.

I think that the Widebody is no longer in production. I have 2 kits, one flying and one NIB. I have read where CA Models calls the Epsilon the successor to the Widebody. I have the Epsilon kit as well.