I'm playing around with a different kind of pull-pull system for the elevator of a 60 sized warbird. The tough requirement is that it has to be totally internal. A picture says a thousand words... please see the attached diagram.

I posted the same question in the Warbirds forum and got no response. Maybe you guys can tell me if this idea will, or will not, work.

I don't see any reason why your system wouldn't work, if carefully rigged. The easy way to find out, before testing it on a flying model, is to simply build a mockup on your workbench and see for yourself.

There's a lot of good advice available on this forum, but nothing beats trying it yourself.

Juice,
Why not just eliminate the pulley and use a pushrod from the servo. Can't see any advantage to using a pulley, just seems like something else that could break. Also unsure how the surface would move in relation to the servo.

When the servo pulls the cable connected directly to the torque rod arm, the arm moves forward and the elevator deflects downwards. When the servo pulls the cable that goes around the pulley then to the torque rod arm, the arm moves backwards and the elevator deflects upwards.

The main reason why I'm thinking about this is for weight savings. Every ounce counts, right? The pushrod alternative requires a 34" length of (aluminum?) pushrod, guided to the tail with a 24" lengh of pastic tube. Although I haven't done any weight comparisons yet, I would think that the weight of two 34" lengths of pull-pull cable plus the weight of the pulley would be much less that the weight of the pushrod assembly. If not, then the pushrod method is a much safer and proven way to go.

Juice,

In principle, this system would work fine. The only problem I see is that you need to keep the cable in tension. If you don't, you will have some slop in the system that you definitely don't want. If you do, your servo will be bearing that load all the time. I don't know whether you want the drivetrain of your servo to be bearing tension all the time. It would be possible to have a static pulley on the servo end, and drive the cable the same way you couple to the elevator horn. It is still not a slam dunk to maintain the correct tension in the cable. I would be more comfortable with rigid rods. If you are really concerned about weight, carbon fiber tubes could be used. To get everything internal, the way you are with the pulleys, you could have the two rods coupled to a pivoting bell crank where you currently have the aft pulley, and couple one of the rods to the elevator control horn. This control horn would then need to accommodate the movement of the rod, which is the advantage of a pulley and cable.

Good luck.

banktoturn

Juice,

I made a mistake in my post. In your original system, the connect point between the cable and the elevator control arm will need to move up and down as the elevator is deflected, while the cable wants to keep traveling in its straight line. You could just allow the cable to stretch a little bit, but I don't like that idea. You either need the pulley to have the torque rod as its axle, or use rigid rods connecting to symmetric control arms on the top and bottom of the torque rod. If you simply don't have clearance inside, you might be able to do something with a bell crank in the back, and a coupling between one of the rods and the elevator control arm, but it may be difficult to get the motion of the push/pull rod and the elevator control arm to be identical. Maybe a simple system with one push rod, and careful attention to avoid flutter in the elevator?

Good luck,

banktoturn

could you just use a simple control horn on the inside and run your wires to it?

Try this pic out. Sorry, it's a little basic.

banktoturn:

You bring up a few things that I haven't thought about. I need to sit down and do some math to determine when slack or tension occurs, and how much.

MHawker:

I would do that in without any second thoughts if I could. But I can't, because there is not enough clearance above the elevator torque rod for the upward control horn. Nice try though.

Ahhh, didn't consider that. I think I would go with the single push-pull rod.

Good luck. Try to keep it simple if you can.

Mike

Did you consider a pushrod out of carbon fiber, these are stiff and very light. I understand you want to save weight but I doubt the weight of a pushrod would make much difference. It like guys who spend extra money for a lightweight fiberglass cowl only to have to add ounces of lead to the nose of the plane. I'd try to build a simple, strong and low maintenance linkage rather than trying to save an ounce or two.

Well... I finally did the math (in Excel) do find out how much tension or slack will be produced on the cables when the elevator is deflected up or down. The vertical location of the pulley on the tail block make a huge difference.

In the ideal setup, to keep both cables under equal tension, the amount of pull from one cable should exactly equal the amount of slack given by the other cable.

In a less ideal but very acceptable situation (positive Ackerman), more slack is given by the non-pulling cable. Only the pulling cable is under tension caused by the elevator wanting to go back to the neutral position in flight.

My results give me a slightly unacceptable situation (negative Ackerman). Not enough slack is given by what should be the non-pulling cable. For example, for 20 degrees of down deflection, the cable that is connected directly to the elevator horn is pulled by 0.234 inches, and the cable that goes around the pulley give up only 0.232 inches. That means something in the loop has to be stretched by 0.002 inches.

What do you think? Is 2 one-thousands of an inch close enough to ideal, or is it not acceptable? I am using Du-Bro's nylon coated pull-pull cables and I'm not sure how much they will stretch.

Juice

Giant Scale,

This linkage idea is not as simple to set up and it will take more time and planning up front. But I don't mind too much becuase I enjoy the building process as much (if not more) than I enjoy all the flying. And once the building phase is done, and if I can somehow get over this current problem, I think that this pull-pull linkage is will be just as strong and low maintenance as a regular pushrod linkage.

It's always a good idea to save weight behind the CG. I especially want to save weight at the tail because I'm adding a retractable tail gear (pneumatic) and sequential landing gear doors and a cockpit (all behind the CG).

And if I ever have to add weight to the nose, there are alternatives to dead lead. Two options I have in mind if my Corsair end up tail heavy, is an on-board glow battery or operational cowl flaps.

Juice

Juice:

how about a cable system as far back as you can go then use a belcrank to transfer the motion to a short pushrod???

Charles

A few ww2 warbirds were done this way including the ME 109 I believe.

I might be missing something but instead of a bell crank on your connector rod why not the pulley; it appears to have enough room to house the pulley you have drawn and perhaps a small inspection hacth would allow maintenance.

Let me know it that helps or if I did not explain clearly

to use a pulley instead of the bellcrank you would be relying on a friction connection from the cable to the pulley, one slip of the cable in-flight and your elevator trim would be way off (not a good thing) if you attach the cable to the pulley you have created a round bellcrank,more weight and more difficult to make.

I already went ahead with the original pulley idea behind the torque rod. It's working fine. The only trouble was finding the right pulley.

I bought an assortment of small plastic pulleys at a local science shop. I found that pulleys smaller than 1/4" in diameter are too small for Du-Bro's nylon coated pull-pull cables (permanently kinks the cables). Pulley's larger that 1/4" are too big to fit into the cramped space at the tail. I tried to fit in my only 1/4" pulley, but it was a little too thick. So I tried to grind it thinner with the Dremmel tool and then ruined it enough so that I would not trust it in flight. I still used it to fit together and try the system. I will be replacing it with a custom made aluminum pulley that at friendly club member is making for me next weekend.

I have pictures of the system with the plastic pulley, if anyone is interested.

CPILCHARD: That is a good idea, but a little to late. I probably would have used your idea instead. But thanks anyway.

Josh

Juice:

I'd be interested in the pictures of your setup. Solving these types of problems is one of my favorite parts of the hobby.

Glad things worked out for you.


Charles

Here are the pictures...

The plane is the .60-size Top Flite Corsair. This is my setup for trying out the pull-pull system on the elevator (as we've been talking about), as well as the rudder.

At this point (the picture was taken two weeks ago), if it was determined that the pull-pull systems would not work, I could still revert back to the standard pushrod systems.

This is the top view of the elevator torque rod. I tack glued some scrap balsa to hold everything in place temporarily. The pulley on the right is sandwiched between two small eyelets screwed into the tiny former at the tail (which is taped on a hardwood block in the picture). 1/16" music wire runs through the pulley's axel and is soldered onto the eyelets.

The cable that goes directly to the servo arm almost brushed by the rudder torque rod on the left (it;'s close, but it doesn't touch). The cable that goes around the pulley is routed along the side of the fuse inside the white plastic tube until it clears the rudder torque rod.

Here's the side view. The lighting is bad... hard to see the black clevises.

Anoter view...

The elevator servo is on the left. The rudder servo is on the right.

I had to calculate the exact locations where the cables would cross the formers. The elevator cables won't move much laterally. The rudder cables will.

Juice,
You've done good work here! Another benefit to the single-point actuator rod on the torque rod is that there is no pressure on any bearing surfaces. A pull-pull with the standard top & bottom cables would need a bearing surface of metal on the leading edge of the torque rod to prevent slacking of cables with use.

Now a real challenge... how can we actuate the flaps & ailerons with the controls completely internal? Mine is a .60 size Curtis P-40 model.

Obviously, what we're trying to do here is hide those ugly control horns.

Fred

Juice, I have a pushrod system that I used on a 60 size Sportster 15 years ago and it is still working great. What I do is to run a one-eigth dia. wood dowel through a standard pushrod housing,support it well and keep it realitivly straight. If it can't bend it can't break! This is lightweight, strong,has no radio problems,and doesn't cause trim changes because of temperature. On each end of the dowel I epoxy on a piece of perforated brass tubing with a short length of threaded rod soldered on.Use a clevis,balljoint,or whatever to make the horn and servo connection. Sporty