Alternative Elevator Control System for F3A models
By Troy Newman updated***7/2004
The following information is provided as a How to Article, nothing more. I am presenting it as a result of requests by other modelers to see the method. The author assumes no responsibility for any failures of the method show. I have used this method now for a long time and no failures. The installation and the attention to details are critical in the trouble free operation of this setup. In essence we are dealing with glue joints, and builder manufactured components. Its up to the builder to take every precaution he feels needed and assumes all risk of the operation. Read these instructions entirely before you dive into building this thing. There are some options given later in the directions that may fit your needs better and your choices early on will restrict you options later. I give the method I use. You can adapt this to anything you like.
In my opinion the absolute best elevator control assembly is designed to be light, strong, and allow the least amount of differential between the elevator halves. The following setup is not my design, but I have adapted a couple of different methods of mounting to insure that the best solution for me. I have done this same setup on 6 planes and just finished it on a 7th. Since 1999 I have been running this setup in F3A aircraft with hundreds of flight a year and zero failures and zero maintenance. You can also with careful install put this in a model that is already done and painted.
In comparison to this I have a couple other systems to address and why I don’t use them. I will only deal with single servo applications because I like a single servo. Any of these setups including the MK Bellcrank can be driven with 2 servos. Two servos give a weight penalty not only in the servo but the extension wire. You would be surprised at how much a 1 meter extension weighs. There is also the issue of matching the servo travels. This has become almost a non-issue with today’s digital servos. I know several top flyers using the JR 3421 digitals one on each elevator. These are great and provide a good solid connection. Since they are digital the centering and the holding power make them perfect in the dual application. I found I liked a single servo better!
First of all many people like cables it’s a pull-pull setup and its very light. Another big advantage to cables is the fact that it is infinitely adjustable. You are not limited by the 1 turn of a 2-56 thread in adjustment. If done properly the connection is solid and feel is great. The next plus for cables is the fact that you can tune all 4 travels of the elevator exactly the same throw at the end points. This can be done easily using the length of the control horns and comes out dead on.
I have not used the Kevlar type cable setups. I choose 30-40lb test nylon coated steel fishing line from the fishing shop. It comes in 30ft rolls for about $2 and the little crimpers are about the same $2 a pack. You can do many many planes with this $4 purchase. Now for a Cable type system to be effective in a model you have to make sure that the line is absolutely a straight line from servo to the control horn. This means exits in the fuse must no touch the cable and be totally free of any friction. Anything touching the cable will cause it to fray and fail.. You also have control horns on top and bottom of the surface. This is pretty easy since most people choose a 6-32 bolt as the control horn. So you just leave it run through and attach top and bottom.
Cables have many good sides, but here are my downsides. It doesn’t look as clean and as neat. Function before form this is OK. Also you have to make absolutely sure the cable is no riding on anything inside the fuse like the fuse exits. This will cause the cable to fail early in its life. I have had some cable failures not many and not recently but they have failed. The key here I think is using eyelets rather than rigging couplers. The eyelets allow the cable to loop around the connection rather than kink. The kink will cause a failure. SO in this case it is again not as clean and neat a setup. The tension of the cables is critical. If too tight it will “pop” into place and cause grief with your servo trying to center and also causes side loads on the servo shaft.. While too slack of a cable will cause a mushy feel. I have found that as the cable ages with the loads we apply they will stretch slightly and become loose. So another drawback is the fact that you have to keep checking them and make sure the tension stays constant. I know how we measure cable tension in the mechanical engineering world, one way is to deflect it (push in the middle) and measure the distance it deflects. But what is a good number? The other problem is that the cable could be stretching under loads. Since there is a cable going to each of the elevators top and bottom you are stuck with 4 different cables with possible different lengths all be them small differences. You also have 4 sets of connections that are not done exactly the same in the same spots. This can affect the stretch and you can get different tensions on all 4 cables. So you can get differential in theory based on the material and the methods used to secure the cables. I want those elevators going up together no matter if I have huge loads on them or not.
This says I will bank the cables and look for better ways. I want the best solution and cables may be it for me but lets look further. Not only that but the cleaner looks are important to me. I always have trouble getting the exit hole right on the cables, and this becomes critical. A sure fire way to make it right is to have the hole oversized. This affects the looks. Personal Preference!
MK bellcrank method
The MK bellcrank is a wonderful little device. Ball bearing sockets make the transfer of forces friction free. It works well. But you have to abide by some rules. You must use a stiff pushrod to serve the middle. I recommend CF pushrod like sold by Central Hobbies with the titanium ends. These get used for the pushrod devices both serving the middle of the MK and as the outputs to the Elevators. I choose the 5/32” rod for the output rods and the 3/16” for the input rod. This works very well. Another thing is support the input pushrod or have a very short run at it. Otherwise the loads could cause this to bend and the solid connection will not be there under large loads. At least in theory the two sides will deflect the same amount as the rod in the middle bends slightly so you should be good to go. Then you have lots of connectors. Some guys have converted the Mk to cables pull-pull inside the fuse and pushrods outside to the elevators. This is in my opinion not the best of solutions because you have no way to watch those cables to look for wear and you are getting the same tension issues as before. Next is the device is made of plastic, however strong that maybe it does flex a little. So when using your ball links and stuff don’t use the standoffs this will make the connection have a moment arm larger and will cause the arms to flex even more. It would be a great little thing if it was aluminum but the price would be higher. Also you need to use ball bearing connectors everywhere. This insures free flow of all the connections and eliminates the friction in the system. You need a total of 6 connectors, which means using the top of the line MK stuff with the ball bearings for the best setup you have about $8 per connector for a total of $48 in connectors. Add in the price of the CF pushrods with all the titanium ends. Say it is another $25-30. Remember there are 3 separate pushrods you have to setup. This combined with the $30 for the Bellcrank itself means you have a $100 elevator setup. Now some people choose the Fiberglass DB pushrods. They are black and some guys are convinced they are CF and they are less expensive than the Central Hobbies CF ones baby. Well I have used them in the past and the ones I have had including the black ones are not CF rather they are black colored fiberglass arrow shafts. They also weigh more than the Central Hobby stuff and not nearly as rigid. So I use them on 60 sized sport stuff. The $100 setup is a bit pricey but will work superbly if installed right. You will never feel the elevators getting differential even though when you flex them with you hands you can make differential. I guess our flight loads are lower than my greasy palms. This entire $100 setup also has a weight to it. I don’t mind weight because it’s needed for strength but have to watch for excessive weight gains. I weighed all the MK stuff including the support plates in the fuse the screws to install it and all the pushrods made up with the connectors. The weight was 4ozs this seems high to me. Now I used the MK device for a couple of years and felt it was the way to go. I had nothing better at the time so it was the best solution. But its weight was a killer and so was the price tag. So I banked it for another option.
My Preferred Method
Now the long awaited method I use. This came to me from Scott Anderson of Dynamic Aircraft. Scott is a master builder and has built championship level airplanes for many of the top guys. Scott also builds TOC aircraft. His work is among the best that I have ever seen. The work is light, strong, and detailed. From what I understand Scott got the idea from a long time pattern flyer Tony Frakowiak. So I want to give as much credit for this idea away. It works great. This method requires much more work to install than the MK or the cables. And it takes doing it at least once to learn the technique of installing it in the fuse. But the second time its cake and you need only line up your servo and your exit holes. The pushrod setup then spans the gap. The extra work is intimidating and can make you stare at it for hours thinking about how it is suppose to work. But when done it is lighter than the MK. Only 2ozs, and its stiffer in the differential category too. Totally mechanical, and if installed properly is friction free. I have some pictures below that show it completed….So you should not have that much head scratching
First of all the basic layout is (2) solid Carbon Fiber rods running the length of the fuse to each elevator. These are solid rods not hollow….They are 0.070” diameter or the same size as a 2-56 pushrod. So they are pretty small. These rods are available from Central Hobbies as the D.E.P.S. kit Dual Elevator Pushrod rod kit. It comes with all of the fitting and little stuff you need. In the past I have purchased all this stuff separately from various sources. Now it is all packed as a kit. These can be found at www.centralhobbies.com
Ok getting started, you wrap sewing thread around the balsa stick and the plastic housing with the CF pushrod inside of it. Now don’t warp it too tight as the plastic will pinch the rod inside and cause more friction. So your goal is to get the plastic housing attached to the balsa stick. Do this for each pushrod. The wrap is done like a fishing pole with close together wraps for about an inch or 2 then looser for 2-3 inches then tighter wraps then looser. All the time keeping just enough tension in the thread to keep the Pushrod attached to the balsa stick and no more. Once this is done you hit the sewing thread with thin CA glue. The balsa will soak it up like mad and will stiffen. This is why a softer piece of balsa will work. You don’t need to get all of it just make sure that the thread and the pushrod are not moving on the balsa stick and there is no way for it to come off. These sticks don’t move and are glued into the fuse as the guides for the CF rods inside the tubes.
Now this is where I vary from the designer’s idea a little bit. I measure the length required to build a ladder from the front mounted Elevator servo to the exit holes in the fuse. I build a ladder type structure attaching the pushrod assemblies to each other. This is done in a big “V” with the Plastic housings on the outside of the “V”. See the attached picture #1 of the inside.
Picture #1 Ladder built and glued inside the fuselage. The horizontal Balsa former above the ladder is part of the stab install. You should have your stab installed already before you install this pushrod setup. I leave the tail post open so I can get in there and work. But the Stab is 100% installed. I use removable stabs but glued in solid stabs are fine also…The pushrod doesn’t care. The Vertical former with the hole in the middle is located in the fuse ahead of the stab slightly and keeps the fuselage from twisting in front of the stab. This former also keeps the fuse sides from flexing ahead of the stab and causes the fin to be much stiffer also.
The “V” is the width of the fuse at the exit holes…pretty easy dimension to get. Let the Plastic guide and the pushrod overhang the balsa sticks a bit. They will exit the fuse and you want that plastic guide to be cut flush with the fuse sides making a friction free guide the entire length while inside the fuse. I use ¼ sq balsa for the cross pieces of the ladder. I build the “V’ up and glue it all with thin CA. Then take some ¼ balsa, light stuff say ¼” x ¾”, and make the supports that you see holding it in position in the fuse. Leave the assembly about 3” short of the Elevator Servo connection. The reason is you have to attach the pushrods to each other, and you have to have some sort of connector to attach to the servo arm. I’ll discuss the attachment to the servo later.
For a friction free setup the pushrods cannot have any bowing or bending from the fuse exit to the elevator control horn, or from the center of the fuse connection shown above to the servo arm. So it is very important to locate your fuse exits in the perfect location. I usually have my stabs done and completed. The stab mounted on the fuse and the control horns already on the elevator halves so I know the proper location. You also should have your elevator servo mount and servo with arm installed so that the angle of the ladder is perfectly setup as a straight shot inside the fuse. This is the most critical and most difficult of the install. Make your fuse exits slightly oversized to allow for a perfect location. Once you have the locations and everything trial fit, you’re ready to make the cross supports to hold the ladder inside the fuse. Oversized is good then sand them to fit further down the back of the fuse if needed. These are easy to make and need not be perfectly placed. Just make sure you are close to mid span and attached up front near the servo. The ¼” square balsa sticks with the Plastic sleeves attached are continued all the way back to the exit holes and actually glue to the fuse at this location. This gives a really solid support where the rods exit the fuse. This method gives 3 supports over the entire length of the pushrods while inside the fuse. Again refer to the picture #1 above for the installed setup. You can use more supports but its not needed. I would not use less supports than the 3 shown above..
One other note when cutting off the balsa sticks for proper length leave the Plastic sleeve and the CF pushrods alone and leave them really long. This will aid in future steps and setup.
When you have the ladder glued into the fuse, use extra epoxy with Micro balloons to fill the slot at the rear of the fuse for the pushrod guide exits. This makes them solid and you can cut the Plastic guides and sand flush with the side of the fuse. Idea here is just to make sure the Plastic tubing is totally surrounded or incased in Epoxy. You are not trying to fill the slot for painting just make sure that the Plastic sleeve is not going anywhere. Cut and sand the Plastic and any epoxy flush to the fuse side. Dips and depressions should be filled before primer and paint using lightweight auto body filler. I use Polyester glazing putty…Dries very quick and sands like balsa. Make sure to keep the Plastic sleeves clear of epoxy or filling putty.
Then you are ready to paint the fuse. Don’t use any special prep to your fuse exits and don’t worry about primer or paint clogging them up. Plastic is smooth and nothing sticks to it, not even your eggs. So when you’re done the little paint that gets inside will come right out.
Fuse is all done painted and its time to hook everything up.
Servo connection up front.
There are a couple variations on this connection and everybody I talk with about the connection has a different take. The first setup and the original design I copied was the ladder was straight, meaning the two sticks were parallel. The CF pushrods that you see in the picture #1 above were then attached to the fittings (I’ll show the fittings and discuss them later when doing the elevator horn connections) and then a piece of Plywood, maple, or even Aluminum was used to get the two sides tied together. This way only one servo would drive the cross bar and the two longer pushrods were follow. This setup of the parallel sticks works best for the absolute friction free setup. The “V” like I use is a little tougher to install but is lighter, and has less hardware and connections. If you install it right the result is just as friction free but tougher to measure everything and setup. The parallel sticks are just cut to length after they are fitted. The “V” requires more work on the setup and assembly of the ladder. Either way works well. Of course the parallel setup can also offer a dual elevator servo option. This allows the servos to be upfront and not affect the balance of the plane. Again I would only use digital servos on a dual elevator setup. The “V” setup can also allow you to use one servo but connect the two separate CF rods to the servo arm. One on top of the arm and one on the bottom with a single bolt going through the ball links. This is a good choice but requires a very good setup and a little more time in the layout and building of the ladder, as both sides need to be exactly the same since they are running to the servo independently.
I use the following method its light, simple and has survived crashes still intact. I use a piece of Central Hobbies 5/32” Carbon fiber pushrod about 3” long and attach it to the Two 0.070” solid CF rods with Kevlar thread and thin CA. I wrap about 1” of length. The 4-40 Titanium fitting below is not glued in place yet just sitting to show the connection for clarification purposes. The 4-40 fitting gets glue in place later on….
Picture #2 (2) 0.070” solid CF rods attached with Kevlar thread and thin CA glue to a Central Hobbies 5/32” hollow CF tube. The 0.070” rods are still 48” long and the 5/32” pushrod tube is about 3 inches long.
Leave the 5/32” hollow CF tube long for now. Get the servo 100% mounted and ready for action. It really will not matter whether it is mounted on its side or not. I choose a side-mounted setup as shown in a later picture. But the fact is it really is of no consequence.
Trial fit the pushrods in the sleeves. You may need to clean out the paint that is in the Plastic sleeve. This will usually just pop right out when you slide the pushrods in. If not. Use a small rat-tail file to open the exit up until its friction free again.
Leave the elevator horn connection alone for now. The pushrods are long and need to have one connection setup perfectly before you go to the other end.
Make sure that you have about 1” of travel each way out of the servo while pushing the “Y” pushrod. Its important there is no binding of the “Y” as the separate rods enter their respective Plastic sleeves. Its OK to have a little extra of the dual pushrods extending out past the balsa sticks and Plastic sleeves but don’t make it a really long run. These rods can flex a little so keep everything as close as possible but still allowing for friction free movement. Cut the length of the 5/32” Central Hobbies pushrod off until everything is the proper length. Remember you have a ball link and a Titanium end to glue into the now single side of the “Y” connection. The way I do this is to test fit everything together. I actually thread my ball link onto the Titanium fitting and slide the fitting into the Hollow CF tube. Everything is attached to the servo arm and the Elevator servo powered up and centered. I then move the servo its maximum amount of expected travel and look for binding. I make sure I have just a little extra travel room in case needs change in the future and I need more elevator throw than expected. Once all the parts are dry fit and assembled make sure everything is the proper length. If all is correct epoxy the Titanium end into the 5/32” piece of Hollow CF pushrod. I use 30 min Epoxy and make sure I have a good lather of it inside the Hollow pushrod. Wipe any excess of with paper towel. While the glue is all still wet I hook everything back up to the servo just as if it was ready to fly on this end of the pushrod. These connections will hold everything in place while it cures.
Elevator Control Horn connections
Only after the 30 min epoxy is cured. Go to the elevator side of the connection.
On the Elevator side of these pushrods you now have the 0.070” solid CF rods sticking out about 10-12” past where they need to be. You are going to cut them and attach a external Titanium pushrod end.
Now use the same technique as up front. Install the solder links whether they are the stainless or the brass versions onto the clevis. Install the clevis on the elevator control horn. Once this is all connected just as it will be in flight. Turn your Elevator servo back on and make sure it’s centered. Use masking tape to hold the elevator halves centered, mark the location to cut the 0.070” solid CF rods. Use a Dremel cut off wheel and cut the rods slightly longer. Now test fit everything together trimming the CF rods back until they fit perfectly inside the solder link but have no gaps inside the connection. Clean the CF pushrod with alcohol and take some 100 grit sand paper and just scruff it up a little...not too much as you will cut the Fibers and make the pushrod weak. Just scuff it lightly. Now use 30 min epoxy to glue the pushrod inside the solder link. Make sure you have a really good lather of epoxy inside the hole and good coverage. Once all in and covered good with glue, wipe any excess epoxy off the connection from the outside. Leave a small bead of glue at the joint. It should look a little like a Plumbers sweat joint. Let everything cure!…..and its all done and connected.
If you make a mistake and the lengths are not correct. You can remove the pushrod ends and re-install them. To change the fittings, remove the clevis from the fitting. Take a Bic lighter and hold it under the fitting to heat it up. Once hot, the epoxy will be liquid, take a pair of pliers and just slide it off. The epoxy will soften with the heat and the fitting will slide right off. Clean the old epoxy off while still soft using 100 grit sand paper and then follow the steps above the re-install a new fitting.
The finished product looks like Picture #3 showing the elevator servo connection. The picture below shows a JR 8417 digital servo side mounted. The Servo arm is the Hangar 9 Aluminum Servo arm part #HAN3431. This is the part number for the JR servos. The Ball link is a Hangar 9 4-40 threaded ball link with a 2mm screw going through the servo arm. The Titanium fitting going into the CF pushrod is the 4-40 fitting for the 5/32” Central Hobbies Pushrod.
And Picture #4 is the Elevator Horn connection side. This picture shows the MK medium Aluminum control horn, and the MK 2mm ball Bearing clevis. The silver link is the Titanium external pushrod end in the DEPS kit. And the connection is 30min Zpoxy resin.
Some more info on Servo choices….
And my choice for elevator servo on a single setup is anything with about 80-90oz-in of torque. This is overkill for sure, but You can never have too much power. The JR 4131, or 8101 are great servos in the analog world. They offer very good precision and a strong power curve. If you want to “GO Digital”…I use the JR 8231, JR 8417, or the JR 8411SA. The 8411SA is a superb choice for this application. My opinion you can never have too much servo. Especially on a F3A model you can never say the servo is too strong or too precise. I also use the Hangar 9 Aluminum Servo arms. These are rigid and do not allow any flexing.
Now I use this system exclusively right now on F3A models for Elevator control. Someday I might find something better. It is not the easiest thing to build and install. But so far it’s the lightest and stiffest connection I have seen. You have to grab the elevator halves and try to flex them opposite of each other to really appreciate the benefits of the this pushrod setup. As for Radio interference issues because of the CF rods, I have had none. I Fly JR PCM “S” receivers in my F3A models and run the Antenna inside a tube inside the fuse. I have also flown JR PPM “FM” Receivers with no troubles. You mileage may vary but I see no issues using the CF rods in the fuse.
Well I think this sums it up really well. Its long but covers the bases. The opinions above are my own and do not reflect on any business or company providing materials. Some modelers with disagree with my methods or choices. These are my models and I make the choices. I tried to give some of my reasons why I make the choices I do. The advice given previously is worth exactly what you paid for it. Just that: Opinion and Advice!