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Hinging Control Surfaces
Easy to do and to mess up
Text, photos and video by Tom Hintz
Posted – 5-24-2017
In this How-To we look at procedures that too many consider to be more nuisance than critical. The reality is that a failure at the hinges or control horns can easily spell the end of your plane. Glue-bound hinges can produce excessive loads on the servos making a failure of the servo or hinge a definite possibility. The extra load on the servos can dramatically increase the current draw that can run the battery packs down much sooner than anticipated. It just makes sense to me to take the time to do these installations right the first time.
Epoxy in its slow-set formulations has been the traditional go-to glue for hinges of all types except for the purpose designed, one piece CA hinges frequently used on smaller aircraft. For this How-To I will be installing the very popular Robart-style pin hinges that come in the QQ YAK 54 35CC ARF kit. However, the flat plate-type hinges can be installed using the same epoxy and procedures described here.
I know some folks have been using fast-acting CA glues to install hinges of all sizes without significant rates of failures. On this point, I opt for the long-term security offered by epoxy, not the speed of installation of CA glues. You will have to decide which adhesive best fits your willingness to take a chance with an expensive aircraft. In this regard, I can’t afford not to use epoxy!
Control Horns First
I always install glue-in control horns prior to doing the hinging simply because it is easier with the control surfaces unattached. Also, I use the same long-set epoxy for installing glue-in control horns for the durability that adhesive provides. Other than not using enough epoxy in the cavity the control horns go into a frequent mistake people make is not roughing up the surface of the control horn that contacts the adhesive. I’ve seen people using fine sandpaper to “break the shine” on the glue surface but I like to get a bit more violent than that. I use a woodworking rasp that produces tangible scratches in the surface that give the epoxy something to get into and dramatically increase the bond strength. I have had control horns installed this way tear sections of the control surface out during exceptionally hard crashes with no hint of a failure at the joint itself.
When dual control horns are used on a surface I install the linkage “ends” between them as they will be in the finished plane while the epoxy sets up. This locks in the correct spacing so there is no miss alignment built into the control horns that could contribute to the breaking down of the glue joint.
Divide the Job
Even when using long set epoxy, the large number of hinges used on bigger planes can have you rushing to get done before curing begins. Rushing is never good so I divide the job up into at least two parts. Sometimes I make the rudder installation a separate job, especially when the tail wheel is being epoxied in as part of that installation. I can mix up smaller batches of epoxy way easier than I can cut miss aligned hinges out of a new plane.
Straight and Square
Whether you are using pin or leaf style hinges the pivot point must be parallel with the edge of the surface they are set into. Get either type of hinge out of alignment and it can create a significant hard spot that the servo must overcome each time the surface is moved. That same hard spot can make it nearly impossible for the servo to be precise when returning to the neutral position. I move the control surface through its full range of movement right after setting the hinges and before I tape the surface in place to let the epoxy set up. If a hinge or two are out of alignment you can feel the resistance at some point in the movement. This is another reason to use slow set epoxy. It gives me time to make these small adjustments that can make the installation bind free.
Tape it Up
Epoxy is just messy to use. I have tried being careful with the application but that never worked well. I finally got out my painters masking tape and put a strip down the edge of the surfaces being hinged, keeping the tape close to the holes into which the hinges are installed. One inch wide tape seems to be the best for this job and my less than mess-free technique.
I have also found out the hard way to keep plenty of paper towels handy along with a spray bottle filled with alcohol for cleaning up errant epoxy. Even if you don’t drop epoxy during the process some will undoubtedly squeeze out around the hinges during installation and we need to clean that up before it sets.
After installing the hinges, I wet a piece of paper towel and scrub the hinge line from both side before removing the masking tape. Then, after removing the tape I go back over the hinge lines from both sides with more paper towel soaked with alcohol to finish cleaning up the excess epoxy.
Key to getting a solid bond between the hinge and wood is to apply a coat of epoxy to the inside of the hole as well as the leg of the hinge. I have tried all sorts of glue syringes and nozzles but nothing works better than a simple popsicle stick with one end trimmed down to fit the hole. Using this simple tool, you can get a good coating of epoxy throughout the hole interior so when we insert the hinge leg, also coated with epoxy we are all but guaranteed a solid installation that can withstand the loads of high energy maneuvers.
Since most of my planes use a pull-pull system to drive the rudder I start hinging there. It is easier running the cables through the tail section while the fuselage is still essentially empty. With the rudder servo installed we can also cut the cables to length and install them with a proper amount of tension.
Another point of installing the pull-pull system now is that it makes other installations easier. Many planes have elevator servos located near the horizontal stabilizer so with the pull-pull cables already in place it is easier to route the servo cables to be sure they do not rub on or otherwise interfere with the pull-pull cables. In the case of many warbirds, you may be installing a retractable tail wheel that often has its own set of pull-pull cables in addition to the rudder cables for steering. Getting all this equipment installed and adjusted now is the prudent thing to do.
After setting the hinges on a control surface, cleaning up excess epoxy and removing the masking tape we need to make sure everything is fully seated and that we have full travel of the control surface in both directions.
I have returned to an airframe after giving the epoxy plenty of time to cure only to find that one or more control surfaces had slipped out of alignment. Whether this movement was caused by gravity or trapped air pushing the hinge back out of the hole, it meant I now had a lousy hinge job that will make the plane less responsive. There is a simple answer to this.
Strips of masking tape across the hinge line, on both sides of the control surface will hold it in position while the epoxy sets up. Make sure that the surface is in its final position before applying the tape but don’t “crush” the surface trying to close up a gap at the hinge line. If we fit the hinges correctly in the early stages of the hinging process this should not be an issue.
I tape the surfaces in their neutral position and check the hinge line gap and alignment of the hinges before moving on to the hardest part of the hinging process for me. Setting the epoxied and taped assemblies aside and leaving them alone until well after the epoxy has cured can be tough but necessary.
Flex and Lube
After the epoxy is fully set we can remove the tape from the control surfaces and move them through their full range of motion to be sure there are no binds from errant glue. If you paid attention when cleaning the hinge line earlier this should not be an issue.
I have found the occasional thin film of cured epoxy on a hinge joint but that just pops off when the surface is moved up and down. If there is a little glue remaining I use the point of a hobby knife to pick it out of the joint. When I am satisfied that I have all the glue out of the joint I apply a drop of plastic-safe lube to the hinges before working the surface up and down to spread the lube throughout the hinge pivot. I usually buy lubes meant for use on RC cars because they have lots of plastic parts and the lube is designed to be compatible with them.
Make Checking a Habit
Because of the loads we apply to control surfaces and the hinges that support them it is a good idea to check them from time to time. Look for hinges that are moving within their holes, possibly from the wood compressing around the hinge or a less than perfect glue bond breaking down. I have been able to repair slightly loose hinges with medium or thin CA. That way I don’t have to remove the whole control surface or cut the hinge out of the wood. And if the looseness isn’t severe I can usually fix it at the field.
Hinging is an important part of assembling aircraft whether it is an AFR or stick built. There are few things that can influence the performance of our planes more than a bad hinge job. Taking the time to do it right results in a better flying, more predictable plane and one that is likely to have a long, fun life as part of your armada. Doing it right doesn’t take much more time, money or materials. All it requires is for you to care about doing it right.
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