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Big hinge gaps can lead to poor response and even failure of a control surface through fluttering. The answer is easy though and we show you how in this story.
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Sealing Hinge Gaps

Enhance performance and integrity

Text photos and video by Tom Hintz

Posted – 12-3-2015

It seems that most people think of sealing hinge gaps as a purely performance-related modification to their RC aircraft. While closing the hinge gap can increase control surface efficiency it can also prevent a high speed flutter that can cause a failure of the hinges or the surrounding structure.

I don’t claim to fully understand the physics involved here but I am sure that air flow can be very different than we think. Leave a gap between a control surface and the airframe and air passing through that gap can disturb the airflow enough to make the control surface “flutter” which is simply a vibration that can be violent enough to damage or break the control surface or the hinges holding it. Either way many experienced builders consider sealing hinge lines to be a standard step on the way to completing an airframe.

The good news here is that along with building in narrow hinge gaps we can fix this problem easily with strips of covering material. Naturally this is best done while assembling a new plane but it can also be done later if we thoroughly clean the surfaces to which the covering will be applied.

Sealing Material

Some ARF kits include a strip of clear covering material meant for sealing hinge gaps. Any color covering film can be used but most try to match colors with the control surface or airframe. We usually apply hinge sealing covering to the underside of the plane so the impact on appearance is reduced.

Clean the area (left) a few times to be sure the oils are gone. I use my covering iron (right) set on roughly 50% of its heat to stick the covering rather than shrink it.
Click images to enlarge

There are a number of adhesive-backed covering alternatives and special tape meant to be used as the hinge itself on smaller models. I have used this tape for sealing hinges and it worked fine, until fuel residue loosened it up. I have also seen hinge gaps sealed with adhesive-backed covering from the same manufacturer as the covering on the rest of the plane with good results. I should note that I have been able to mess up all of these alternatives at one time or another so you may want to investigate all of the possibilities before choosing the best one for your situation.

Clean It Up

As with any covering procedure starting with a clean, oil-free surface is crucial if you want the material to stay put. I have found plain alcohol to be a great cleaner but you need to check compatibility to be sure any cleaner works with your covering without damaging it. Remember that the oils and fuels we use can be stubborn so I repeat the cleaning process two or three times to get the best bond possible.

Control the Heat

Another way to mess up this application is to use too much heat. Most modern covering material uses an adhesive with two temperature reactions. At roughly half of the available heat on covering irons the covering material adhesive is activated. It is important to finding this heat range for your covering so a little trial and error on scrap material might be in order before working on your model.

The is a tendency to use too much heat during application of covering material. If you get into the higher temperature ranges that cause the material to shrink it does not stick to the airframe very well. I also had to get this dual heat concept through my head but I finally did and my covering tasks are less frustrating because of it.

Application

Cut consistent width strips (left) to make this job easier. the needed width depends on the size of the plane and its control surfaces. When done check to be sure you have full throw capability (right) with no binding.
Click images to enlarge

I cut strips of the covering material using a straight edge to insure a consistent width. Exactly what width to use depends on the model you are working with. Most try to keep the sealing material within the “valley” created by the edge of the control surface and where it attaches to the airframe. I am not aware of any benefit to extending the covering out of the hinge area and onto the surrounding surface.

This should be obvious but to be sure the sealing material does not limit movement of the control surface we want to deflect it fully to open the hinge line where the covering is to be applied.

The hardest part of applying hinge sealing covering is getting it in without wrinkles. Once again I fall into the mediocre part of the skill range in this regard. I have found that tacking one side in place and then tacking the other side down before going back over and sticking the full length down seems to yield a better job for me. You may find a way of installing the hinge sealing material that gives you a better shot at a no-wrinkles job.

Once the entire strip is sealed in place move the control surface through its full range of motion to be sure the new covering is not trying to limit it. If that checks out hook up any linkages you removed and check the surface movement once more just to be sure.

If you normally use sealer on the edges of your covering apply it to the edges of this hinge sealing strip as well. Other than that, if you had the surfaces clean there really is no further maintenance other than checking these strips whenever you clean the plane up.

Video Tutor

Usually there is no huge change in flight characteristics due to sealing the hinges but be ready for it anyway. The only thing I have noticed after this installation was a slightly more aggressive elevator on my Aeroworks 50cc Edge 540. On the RedwingRC 50cc YAK 55 used in this story everything was so aggressive to start with I did not notice any change in how it flies. In any case we have reduced the chances of control surface flutter by a bunch, assuming the rest of the linkage is correct.

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