As I stated in last month’s article (but it needs to be emphasised again) - Your airplane, and how it performs in the air, relies completely on the aerodynamic flows and forces placed upon it. The way it performs and flies is based on the inputs to the control surfaces and the airflow over them, which in turn enables the airplane to maneuver precisely, smoothly, and safely in the air. How aerodynamically tight those controls surfaces are, how firm they are when forces of flight are applied to them, how smooth they operate, and how safe they are hooked up – all determine the aircraft’s performance, it’s safety, and the longevity of the model.
How many times have you seen a model that shows up to the field with loose or sloppy connections between the servos and control surfaces? Or how about the large gaps in hinge lines of the control surface? Or even hinges that are actually loose or misaligned?
The result can be that you have a model that will be difficult to trim, and one that can constantly change attitude when the speeds increase or decrease.
One thing I’ve learned over the years is that the response of your airplane relies a lot on the way your control surfaces are hinged, the way they are installed, and the amount of hinge gap there is. Weak or misaligned hinges will allow your control surfaces to flex, or flip from one direction to the other abruptly, which minimizes control crispness, or unnecessarily increases the amount of control input (which makes it more difficult for new flyers to learn). It can also make the model difficult to trim. This is especially true with higher performance and fun-fly airplanes.
Having large hinge gaps is a NO-NO! And I do blame some plane manufactures for setting a bad example for this as well. Just because your ARF has large hinge gap from factory, does not make it acceptable, or proper!!
Aerodynamic flows are disturbed; turbulence can be generated, and in extreme cases, can render the control surface almost useless or non-responsive. If you are going to spend time and money on your plane, take a little extra time to reduce hinge gaps and ensure the alignment is the same across the surface. The airplane will fly better and more precise. Instructors need to be diligent about this, as training a new flyer will go a lot smoother and will require less time, with less visits to the field, if the plane flies true and is able to be trimmed properly. It is also the job of the instructor to set a good example, and to maintain safety at the field. In some cases, it is just better to show the student the problem/issue, and have them go home and fix it - to return another day; rather than risk flying an non-aerodynamic model.
Excessive Hinge Gap
A large hinge gap does two to three things:
1. It allows for an extremely low pressure between the control surface and the wing/stab/rudder (wherever the control is located), thus creating uneven airflow over the surface, and in some cases allowing the boundary layer (the thin film of air on the surface of the wing that does not move) to separate prematurely. This creates further drag, can cause a loss of adequate airflow, and compounds many other drag related issues. But that is back to theory of flight lessons, and not the purpose of this article.
2. When the control surface moves, it also disturbs the airflow over/under the control surface by allowing airflow to flow through the gap when the control surface is moved. (See illustration below).
This will also cause the control surface to be less responsive, not something that is desirable!
3. It may cause some aerodynamic flutter. (But - This will be discussed in an upcoming article).
So take the time to reduce the gap on the hinge lines. You and your plane will love you for it. Hinging your control surfaces can be a bit time consuming, so here are a few guidelines to follow when doing them.
Sealing Hinge Gaps
A sealed hinge line prevents air from flowing through the hinge line. It is preferable to seal all hinge lines with tape or an iron-on film or fabric.
The seal can be a piece of packing tape adhered to the bottom of the surface, or iron on type of covering such as Monokote, or even TrimKote (clear is available). It is easy to do. Try it on a plane you have now that does not have sealed hinges and you will see how much a difference it makes. If you like the response your plane currently has then you will find that you can reduce the control throws to get the same response after sealing the hinge lines. This means less of a load on the servo and less drag to perform the same maneuvers.
To Seal a Hinge Gap
Only one side of the hinge line needs to be sealed (Usually the bottom) assuming the seal is not the hinge as when using Monokote hinges. Remember - if you are flying Nitro Models - tape is not fuel proof and could make a slimy mess after a while.
Thoroughly clean the surfaces so the tape will adhere properly. Cut a piece of packing tape/Monokote a little longer than the length of the hinge line. If Tape - Stick the tape to a clean piece of glass.
Tape/TrimKote - Cut the tape so that it is approximately 1-1/4" wide. That width will allow about 1/2" to be adhered to both surfaces. Deflect the control surface all the way and push the tape/TrimKote into the hinge line. Burnish it down to both surfaces, flex the surface back and forth to ensure it does not bind.
For Monokote apply similar to the Tape, except you will have to iron the surfaces down with the control surface fully deflected.
For long surfaces, such as strip ailerons, it is sometimes easier if you have a helper. Get a straightedge and put the non-stick side of the tape/TrimKote over the edge so that it is centered lengthwise along the edge. With each person holding one end, push the straightedge into the hinge gap and then rock it back and forth to stick the tape to both surfaces. Remove the straightedge and burnish down the tape/TrimKote.
In addition - Here are a couple of links to view a U-Tube Video on How-To seal a hinge gap…
You need to consider how many hinges you will require per control surface – and rarely have I seen two as the right number. Though, I have seen many new .40 sized trainers and sport models show up to the field with only two hinges attaching an entire control surface (yes even ailerons). If the control surface is more than 5 inches in length, then three hinges is a bare minimum. Otherwise - use your good judgement. You should consider the following:
Try not to allow your surface to flex between hinge points. You can pin the control surface with T-pins where you are considering putting a hinge. If your control surface can flex significantly between hinge points, then shorten the distance between them.
If a hinge breaks or fails, would it cause a catastrophe, or are you still safe to return to the field?
Is your model a racer, a pattern ship, a trainer? The faster the model, or the more aerobatic it is, then more hinges are required.
Are you placing the hinge is an area that is mechanically sound/strong? This applies mostly to stick built surfaces, but needs to be considered for all, and this should be considered prior to covering, as you may have to add material to the area in order to secure a hinge. (See illustration below)
5. Also, now is the time to consider type of hinge you will be using. There are many different types available.
Types of Hinges
Flat hinges come in a variety of sizes for all types of model airplanes. Only the smallest of model aircraft have surfaces that are too thin for a flat hinge. Very thin surfaces can use Monokote hinges, or thin Mylar. There may be other ways of hinging thin surfaces as well, but you would have to talk to someone who is more familiar with micro-flight than I am.
Polypropylene Hinges (Also called Poly or "Live" Hinges)
This type of hinge is an excellent choice, easy to install, and very long lasting. They almost always have a bit of a stamped hinge line.
I use poly hinges most of the time. The good thing about poly hinges is that they last forever and tend to be very tough and have no play compared to a pinned hinge.
Before you install poly hinges, you should flex the hinges back and forth a few times to loosen them up and provide a crease along the hinge.
There can be a drawback on these types of hinges, as they are a tad stiffer, and due this extra stiffness, they tend to prevent the ends of the surface farthest from the control horn from having the same amount of free throw as the surface has nearest the control horn. This behavior is mostly noticeable on longer surfaces such as strip ailerons using torque rods near the wing root. It can be reduced by loosening them up more with the creasing explained above.
On the other hand, if a small amount of resistance from the hinge prevents the surface from deflecting properly, then air loads in flight will probably do the same thing even with a pinned hinge. In other words, this is probably more of a problem with the control system than the hinge system.
These hinges are very easy to install with epoxy or CA, but back in my F-1 days I discovered a super way of installing them that is fast and amazingly strong.
· First - Cut two pieces of masking tape the width of the hinge and fold over the ends. (See Illustration)
· Second - Insert the hinge into the pre-cut hinge slot to the hinge line.
· Third – Apply a few drops of CA to the top and bottom of the hinge.
What you will notice is that the CA is sucked into the hinge slot by capillary action. What you don’t see inside, is that the CA is absorbed (by Capillary Action) into the masking tape on both sides, securing it not only to the hinge, but also to the balsa wood surrounding the masking tape. It is now secured for good! And the voice of experience will tell you that if you have to remove that hinge – just forget it… it will not come out without damage to your structure!
· Fourth – When you push the control surface in place over the hinge, it is a good idea to put a little Vaseline along the hinge line and top and bottom of the hinge. This way, if any CA gets on those surfaces, you won’t be gluing the control surface to the hinge line.
You will notice that when you push the control surface onto the hinges, that you are able to push it right up to the hinge line, reducing the hinge line to almost nothing. So – Point – A tight surface also allows a Capillary effect on the hinge line for the CA. And CA works best with tight surfaces and with Capillary effects – so keep this in mind when using this method, or you may find how far that one or two drops of CA can travel… :-)
This is a type of flat hinge that has an actual pivot point - often a steel pin, or small diameter cotter pin. At times when I use flat hinges. If you look at any full-size aircraft, you will notice that the hinge line is partially inset into the control surface. The only type of hinge that allows this same setup in a model aircraft is a pinned hinge (flat or Hinge Point). There is a minute amount of play in the hinge, but very minimal. Installation requires that you put Vaseline on the used e hinge itself as well as around the hinge during install to ensure no glue gets into the hinge itself. CA can be used, with caution, similar to the installation of Polypropylene hinges. But you do risk a greater chance of CA leaking into the pinned hinge, and then you may be faced with trying to get it out, and likely that won’t happen without structural damage. You may be better off using epoxy for this type of hinge.
CA (Cyanoacrylate) Hinges
Some say that these hinges are made for lazy people. Perhaps, but they are not the best choice for hinges. CA Hinges are made from a strip of Mylar with a piece of thin cloth or paper laminated to each side of the Mylar. They are called CA hinges because they are intended to be glued in place using cyanoacrylate (CA) The porous laminate is what the CA actually adheres to. This is similar to the method I described on Poly Hinges. However, epoxy also can also be used, but that is not what they were designed for.
The only advantage CA hinges have over other types is that they are so thin that you can shove a hobby knife into the surface and that is your hinge slot. **Capillary action again…** Other than that there are so many things wrong with them that it is hard to remember them all.
The first thing that comes to mind is that they require you to deliberately leave a larger gap between the flight and control surface than would be necessary with any other type of hinge. This gap is necessary to prevent these hinges from breaking.
The larger gap and the thin, flexible hinge material combine to allow the surfaces to flex up and down - sort of like what a belly dancer does by panning her head side-to-side. Control surfaces are not supposed to be able to move like that, but it is a fundamental and defective property of CA hinges.
Second, and more importantly, CA hinges have the highest failure rate of any type of hinge that you will ever use. If you have flown any amount of time, you have likely seen or experienced failure of these hinges. The fact remains that CA hinge failures are reported frequently, to include everything from the laminate delaminating, and in some competitive RC venues, they are discouraged, or not allowed.
Pinning Your Hinges
Some would suggest that you always pin flat hinges to be mechanically secure. Please - Do not confuse pinning hinges with pinned hinges. Pinning refers to using a toothpick to secure the hinge to the flight or control surface with the pin centered in each half of the hinge. I have rarely ever pinned a hinge myself, but below is how you do it if you choose to.
One way to do this is to glue in the hinges in place and attach the control surfaces. After the glue is set up, drill a 3/32" hole all the way through the surface and the hinge.
You then push a round toothpick through the hole and then use a single edge razor knife to carefully trim off the excess. A drop of thin CA will secure the hinge forever. You can either put a drop of fuel-proof paint over it to match the covering or, if using iron-on covering, put a scrap between two pieces of paper and use a hole punch to make dots that can be ironed over the pins.
A pinned hinge will never pull out without taking other parts with it. The choice is yours if you do this or not, but you better be sure that you know how to glue a hinge in properly if you don’t pin them.
Hinge Points were first released by Robart and were an instant success. They are strong, reliable and very easy to use. Rather than cutting a slot for a flat hinge, you simply drill a hole using an alignment centering guide tool. Hinge Points are a favorite type of hinge and the first hinge many modellers use unless there is a specific reason why they cannot use them.
For example, a narrow, highly-tapered surface, such as a strip aileron, may make using Hinge Points a poor choice. The Hinge Point may only be able to partially inserted before it goes through the outside faces of the control surface, thus requiring the Hinge Point to be shortened significantly.
Although Robart states you can cut a Hinge Point down to 2 barbs, I would not trust the life of my model to this. In cases like this I use pinned, flat hinges.
Rather than repeat Robarts guides on the installation and use of Hinge Points, here is the link to view their How-To video.
It can be tricky to get the slots for flat hinges centered and straight in the hinged surface. Imagine the front door to your house - with one or more hinges misaligned, crooked, or with a big gap… You can imagine how difficult it would be to open and close your door without cracking or severely straining something, or with wind blowing in. J You want things to be aligned and strait so that it moves freely, and with little effort, and no gaps – the same as the control surface of your model.
Proper centering of the hinges in a straight line across the control surface extremely important. Hinges that don’t line up along a fixed linier point of rotation can cause binding, and also create a situation where the control surface cannot stay in a neutral position, or stay centered. Alignment is not just for the rotation of the hinge line, but also the angle of the hinge. (See below)
Another common problem is that the slots can get cut at an angle when viewing the slot straight on. It is not always an insurmountable problem, but you have to be careful not to let the wood grain change the angle of the slot tool as it is pushed into the wood.
There are a lot of centering tools to aid in cutting your hinge slot or hole. However, some of the available hinge slotting guides do not always resolve the problem because they are made from soft, flexible plastic. If the slot is not located properly on the first attempt, then more material sometimes, must be cut away using a hobby knife so the hinge will align properly. This leaves a gaping hole instead of a nice tight slot which is not desirable, so you will have to move the location of the hinge.
This problem simply does not exist with the Robart style Hinge Points because a Hinge Point can be rotated into perfect alignment.
Du-Bro has some of the best tools for centering and aligning your hinge slot. The tools attach to your X-Acto blades or you can purchase some with handles from Du-Bro. The tools are easy to use, and they keep your hinges aligned and centered to the tailing-edge and your control surface. Simple use as directed.
I hope this helps provide a few pointers on the How’s and Why’s to hinging, alignments, and sealing those hinge gaps. This article was supposed to also include connecting your controls to the Servos, but for the purposes of length, we will cover that in the next article.
Vincent Mulhall MAAC 11134