Originally by Dean Pappas for Sport Aviator
Sealing the hinge gaps is important. It is one of the simplest things in the world to do, and it can fix all kinds of problems. Note: Some older planes that use cloth or sewn hinges do not have gaps that need to be sealed.
A technique can be done with tape or iron-on covering for park flyers and small models weighing 4 pounds or less. Short lengths of covering are ironed together, sticky side to sticky side, with roughly 1/8 or 1/4 inch of overlap. The pieces are ironed to the top and bottom of the fixed surface, in an alternating fashion, and each piece is fed through the hinge gap in an “S.” After a little work with an iron, you have a gap-free hinge. It’s light, simple, and economical. This technique combines the seal and hinge into the same place and is not recommended for larger models.
Many modelers use an iron-on plastic covering for at least the wings and tail feathers. Even with trim schemes that cut across the hinge lines or color changes from fixed to moving surfaces, we can do a pretty job with the same covering material. To make a seal that does not tighten and sag when the controls are moved, we have to make an “S” seal as with the hinges above. You can even use different colors in each half of the “S” bend to match the colors on the top and bottom of the airplane.
The beauty of the “S” seal is that it does not tighten and bind the control surface—even at 3-D control throws. Clear iron-on covering can also be used if there are too many color changes near the hinge line. For painted models, you need to seal with clear tape. Pliable, clear vinyl window-sealing tape typically works well for this purpose. We recommend “Blenderm”, a 3-M product. This works extremely well in sealing the hinge when applied to the bottom of the hinge with the control surface fully extended up. Properly constructed and mounted, there isn’t much of a gap in the first place.
How to Seal the Aileron Hinge Line with the “S” Seal
- Cut a credit card-sized piece of 1/32 plywood to apply the seal. Make it just long enough to reach from hinge to hinge.
- Wrap a piece of the tape, sticky-side out, around the card and keep it taut with your fingers.
- With the aileron bent up against the stop, stuff the edge of the card as deep into the underside of the hinge line as you can.
- Stick the tape to the wing and aileron by rocking the card, and leave the free ends.
- With a sharp knife, cut the free ends off just inside of the corner of the beveled edges.
Why Do We Seal the Aileron Hinge Line?
We have to review a bit of theory to answer this question. We don’t need Bernoulli or any of that fancy stuff; airplanes fly because the wing pushes down on the air and the air pushes back up against the bottom of the wing. This is a bit of an over-simplification, but that’s okay for right now.
The high-pressure air on the bottom wants to leak upward through the aileron hinge gap. The effect of high-pressure air leaking out from under the wing, through the gap between the wing and aileron, is bad. This leakage causes a loss of lift and hampers good roll control.
The problem of aileron hinge-line leakage gets worse when the airspeed is low and the angle of attack is high, and it gets even worse when aileron is drooped. High angles of attack result from pulling “G”s or from flying slowly. As the angle of attack increases, the leak worsens.
The leak is further worsened when you apply aileron control. The depressed aileron forces the air downward so that the local air pressure is even greater. The leaking air squirts out as a “sheet” that eventually breaks up and joins the airflow past the wing. Until it breaks up, that sheet of air looks like an aileron pointed the wrong way. It’s not made from wood, but it is real.
Another problem that can occur with ailerons is adverse yaw, which is yaw in the opposite direction of the desired roll. Sealing the gaps gets rid of the leakage problem and reduces (but not eliminates) adverse yaw. It also makes the ailerons more powerful, so you can reduce the aileron throw and still get the same control effectiveness.
Aileron seals have no bad effects that I am aware of. They can actually have good effects, such as saving servo power, preventing flutter, and making the airplane behave better during takeoff and landing.