RC Airplane/Control surfaces and linkages
Control surfaces are movable surfaces that control the airplane. These are connected to servos with linkages. Getting these made carefully and accurately is essential to the success of your model.
Types of control surface
- Aileron - on the wings controlling roll.
- Elevator - on the horizontal tail controlling pitch (up and down).
- Rudder - on the vertical tail controlling yaw (left and right).
- Flaps - on the wing, but change the wing camber to increase lift (and drag) for landing.
- Canard - a horizontal surface at the front of the plane that controls pitch. Much less common than elevators, but does the same thing.
Some of them are assigned multiple names depending on how they are used.
Control surfaces for RC planes can be made from many different materials such as balsa wood, plywood, foam, corrugated plastic and sheet plastic. The structures should be strong and lightweight and have at least a portion of the surface that is solid so that a control horn can be attached.
Hinging and movement
Control surface hinges can be made out of various materials. Larger planes use traditional style hinges made from combinations of nylon, metal, fiberglass and plastic. Very small or lightweight planes will often use nothing more than clear plastic tape (packing tape) that is stuck to both the control surface and the frame. This requires that the opposite side of the joining control surface edge be angled to allow movement in both directions. Midsized planes use smaller hinges similar to the ones use on larger planes, or will utilize CA hinges (flexible sheet plastic covered on both sides with meshy cyanoacrylate material to allow for glue wicking) that are simply inserted and glued into small slits in the joining edges.
Because of the available space in larger planes in the wings and fuselage, servos are usually placed as close to the control surface as possible, which results in much better accuracy. It does not require much or any routing of control rods or lines through the fuselage.
Usually, lack of internal space in smaller planes often requires servos to be mounted in the main cavity or on the outside of the fuselage, requiring longer control rods and the necessity to route them through the covering of the plane, in some fashion. It is sometimes necessary to put servos as near the centre of gravity (C of G) as possible to balance the model correctly.
Control linkages consist of levers, joints and other moving parts that allow a servo to move a control surface. There are hundreds of available options ranging from various materials, to different sizes and weights, depending on what is needed for operation and flight requirements. For a hobbyist building from scratch or from a kit, proper selection and assembly of these mechanical components can be a great challenge rewarding.
If you have bought an ARTF (almost ready to fly) model, it will be likely that control linkages are included with the kit. Be careful though, the linkages included with many ARTFs are terrible plastic moulded jobs, which have a tendency to pop open or break in flight! If need be, it might be worth browsing the shelves of your local model shop to find some better quality linkages.
Control arms and horns
Control arms are a type of lever attachment on a servo crankshaft. The attachment may have one or more arms to allow for multiple positions and control rods. Some attachments that have multiple positions are circular. It's the control arm that transfers leverage from the servo to the control rod.
A control horn is on the receiving end of the servo leverage, either being pushed or pulled. The control horn is simply a lever that is mounted onto a control surface, perpendicular to the edge of the control surface, that connects to the end of the control rod allowing the surface to pivot on a hinge point.
Control rods or wires can be made from various material such as plastic, metal, carbon fiber and fiberglass. What is used is often determined by the size and desired weight of the plane, what is being controlled (aileron, retracts), and the appropriate linkage types. Control rods are often attached to the control horns and other levers by clevises, ball joints, or other specialized adapters. Small craft and park flyers often use a simple z-bend (bend in the wire the shape of a "Z", all angles being 90 degrees) to attach to a control horn.
Larger RC airplanes allow for a wide variety of options, materials and techniques when assembling control linkages. The larger and thicker control surfaces, potential flight weight and fuselage cavity size make it possible to not only choose from very strong, easy to work with materials, but they can also be obscured from sight with ease.
It is common on these larger planes, since more torque is required and faster speeds are reached to have control rods made from aluminum, carbon fiber or fiberglass. Linkages are often made by soldering, clevises, ball joints, or some combination thereof. Control horns are usually bolted straight through the control surface provided strength and durability. Larger planes will often have retractable landing gear connected to one or more servos using the same kinds of linkages and components.
Small planes and park flyers often have more restrictions. Materials must be lightweight (plastic, thin wire or cable, aluminum, fiberglass) and as small as possible. Small tube wire guides are often used to encase the flexible wire or cable to its control surface destination. There are many available fuselage opening components to allow the rods to neatly penetrate the covering and some can be made from household materials.