Newcomers
Bob Underwood Box 40, St. Peters MO 63376
Most models, except Free Flight (FF), have movable surfaces to control the flight path. In beginning‑type Control Line (CL) models, the elevator is the primary moving surface; Radio Control (RC) models generally have three or four surfaces (elevator, ailerons, rudder, throttle). Newcomers often treat how those surfaces are actuated as an afterthought — they shouldn't. Since, with aircraft, there is at least one landing for every takeoff, make sure the landing is on the wheels—not vertical, inverted, or otherwise strange.
Hinges
Things that swing (elevator, ailerons, rudder, throttle arm) must be hinged so the control input moves freely. There are many hinge types:
- Simple rectangular fabric/nylon hinge material (thin, forgiving of sloppy installation).
- Metal‑pinned flat nylon hinges.
- Peg‑type hinges with metal pins.
- Full‑length strip hinges that run the entire span of the movable surface.
For the average trainer, simple hinge material is usually sufficient. Newer flexible hinge strips can be slid into a narrow slot cut into the mating surfaces and seated with a thin cyanoacrylate (CyA) adhesive that will wick into the joint. This tends to create a slightly stiffer hinge action, which can be beneficial — a really loose hinge can contribute to control‑surface flutter and servo loading.
Tips for hinge installation:
- Ensure hinge pins cannot slide out and that both ends of the hinge are securely attached.
- Keep hinges aligned across the span; misalignment can cause flopping, binding, extra strain on pushrods, and erratic control surface movement (this is especially noticeable on long strip ailerons).
- Minimize the gap between the movable surface and the fixed portion to improve control without requiring excessive surface movement. To reduce the gap: either round both mating surfaces and place the hinge at the center of the arc, or leave the fixed surface flat and sand a V‑shaped edge on the movable surface, centering the hinge at the V apex. Use a sanding block and care to find the center for correct hinge alignment.
- After hinging, move the surface through its range and tug firmly to ensure hinges are seated. For wide flat hinge material you can pin the hinge by drilling through the surface and hinge and gluing short dowel or toothpick pieces, then snip and sand smooth.
Pushrods and Bellcranks (CL)
A hinged surface must be pushed and pulled — this is done with a pushrod. On CL models:
- The bellcrank is a T‑ or triangular‑shaped piece (nylon or aluminum) anchored on a pivot near the center of gravity (CG). The bellcrank translates the motion of the two lines into linear movement of a pushrod.
- The pushrod runs from the bellcrank to the control horn mounted on the elevator (or other movable surface). On trainers, pushrods are typically music wire and should be supported along their length to prevent bending. Profile fuselages usually make support easy.
- Attachments vary: simple Z‑bends in the wire, clevises, snap sockets, or small fittings with set screws. I prefer having one adjustable end (usually the control horn end) because getting two Z‑bends exactly right is difficult. If the bellcrank arm has several holes and the control horn does too, using the control‑horn hole closest to the pivot and the bellcrank hole farthest from the pivot produces less elevator throw (avoid excessive throw — over‑control is problematic for trainers). Trainer plans usually suggest recommended throws; err on the low side if unsure.
Obtain a copy of the Competition Regulations when setting up and fitting lines. Study the attachment methods and recommended line diameters and pull‑tests; when in doubt on wire diameter, err slightly larger.
RC Considerations
Many of the same principles apply to RC models, but the bellcrank is replaced by servos:
- Pushrods in RC are often wood (hard balsa or spruce 1/4" square) with short wire ends, tube‑in‑tube linkages, or carbon‑fiber arrow‑shaft rods with plug‑in fittings.
- Methods for securing wire ends in wood include bending the wire 90° and embedding it about an inch in the wood, wrapping with thread or shrink tubing and epoxying.
- Tube‑in‑tube systems need the outer tube anchored near each end (roughen and glue) and sufficient inner tube extension beyond the outer tube for servo/control movement.
- Prevent pushrods from flexing under load by supporting or anchoring them at intermediate points.
- End fittings include nylon or spring‑steel clevises, round fittings with set screws, Z‑bends, and snap‑on nylon sockets over steel balls.
- Throttle linkages often must route around fuel tanks; small nylon tubing with a flexible cable or dedicated throttle cables work well. The same flexible cable types can link a rudder servo to nose‑gear steering (you may need to create bends to match steering arm geometry).
Most trainer kits provide guidance on hinges, pushrods, and fittings and may suggest proprietary parts; follow the kit guidance where appropriate but apply the basic principles above.
Terms (Illumination)
- Bellcrank: A T‑ or triangular‑shaped piece (nylon or aluminum) in a CL model that translates the movement of the two lines into linear movement of a pushrod.
- Control Horn: A nylon or metal arm attached to the movable surface; the pushrod connects to it to provide push/pull motion.
- Clevis: A U‑shaped threaded attachment that screws onto a pushrod end and secures with a pin through the servo arm, control horn, or bellcrank.
- Profile Fuselage: A single slab of balsa used for CL trainer fuselages; it looks correct from the side but is thin in depth, allowing quick construction and easy access to control mechanisms.
- Strip Ailerons: Narrow strips of balsa running nearly the full span of each wing half; special mounting hardware and short pushrod runs are common for these.
Transcribed from original scans by AI. Minor OCR errors may remain.
