Hinging for IMAC and 3-D
By Jason Noll
Any size model will benefit from having a tight hinge line.
Correctly hinging and gap-sealing your International Miniature Aerobatic Club (IMAC) or 3-D airplane with Robart or hinge-point-style hinges can be a frustrating experience. I hope this article makes it easier for you and helps you through that stage of building an airplane or assembling an ARF.
This method will set you up for a great-looking hinge line and will allow you to do a terrific finishing job on the gap-sealing process. There are many ways to achieve these goals, but the one I will share with you works best for me.
I like to use Robart hinge points because they provide a strong, slop-free joint that I so often see broken on models that were set up with flat-style hinges. The inboard or outboard portion of the surface is where the torsional loads are highest, and I’ve learned that the hinge points hold up better. All that combined with ease of gluing the hinge-point-style hinges in place makes this method an excellent way to end up with an excellent-looking aircraft.
I recommend using the Robart 3/32" Hinge Point (item 305) on .10- to .40-size models, the 1/8" Hinge Point (item 307) on 1.20-size models, and the 3/16" Super Hinge Point (item 309) on 30–40% models.
If you are unfamiliar with the gap-sealing method and its benefits: a "gap" in your model’s control surface is the distance between the control surface trailing edge (TE) and the flying surface leading edge (LE). This gap allows air to travel through while you are flying, creating a high risk of flutter — a violent oscillation of your aircraft’s control surface that can cause it to depart from the airplane.
Sealing the hinge gap will improve control-surface effectiveness. By not allowing the air to pass through the gap, you are essentially making that air work against your control surface, allowing it to be more responsive and more efficient. The original article includes photos showing correct and incorrect gaps.
Making It Easy
You will need the following tools to do this job:
- Drill
- Motor tool
- Drill bit (11/64" is right for the 3/16"-diameter hinge points used in this demonstration; a slightly larger bit may be required depending on the wood’s hardness)
- "Pine tree"-shaped sanding bit
- Soldering iron with an old bullet-point tip
Procedure:
- Mark the centerline on the LE of the control surface (aileron, elevator, or rudder) and on the TE of the flying surface (wing, stabilizer, or fin) if it’s not pre-beveled. For this article I pre-beveled surfaces; that will act as my centerline. Mark the desired hole locations for the hinge points.
- As a rule of thumb, on 35% airplanes place a hinge every 4 inches on the ailerons and every 3 inches on the rudder and elevators. For 40% airplanes place a hinge every 5 inches on the ailerons and every 3 inches on the rudder and elevators.
- Carefully center-grind each marked location with the point of the "pine tree"-shaped sanding bit. This creates a small pilot hole to follow up with the 11/64" drill bit. The pilot hole prevents the drill from wandering or sliding off center; otherwise that hinge may not line up and may even cause binding when the control surface is moved.
- Drill the center marks to the desired depth. If possible, run the bit all the way in, stopping just shy of the chuck.
- Relocate your motor-tool bit and push it in slightly farther to allow for half of the hinge joint to sit just below the surface. This will minimize the possibility of a visible gap between the surfaces. Be careful to make the depth the same on all points. If you sink the bit too far, it could cause binding and reduce the effective gluing area. It is crucial that the pivot point on all the hinges align.
- After you have made all the hinge points, cover the model. If you are building an ARF you will start from here. Do not try to cover down into the holes with your iron or attempt to cut out with a knife.
- Once the model is covered or the ARF is out of the box and you are at the hinging stage, locate the soldering iron with the old bullet-point tip and get it warmed up.
- Use a pen to mark each of the previously drilled holes hidden below the covering. Carefully sink the soldering iron into the holes. Give it a twist or two and move it around slightly if your iron tip is smaller than the hole diameter. This will create a nicely cleaned and sealed hole.
- Once all the holes in the surfaces are prepped, glue the hinge points in place accordingly. Be sure that the hinge joints are pushed together tightly to eliminate any possible gap. Monitor them during the drying process. Sometimes they slip a bit; the epoxy heats up and thins as it cures, creating almost a lubricant.
Gap-sealing the hinge line:
- Measure the distance between each pair of hinges. Cut a strip of covering roughly 1 inch wide, depending on the size of the bevel. Cut individual pieces to match the measurements you made previously.
- Remove the backing from the strips of covering. Fold each piece in half and make a sharp crease. Place the folded piece in the crevice of the bevel and between the hinge points.
- Be sure to have the surface fully deflected when ironing the folded piece into place. Control-surface travel will be limited if the surface is not fully deflected, which will lead to the gap seal’s failure because of excessive force applied by the control-surface servo. Continue to fill the spaces between the hinge points and the ends of the hinge line.
You’re finished! Now you have a clean, airtight control surface that will positively affect your flying and last the life of the model.
Jason Noll [email protected]
Sources
- Horizon Hobby
4105 Fieldstone Rd. Champaign, IL 61822 (217) 352-1913 or (800) 338-4639 www.horizonhobby.com
- Robart Manufacturing
Box 1247 Saint Charles, IL 60174 (630) 584-7616 www.robart.com
Transcribed from original scans by AI. Minor OCR errors may remain.
