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Installation of hard points balsa sheeted foam surfaces... More than one way to skin a cat.

Discussion in 'Scale Civillian and WarBirds' started by orthobird, Feb 25, 2016.

  1. orthobird

    orthobird 150cc

    Would like to illustrate a solution to a problem encountered, in some, model airplanes with balsa sheeted surfaces (wings and stabs) and the use of dowels glued into the foam/sheeting and LE of the surface.

    One of the problems encountered, and that I have seen, is the overtime, softening or loosening, of the foam around the hard wood dowel.

    To give some background, allow me to explain:

    Installation of a control horn on a foam elevator or aileron, that will be used in a 35% or 40% gas airplane, requires that the control horn, somehow, be fixed to the surface. Foam alone will not be able to withstand the forces placed, and therefore, some type of hard point is employed. This will be glued into the foam.

    Some kits, in the market, will suggest the use of a 5/8" round dowel, that can be glued onto the back surface of the leading edge, and by 1st drilling a hole, with a Forstner bit, into the foam, right on the edge with the balsa LE.

    This is an example of this technique:

    a round hole is 1st drilled or cut out, that will have contact with the balsa Leading edge.

    this is to demonstrate the nice hole in the surface LE area.


    After this, then the dowel is pushed in, with glue, and it should touch the top balsa sheeting, but not go thru it.

    once cured, it is sanded flush, and then drilled.


    An 8-32 screw is then passed thru, and the control horn is threaded on.
    0594.jpg 0596.jpg
  2. orthobird

    orthobird 150cc

    Sorry, last night, I could not finish, as I WAS on call. Ok, back on topic.
    I once had an older airplane, that had this set up, and I noticed that the bolt, had a significant amount of "sway" forward, backwards, and even side to side, in the foam. Meaning, it was not the bolt loose in the wood dowel, but the wood dowel in the foam.

    Likewise, I was at an IMAC contest last year, when one of the pilots, with same type of set up, noticed this on his wings. This pilot flew Intermediate.

    I recall him and his father spending the night repairing it, the next day, he won the contest.

    In any event, seems to me, that over time, and repetitive use, and more particularly with more violent or extreme use, over time, this is an area of weakness.

    I have, over the years, attempted to counter this, by doing something different.

    Here is my 1st version of what I have done in the past, and this is for a different type of control horn. If you do not want a bolt, and prefer the G10 material double horns, then this is a different way of making a hard point.

    Here it is, I hope no one gets hungry while I am explaining this:

    1. Begin by making a sandwich, of thin plywood - balsa sticks - thin plywood.
    The three balsa sticks are gauged to be gapped the same width as the control horn spacing.


    2. I use two control horns to maintain uniform spacing, then adhere with CA glue.


    3. I made a long one, and this can last for making 2 airplanes.

    4. Appears they are parallel. Hold the mayo....

    5. this is the way it works. It will be in this position when it is adhered to the leading edge of the control surface.

    6. time to adhere the top part.

    7. Once it is cured, Testing of the fit. the Sandwich is ready.

    8. Cut out the balsa and foam only on the undersurface, do not get into the top surface with cutting. the key is that the front of the "sandwich" has intimate contact with the back side of the leading edge. Glue it in place with your favorite wood glue (polyurethane, epoxy, titebond).

    I will say this, if you use epoxy, and it gets into the slots, it will be a bugger to remove.

    9. Sanded it flush with the bottom balsa sheeting. As you can see, mine got some poly-u glue in the slots. No problem. Get a dremel and I "cored" out the slot again, Very easy.


    10. Test fitting of the horn, and you are done.


    11. Another view.

    final result once beveled and sanded smooth.

    You can also paint it any color you want. One point, here, it was not glued in, but when it did get glued in, you can "shim" the slot, so that the control horn holes are directly in line with hinge gap, or, you can core the slot out more "back" so that the holes of the control horn are in line with the hinge gap.
  3. orthobird

    orthobird 150cc

    Here is a third way. If you are a traditionalist, and want to use the "bolt" system, then I would employ this. To further explain, it is my opinion, and I am not an engineer, and have no training in aeronautics or anything close to that. However, when I was in college, I did take one year of physics with labs (2 semesters of each). But that does not qualify me. However, it is my un-educated opinion, that if you use a wood dowel, in foam/balsa, and the dowel is round, 360 degrees. There is maybe a 5 degree arc of the dowel, that will have point contact with the back side of the balsa leading edge. It was my theory, that if this could be made into a larger surface area, then a higher degree of bonding, and possibly, a more durable and predictable construct.

    Here we go:

    1. Here are what you need for 4 hard points: 4 hard wood rectangles, 8 balsa rectangles, the balsa is 1/2" thick. The hard wood is 1/4" thick. There is a 5/8" hard wood dowel.

    2. going to make a sandwich again. Using titebond for this step.

    3. top of sandwich is on. once this cures. all 4 sides are sanded smooth and square.

    4. marked out the location for the hard point on the aileron under-surface. the hard point sandwiches are curing.


    5. once the hard points were cured, I used my drill press, and a 5/8" Forstner bit to core out the hole on the hard point. As you can see, I am coring it out, again, so that the hard point dowel, will be as near to the leading edge as possible. Just like the original type of set up.

    6. I then mark the location for the hard point, and delineate it with a fine point sharpie.

    7. I then cut this out,

    8. Then use a foam cutter, to the desired depth.

    9. here it is after melting the foam down.

    10. I then measured the distance from the top surface to the bottom surface of the sheeting, and cut the wood dowel, with an extra 1 cm. I then placed the dowel in the cut out area, and marked where it would be flush with the bottom surface. You want the wood dowel to have contact with the top surface sheeting, but not thru it. I did core out the foam inside the hole down to the top surface balsa sheeting.

    Following this, the whole construct was glued in and allowed to dry.
    IMG_0540.JPG IMG_0541.JPG

    11. Once it cured, the area was sanded flush with the surface of the balsa sheeting. The hole for the bolt can now be drilled into the wood dowel.

    12. I intentionally oriented the grain of the balsa to be perpendicular to the leading edge.
  4. orthobird

    orthobird 150cc

    Here is a 4th way of doing it. I love these, but they are expensive (but makes it the easiest way to go, so probably evens out!!!):




    Snoopy1, Luchnia, 49dimes and 8 others like this.
  5. BalsaDust

    BalsaDust Moderator

    Awesome right up Cam.

  6. +1
    Excellent thread.:way_to_go:
    49dimes and orthobird like this.
  7. :shake: Nice tips!
    49dimes, pawnshopmike and orthobird like this.
  8. Dirt

    Dirt 50cc

    Very nice Cam!!
    orthobird likes this.
  9. oh58skid

    oh58skid 50cc

    I love the JTEC control horns! I have them installed on three 40% airplanes, easy to install and trouble free.
    orthobird likes this.
  10. Snoopy1

    Snoopy1 640cc Uber Pimp

    Again thanks for taking the time to help and educat us. Great it helps in building a better plane.
    Robotech and orthobird like this.

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