deatonbt

Hey All,

I am trying to decide what diameter dowel to use on a wing or maybe metal rod. It is an 84" span foam wing with a flat bottom airfoil on an ME-109. It is going to come in at about 18 lbs. with a Zenoah G-38 for power. I will be doing lazy acrobatics and no outside loops.

Brian

speedracerntrixie

My Feedback: (29)

For this type of wing attachment I have been using CF rods for years. On that size of an airplane I would use nylon bolts all the way around.

All Day Dan

My Feedback: (5)

Here's what I do. It's a 5/16 wing dowel and 1/4 inch nylon screws. Planes are 50 cc at 20 pounds. Dan.

deatonbt

For some reason, I had never thought about using plywood glued to the leading edge of the wing to support the dowel on my previous foam wings. I have been drilling the hole in the wing larger than the dowel and depending on Gorilla glue to expand to get more area for support in the foam. Plywood is probably stronger and a lot less messy.

BMatthews

For this larger class and weight of models the method shown by All Day Dan is far more suitable. Other than the smaller and lighter styles glueing into foam is a poor gamble even if you think that the glue is exanding out. It doesn't expand THAT far.

Frankly for a model of this size and weight I'd go with nylon bolts over dowels and rubber bands. The final result will be far neater and actually results in reduced loading in the areas where the rubber bands bend over the wing and around where the dowels would be. This is because the rubber bands need to be preloaded to a high stress level where bolts need only be snugged down to a light degree of preload. With models over 12 to 15 lbs weight it pays to begin thinking more in terms of full size engineering solutions and leave the traditional model solutions for those examples that are more in scale with the more typical term of what is a "model airplane".

Much of this relates to anticipating stress points and using matierals to spread that stress out over a wider area of the "softer" structure. For full size this implies locallized multi layer gusseting to produce "hard points" where the plates and angle stock of these gussets spreads out the load over a wide spread portion of the thinner skins and stringers. Dan's plywood plate spreading the load out into the leading edge strip wood and hence into the skins instead of relying on the foam is an excellent example of this idea.

Rodney

All Day Dan's way is very good. You can also use 10/32 nylon bolts rather than the 1/4/20's as they are more than adequate. In fact they are better if you wind up in a crash as they will shear easier than the 1/4/20's which is what you want in a crash. Unfortunately, the shear strength of the 1/4/20's is greater than their tensile strength a bad thing.

deatonbt

I should have mentioned that the dowel was for the leading edge. I've always used a front dowel with nylon bolts on foam wings. I don't like using rubberbands on any model because of the mess and the way that balsa can be crunched by them on the surface of the wing. I used a dowel on an 84" span American Eagle Spitfire with no retracts that came in at 13.5 lbs. With a fiberglass fuselage and foam wing, there just isn't much to the airframe that adds weight. I used Solartex and Rustoleum to finish the plane with no surface detail. The wing took quite a bit of abuse including a landing in tall grass that spun the plane sideways with no damage. I only used cloth at the center and didn't put in a spar. I couldn't remember the size dowel that I used on the plane. The plans that I am using now are for a built up wing with a tab drawn from the leading edge but give no other dimensions. I got a foam wing cut because they are easier and quicker to sheet and to get a straight wing.

Gorilla glue works really well with foam. It is the consistency of syrup and easily fills voids. It expands 3 to 4 times in volume as a foam in an area that is not clamped while drying and has a hardness that is difficult to sand. What I have done before is drill a hole that is larger than the dowel and then sand the hole larger at the leading edge down to the foam with a bevelled angle. Put tape on areas that don't need glue on them, and partially fill the hole with glue. Then slide in the dowel and wipe away any glue that may get close to the top. The glue then foams over the top while drying and must be sanded away to the shape of the leading edge. The dowel is then in a substance that has a larger contact area with the foam to better resist crunching it as compared to a slip fit. There have been times where I didn't get a flat surface while sanding in dihedral. The glue worked very well in filling the void.

Here is a picture of the plane. It is kind of a pieces parts kit with the fiberglass fuselage purchased from one source, the wing another, and the plans and canopy from another. The wing isn't mounted yet. The tail surfaces are balsa. The vertical stab is sheet balsa, the horizontal stab, evelator, and rudder are built up.