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Analyzing the Foam Wing
 
Author: Clay Ramskill

ANALYZING THE FOAM WING

by Clay Ramskill

Use of the foam wing in model aircraft construction has been around now for some years. This type of wing has some really great advantages, but has NOT "taken over" model plane construction. Some kits and designs use a foam wing, some do not. Interestingly, most ARF (almost ready to fly) kits have even stayed with labor intensive "built up" wings, with conventional rib and spar design. Why?

Oddly enough, the list of advantages is nearly equaled by a list of disadvantages. Weighing these points against each other, we find that personal preferences - on the part of a manufacturer, a designer, or builder - may be the deciding factors, every bit as much as technical considerations.

The principle behind the foam wing is pretty simple. We just use the SKIN of the wing for all the load bearing strength. The skin can be very thin plywood, balsa, obeche or other exotic woods, or wood plus fiberglass. The foam is there to lend the proper shape to the skin, and to stabilize it from deformation under load - such as buckling under compression. The actual strength of the foam contributes very little to the bending strength of the finished wing.

Just using wingskin to carry all the loads may seem inadequate until you realize how much of it there is. Consider a wing sheeted with 1/16" balsa, a very common composition. Going from the leading edge back to the trailing edge, every inch of the 1/16" balsa is the same amount of balsa as a 1/4" square spar! If our wing has a chord of 10 inches, that would be the same amount of balsa as TEN 1/4" spars! Or five 1/4 by 1/2" spars. Or whatever; -this is plenty of beef. And note that all the beef is where it does the most good - right out at the outer periphery of the wing.

Let's examine some of the foam wing advantages:

Shorter building time. This may be arguable, but most builders feel that building a foam wing is quicker, less tedious, than conventional construction.
Manufacturing ease. Once the proper cutting jigs and equipment is on hand, foam cores can be cut in great numbers with minimal effort. The foam wing certainly requires less work in drawing plans; the whole concept is far simpler.
Strength/Stiffness. This is an area where the foam wing really shines. Foam wings tend to be very stiff - they resist bending and twisting far better than the conventional design. For high performance aircraft, this is essential; most pattern planes, larger aerobatic planes, and ducted fan/turbine planes use the foam wing exclusively.
Accurate, Smooth Shapes. A conventionally built up wing that is not fully skinned will have slight irregularities where the open spaces meet solid structure. And the covering will have slight concavities in the open areas. The fully skinned foam wing will be smooth, and accurately follow the airfoil shape throughout the entire structure.
Tapered Wings are easily Reproduced. All that is required for the foam wing is the shape of the root and the tip airfoils to accurately scale everything in between.
Washout or Twist is Easily Incorporated. Positioning of the root and tip templates during foam core cutting will determine any twist or washout in the final completed wing. Aerodynamic washout (ie. progressing to a more stall resistant airfoil shape out toward the wingtips) is just as easily accomplished in the cutting process.
Foam Wing DISADVANTAGES:

Weight. The completed foam wing will generally be slightly heavier than a comparable wing of conventional construction. This will vary with the choice of wingskin weights, the type and amount of bonding glue used, and other construction considerations.
The technique of building a foam wing is very different than conventional wing construction; this is only a problem if the builder is intimidated by learning new building methods.
Covering. Applying covering requires more care - the foam core or the bonding agent may be damaged by the use of too much heat in the covering process. If the model is to be painted, however, this is considerably eased by the full skin.
Hard Points. Attaching or mounting landing gear or servos to a foam wing is trickier - there just isn't much in the way of solid structure to bolt on a landing gear, for instance. Properly designed mounts must be built into the wing to carry the appropriate loads.
Repairs? A damaged foam wing may be difficult to repair - there are techniques to do this but its more difficult and time consuming than just splinting a couple of ribs or spars on a conventional wing structure.
SO- are foam wings for you? Like so many other things in the world of model aviation, it all depends. It depends on just how strong your model must be. The importance of a totally accurate shape. How heavy is permissable. Your building preferences. Or, maybe that's just what came with the kit!


 


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