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Building Techniques/ Hybrid Foam Wing

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If you have the need to calculate the density of your balsa or foam here is a great applet that was designed for modelers. You will be taken to a different web so use your back button to return to this page.



Welcome to the page that will cover the varied building projects that will take place this winter in the Winship Models shop. The projects will hopefully cover a few 1/2 A airplanes along with a Hostetler Edge 540 and any thing worth mentioning in between. For More Tips Visit The Building Techniques Page.


Joe Hartley's Balsa USA EAA Bipe under construction. Balsa USA airplanes employ conventional construction which Joe seems to favor. Notice the aliphatic resin glue on the bench. Joe is going to power the EAA Bipe with a Saito 100 and apply scale effects such as pinking tape and a dope finish.


The Miter Cutter

If you like to scratch build you will need one of these miter cutters and sanders. This tool is available from Just R/C Planes in Quincy KY. The tool is adjustable for any angle and has the degrees laser etched into the tool surface. At the top of the tool there is a 90 degree angle already set up so you don't have to change the angle to get a 90 deg cut. At the right of the board is the sanding block that rides in a groove for sanding the sticks for a perfect match. The cutter comes with a saw also. Just R/C Planes email



The Davis Akro Pro hybrid wing. This wing was designed to incorporate the best of both types of construction  (foam and built up). There are advantages of both methods of construction that I liked so I decided to construct a wing combining all of the good ones. 

Built up wing advantages are lightness and the scale look of ribs (depends on airplane).  Foam wing advantages are mainly accuracy and speed of construction.

Once the core airfoil was cut I cut the tube hole and epoxied the tube in place. After the tube was in place I cut a slot in the foam down to the tube on the top and bottom to place the 1/4 inch end grain balsa. The balsa was placed end grain to keep it from crushing and was capped with a molded in carbon spar that sat flush with the foam core.  A trough was cut to accept the carbon tow and epoxy that extended from the root to the tip. The carbon tow was gradually decreased in cross section towards the tip, this was done to avoid a stress riser and to save weight. End grain balsa was also added to the core where the aileron control arm would locate. (The carbon spar and end grain tube caps cannot be seen since they are buried in the foam but there is a rough drawing of the wing tube, end grain and carbon spar below. This wing was built before this web site was even thought of so this article is in retrospect).

The grooves for the ribs were cut with a modified soldering iron tip with a depth gauge attached. The wood strip ribs were added to the grooves and glued to the spar after which they were sanded flush with the top surface of the foam. This is where the real savings in time takes place since the ribs do not have to be plotted or cut out. By placing the strip ribs in the foam grooves you could literally place the ribs at any angle, warren truss or geodetic construction could result effortlessly. If you were building a wing where you didn't want any foam left you could run the strip ribs between the leading edge and trailing edges with carbon fiber reinforcements as a spar. Once you start thinking outside of the box many options become available.  The wood leading edge was glued to the leading edge of the foam wing and shaped to match the contour of the airfoil.

The next step was to vacuum the sheeting that you see on the wing down to the foam using epoxy as the adhesive. The sheeting overlapped the strip ribs about 1/4 inch and completely overlapped the leading edge. By placing the sheeting over the leading edge you gain some strength and hide a seam. At this point when the wing is bagged there is still foam between the false ribs. Once the wing was cured and taken out of the bag  the foam was removed from between the strip ribs and the vertical grain supports are added between the top and bottom strip ribs. The rib caps are added at this time to match the sheeting.

The back portion of the wing is cut away so that a full depth and full span trailing edge (which goes in front of the aileron) could be added to the wing. At this time the aileron was capped with a leading edge and trailing edge along with the end caps.  I wanted the trailing edge of the wing that was in front of the aileron to extend the full span to get away from a stress riser at the root of the aileron cutout on the wing. The trailing edge of the wing at the root was also glued back on the wing at this step.

For extra lightness the leading edge could be cored out after bagging.  All coring has to be done after the bagging process since the cored wing will not hold up under any vacuum, tried that to see what would happen, won't mention the results here. 



Tube Spar Cutaway View


Close up of the wing strip ribs and vertical stiffeners. The cap strips were added after vacuum bagging.


The first projects on the bench this winter as a warm up are two 1/2 A airplanes, the Kaos and a Stick.   Both plans were purchased from RCM plans service and are very simple to construct. With all of the cheap pre-fabricated ARF airplanes out there it is easy to forget about the scratch building possibilities out there which are as endless as your imagination or maybe the plans that are available.  So stretch the envelope and take a hot cup of coffee, or if you are my brother in law a 24 oz. Mountain Dew and head for the shop.

Pictured above is the 1/2 A stick fuselage resting on the vacuum bagged wings.  The wings at the top of the picture are 1/2 A Kaos wings ready to bagged.  If you would like more information concerning vacuum bagging foam wings you can click here to go to our sister web site that covers this topic. Please use your Back button to return to this page since you will be on another web site.  This project is just getting started as of 11-29-01 so there will be more information on a weekly basis.


So my brother in law doesn't feel left out I have included his 1/2 A Kaos wings in the vacuum bag.



When vacuum bagging foam cores you will need a vacuum that can be regulated such as the ones that Aerospace Composites sells.  They have a great pump with a vacuum reservoir that can be regulated.  If the vacuum exceeds around 8 inches of mercury on one pound per cubic foot foam the core will end up crushed or distorted.

Epoxy such as Z Poxy or Smooth and Easy are great for bagging cores. Any good laminating epoxy will work equally well.  How much does the epoxy weigh? I am a firm believer that if done right you will have a better bond with epoxy and vacuum bagging than any other system or method.  The sheeting for the Kaos was weighed before and after the epoxy was added to see how much weight was added in epoxy.  The numbers here can be translated to any wing since we are dealing with square inches.  Total sheeting area for the Kaos wing was 342 square inches.  7.2 grams of resin was used to coat the wood.  This works out to approx. .021 grams of epoxy per square inch.  If the epoxy is applied correctly the .021grams will be about right.  When applying the epoxy to the sheeting drizzle a bead of epoxy across the top of the sheeting and squeegee the epoxy so that there is no gloss and the wood looks dry.  There will be enough epoxy forced into the grain to adhere the sheeting to the foam.  We have done numerous tests on different epoxy films and the amount it takes to adhere the wing skins under vacuum is very little due to the vacuum pulling the epoxy out of the grain to make contact with the foam.  If you have a vacuum system try to experiment with different epoxy viscosities and see just how little epoxy it takes, you will be surprised.  All this is taking into consideration that you have a smooth surfaced well cut clean foam wing. Remember that 7Hg to 8Hg is all that is needed to vacuum the skins down to 1# white expanded bead polystyrene  foam.


Before putting the cores into the vacuum bag we use masking tape to secure the sheeting in place.  The tape is applied loosely so the core can be moved around if needed before placing it in the bag.  The grain runs parallel to the leading edge and the "C" grain is placed at the trailing edge of the wing.  Since the "C" grain doesn't like to bend it is better suited to flat surfaces.  C grain can be identified by the speckled appearance and stiffness.   When gluing the sheeting together We use masking tape that runs the full length of the seam and flop one of the sheets over the edge of the bench thereby using the masking tape as a hinge.  CA glue is used sparingly to the seam so that when the sheets are brought together there is little if any seep out of the glue.  Sanding the sheeting is done with a large 1/4" aluminum pad about 8X10 inches with 100 grit.  100 grit might seem like overkill but it works well since there is so much surface area.  If there is any CA glue the pad will take it off easily.  CA allows you to make up the sheeting without having to let dry overnight.


An Ohaus gram scale is invaluable for building light and weighing epoxies etc.  You can make good choices when you know what the materials you are building with weigh.  The scale pictured here weighs in the tenth of a gram.  Electronic scales are good but not as accurate since they tend to round off to the nearest gram.  This scale never needs batteries either.



The Half A Stick on workbenck for alignmentThe 1/2 A Stick is coming along. Notice the cross hairs on the glass bench top, they were drawn on masking tape with the aid of a carpenters square and are used for alignment purposes. The plane is aligned according to the lines drawn on the masking tape and not the tape itself. The leading edge of the horizontal stab was aligned with the stab line and the fuselage was aligned with the fuselage line running perpendicular to the stab line. When the wing is ready to mount we will add another line to match up with the trailing edge of the wing. When transferring a line from the table to the trailing edge above the bench top a square will be used. The wing incidence can also be set off of the glass bench top by measuring the centerline at the tip from the glass.  The beauty of this plane is the way it can be assembled with the fuselage sitting flat on the bench.