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Old Jul 20, 2011, 06:10 PM
jackerbes is offline
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USA, ME, Ellsworth
Joined May 2008
18,110 Posts
** Blu Baby 33" KFm3P Polyhedral Tip Wing Build **

Blu Baby 33 KFm3P Polyhedral Tip Wing Build - Part 1

I recently made a KFm2 wing with polyhedral tips for use on my KFoenix Stick and flew it in RET configuration (Rubber, Elevator, & Throttle, no ailerons) and was really impressed with it. It had excellent tracking and control and good rudder authority, which had been a problem with the stock Slow Stick wings. The wing seem to "lean into" the turns nicely as speed increased and as turns were made more aggressively. It was almost as if I were flying a wing with ailerons.

A video of one of the early test flight can be seen here:
KFm3P Wing Test on Blu Baby 33 Foamie (6 min 40 sec)

To the build...

The flat center section/polyhedral tip wings are a quick and easy build and have better inherent strength than the two panel wings with dihedral that have a center joint. And I've also found that the flat wings like this have much less need for a spar because the multiple and full width gluing lines create a monocoque structure with very good strength. As an example, the 48" span 11-1/2" chord wing I made for the KFoenix Sticks has two 1/4" dowels (36 grams for the two dowels, and only about $1.50 in cost) and probably has more inherent strength than it will ever need.

So I wanted to get a BB 33 back into a better advanced trainer and sports flyer configuration and decided to make a 33" KFm3 wing with polyhedral tips for it.


1 24" x 48" sheet of FFF (Dow Protection Board III insulation, from Lowes or Home Depot)

1 3/16" x 36" wooden dowel (any wood will work)

Gorilla White Quick glue (any PU glue will work)

Clear package sealing tape (Duck brand "Standard" weight is good)

Colored package sealing tape for final covering/decoration/improved visibility in flight (final covering and colored tape is optional, but it contributes to strength and is very eye pleasing).

Clear Duck tape (fabric reinforced tape, any color would work but I would not be caught dead with a RC plane with the standard gray or silver tape on it)

Bamboo skewers for trailing edge reinforcement at wing center

Optional parts:

1/8" wood dowel or 2mm fibreglas rod for leading edge reinforcement

The numbering on the procedures is keyed to the numbers on the images.

01 - Use the standard BB 33 KFm3 wing templates and cut the two wing panels in one piece with no center joint. This can be cut with the skin side up or down, the skin side will be down as the wing is assembled.

Also cut the other two rectangular pieces of foam seen in the photo, their sizes are:

3-3/4" x 33" (KFm3 50% strip)

2-1/2" x 33" (KFm3 75% strip)

3/4" x 33" (filler strip)

The sizes of those and their purposes are explained below.

I have modified the 50% and 75% KFm2 strips a little to turn them into straight line cuts. Instead of the KFm3 steps running from 50% and 75% at the root chord to 50% and 75% at the tip chord, they run as a straight line all the way across the wing from the 50% and 75% points on the tips.

This makes the two KFm strips a wide rectangle in shape instead of having a wide or nearly flat "V" shape at their bottom edges.

The KFm3 Sport wing is 7-1/2" wide at the tips so the 50% and 75% lines are located 3-3/4" and 5-5/8" back from the leading edge. The two rectangular pieces of foam you cut above are used as follows:

3-3/4" x 33" (leading edge and back, back edge forms the 50% step)

2-1/2" x 33" (the front 5/8" supports the 50% step, the back edge forms the 75% KFm3 step)

The 3/4" x 33" filler strip will be under the KFm3 50% strip up near the leading edge. This filler supports the 50% strip against crushing and contributes to forming the upper wing surface into a nicely curved shape down to the leading edge. It will be glued to the wing panel and the KFm3 50% strip to form a full span line of structural strength across the wing.

02 - All of the wing parts are seen in this photo, from the top down they are:

- KFm3 50% strip

- Filler strip

- 3/16" x 36" wooden dowel (Crafts section at Wal-Mart)

- KFm3 75% strip (also supports the 50% step)

- Wing panel for KFm3 Sport wing

The wing panel can be made without the templates from piece of foam that is 33" wide and 8" high. Simply mark two points 15-1/2" in from each end (that leaves a the 2" section in the wing center that is over the fuselage), then mark two points 7-1/2" back from the leading edge on the wing tips. Cut a straight lines from the marks on the wing tips to the marks near the wing center and that is the one piece wing panel.

03 - The wing panel is skin side down, the KFm3 50% strip is skin side up, decide that arrangement now and turn the KFm3 50% strip over skin side down. Cut 45 degree or so bevels on what will be the leading edges of the wing panel and the KFm3 50% strip when the two are taped together.

The bevels allow the edges of the foam to bend and curl downward a little as the pieces are taped and folded to form the leading edge. The leading edge will be slightly rounded "V" and not sharp. A sharp leading edge is not wanted or needed on a slow flying plane like the BB 33 and a sharp leading edge is also be easily damaged.

04 - To join the wing panel and KFm3 50% strip, lay a 36" or so long strip of the clear package sealing tap out with the sticky side up. I tape the right end down and the weight of the roll of tape keeps it straight and flat.

Lay the wing panel so it is on the bottom 3/4" of the tape and press it down and into good adhesion with the tape. That is what has been done already in this photo.

Note the faint lines drawn across the wing panel at this point. The two lower lines are drawn with a fine tip Sharpie across marks that were 3-3/4" and 5-5/8" back from the leading edge at the wing tips. They are the eventual locations of the 50% and 75% steps.

The line up near the leading edge is 3/4" back from the leading edge and is where the front edge of the filler strip will be when it is glued to the wing panel.

05 - The 50% KFm3 strip has been laid on the upper half of the exposed tape with a even gap of a little less than 1/4" all the way across. I know from previous experience that the gap needs to be that wide for foam of this thickness and I used the 3/16" dowel that will be the main spar to judge the gap and keep it even all the way across. The spar is not glued there, it is just to keep the gap right and even.

06 - A close up of the 3/16" dowel being used to judge the gap width and keep the gap even all the way across. I lay the dowel (that is a 1/8" dowel seen there) on the tape and just touching the wing panel leading edge bevel, then bring the KFm3 50% panel down and into contact with the tape so it is just barely or not quite touching the dowel. Then I remove the dowel and press the foam down into contact with the tape.

07 - Cut the tape even with the foam on both ends and turn the joined panels over. Smooth the tape into contact with the skin side of the foam. The adhesion of the tape to the skin is very good, you are unlikely to be able to separate them without damaging the foam, removing the skin, or both.

I put one or two more strips of tape over the fold area to strengthen it more. If I use one piece I just center it on the first strip. If you use two strips I arrange one so it is 1/2" above the upper edge of the first piece of tape and the second so it is 1/2" below the lower edge. That will give you either two to three layers of tape on the leading and, for a very small amount of weight added, you will have a very durable leading edge and a stronger joint at the fold.

08 - Then turn the wing panel back over and drop the 1/8" dowel into the gap between the wing panel and the KFm3 50% strip. The lift the 50% strip up to check the tension on the tape. If the gap was about right the foam will come up to just slightly beyond vertical when you start to feel some pulling tension on the tape. The weight of the utility knife leaning on the foam is holding the foam back as seen there

If it all feels about right, lay a board that is about 33" long against the foam and fold it all the way back until it is laying flat on the top of the wing panel. It will resist folding back and some force will be needed but this is the step that finalizes and forms the leading edge.

09 - I put weights on top of the board here so I could take this photo. Also, as you can see, the 75% strip was laying on top of the wing panel when I folded the 50% strip back so it is resting on that instead of the wing panel. But that is fine, it does not need to be any further down than that.

Allow the KFm3 50% strip to come back up and fold it to the front and out of the way for now. The next step will be to glue the filler strip to the top of the wing panel.

10 - The line I drew on the wing panel to mark the back edge of the filler strip was spritzed with a fine spray plant sprayer. Spritz the area where the filler will be in contact all the way across, you can't get things too wet for PU and the glue will not work worth a darn if you do not use water.

Lay the filler strip bottom side up, put a small bead of PU glue down the length of it, and then spread it evenly across the filler strip with a flat flexible plastic spreader. I use spreaders cut from the plastic snap-on lids of food containers. When spread evenly there will a slightly shiny appearance across the width of the strip.

Position the strip even with the line, press it into contact with your fingers, and then weight it down all the way across the strip. Make sure to get weight all the way across and on the ends or the foaming action of the PU glue will lift the strip up. Check it several times in the first few minutes to make sure the strip does not slide out of contact. If you don't do that the "crooked gluing with PU fairies" will sneak in and push the stuff around to make you look bad.

Give it about 10-15 minutes (with Gorilla Quick PU) and you can remove the weights and get on with the job. Any foam outs or ooze here are not a concern. They are handy for telling when the glue is set. When you can push the foam outs down flat with your finger and none of it sticks to you it is set up enough.

To be continued...

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Last edited by jackerbes; Feb 01, 2012 at 07:25 PM.
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