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Nov 09, 2010, 11:06 AM
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Building a 32" KFm3 Flat Wing w/Ailerons

32" KFm3 Flat Wing Build - Part 1

This describes how to build a flat (no dihedral) KFm3 wing for the Blu Baby 33 or a similar sized plane. No dimensions are provided, this wing can be build in almost any size and other materials and the construction would be about the same.

The material used is the material commonly called FFF (fan folded foam). It is sold as Dow Protection Board III extruded polystyrene foam insulation in bundles of twenty-five 24" x 48" fan folded sheets that form a 48" by 50 foot long strip.

The wing is based on the plans in the "Blu Baby 32 inch KFm3 sport wing FULL.pdf" document in the Blu Baby thread and the plan can be downloaded here:

This wing is 32" wide and the chord is 8" at the wing center and 7" at the tips. The wing area is 240 square inches.

In the original plan the wing is built in two panels and with dihedral, some minor changes were made from the plan to build it as a flat, wing. The changes from that plan are:

1 - This wing is made with one piece panels for the wing panel and the two KFm strips (there is no center joint in the wing).

2 - A filler strip is added to the wing beneath the 50% KFm strip. It is located between the leading edge and the 50% step. It adds a glue line across the width of the wing for additional strength and resistance to twisting and bending forces, and provides some resistance to crushing in the forward half of the wing.

3 - The KFm 50% and 75% steps are located at those percentages of the tip chord and run straight across the wing instead of going to 50% and 75% at the root chord. That makes the KFm steps slightly further forward at the root chord. The are located at 44% and 66% of the root chord.

4 - A 1/8" wooden dowel is embedded in the fold at the leading edge of this wing as a leading edge spar and for protecting against damage. No other spars are used.

5 - The depth and length of the ailerons was reduced from 1-1/2" x 14" to 1-1/4" x 11". I think that will provide plenty of aileron authority for this flat wing.

This wing is 3/4" thick at the 50% step (the thickest point in the wing) and that is 9% of the chord. The steps are about 1/4" high (the thickness of one layer of FFF).


The numbered steps in the build description are keyed to the numbers on the images should you download them. Otherwise they can be used for a reference in questions or comments.

01 - Cut All FFF Strips - One 24" x 48" sheet of FFF will build this. Only other material needed is a 1/8" wooden dowel for the leading edge spar and some clear or colored package sealing tape. That dowel can be omitted for a small weight savings if preferred. I used Gorilla Quick polyureathane (PU) glue.

Make a light clean up cut on one of the 48" sides of the FFF to get a clean square edge where the sheet was separated from the bundle. I use a sheet rock square to make the clean up cut at a right angle to the 24" side.

Four pieces of FFF are cut as follows:

1 - 48" x 8 for Wing Panel

1 - 48" x 3-3/4" for KFm 50% Strip (this is 1/4" over sized, it will be trimmed later)

1 - 48" x 2-3/4" for KFm 75% Strip (1" is provided for overlap and support of 50% strip)

1 - 48" x 1" for Filler Strip

02 - Parts Arrangement - The final arrangement of the four strips will be as seen here. Preserve the plastic coating on the FFF as much as possible and keep it on the outside surfaces of the wing. As seen here the plastic skin is on top of the two KFm strips and on the bottom of the wing. The area from the 75% step back to the trailing edge is bare foam.

The plastic skin will be removed from the top of the Filler Strip and where the KFm 50% strip overlaps the 75% strip. This is to get better adhesion when the two layers of foam are glued together.

The lines seen drawn on the wing panel are the 50% and 75% KFm step lines. In this case the lines are located at 50% and 75% of the tip chord (3-1/2" and 5-1/4" back from the leading edge).

03 - Cut Trailing Edge Taper - Make a mark at the wing center (16" from both tips) and another mark 1" up end of the tip from the trailing edge and cut the tapers on the back edge of the wing panel. Cuts are made with the small snap-off tip utility knife seen in the photo (yeah, broken handle and all). When cutting foam keep the knife blade inclined back at an angle as much as possible and it will never snag the foam and cause rough cuts. If the blade gets too close to vertical, it will snag on the foam and leave a rough edge.

04 - Cut Bevels - Use a straight edge to cut bevels on the wing and KFm strip. Incline the knive back about 30-40 degrees or so, incline it away from the cut, and follow the straight edge as seen here. The piece of scrap the knife is stuck won't be there of course, that is just for the photo.

05 - Cut Bevel on Wing - With the wing laying bottom down (skin side down) cut a bevel on the leading edge of the wing.

06 - Cut Bevel on KFm Strip - With the 50% KFm strip laying top side down (skin side down) cut a bevel on the leading edge of the KFm strip. The two bevels will form a "V" with the open end facing forward when the KFm strip is laid on top of the wing panel. The 1/8" dowel will rest in the "V" and be centered on the leading edge when the wing is assembled.

07 - Mark Step Locations - Mark the 50% and 75% step locations on the top of the wing panel. Make marks 3-1/2" and 5-1/4" back from the leading edge and draw lines connecting them. A fine tip Sharpie is good for this.

08 - Step Locations - The KFm step location lines are seen at the center of the wing, they are still the same distance back from the leading edge but are no longer at the same percentage of the chord because the wing is wider at the center. This will affect the flying ability of the wing very little, the straight step line was used to simplify the building of the wing.

09 - Prepare Tape for Leading Edge Join - I like Standard grade Duck brand clear package sealing tape for this (Wal-Mart usually has it). That is a 2 mil (not Heavy Duty) grade of tape. Lay a 36" or so strip of it on the bench with the sticky side up and tape the end down on the right. On the left, stick a metal washer to the end of the tape where it is hanging over the end of the bench to hold it in place flat.

Take another 36" piece of tape and stick it to the first strip aligning it with the center of the tape so they overlap half way and are stuck together. Then take a third strip and repeat that putting it above the second strip. At the end you have a 4" wide strip of tape with the first strip underneath it and centered on the 4" width. The 4" wide strip is seen in the photo here.

10 - Attach Wing Panel to Tape - Center the wing panel on the tape and align it just a little below the center of the 4" width of tape and lay it on the tape. Press down on the FFF at the bevel all the way across to get some adhesion and then cut the tape free at both ends of the wing panel.

To be continued...

Last edited by jackerbes; Feb 01, 2012 at 07:33 PM.
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Nov 09, 2010, 11:15 AM
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32" KFm3 Flat Wing Build - Part 2

32" KFm3 Flat Wing Build - Part 2

11 - Position Dowel on Tape - Hold the 1/8" dowel on the bevel and let it roll down and into contact with the tape.

12 - Put Dowel Against Bevel - As seen in the photo, the dowel should be laying on and adhering to the tape and up against the bevel on the wing panel.

13 - Attach KFm Strip to Tape - Hold the KFm 50% strip up above the tape laying back at about a 45 degree angle, center it on the wing panel, and get an even gap all the way across in front of the dowel that is about the same width as the dowel. Then lower the KFm strip down onto the tape and lay it back. It will look like the photo when the KFm strip is laid back.

Important! If the gap at the dowel is too narrow the tape will be too tight when the KFm strip is folded over and down onto the wing. The gap seen in the photo, with the KFm strip laying all the way back, is about right. It is the same as the dowel's size and even all the way across. Press down across the FFF at the bevel to get some adhesion to the tape.

14 - Check Tape Tension - To check the position of the KFm strip on the tape, lift the 50% KFm strip up until it is vertical. If you feel a very slight pulling tension on the tape as it gets to the vertical position as seen in the photo, it is right. If you feel tension before it is vertical or very much tension when it is vertical, it may be too tight and the tape may come loose or tear when the 50% KFm strip is folded all of the way down to meet the top of the 75% KFm3 strip.

If every thing is right, lay the strip back down and turn the wing panel and attached KFm strip over. Use your fingers to smooth the tape into full adhesion on the plastic skin and smooth any wrinkles. If any air bubbles are trapped they can be vented with the tip of the utility knife. Then turn the wing back to bottom side down.

15 - Mark 50% Line on 75% KFm Strip - Lay the 75% KFm strip on the wing panel and align it with the 75% line. Then make marks on both ends of the 75% strip in line with the 50% line on the wing panel. Draw a line across the 75% strip and is where the 50% strip will be trimmed for final positioning of the step. In the photo the marks had been made and the line has been drawn.

16 - Peel Plastic Skin in Support Area - The front inch of so of the 75% KFm strip is the support for the 50% strip. It also is a key part of the strength across this wing. But PU glue does not stick to the plastic skin very well so the plastic skin will be removed from the area of the 75% KFm strip where the 50% strip rests. This is important to getting a good line of glue adhesion across the wing. Take the tip of the utility knife and make the shallowest possible cut in the plastic skin along the line just drawn. You'll feel the knife tip break the skin and you don't want to go any deeper than that. Cut the line all the way across the 75% KFm strip.

Pick the corner of the plastic skin up and pull it up slowly keeping it inclined at a 45 to 60 degree angle to the foam. If you get too close to vertical, the plastic will start tearing at the pin holes. The peeling has just been started as seen in the photo.

17 - Glue 75% KFm Strip - The 75% KFm strip will be glued in place about as seen here. It wil be algned with the 75% line on the wing panel and the 50% lines on the wing panel and the 75% KFm strip will also be in line.

18 - PU Gluing Essentials - Seen are a bottle of Gorilla Quick PU (resting upside down in holder to keep it from setting up or "skinning over" in the bottle), two plastic glue spreaders cut from snap on can covers, another can cover to lay the spreaders on, and a water spritzer bottle. Spritzing with water when using PU glue is not optional, it is an important part of making sure it foams up and works well.

19 - Applying PU Glue - A typical application is seen. I run serpentine beads a distance apart and then spread the glue with a credit card like spreader. This glue is ready to be spread, when spread if you hold it to the light you should see a lightly frosted/shiny appearance where the glue is spread. Too shiny is too heavy an application (needs more spreading) and dull bare foam is not enough.

20 - Spritz, and Align Parts - Spritz water on all of the area that will be in contact with the part that has the glue spread on it, can't have too much water. PU glue is very slippery at first, make sure parts are aligned and press down with your fingers and hands to get the areas to come into contact.

To be continued...

Nov 09, 2010, 11:20 AM
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RogueTitan's Avatar
Hey Jack

Nice to see you back with a new design to share with the rest of us Air Heads
Nov 09, 2010, 11:24 AM
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32" KFm3 Flat Wing Build - Part 3

32" KFm3 Flat Wing Build - Part 3

21 - Clamp or Weight Parts - I placed flat board on top of the part and place the necessary weight on top of that, more will be added to the short exposed area on the left end. Check the alignment marks several times in the first minute or two or it might slide out of alignment.

22 - Let Glue Set Up - I spritz water on the spreader and lay that aside on the can lid so I can tell how the glue is setting up. The excess glue on the spreader will foam up start setting and when you can touch it and deform it without it sticking to your skin you can work with the parts again. If it sticks to your skin give it more time. The 15 minutes is OK for parts like this that will not spring apart or pop apart from pressure in the assembly, you can unweight them in 15 minutes or so. For anything that has more pressure and wants to pull on the glue, I give it an hour or two.

23 - Glue Filler 75% KFm and Filler Strips - The KFm 75% strip and the Filler strip have both been glued into position in this photo. The plastic skin has also been removed from the top of the filler strip also in preparation for it being glued to the 50% KFm strip when it is folded back.

24 - Parts Arrangement - This is the tip of the wing with the parts glued in place. The slight amounts of stagger or overhang on the dowel and the ends of the parts will be trimmed even later.

25 - Trim 50% KFm Strip - The 50% strip has to be folded back two times. The first time will be to transfer the marks from the 50% line on the wing panel to the 50% KFm strip. Then the strip will be trimmed, folded down again, and glued into place.

I hold a board against the back of the 50% strip to help it fold down evenly and I also have some weights handy to place on top of the board once it is folded down.

26 - Fold and Mark 50% Strip - With the 50% strip folded back I make a mark on the 50% strip in line with the 50% line that is on the 75% KFm strip and the wing panel. This is so I can trim the 50% strip to be even with the line.

27 - Check Leading Edge - Look at the leading edge and make sure the dowel is centered, that the tape is holding, and and that everything looks OK there. The combination of the tape and beveled edges on the FFF really pulls itself together into a very straight, smooth, and durable leading edge.

28 - Trim 50% Strip - With the 50% strip unfolded again, use a straaight edge to trim it even with the marks made when it was folded. This will put the step right on the 50% line and make sure it is straight.

29 - Refold and Glue 50% Strip - PU glue was applied to the top of the Filler Strip and the area where the plastic skin was peeled off of the leading edge side of the 75% KFm strip. The back of the 50% KFm strip was spritzed with water, and it was folded down on top of the other strips and weighted down.

A visual check was made to ensure that the FFF panels were in contact all the way across, another board and some more weight was added on the right end to get the assembly evently weighted all the way across.

30 - Check Leading Edge - A quick check of the leading edge and everything looks good there too. I'll give this a couple of hours to set up or even overnight because the strip want to spring up, even when the glue sample is completely dry and cannot be crushed with my fingers, might be too soon. If I'm working in the evening, I would do this last thing and call it a night.

31 - Finished Wing Assembly - That finishes the wing. I use a razor saw to trim the dowel ends and a sanding shingle with 60 grit open grit sandpaper to sand the ends of the wing flat and vertical. This is the point were it starts getting exciting! This all foam wing has a very nice combination of stiffness and resistance to bending forces and the strength is very well distributed. The wing weighs 85 grams or 3 ounces at this point.

I'll follow up with more details on finishing the wing in my next post.

Nov 09, 2010, 01:29 PM
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Dickeroo's Avatar

You're a true champion of communication.

Jack... you are one of the best when it comes to clear, concise wording. Once the work is done it may look easy to do, but it is not. Communicating ideas without adding layers of fat is not easy to do. You, however, are a master at presenting ideas that are easy to understand and comprehend. I salute you.

Dick Kline
Nov 09, 2010, 08:34 PM
jackerbes's Avatar
You are too kind Sir!

Let me know how your wing turns out.

I got the ailerons on today and the servos mounted, the winds are killing us here but I may get a chance to fly it some day...

Last edited by jackerbes; Nov 10, 2010 at 12:19 PM.
Nov 10, 2010, 12:26 PM
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32" KFm3 Flat Wing Build - Part 4

32" KFm3 Flat Wing Build - Part 4

OK, we finished up with the bare wing at 85 grams, now I'll fit the servos and finish it up.

We can agree to disagree on how to best do this but with HXT-900 9 gram servos costing about $4 each I am not the least bit inclined to consider the cost of a second servo a factor. And when I look at the short, direct, and well centered (on the aileron) linkages, I consider it the best choice. It looked better and has fewer snag points too.

32 - Layout Servo Pockets - The servo pockets will be just forward of the 50% step and will put the servo arm 8" in from the outboard tip of the wing. The servo case will be to the inside of the arm. Lay the servo on the wing and trace it's outline.

33 - Router Cut Pockets - Put your surgical mask on, FFF is not good for the lungs. I plunge cut the corners with a Dremel cutter and remove the top two layers of FFF down to the wing panel. You'll be able to see and feel each layer as you encounter it and the material will be removed quickly. If you're new to it, a little practice on scrap is good. Note that the pocket location will open up access to the inside of the wing, that will allow us to run the servo leads through the wing for a very clean installation.

34 - Dremel Cutter - This is the Dremel cutter I use, it is a coarse 1/4" cylindrical bit intended for wood working. It works wonderful on foam and has a service life of at least 35 years (that is how long I have been using mine).

35 - Drill Servo Lead Access - Mark the center of the wing, and hold the wing up to a light to see where the forward edge of the 75% KFm strip is and mark that. Use a tube drill (that is a 5/8" one there) to make the hole.

The end of the tube has small teeth on it, hold it on the foam inclined at a slight angle, located in a "V" formed by your fingertips (to keep it from "walking"), and rotate the tube with light pressure to cut the skin. As soon as the skin is cut make the tube vertical and rotate it with light pressure to make a hole.

I drop or push a string into the wing at the servo pocket, then catch it with a piece of wire with a hook on the end, bring it to the access hole, and pull it out. Do two strings, one from each pocket and tape them down next to the hole with masking tape.

36 - Mount Servos - If your servo leads will reach and hang out of the hole they are long enough (the HXT-900 leads were plenty long enough). If not, add an extension to the servo lead and put a narrow wrap of electrical tape around the joint where the servo lead enters the extension socket so it will not pull apart later.

Test the servos, center them, and mount the arms so they stick up at a right angle to the housing. Drill the holes in the arms out so that your push rods will fit easily but without play (I use .047" or .052" wire and a drill that size too).

Tie the string lightly to the servo connector and pull the lead down into the wing and out the hole. You'll still have enough slack to work with the servo as it is mounted. Test fit the servo by dropping the back end where the lead comes out down into the wing and then pushing the case back and down in. The edge of the case should align with the step and the servo will be even with the top of the wing. The case is effectively part of the step now.

Lift the servo out, lay it aside, and put a Dime sized blob of hot melt glue down in the wing centered in the servo pocket. Immediately put the servo back again (pulling on the strig as you do that) and press it down just until it is even with the top of the wing and the back of the step.

About Hot Glue - There is one best hot glue for building foamies. It is the Arrow Brand #BSS6-4 Super Strength Slow Setting Hot Melt Glue. Get it at Lowes or Home Depot. When used with a regular temperature (not low temperature) hot glue gun it is the most reliable glue you can find. When SSSSHMG is used at the right temperature, you will see a slight wisp of steam or smoke sometimes, will have plenty of working time to position parts and even shape or form the glue (a moistened finger is good after glue has cooled a little). If you use the cheap Chinese junk, it will not work as well and I don't want to hear that my process does not work.

37 - Servos Mounted - There it is, two perfectly mounted servos!

38 - Leads Ready - And two easily accessed leads ready to use. In use you will need to put a "Y" cable on the two leads and plug that into the Aileron channel on your receiver. When the "Y" cable is put on it will extend the leads again, tape the fittings together to avoid losing a lead up inside the wing. Any excess cable length can be pushed up inside the wing later. I like 6-8" or so hanging out so I can work with the wing in mounting it and I just fold that up in the cabin or fuselage when I mount the wing.

To be continued...

Last edited by jackerbes; Nov 10, 2010 at 12:33 PM.
Nov 10, 2010, 12:32 PM
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32" KFm3 Flat Wing Build - Part 5

32" KFm3 Flat Wing Build - Part 5

Now we'll cut the ailerons out, mount and hinge them. and finish up the linkage. I made the ailerons 1-1/4" wide and 11" in length measured from the outboard tips in. That should be plenty of aileron authority for a high winged aileron trainer/sports flyer, I'd be surprised that very much of the available deflection will be used in flight.

40 - Layout Aileron Cuts - The aileron layout is marked and drawn on the back edge of the wing.

41 - Cut Ailerons - The ailerons are cut with the tools seen here and as previously described. Avoid crossing cuts at the corners, keep the knife inclined back at an angle, and it all goes nicely

42 - Bevel Ailerons - The edge of the aileron, and the wing where it meets it, will be beveled to form an inverted "V" on the bottom of the wing to allow for the aileron to swing down. The hinge line will be the mounting tape and located on the flat top of the wing so no additional clearance is needed for up swing.

I have posted an image here that explains package sealing tape hinge. This is the Worlds Greatest Hinge For Scratchbuilt Foamies (WGHFSF). It is simple to master, works perfect, and is easily mastered. Try one on some scrap first if you want some practice.

Lay a straight edge on the bottom (plastic skin side) of the aileron and cut a 30-40 degree or so bevel on the edge that will meet the wing.

43 - Bevel Aileron Cutout - Lay the wing on the corner the bench so that the area from the 75% step back is flat on the cutting mat. The plastic skin side will be up, lay a straight edge along the edge of the aileron cutout and make a 30-40 degree bevel on the wing.

44 - Mount Ailerons - As seen here the wing is bottom side up with a half exposed strip of tape is already attached to the top surface and laying sticky side up. There two short pieces of light cardboard laying on the wing as the spacers for the line. The aileron will be laid on top of the spacers with the edges of the two bevels aligned vertically.

45 - Tape Aileron to Wing - The aileron is almost in position here, that will be aligned and held in position and the tape will be lifted up to vertical and pressed into adhesion with the wing bevel, then on around and onto the bevel on the aileron. Use your finger tips to get the tape in contact and then lay the tape on the aileron smoothing it forward.

To finish the WGHFSF hinge turn the wing over and let the wing and aileron lay flat on the bench. Use a tongue depressor with smooth rounded edge to press the tape down into the bottom of the "V" and back against the bevels on the sides.

Then turn the wing over and you see a narrow line of exposed tape. Pressing on the back aileron edge will push the tape up into an inverted "U" on the top of the wing. Lay a strip of tape that is the width of the aileron on the wing above the aileron, push on the aileron to get the gap/exposed tape line even, and lower the tape down across the gap and onto the aileron. Work the tape down smooth, fold the aileron up on to the wing and smooth it on the bevel, and you will have just created the WGHFSF hinge.

46 - Locate Control Horns - Use a square or corner of a piece of paper to located the control horns. The line for mounting the hors is in line with the servo horn and at right angle to the KFm step.

The control horn seen there is made from a Dollar Tree store dental flosser. The drill has a 1/4" tube drill in it and will be used to drill two holes in the top of the aileron. The legs of the control horns will be set in hot melt glue in the holes. When I drill the holes I just drill the tape on the top layer and the foam and pick the foam out of the hole leaving the tape to close the hold on the bottom.

For the horns to work properly, the hole in the control horn needs to be directly above the hinge line, not in front of or behind it.

47 - Mount Horns and Linkage - Seen here the horns are glued in place, Mini EZ connectors have been added to the horns, and the linkage to the servos fitted.

48-49 - Check Aileron Action - This all looks good, I'm getting about 45 degrees of up and down travel, this will be a candidate for low rate setting on the TX, I can tell already.

I am checking the ailerons here with a servo tester that will test two servos at a time. With that I can rotate a knob to see the same directions of travel and travel limits I'll see with the TX set to defaults. Very handy little thing to have in the shop.

The few remaining touches will be to add the protectors on the wing center at the leading and trailing edges where the rubber bands will cross, maybe add some tip plates to the end of the wings, and some colored tape at least on the bottom of the wing to help with inflight visibilty.

I'll fill you in on the details later...

Last edited by jackerbes; Nov 14, 2010 at 05:02 PM.
Nov 10, 2010, 02:31 PM
jackerbes's Avatar

And it's ready for a test flight...

So I got the edge reinforcements on, added some colored tape to the bottom of the wing for in flight visibility and I think this ready for a flight test.

I got the last BB 33 I built out of the rafters, the BB 33 built with a full length monobloc fuselage and tried the wing on that and it looked like a go.

Got a receiver in the plane got all the servos tested, the venerable old Tower Pro 2410-21 is still running fine and churning out 100-120 Watts or so on a 1100 mAh 4S A123 pack, so we're ready for a test flight.

If the winds we are getting ever back off a little bit...

The wing weighed 115 grams/4.1 oz. as you see it here, the AUW of the plane with a the 4S A123 pack is 585 grams/20.6 oz. The wing has an area of 240 and will be flying at about 12.4 oz./sq.ft.

We'll follow up with a report on how it goes.

Last edited by jackerbes; Nov 10, 2010 at 09:52 PM.
Nov 13, 2010, 07:37 AM
jackerbes's Avatar

Maiden Flight Report

OK, I got this wing out and flew it yesterday on Old Yeller (BB 33). Unfortunately the hat cam let me down for some reason and did not produce a usable video.

I added a little weight to the nose (1/2 oz. or so) to get the CG right at 3" back from the leading edge. First launch was uneventful other than I happened to choose a nearly depleted battery that had missed the recharging drills.

But there was enough power for a slow and low circle back to a gentle landing, sort of a extended circular test glide. Low power test glides are a good thing!

The next launch was with a fresh battery and it flew away nice. A little bit of jerking around as I found the sticks and quickly learned that this wing has some pretty good aileron authority, more than I anticipated. I dropped back to low rates (50% all around) and quickly got it trimmed out.

I found the plane to be a pretty snappy flier as far as aileron response but still easy enough to control. It is no longer a beginner's plane, the best results will be with a calmed down pilot and more gentle touch.

The aileron control was very good, it soon became easy and instinctive to start and stop turns and rolls. As I expected, I had to be responsible for stopping anything I started but it was not too demanding as I let the subconscious take over.

Rudder only turns were surprisingly good, it seemed to induce a little bit of roll too. A coordinated rudder/aileron turn at a gentle rate was very nice and usually eliminated any need for elevator to maintain altitude. Big gentle circles and flat figure eights were very nice.

I was interested in how the roll rates would be with the flat wing found that to be quite nice too. From level flight at full toot (90-100 watts or so) half stick travel or so on ailerons would put it into a nice series of rolls. The rate was pretty quick, like one per second or so. I couldn't time some elevator in at that rate so I would get a series of two or three slowly descending rolls before I'd chicken out on diminishing altitude and stop it.

There was some "barrel roll" aspect to the rolls but was about what I would expect to have with the high wing and slab sided fuselage I think. This will turn the BB 33 into a good aileron trainer though, no doubt about that.

Vertical or near vertical climbing rolls to either either direction were easy and controllable.

It inverted very nicely and only needed a small amount of down elevator to stay in easily controlled inverted flight. Loops, and half loops, Immelmanns and the like were easily accomplished.

The wing loops easily either inside or outside and when I went back to high rates it would do very tight and continuous loops. All the cheap thrills were there.

All in all, a nice flyable wing and a step forward in the fun and skill levels for the BB 33.

So I celebrated my success by losing Old Yaller in the sun while I was in a full throttle fairly low approach. I just couldn't visually recover or decide what the attitude was and I augered it in a full throttle, burying the motor in the soft muddy ground, destroying a prop, and breaking my new wing!


The blindingly bright low sun on a winter day in Maine can be a heat breaker when it is right behind your favorite approach.

Nov 13, 2010, 07:48 AM
jackerbes's Avatar

How to repair a FF wing...

The breaks radiated forward from aileron cutouts as the plane came to a sudden stop and the wing lifted and moved forward. There was a crack all the way across the two FFF layers thick area at the step.

I spritzed the cracks with water and used an acid brush to work some PU glue down into the cracks well, especially at the break along the step.

The plastic skin and colored tape on the bottom side of the wing had acted like a hinge for the break and probably is all that was holding the pieces together when the dust settled.

I gave the PU a minute or so and then blotted all of the surface water off with a paper towel. Then I put strips of clear package sealing tape over the cracks where the foamed up PU was. That was smoothed down and the wing was weighted flat for 30 minutes or so.

And it is good to go and ready for the next flight!

Note to the pilot, keep your plane and eyes away from the sun! There are no Huns lurking in the sun in Maine, you can ignore it!

Nov 13, 2010, 01:01 PM
Registered User
philipa_240sx's Avatar
Hi Jack,

Glad to see you had some success with the KFM3 wing.

I didn't have much luck with my KFM3 wing, but I notice some differences that may have accounted for it:

1) The BB33 KFM3 wing has ailerons placed much closer to the 75% step.

2) I had approx 3 deg dihedral.

Basically what would happen is the plane would tend to pitch up when banking. Possibly stalling the wing and ailerons. The end result was mushy response and I would have to bring the nose down, then try again. The CG and incidence were spot on so I could never explain the phenomenon.

I notice my BC12D Taylorcraft with a near flat KFM2 aileron wing is much more well behaved. Nice turns are a snap with this wing and only a touch of rudder is needed for a nice co-ordinated turn.

Thanks for the insight. Perhaps I will try another KFM3 winged BB in the future!
Nov 13, 2010, 02:47 PM
jackerbes's Avatar
Originally Posted by philipa_240sx View Post
Hi Jack,

Glad to see you had some success with the KFM3 wing.

I didn't have much luck with my KFM3 wing, but I notice some differences that may have accounted for it:

1) The BB33 KFM3 wing has ailerons placed much closer to the 75% step.
Steps too close to the aileron is often cited as the cause of poor aileron performance but I'm still not sure that it is the only factor in the poor performance. It might be a combination of things that are at work here. I seem to notice the following are present when wings are cited as "poor performers":

1 - There is dihedral

2 - There is an open hinge line close to the step.

I built one poor performing KFm3 with ailerons and realize now that it also had an open aileron hinge line. When I built the second I had moved on to using full length tape hinges that effectively sealed that open hinge line. And that second wing worked much better. In that type of hinge (image attached) the hinge line is on the top surface of the wing and the inverted "V" hinge opens to the bottom.

I sort of wonder now if having a smooth surface from the step and to and over the aileron hinge is not a factor. Like it promotes smoother air flow or something?

2) I had approx 3 deg dihedral.

Basically what would happen is the plane would tend to pitch up when banking. Possibly stalling the wing and ailerons. The end result was mushy response and I would have to bring the nose down, then try again. The CG and incidence were spot on so I could never explain the phenomenon.
That 3 degrees works out to about .84" or almost 7/8" of an inch per tip and that is not a lot of dihedral but I wonder if it isn't still a factor too. I don't recall my not so good KFm3 as being that way or that bad.

Did you notice if the aileron performance seemed OK when coordinated rudder/aileron were used and if you were flying more in a "real airplane with passengers in the back drinking coffee" style? Or was is that the disappointment was not more that it would not fly well on ailerons only (i.e., when flown more in the "yank and bank" style)?

I notice my BC12D Taylorcraft with a near flat KFM2 aileron wing is much more well behaved. Nice turns are a snap with this wing and only a touch of rudder is needed for a nice co-ordinated turn.
And the BC12D has similar dihedral to what the KFm3 did? And the same kind of hinges the KFm3 did?

Thanks for the insight. Perhaps I will try another KFM3 winged BB in the future!
I encourage you to try it. If we keep messing with this we may figure something out here. If you or I took the trouble re-do the hinges on the lower performing KFm3 wings with sealing tape hinges it might tell us something. Like maybe the smoother air flow makes better ailerons or something.

Then there is the fact that our expectations can be a factor and differ too. I tend to just take whatever I get unless I find or feel a need to pursue getting something that was not there.

For example, I consider it normal to use coordinated controls on a high winged plane with ailerons. It is just the nature of the beast. The business of messing with differential ailerons never came up with me because I don't try to get "yank and bank" performance out of a "don't spill the coffee" plane.

So I was quite pleased when the 48" flat SS wing turn out to have great aileron response without turning into something that was hard to keep in smooth and level flight. I credit at least some of that to the KF wing but I also had a similar experience with the flat wing on the SPADET LC-40 I built.

The SPADET has a flat-bottomed Clarke-Y kind of wing and it was very pleasant to fly on ailerons only or with rudder and ailerons. The SPADET had a high wing too and the rolls were not very axia. But I think axial rolls and high wings exclude each other at least to some extent.

My wing was made with the KF step lines straight across the wing from 50% and 75% of the tip chord. So that puts most of the aileron a little further away from the step and that might be a factor.

But if you build a flat KFm3 like this one and don't get enough roll rate out of it I would be *very* surprised. I was somewhat taken aback with what I found especially since I had reduced the width and length of the ailerons a little.

Nov 13, 2010, 03:24 PM
Registered User
philipa_240sx's Avatar
The KFM3 on the BB had full length taped hinges with the V on the bottom. See photo below. I needed to add some rudder for co-ordinated turns. Bank and yank works, but not as pretty and resulted in the upward pitch/ballooning sometimes. Roll performance on the other hand was good... no problem pulling barrel rolls so long as you had some airspeed.

The KFM2 on the BC12D has simple 'CA style' hinges... about every 3-4" and the hinge V is on both sides. They are not taped. The wingspan is a lot longer (48"), only 1/2" dihedral at each tip (1.3 deg). The ailerons however are much smaller and the plane does not have the authority for rolls. It'll come up on knife edge, but refuses to roll over easily.

It's really hard to compare these 2 planes and the differences of KFM2 vs 3. They are so different in design, size, etc.

Another thing that maybe points to an issue with the BB33...

Just before I retired the plane, I was having issues with pitchiness... just like you would get with a tail heavy plane. But nothing changed, and the plane flew better when new. I wonder if there was underlying damage that I couldn't see... it was completely covered with colored packing tape after all.
Nov 23, 2014, 03:36 PM
Registered User
Working on building one of these now. At what point are you trimming the length of this thing to 32"? Looks like you're building it at 48" all the way through.

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