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Author Topic: Keil Kraft Cub  (Read 486 times)
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pedwards2932
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« on: May 20, 2020, 05:13:25 PM »

Wanted to try a small towline glider.  The Cub seems to fill the bill....it is designed either for tow or jetex.  This is my first try at a build log and thought I would demonstrate my process.  I use a 2.5 watt laser to cut my parts.

First pic is where I pull the plan pdf into Inkscape and trace parts.  Second pic is the traced parts that I export as a .dxf file.  Third pic was the .dxf file pulled into the laser program I use to convert to a gcode file which my laser can use to cut.  In the fourth pic I made a test cut to make sure the parts looked okay.  The last pic is the laser cut balsa.  I don't think I got the focus exact enough as the kerf is larger than I like and it didn't cut all the way thru.  I was using scrap balsa and it was a bit harder than I like.
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martynk
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« Reply #1 on: May 21, 2020, 03:56:37 AM »

What %age power and speed did you use? Were you using gbrl laser mode?

Martyn

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RalphS
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« Reply #2 on: May 21, 2020, 05:25:08 AM »

Thanks for the process explanation simply put.  I appreciated the Silhouette cutting information that, I think, you pioneered on this forum.  Someone has to push different methods.  Keep it going.
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pedwards2932
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« Reply #3 on: May 21, 2020, 07:58:54 AM »

I used full power and 300 mm/min, my laser is 2.5 watt.  The balsa was denser than I usually use and I don't think I had the focus perfect.  I have printed a new air assist system that should help.  With harder balsa it usually works better to use 2 passes at a higher speed....I just got in a hurry.
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pedwards2932
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« Reply #4 on: May 21, 2020, 08:13:27 AM »

I use a magnet system for assembly.  I got some sheet metal and glued it to a sheet of 3/4" plywood.  I have found magnets at various places but Harbor Freight and Walmart seem to have the best prices.  The magnets do a great job holding parts at 90 degrees.  Since this is my first assembly of these parts I try to make notes to improve the cut files.  The ribs were a little to small vertically so I will fix that in the cut files.  You need to make sure the leading edge and trailing edge are a little larger than the pieces you are gluing to them to allow for sanding.
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pedwards2932
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« Reply #5 on: May 21, 2020, 02:03:17 PM »

Since I have gotten my 3D printer fine tuned I have been making some helpful tools.  First pic is of some sanding tools.  These really work well at getting at hard to get areas.  Second pic is a cutting tool that I made using a single edge razer blade.  Good for 90 degree cuts as well as angled cuts.  Third pic is a balsa stripper that uses scalpel blades.  I have a commercial version of this as well but I liked this design as it is easy to adjust the width.
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pedwards2932
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« Reply #6 on: May 22, 2020, 10:17:42 AM »

I have the bones assembled.  Everything fit nicely.  I think I am going to take some of the parts and align them up to where they go on the pdf plans.  This will allow me to adjust them a little to get precision fit.  I will only do this if I plan to build another one as everything seemed to go together okay.  I had used the parts pdf for tracing and I am not sure how accurate it was.  You have to allow for some kerf when you use the laser.  Building another will depend on how well it flies.
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pedwards2932
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« Reply #7 on: May 23, 2020, 08:44:30 AM »

I have everything covered now.  An advantage of using a magnetic board is you can wet the covering and use the magnets to hold everything flat or you can add shims for wash out/in while the tissue is drying and shrinking.  I have left everything flat for this.  I use glue stick to attach the tissue and use a wet cue tip to go over the tissue where it is glued.  I let everything dry for a day before shrinking.  First pic shows using magnets to hold down while everything shrinks.  Second pic is everything covered and shrunk ready for clear coat.
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pedwards2932
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« Reply #8 on: May 24, 2020, 08:22:08 PM »

I sprayed everything with Krylon clear 3 light coats.  I installed toothpicks for wing and stab hold down.  This was according to the plans but it called for matchsticks.  If I build another I think I would glue the tail section on instead of the rubber bands.  Next I need to figure out the windshield.  I think I will use the craftcutter to cut it out.
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pedwards2932
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« Reply #9 on: May 25, 2020, 01:27:00 PM »

I did the windshield today.  I used my craft cutter to cut it out. First pic is where I traced from the plans.  I found that the plan trace wasn't very accurate so I took the windshield tracing and modified it so it would fit better.  Second pic is the ecal program I use for my craft cutter this allows me to place the cut anywhere on the mat.  Third pic are some clear plates that I get at the Dollar store they are fairly thin and cheap. Forth pic is my Sizzix craft cutter with the clear material on the platen.  The platen is real sticky and it holds what you are cutting in place.
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pedwards2932
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« Reply #10 on: May 25, 2020, 01:32:48 PM »

First pic shows the clear material has been cut.  It wasn't completely cut thru but good enough to pull it out. Next pic shows the piece cut out.  Last pic shows the windshield has been glued in place.  I had to make minor adjustments to the cut outs for the toothpicks.  I used Aleene's original tacky glue....it dries clear and holds tight.
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pedwards2932
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« Reply #11 on: May 25, 2020, 05:42:59 PM »

I got the windshield install so I can call this done.  Need to balance the model and do some test gliding.  I have only done  a towline launch when I was a kid.  Any ideas on the best way to launch this plane?
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LASTWOODSMAN
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« Reply #12 on: May 25, 2020, 06:48:42 PM »

     Hi pedwards.     Your KK Cub is looking very good.   I am no expert,  but I would do a lot of little hand glides first,  on good grass, and try different angles to the wind,  first.   I tried "Hi Start" Tow Line Launch,  with a wire Tow Hook, sticking out of the front bottom of the glider's main skid.   After losing the wire tow hook,  I made a rubber Catapult, and carved out a notch for it in the main skid.   I like Catapult Launching a lot better.   It is quicker,  and handy to bring along,  especially if you are just out for some fun in a small park.
     I have some pics of my setups and Flight Reports in my Osprey glider thread here  
https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=23913.225

around about Replies 235, 240, and 252.    Like I said,  I am no expert,  but they do fly quite silently ...  Smiley

Pic #4495     Stretched out Ready to Launch    
Pic #4417     High Start Tow Hook
Pic #4571     Catapult and carved "Notch" in Main Skid

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Richard
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OH, I HAVE SLIPPED THE SURLY BONDS OF EARTH ... UP, UP THE LONG DELIRIOUS BURNING BLUE ... SUNWARD I'VE CLIMBED AND JOINED THE TUMBLING MIRTH OF SUN-SPLIT CLOUDS ...
pedwards2932
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« Reply #13 on: May 25, 2020, 07:07:02 PM »

I plan on doing a lot of test gliding.  I don't think this plane would handle a catapult launch without a redesign of the tow hook.  I have read your exploits with your glider with interest.  At any rate we will see how it goes.  Since I have all the files set up it will be easy to build another an make mods as needed.
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LASTWOODSMAN
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« Reply #14 on: May 26, 2020, 11:22:31 AM »

     Well, ...  now I  am curious as to the design of your towhook ....    I actually have never pulled the rubber, of the Catapult launcher, all of the way back.  I was warned a couple of times about doing that.   Shocked   Do you have a pic of the plan ?  - what is the wingspan? what is the degrees of wing incidence?   what is the decalage?
     I am looking forward with interest - especially to see how much  nose ballast weight you have to put in it to balance it,  and where you are going to stick the ballast.   I would suggest just gluing the tailplane on, to save on the ballast weight added to the nose.
     Anyways,   good luck with it all.    I'm looking forward to your test flights.   Smiley

PS    For what it is worth,  it seems that the force of launch, is about the same, for both my Catapult launch, and my little High Start Tow Line launch.  Neither ripped off the wings ...  Roll Eyes  yet.

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Richard
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OH, I HAVE SLIPPED THE SURLY BONDS OF EARTH ... UP, UP THE LONG DELIRIOUS BURNING BLUE ... SUNWARD I'VE CLIMBED AND JOINED THE TUMBLING MIRTH OF SUN-SPLIT CLOUDS ...
pedwards2932
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« Reply #15 on: May 26, 2020, 01:28:37 PM »

Nose weight is 3.2 grams.  All up weight is 17.6 grams.  Wingspan is 18".  Wing incidence is +3 degrees and decalage is +1 degree.  I just cut open the covering on the fuselage on the lower front bay and used clay.  Once I am sure of the balance I'll recover it.  The hook is just a 3/32 spar in the second bay with a hook made from a paper clip.  I noticed when I was test gliding there is a slight warp in the wing on the right side that was giving it some wash out.  I set it up on my build board with magnets holding it down to get it straight.  Seemed to work.  I think if I build another I will use stiffer spar balsa.  I may glue the tail on if I build another.  The plane seems to glide nicely so we will see how it does when I launch it.
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pedwards2932
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« Reply #16 on: May 27, 2020, 01:55:00 PM »

Had a chance to test glide.  I haven't closed up the forward hatch where I was putting in the weight.  Seems to be okay for now......need to try a high start to see if I can get it to work.  Need to do some research on that.  I use a clip that holds my phone on my hat to video these.  You can use voice commands to get it to start.

https://youtu.be/WsdSc6xRWlc
https://youtu.be/SI1SCWemxIE
https://youtu.be/nBqVQnI8QSc
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