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Old Apr 27, 2010, 02:54 AM
smithy
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Cornwall, UK
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SU25K Frogfoot

Well i've decided to move the build thread over here. Progress so far, started laying up 1/16 balsa skin but made a right botch up of it so i have removed skin from nose section and will redo it using the strip planking method. The nose shape was all wrong. The next one will have moulded nose cone aswell as moulded intakes as there are some tight curves in these areas. Must admit i've never done strip planking before so here goes nothing!!

Phil

http://www.rcgroups.com/forums/showthread.php?t=1208235
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Old Apr 27, 2010, 11:38 AM
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Ed Waldrep's Avatar
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The way to get smoothly flowing curves is to use long (lengthwise/horizontal), continuous strips of wood that run over several bulkheads. That way the natural bend in the wood will give you your curve, and the skin will be stronger because it's a continuous peice vs having a butt joint in the skin at each bulkhead. Stip planking is what it's called.

Why use strip planking? It's best used for a compound curve, a curve that curves in two directions, like often found on fighter's nose. As said before if your bulkheads are correct it will give you a natural curve from the bent wood, if you glue the strips correctly. If it's a simple cylinder, like say a roll of toilet paper, then bending big balsa sheets is easier that using multiple strips (less joints to sand).

What I would do in this case is run a straight, wide sheet starting behind your intakes (maybe to the next bulkhead behind the intake or wherever the side sheeting ends) and run it forward all the way to the nose (actually a bit longer by say 1/4 " to give you extra to work with). I would make lengthwise cuts in this sheet in the front to cut it into strips, running from the front end back to what I assume is bulkhead F3. So starts as a solid sheet in the back, and at the front the first third of it or so is cut into strips, which are STILL ATTACHED to the main sheet. Once you get the sheet glued to the bulkhead near the intake and F3 and whatever bulkheads are in between, you would wet the strips with water or ammonia and bend them inward to touch F2 and F1. You might want to clamp them down and let them dry before gluing because the balsa will shrink as it dries and the gradual bend my disappear (ask how I know this!). You may need to trim the balsa some because of the smaller radius of the nose, you might end up with very long and skinny triangle shaped sections trimmed out of the strips.
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Old Apr 28, 2010, 03:01 AM
smithy
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Cornwall, UK
Joined Dec 2004
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Ed, brilliant, thanks for the great advise. Always good to get tips from the guys who know what there doing. So your saying its fine to use sheet balsa on the relatively large flat areas then strip when it comes to the tighter curves like the nose and intakes. The Froggy has some large side areas but gets real tight around the front of the intakes. Also, what thikness balsa is best, i was using 1/16 but read somewhere that 3/32 is better as it gives you more material to sand down without sanding through it.
Any details on the F-18? You said it might be a kit.

Thanks, Phil
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Old Apr 28, 2010, 04:37 PM
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St.Catharines, Ontario
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exactly Phil. like I told you in my last post of the other thread, you can plank the way you were in the mid section, but when it comes to the tail and nose, strip planking is the way to go. if it were me, I would be tempted to strip plank from F6 forward to F1 and then carve/sand, or mold a nose cone. I don't remember what the tail looks like, so you may have to do the same as the nose, or you might be able to strip plank right to the tip of the tail. I would say 3/32 is your best option for strip planking, although I've used up to 1/8th and it's worked quite nicely. it just takes more effort to plank with thicker balsa. as suggested, you can use ammonia or water, but I use water in a spray bottle. personally, I hate the smell of ammonia and don't use it because of that. LOL. each has their own methods, which is why a forum is so great...........lot's to learn from those willing to provide the info.

another method I have tried and works well, is elastic bands as clamps. just make sure they are tight enough, without being overly tight. start with your side pieces, cut a bit wider as suggested and glue to the formers. then put elastics on them at the formers and allow to dry. if the elastics dent the wood, you can simply spray the wood with water and the dents will pop back out, or use a cloth buffer between the elastic and the wood............that's the beauty of using balsa. after that you can glue top and bottom pieces on and repeat the clamping, then all four, 45 degree angles and repeat the clamping. then it's just a matter of cutting fill-in strips for the gaps.

what I have done in the past, is to lay the balsa on the table and sprayed the outer sides with water and allow it to soak in for a bit. this keeps the glue side dry and gives the balsa some flexibility before you plank the plane. just a spray or two along the length from a spray bottle is enough. wait a few minutes and plank away. lol. you can use CA if the curves aren't too severe, but if they are, then white glue is a good option. it's lighter than epoxy and strong enough for the job. there's some good white glues available these days, that dry very quickly, in the matter of an hour or so.

Rich
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Old Apr 29, 2010, 07:00 AM
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Arngeir Blakseth's Avatar
Molde, Norway
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3/32 or even 1/8 is what I would use. Leaves you with more material to sand down to get the surface really smooth, and it also helps avoid the starved horse look as it won't sag as easily between the formers. If weight is an issue, be picky when buying balsa or if you already have a big stack of sheets handpick the ones to use for planking.
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Old Apr 29, 2010, 02:02 PM
smithy
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Cornwall, UK
Joined Dec 2004
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Thanks guys, great advice. As i started with 1/16th sheet im going to carry on with this build. Ive made a few design changes for the next one so this is really just a rough prototype to test flying ability. If this turns out ok i will concentrate on scale appearence and building technique for the next one, which by the way is slightly different as it will be an SU39.

Phil
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Old Apr 29, 2010, 02:04 PM
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cool. I like the look of the bigger surfaces. must have been a blast to throw the real one around. LOL.
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Old Apr 29, 2010, 02:22 PM
smithy
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Cornwall, UK
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Yeah, i wouldnt mind a go in one. Ive seen some video of a 39 and it seems more agile than the 25, must be like you said, larger flying surfaces.
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Old Apr 29, 2010, 02:30 PM
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my guess is it's kindo like any 3D flyer....................stick bigger surfaces on it and it's gonna be more responsive, up to a point where too much is too much. LOL. that plane must haul around the sky though with those HUGE surfaces.
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Old May 22, 2010, 03:43 AM
smithy
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Cornwall, UK
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Im getting to the stage now where i need to start sanding part of the fuselage and wings. What are you guys using for filler? Is there a lightweight filler for balsa or would any wood filler be ok?

Phil
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Old May 22, 2010, 11:07 AM
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go to your hardware store and see if they sell a lightweight spackling compound. I suppose you could use polly filla, but it's a tad heavy or heavier. if you can find a water based one, those tend to be a lighter weight too.

Rich
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Old May 22, 2010, 03:22 PM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Uden Volkel, Netherlands
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Quote:
Originally Posted by jumpy View Post
Im getting to the stage now where i need to start sanding part of the fuselage and wings. What are you guys using for filler? Is there a lightweight filler for balsa or would any wood filler be ok?

Phil
I don't use filler on the planking. I use CA on the inner sides of the strips and sand the fuse smooth. I wipe of the dust and glass the fuse. The epoxy+dust in the seams will be the best filler you'll ever get.

Nice project!
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Old May 27, 2010, 12:42 PM
smithy
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Cornwall, UK
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Some more pics of progress so far. Nose section is completed, just needs sanding and filling to shape. Who said planking was theraputic?? I was going to use mini servos mounted at the rear for rudder and elevator but with the weight of 2 minifans and skinning yet to be done i have now opted to mount standard servos up front with snakes to rear controls to keep weight forward, hopefully the cells will also move some more weight forward aswell. By the way, how far from the fans can you have the cells? Is it better to have longer battery to controller leads or longer controller to motor leads?

Phil
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Old May 27, 2010, 01:44 PM
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Quote:
Originally Posted by jumpy View Post
Some more pics of progress so far. Nose section is completed, just needs sanding and filling to shape. Who said planking was theraputic?? I was going to use mini servos mounted at the rear for rudder and elevator but with the weight of 2 minifans and skinning yet to be done i have now opted to mount standard servos up front with snakes to rear controls to keep weight forward, hopefully the cells will also move some more weight forward aswell. By the way, how far from the fans can you have the cells? Is it better to have longer battery to controller leads or longer controller to motor leads?

Phil
Cells are put in saddle bag fashion if necessary or doable.

Wire length; Most explain keep your battery side as short as possible but not to exceed 20cm on the motor side and adjust your AWG according to the current draw. So 80amps and above 10AWG.

But this is what I've read. Haven't taken it to these limits myself.
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Old May 27, 2010, 01:58 PM
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Phil

that looks waaaaaaaaaaaay better planked. good stuff.

Max gave some good advice there. I'm not sure what you can or could get away with as far as wire length is concerned, but if you are gonna increase it, increase it on the motor side of the ESC. this all depends how good your ESC is also and I'll tell you why.........I am using a Turnigy Plush60ESC on my Eurofighter on 4S. I believe the stock battery to ESC wire is about 8" or so. I increased that by double and I haven't had a problem yet. the power system gets no warmer than it did at stock wire lengths. I inquired about this and the answer I got was, "with the newer generation of ESC's/batteries/motors, wire length is less critical and the hardware can handle it". although, in the next sentence, he said that it's still better to keep the battery side stock if possible and modify the motor side of the ESC.

Rich
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