Bent Profile-What’s your solution?

[Paul Taylor]

So I decided to build a Pathfinder kit that has been in my stash and stored flat.
Took the fuselage out and it has a pretty good curve to the inside. Not good. I put some apple cider vinegar on both sides and shimmed up the ends last night. Over corrected. Went Back the other way. It was close but started warping again. Trying again with it flat to see if it will hold.
If this doesn’t work I’m thinking maybe sink a CF rod on one side?
Thoughts/ideas?


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[realSteveSmith]

I'd be inclined to use that fuse as a nice template to make a new laminated fuse from several pieces of thinner wood (1/4" x 2.... 1/8" x 4 etc).

[Paul Taylor]

I'd be inclined to use that fuse as a nice template to make a new laminated fuse from several pieces of thinner wood (1/4" x 2.... 1/8" x 4 etc).

I was afraid that is the answer. 🤣

I might go that route if it doesn’t straighten up. The other issue is where to find good balsa. 😑

[Dave_Trible]

I'd be inclined to use that fuse as a nice template to make a new laminated fuse from several pieces of thinner wood (1/4" x 2.... 1/8" x 4 etc).

Yep.  Whatever you do now,  once you starting doping that later the warp will come back.   I have to admit this didn't go where I thought you were going.   From the other thread about storing airplanes.....hanging from the leadouts is asking for it in the kisser about warped fuselages-especially profiles.  You are basically hanging it in the middle and hanging weights on each end.   I hang mine from the tail wheel sort of.   I permanently bind a piece of lead out cable to the tail wheel mount.   It trails a few inches behind the airplane with a loop which I use for the launching stooge.  I also hang the airplanes from this same cable.   The airplane hangs nose down with the minimum of stress on the wings or fuselage and it all stays straight.   In this case just head any problem off at the pass by building straight and hope it stays that way.

Dave

[Paul Taylor]

Yep.  Whatever you do now,  once you starting doping that later the warp will come back.   I have to admit this didn't go where I thought you were going.   From the other thread about storing airplanes.....hanging from the leadouts is asking for it in the kisser about warped fuselages-especially profiles.  You are basically hanging it in the middle and hanging weights on each end.   I hang mine from the tail wheel sort of.   I permanently bind a piece of lead out cable to the tail wheel mount.   It trails a few inches behind the airplane with a loop which I use for the launching stooge.  I also hang the airplanes from this same cable.   The airplane hangs nose down with the minimum of stress on the wings or fuselage and it all stays straight.   In this case just head any problem off at the pass by building straight and hope it stays that way.

Dave

Who ever said you can’t teach a dawg new tricks. Or I’m always learning from yall.

Thanks Dave I guess. 🥴 Now I have to maybe redesign my hangar.
I’m very limited on space in a one car garage. 😞

[John Carrodus]

Hiya Paul
Possibly too late now - you have probably already started on a new fuz. Having faced a similar problem a few times, I would  pour boiling water onto the fuz and or steam the bugger outof it over a boiling small pot of water. Twist past the point of no return and as it dries it will try to, but you will / should end up with a straight fuz.

[Dan McEntee]

  The initial warp is there from  the sawing process. When the blade goes through the log to cut the sheet, it either creates or relaxes stressin the wood and can either warp right away or slowly over time. Stick warp the same way. I learned a trick a long time ago of drawing sticks over a sturdy edge while bending it away from the direction it was warped in. Takes a few practice swipes to get the feel for it. I have done thinner sheet this way also I haven't had to do a 1/2" thick fuselage but it worth trying. For something that thick a slightly rounded edge may be best. It may also take a few laps over the edge to get it correct. When satisfied, let it sit over night and see if it stays, and I think it should You are creating stress in the opposite direction. This is a take off of the old craft where you curl strips of paper over the edge of a pair of scissors to make it curl. I use the same sort of trick to take the curl out of a set of plans that has been rolled up for a long time. Like anything else, this takes some practice to get the hang of it.
  Type at you later,
   Dan McEntee

[Brent Williams]

If you are feeling adventurous, build a foam core fuselage in the manner demonstrated by Pat Johnston. 
Much easier to build one of these stiff, straight and light, rather than rely on a single sheet.

[Howard Rush]

Paul,  I think your problem is the apple cider vinegar.  You should have used balsamic vinegar.

Hope this helps.

[wwwarbird]

 Use it for a pattern and do a built up fuse, lighter, stronger...

[Jim Hoffman]

Consider calling the manufacturer and ask for a replacement of the unusable part

[M Spencer]

Soak it overnight , dry bowed equally opposite . In the hot sun . Or soak it for a week . A few Days. Anyway .

But if it was  only 1/ th in. , Shaving it aft tapered to equal it , if its good hard wood , might get it . Without the water .

Block Sand flat with  60 Wt . initially . perhaps . The Flat off with 80 . But weight a bit , if its playing games , to see it
adheres to orders . Test the bow / resistance , each way , before you go at it , might tell you something .

But Id take it as ' stress relieved ' as is . So its gunna  get it walking about , maybe , however you go at it .

If the FRONT is Straight , taper off the convex aft .
As in put the NOSE FLAT on the bench - Draw a line across aft , at half hight ( 1/4 up if its 1/2 wood ) and see how you go , from there .

[Paul Taylor]

Paul,  I think your problem is the apple cider vinegar.  You should have used balsamic vinegar.

Hope this helps.

Maybe I should learn how to cook. 🤪

[Ken Culbertson]

If this is old hard wood I don't think all the water going over Niagra is going to fix it permanently without some help.  If you like the square look of the slab profile get some .5mm x 12mm CF strips. Now soak it for a while in the bend area then clamp vertically in a vise with the bend away from the bench and pull it straight and anchor it some way so you can work on it.  Let it dry some but not completely.  Rough up one side of the CF strips and epoxy them to the top and bottom.  Drill 4 or 5 small holes along the length of the CF strip and screw it to the fuselage with 1/2" #3 flat head screws.  You can do this after it dries. Countersink them to keep the heads flush or add another balsa cap to hide them.  This will keep the strips from delaminating over time. 

If you want a rounded top and bottom get out your x-acto hollowing tool and carve out a 1/4" channel in the top and bottom after you have pulled it straight (still in the vise).  Epoxy in an arrow shaft (target, the light ones).

If the twist is not too much you could skip all of the soaking and bending, and just draw a center line through the curve, dig the channel, epoxy in the arrow shaft then sand the high sides to match.

Now here is the dirty little secret that no one will tell you about.  A warped fuselage (within reason of course) on a profile means nothing if the nose is pointed in the right direction relative to the wing and the elevator hinge line is "parallel" to the flap hinge line.  You do need to get the rudder perpendicular to the wing, not parallel to the fuselage.  I put parallel in quotes because you want it parallel in flight.  If it is parallel on the ground, it will not be parallel going 50mph around a circle.  I purposely mount my stab with the inboard 1/16" closer to the flap hinge to allow for the tail being thrown out by centrifugal force.

Good luck.  There are lots of ways to skin this cat.  Pick one and get building!

Ken

[Dave Hull]

Just to be different here are a few other alternatives: 

Dry the material out, thoroughly. Set up a resaw fence in the bandsaw. Slit the blank vertically from the tail forward, leaving as much of the front of the fuselage unsawed as possible. Usually the bow in an 8"-12" isn't too bad. Use a thin kerf blade, and don't bother with an official resaw blade--you don't want that. Let the blank "cure" some more. Now pick your poison and glue it back together. If the kerf opened up when sawed and the "leaves" bent out, I would note which side was most pronounced and put some English toward that side when clamping down to your bench. (Shim under the fuse slightly, maybe around a 1/16" unless you got some real bananawood on your hands, in which case add more.) If both sides bent the same direction I would add more English. If both sides opened up but were touching at the tail, and they were even, I'd clamp straight. This resaw technique is often used to make toboggans, so you ought to be able to un-tobogganise your plank....

As far as purely mechanically straightening it in less than a year (a notional timeline) you would need to add compression stress on the convex side. The problem is that you are not neutralizing stress, you are simply added countervailing stresses. They likely won't stay balanced over time and it will pretzel up on you before long.

Another way to use that chunk of wood is to draw a bunch of strategically places lightening holes. Think Warren truss here and really get some weight out. Now go back and skin it with some 3/32" or 1/8" balsa sheet. you might want to plank it at a 45 degree bias. But you need to wet out the whole surface and get it clamped down before the glue grabs too much. A glue roller over the core blank would work well.

There ain't no guarantees when dealing with a non-homogenous material, but you get another try at it before it becomes something other than a fuselage....

PS--Dunno why you were messing with vinegar. The age-old soakant is water with ammonia in it. But water by itself works well. The idea with taking out the warp is to soften the lignin that is binding the wood cells together.

[Ty Marcucci]

I had a Midwest Skyraider with a warped fuse.. After all the soaking, etc, didn't work, I made two shallow vertical cuts with a razor saw about 1/8th inch deep in the middle of the bow.. I filled the cuts with slow curing super glue.Then soaked it and placed it under weights for 48 hours.. That was over ten years ago and it is still straight. 

[Ken Culbertson]

One thing all of these solutions, and they all have worked is that they focus on keeping the warp out over time.  The stress in the wood will relax over time and all of the methods, including mine, do not relieve the stress, they only counter it until it goes away on it's own.  I think the best solution presented here is the one that eliminates it nearly entirely but is nearly as much work as rebuilding from scratch. That is Brent's suggestion to use the Pat Johnson method.  Hard foam or a truss structure from thin stiff balsa strips covered by a balsa or 1/64 plywood skin.  That way 90% of the warp stress goes in the scrap balsa bin.  1/2" to 3/4" on the remaining wood and round the corners of the cutout.  I would add a strip of .3mm CF to the inside but that probably is not necessary to hold off the stress that will still be in the top and bottom.  It adds weight but it also fights wiggle. A heavy tail on a profile can be a SOB as it wiggles in turns.

Ken

[Dave_Trible]

I was afraid that is the answer. 🤣

I might go that route if it doesn’t straighten up. The other issue is where to find good balsa. 😑

As to finding good balsa I've had good luck with USA Balsa.   Finding a perfectly straight piece of 1/2" that long might be hit or miss.   Where things are critical I order two or three extra pieces and choose the best. Flaps come to mind here.  This game is not cheap to play these days........Just figured out an order for enough for a new full contest ship with about a 20% overage for my stock bin- $180.00.   I'm trying to slow down my building activities a little.

Dave

[doug coursey]

As to finding good balsa I've had good luck with USA Balsa.   Finding a perfectly straight piece of 1/2" that long might be hit or miss.   Where things are critical I order two or three extra pieces and choose the best. Flaps come to mind here.  This game is not cheap to play these days........Just figured out an order for enough for a new full contest ship with about a 20% overage for my stock bin- $180.00.   I'm trying to slow down my building activities a little.

Dave

I FEEL YA....IM BUILDING AN IMPACT WITH FOAM SHEETED WINGS AND WITH PIPE AND ROJETT 50 AND WILL BE IN THE $1000 RANGE       

[Motorman]

You could make a vertical cut every 2" and butt joint it back together on a flat surface then sand it flat after the glue cures.

MM

[Ken Culbertson]

Just how big is this warp anyway?   We may be proposing $50 solutions for a $5 problem.

Ken

[Peter in Fairfax, VA]

I have a profile Pathfinder, a Brodak ARF, and it has a hollow, built-up fuse.  Your kit looks different.

[Paul Taylor]

Just how big is this warp anyway?   We may be proposing $50 solutions for a $5 problem.

Ken

It’s was bowed up enough I could slide a carpenters pencil under the middle. Bending inboard.
I got it laying flat this morning. I’ll give it a week and a few days see if it stays put.

And it’s a $40 fix if I have to buy 4 - 1/4inx4x36 to build a new one.

[Paul Taylor]

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[Paul Taylor]

I have a profile Pathfinder, a Brodak ARF, and it has a hollow, built-up fuse.  Your kit looks different.

ARF’s are built using different materials and methods. Sometimes hot glued together.
Kits are all balsa and plywood.

[wwwarbird]

 Don't overthink this, simply refer to Reply #9 here, you could be done with it by now.   

[Perry Rose]

I'm with Ty on this one.

[Ken Culbertson]

It’s was bowed up enough I could slide a carpenters pencil under the middle. Bending inboard.
I got it laying flat this morning. I’ll give it a week and a few days see if it stays put.

And it’s a $40 fix if I have to buy 4 - 1/4inx4x36 to build a new one.

I see your point.  If it can be fixed w/o replacing a bunch of stuff you want to try.  I used to have a heated swim spa.  If I wanted to mold things I would throw the sheets into the hot tub for a couple of hours, sometimes overnight.  Maybe it was the chlorine, but I suspect it was the heat and water that made even the hardest sheets like rubber.  I used it that way to turn 1/4" strips of pine into arches.

Ken

[Steve Helmick]

 Use it for a pattern and do a built up fuse, lighter, stronger...

I'd agree with this to a point. Some guys don't like the looks of the Pathfinder fuselage, and instead build a "StarFinder" fuselage as designed by Norm Whittle. The StarFinder is otherwise a Pathfinder. Brent should be able to post a picture or drawing.

I'm a big fan of using 1/64" plywood on the outer surfaces of a profile fuselage. One of the things you should investigate is what YOUR profile rules are. Ours allow the fuselage to be 3/4" thick. If you put 1/64" ply on both sides of 1/2" balsa, it will be almost as stiff as an oak plank, and a whole lot lighter.    Steve

[Steve Helmick]

Thinking more about this, I would likely cut out the center roughly like the foam area of Brent's drawing, use either foam or diagonals, and 1/64" plywood both sides. While at it, I'd probably add 3/16" balsa layer to bring the thickness up to just under the .750" max allowed by our rules. Check your rules before you add the 3/16" layer!  Steve

[Harold Brewer]

Had exactly the same issue with my Muststunt I.  Inserted a long CF rod into the middle of the fuselage with epoxy and fixed the issue; quick and relatively painless.  That was about eight months ago and still straight.

Brew

[Ken Culbertson]

Had exactly the same issue with my Muststunt I.  Inserted a long CF rod into the middle of the fuselage with epoxy and fixed the issue; quick and relatively painless.  That was about eight months ago and still straight.

Brew

Same here, used two arrow shafts but a larger single in the center should work just as well, maybe better, certainly lighter.  Three years.  Once the balsa relaxes in its new shape it won't warp again.

Ken

[Bruce Guertin]

  The initial warp is there from  the sawing process. When the blade goes through the log to cut the sheet, it either creates or relaxes stressin the wood and can either warp right away or slowly over time. Stick warp the same way. I learned a trick a long time ago of drawing sticks over a sturdy edge while bending it away from the direction it was warped in. Takes a few practice swipes to get the feel for it. I have done thinner sheet this way also I haven't had to do a 1/2" thick fuselage but it worth trying. For something that thick a slightly rounded edge may be best. It may also take a few laps over the edge to get it correct. When satisfied, let it sit over night and see if it stays, and I think it should You are creating stress in the opposite direction.
  Type at you later,
   Dan McEntee

I had a couple Twister 1/2" planks and did this a couple of weeks ago. I drew them over a 3/4" diameter wood dowel. They're still straight. We'll see what they look like after they get some dope on them.

[Brett Buck]

I would be tempted to cut it vertically using jigsaw or bandsaw, leaving it connected at one end (presumably the front), stick a piece of 1/32 with glue on it in the kerf, then block it straight until the glue cures.

    Brett