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Picture of How To Repair
Depron foam profile aircraft have recently become my new obsession.  They're easy to build, parts and pieces are relatively cheap to buy and they're a blast to fly.  It all started when I stumbled upon one man's designs (http://rc.tomhe.net) and I've been on a roll ever since.  I have built and flown several larger (full size and full+) of these planes and they rip through the sky.   They really do look great rolling across the sky and never fail to attract a crowd.

As great as the large ones are to fly I've been on the lookout for one I could throw around my backyard.  Reading through the threads of my favorite build (http://www.rcgroups.com/forums/showthread.php?t=723890) I came across the Mini-F22 and away I went.  Another great build that went together quickly, flew great and gave me tons of fun.  Until.......

All foamies have a single Achilles heal.  They are, well....  they are.....built with foam.  I was flying my Mini-F22 around a local ball field when I came out of a diving turn at full speed and didn't pull up (or couldn't - I'm still claiming that I couldn't but it's a loosing battle) and BAHM!  Nose first into the turf.  If this was a complicated kit build I'd be in tears.  However, being a foamie, I simply picked up the pieces and headed back home.

As you can see in the picture, the nose took the most damage (total loss) and there were lesser amounts of damage in other areas.  From what I've seen most damage on these foamies is from the nose backwards.  Again, not as bad as it may look.  Foamies are resilient.  Heck, I've even seen them fly WITHOUT a nose.  But I digress, they can withstand a lot and even if they do break they're really easy to fix, which is why we're hear after all...
 
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Step 1: Seperate The Good From The Bad

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Let's get started and here's what we're going to need.

     *  Marker
     *  Ruler
     *  X-Acto blade or similar

Hint:  Make sure the blade is SHARP.  Scratch that, make sure the blade is NEW!  Nothing is worse than cutting foam with a dull blade.  Dull blades squash and tear the foam.  DO NOT USE DULL BLADES!  Blades dull quicker than you'd think.  At the first sign of pulling the foam - swap out the blade.  Also, keep the knife at a low angle when cutting.  Hold it at 90 degrees when cutting and even a slightly dull blade will pull and rip the foam.  Hold it at a very low angle and the same blade will slice right through the foam.

I'm going to start off by assuming that there is more of the plane to save than there is damaged.  If the spar is damaged or most of the wing area is ripped or crumpled I'll just gut the electronics and throw them into a new build.  If you want to save your plane you have to be sure there's actually enough to save.

First I'll use my ruler and marker to scribe cut lines between the good parts of the plane and the bad parts.  A good  word of wisdom is to always try and mark at an angle to give you more surface area to glue.   More surface area + more glue = stronger bond.

What I did with this plane is mark lines across the top wing, the main spine, both sides and the bottom deck.  I usually don't worry about minor damage, after all it is a foamie.  I just try and cut away what's ripped, torn and crumpled beyond hope. 

Step 2: Trace Twice, Cut Once!

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Now that you've cut away all the damaged pieces you need to separate them.  Try and be careful not to destroy the slots as they'll come in handy to match up with the stencils later.  Use a sharp knife to gently pry apart the glued pieces.   I usually mark them and put them aside for later use. 

Don't worry if you didn't bring everything back from the field because the only parts you're actually going to need are the pieces you just cut off.  The rest will take care of themselves.

Here's where being a pack rat comes in handy.  Pull out the old original stencils of your build, you're going to need them again (and again and again).  If you're a "use it and toss it" kind of person you're not out of luck.  Just reprint and you'll be good to go.  It just takes a bit longer because you have to cut out the paper stencils again.

I'll take the pieces of foam that I just cut off my plane and place them onto their corresponding templates which tells me how much of the template I'll need to trace onto new foam. 

After I trace the template onto the foam I'll overlay the cut piece, match the position and then mark the cut lines so the new piece will perfectly match its mated piece when assembled.

Do this for all pieces and when you're done you'll have everything you'll need to make a full plane again, even if you where missing the majority of your nose.

Step 3: (Re)Assembly - a.k.a "This seems familiar..."

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This part should feel like old hat.

Dry fit everything together.  I must be terrible at holding a knife because no matter how hard I try my slot cut-outs never come out perfect. 

Sand what you need, make sure everything fits snug without distorting the foam and never force anything.  Once foam gets bent, just once, it looses a lot of it's integrity.  If you traced everything right and are better with a knife than me everything should fit hand-in-glove.

Please don't skip the dry fit.  Once you start with the glue and find that something doesn't fit right, you're toast. 

During the dry fit I've even come across pieces I must have traced incorrectly.  Easier to trace again and make perfect than to try and spit-patch it.

Step 4: Pins And Needles - Oh, And Glue Too!

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If everything in previous steps went well, so will this one.  Glue the pieces together as you would the normal build.  I usually end up using more pins than the original build because of the way certain pieces bend.  Where these bending pieces are usually held in place by all their slots and tabs the cut pieces are usually missing the benefit of those so they'll need a helping hand to keep everything flush.

Besides pins I find that using clothes pins helps keep pieces (wing) aligned during glue.  One thing to remember when using clamps of any kind with foam is that FOAM DENTS REAL EASILY and 3mm foam dents easier than others.  I ALWAYS use scrap pieces of foam when using clamps so the scrap pieces take the dents.  I also use some wax paper underneath the scraps to keep any excess glue from sticking them to the plane.

When done, everything should look perfect.  All the pieces match, alignment should good and everything should look like new.

Step 5: I'm Stuck On Velcro

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Almost done!

When I removed the old pieces of foam I also peeled off the Velcro.  Velcro may not be too terribly expensive but with a bit of glue it goes back on easy and works good as new.  Waste not, want not and all that jazz..  I've transferred Velcro from plane to plane and have never had it peel off or fail.

Soooo, a couple of dabs of glue, on goes the Velcro and I'm one step closer to flying.

Quick note, you might notice a couple of white, oblong patches on the plane.  On some of my splices I use patches cut from standard printer paper dabbed with glue. 
  -  for wing leading-edge damage I'll cut a patch, dab with glue and wrap both upper and lower edge.
  -  for splice work I'll glue a patch onto seems that might see a lot of stress either flying or ... ahem... landing.
Probably overkill, especially for these smaller and lighter planes but they're relatively light and they put my mind at ease..

Step 6: Good As New!!

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No, really!  It flies just like new.  The splices are tight, everything is solid and the C/G is pretty much where it was before.  The one thing I didn't do for this repair is weigh it before and after.  Since I didn't have the weight after it was build and I was missing pieces after the crash weighing it after the repairs would prove nothing.

It's really that easy!  If you can build a foamie you can fix a foamie and as long as the plane isn't totaled it's usually easier and faster to fix it than to build one from scratch.

Although mine isn't painted (yet) this would be the point where I'd pull out my airbrush and match up the paint scheme but that's another story..

Remember, if you can't open it you don't own it and if you can't fix it it's not broken - good luck!

Step 7: The "M&M" Treatment!

The biggest problem with these profile foamies is that they are EASY TO DAMAGE!  No, really!  After only a few flights my 3mm F-22 looked like it has been through the war.  Scratches, gouges, dents, bends, you name it.  Heck, hold it too tight and you leave finger dents. 

On my bigger 6mm foamies it isn't too bad as that Depron is a bit stiffer and stronger.  It still dents and scratches but it takes landings a LOT better.  I've read before that using packing tape on the leading edges keeps them from nicking and I've used it a time or two before with some success but I think it is too heavy for the smaller and much lighter 3mm minis.

The part that takes the most damage is the bottom fuselage (vertical piece) and I have taken to coating the bottom of that foam with some epoxy.  Light (when put on very thin), hard and puts up with a lot of damage.  It worked so well that I started to put it on the leading edges as well as some other parts of my foamies to reduce damage. Again, if put on sparingly it's not too heavy.  Also, packing tape has a tenancy to pull away from foam but the epoxy is on to stay.

This has lead me to try something new.  I used some slow cure epoxy with some iso alcohol to thin it out.  With a disposable brush it goes on like a lacquer with a very thin coat.  At first I just put it on the bottom of my plane to help it survive the landings and then I put it on the rest of the plane to keep the rest from getting damaged.  What I ended up with is what I call an "M&M" finish.  Hard coating on the outside, soft on the inside.

So far it seems to be working rather well.  It didn't add too much weight (flies the same), keeps the scratches at bay (oh, it still won't prevent the hard landings from doing damage...) and it looks kinda cool...  Hit it with a coat of paint (more weight) and it looks like a monocoat or doped finish.  Save some weight and add some color or tint to your epoxy.

The two keys to this step are slow cure epoxy and a thinner of your choice but it does have to be thin or it won't paint on right.

WARNING!!!  DO NOT USE ACETONE AS A THINNER!  Remember when I said, "and a thinner of your choice"?  Just look at the line above, I'm not kidding, I just said that.  I lied.  You can use any thinner of your choice as long as it doesn't eat your foam for breakfast.  Acetone, well, acetone doesn't get along with foam and a it turns out may people choose acetone when thinning epoxy.  Go figure.  Please just test whatever thinner you want to use on a scrape piece of foam first.

One last word of wisdom regarding epoxy.  MIX IT!  I've seen friends mix their dual-part epoxy with two or three strokes and then slap it on.  Nooooooooooooooo!  It really does need to be mixed well, very well.  75 to 100 strokes is good and you can see an absolute difference it its appearance when mixed well.  Well mixed epoxy also cures harder and stronger, last longer and doesn't have that 'tacky" feeling that under-mixed epoxy can have.
dchall82 years ago
WOW!

Thank you for bringing your new obsession to the next level - writing about it.  This is one of those Instructables you don't see coming.  You can't just skim over this one.  You have to read every word to find all the Easter eggs packed in.  Thanks again!!