Warp Woes

Best, Hoovie

There is a problem nowadays getting good 1/2" balsa sheet for the classical profile fuselage. (I'm not sure it was ever easy.)

Good luck!

L.

"If you don't want to work, you have to work to earn enough money so that you won't have to work." -Ogden Nash

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(I know you Stunt flyers will be interested)

So this is better than tying the piece of buttered toast onto a cat?

Hoovie,

I think I can save you some time and frustration here.

Warps tend to have a mind of their own. I once spent nearly a week trying to get a warp out of a piece of 1/2" balsa that was for a Twister (hmmm...was that just a coincidence?) fuselage.

I first soaked the offending piece of balsa in hot water, and then weighted it down to a flat surface, and let it dry for a couple of day. When removed from the flat surface, it looked fine...straight as an arrow.

Within a couple of days, the warp had returned !!!

So this time, I soaked the wood, and weighted it with shims that would give an equivilent bow in the OPPOSITE direction. Let dry for a couple of days, and it looked fine. A couple of days more, and you guessed it....right back to where it started.

Save yourself the time and frustration. Use the warped fuselage as a template to make a new part out of selected wood, and throw the warped piece in the scrap barrel, to be used for some other application.

If you don't have a good piece of 1/2" balsa, i.e. no warps and the correct density, consider laminating two 1/4" pieces (this provides some stability as long as you use epoxy and not a glue that requires evaporation, which can get you right back to the warpage problem).

If you are up to a little experimenting, a foam core or built up fuselage, with balsa or ply sheeting, will provide you with a stiff, light weight profile fuselage. I believe that Larry Cunningham has that fuselage construction written up somewhere, perhaps on the PAMPA site.

Cheers,

This will hold the fuselage straight and give you some aditional stiffness in the rear. Cover with carbon fiber or silkspan and it will be fine.

This airplane had this treatment and it is still straight after 3 years...

Thanks for the suggestions.

Best, Hoovie

Dave,

Online HTML versions of some of the SN articles (including the SN Mo'Best construction article) are posted on the PAMPA web site, try the following link:

http://www.zianet.com/pampaeditor/earticles.htm

The latest Mo'Best fuse construction differs slightly: 1) the vertical formers in the AFT fuselage are no longer used, 2) the fuselage perimeter is from 1/4" x 3/8" sticks (3/8" wide), and 3) aft fuselage sides are planked with 1/64" plywood (versus diagonal balsa sheeting).

It is not complicated at all. It is more labor intensive and expensive (1/64" plywood).

L.

"Under certain circumstances, profanity provides a relief denied even to prayer." -Mark Twain

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So tell me, how do you feel about (mostly) black planes? Are they fun in the sun?

To tell the truth, I really like black. It's pretty visible out here in the New Mexico desert..

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Black, like white, has the advantage of being a neutral color (which goes with almost any other color) and naturally high contrast (also good, IMO).

(As you might have figured out, I like YALLOW too! )

L.

"The first human being who hurled an insult instead of a stone was the founder of civilization." -Sigmund Freud

Thanks, it has been a fun airplane. The tank is a standard 4 ounce Sullivan. I made a separate light ply bracket with slots for heigth adjustment.

just looked at the picture again.. At the time it had a Dubro tank not Sullivan..

Once it is out, continue painting both sides of the fuse at once, of course.

Hoovie-

I'm hesitant to post this, having resolved not to post for a while, especially since the one person who has taught me the most since my return has posted a contrary opinion here. But I think that you can use your fuselage - or at least find out whether it is usable - with a minimum of effort. I have successfully handled this problem on my current profile project; it was warped about the same amount (except S-curved, NOT the fault of the manufacturer!) in the wrong direction and now stands as a finished fuselage that local club members think is pretty nice and still rigid enough.

I keep a 4' x 2' piece of straight, flat 3/4" medium-grade plywood as a sanding board. To it are glued a couple pieces of medium grade (about 180 - 200 grit) garnet paper. I use this especially to sand larger pieces (like your fuselage) that need to remain planar. The plan is to sand the left fuselage side flat by sliding the part along this abrasive surface.

This will likely result in an aft thickness of at least 5/16", with a good likelihood that the thickness near the leading edge of the hirizontal stabilizer will be about an adequate 3/8". As you approach removal of your 5/32" - 3/16" in that area internal stresses may have eased, reducing the total warp somewhat. Since your fuselage is warped along its entire length, if you choose to sand straight from nose to tail, your tail will come out thicker. You will then need to decide whether to offset the engine thrust line back at least to neutral, or choose to sand your maximum nose thickness to a value that allows the right fuselage surface to parallel the left. You could choose Larry's standard 3/8" thickness, allowing use of standard 3/8" x 1/2" maple mounting stock.

If all works out, you will have room to align your vertical stab with the right side (not necessarily at that edge), with room for outside bias or inside camber, if you are of a mind to use this. Rudder of course can be offset regardless - again assuming you wish to use offset or cambering. The plus here is that you can use the flat (planar) left side as a reference frame for squaring your wing and stab with the fuselage.

I would first choose which part of the fuselage you want to be at maximum (1/2") thickness - perhaps the wing root quarter-chord point. Then lay the fuselage on its right side, nose and tail shimmed up at about equal heights - or biased to your preference - so that it doesn't rock. Then using a piece of 1/2" thick stock lying flat on your building surface or sanding board as a guide, draw lines from nose to tail along both sides (will be fuselage top and bottom, when installed in plane) with a fine felt-tip pen. Connect these lines along the nose and tail surfaces so that the entire fuselage blank is ringed by this line 1/2" above the table. I find these lines necessary references after sanding has begun, since all kinds of asymmetry and curvature can temporarily creep in.

Now, flip the fuselage over so that, for you, it is bowed up in the center. Place in on the flat sanding surface and push it back and forth, varying the points of maximum downward pressure to control wood removal, until you have sanded the buttom (left side of fuselage) flat along the line you drew. This process, even without using a razor plane - will take less than 15 minutes. When finished, you can twist the fuselage to see whether it is as rigid as you want.

If the thing comes out too flexible, you still have the option of cutting out a section for interior triangulated truss work. Then you can laminate 1/64" plywood and get a structure much stiffer than soild balsa. In that case though, I don't know from personal experience what risks you take in wrapping the 1/64" ply along the arced right side. Calculations I did for an original design of smallish size for an LA .25 indicated that sheeting both sides with 1/64" ply and using 3/32" ply triplers for the nose was lighter than solid balsa with 1/8" ply doublers. This of course is minus the weight of the laminating adhesive.

Well, again, a post FWIW. If you are like me though and don't like to give up or throw away a useable piece, this has worked for me....and before you local guys chime in about all the extra time I spend on things, please take my word for it: this sanding block operation is not that time consuming! It's the LEAST of the time I've spent fixing things. Now if I could just stop messing things up, I'd have less to fix!

SK

I never throw away a piece of thick sheet balsa either. I have a full Magnum kit box full, where bad fuselage sheets, etc. get tossed, and eventually end up being wheel pants or wing tips, etc. There's some serious embarrassments in there!

But you have to ask yourself how much your own labor is worth. You can put in a lot of effort salvaging a warped fuse sheet, or just go get another one, and toss the warped one into the scrap bin..

WTWTCH? What's the WORSE that could happen? You have to go get another piece and toss the warped one! Repeat as necessary.

Balsa is not THAT precious.

JMNSHO.

L.

"I was with some Vietnamese recently, and some of them were smoking two cigarettes at the same time. That's the kind of customers we need!" -Jesse Helms, US Senator from North Carolina, at a dinner to promote the state's tobacco industry, 1996.