Flap Building Surface

szicree · #1 08-28-2006, 07:39 PM

Back when I built my flaps and ailerons I remember that getting them twist-free was something of a challenge. As I recall, part of the trouble was finding a table that was perfectly free of warps and twists. It turns out now, that for reasons unrelated to this, I am having to build a new set of flaps and am not looking forward to this problem again. What I really want is not so much a straight table, but a straight board of some type that could easily be clamped to the flap. I'm picturing somethink the same size as the flap, like a bookshelf, but very rigid and straight. Anybody got ideas on what could be used? C'mon all you engineering types, what've ya got??

alpinelakespilot2000 · #2 08-28-2006, 08:46 PM

Go to Home Depot or Lowes, get a 4x8 sheet of 1" MDF and split it lengthwise down the center. (They will cut it for you for free with their panel saw... it will make it MUCH easier to get home since it is HEAVY stuff). Also buy a couple pieces of 1"x2"x8ft poplar or some equivalent hardwood that is straight. Cut those into 2 foot lengths and lay them evenly spaced across your MDF pieces. Essentially you're making a sandwich with the poplar laid on edge in between the 2- 2x8 sheets of MDF. Once you glue this together (and throw in a few countersunk screws for good measure), you will have an absolutely flat surface that will require little, if any, shimming. Mine is sitting on 2 saw horses. I can put the sawhorses on both edges and there will be almost no sag in the middle of that surface. If I haven't described this very well, let me know and I'll take a picture for you.
Good luck.

mlw450802 · #3 08-28-2006, 08:48 PM

Quote:

Originally Posted by szicree

Back when I built my flaps and ailerons I remember that getting them twist-free was something of a challenge. As I recall, part of the trouble was finding a table that was perfectly free of warps and twists. It turns out now, that for reasons unrelated to this, I am having to build a new set of flaps and am not looking forward to this problem again. What I really want is not so much a straight table, but a straight board of some type that could easily be clamped to the flap. I'm picturing somethink the same size as the flap, like a bookshelf, but very rigid and straight. Anybody got ideas on what could be used? C'mon all you engineering types, what've ya got??

A hollow core door is quite flat and rigid. You can expand on that theme by ripping a 4x8 sheet of mdf lengthwise and gluing the pieces together, separated by steel 2x4 or 2x6 pieces.
If you take extreme care to make the pieces flat before glueing, this table is unbelievably rigid and strong and will not warp.

The principle here is to create the largest moment of inertia of the structure to make it very stiff in bending. In order to do that, the 2x4 or 2x6 must be placed lengthwise to create a shear web. A few shorter pieces can be placed crosswise to keep the table flat that direction. The stiffness will be proportional approximately to the third power of the separating distance.

-mike

alpinelakespilot2000 · #4 08-28-2006, 09:46 PM

Here's the pic. of mine. I forgot to mention, I recommended poplar or a hardwood because it is usually easy to find a piece that is warp free, unlike almost any 2x4 I've ever bought. I think Van's recommended the method I described but recommended 2x4's. My thinking was that there's no way to get a flat surface if your component pieces are not true. Good luck.

szicree · #5 08-28-2006, 10:07 PM

I really appreciate all the suggestions, but I still feel like making the glued-up deal completely twist free could be a pain. Maybe I'm overthinking it.

alpinelakespilot2000 · #6 08-28-2006, 10:22 PM

Steve-

It sounds complicated, but it's really a piece of cake--took me about 1.5 hours to build. IF there is any twist in it you can easily take it out with a shim on one of the corners and then clamp the surface down if it wants to pop up elsewhere. But I haven't found that to be necessary. Mine doesn't have any twist, even when just sitting unshimmed and unclamped on sawhorses. Just yesterday, before very belatedly prosealing my flap trailing edges, I put my digital level on each end of the flat surface and had 0.0 degrees of twist.

It does help to first have a flat surface to build it on, but even that's not necessary if you're willing to use a shim. (Of course, if you had a flat surface there'd be no reason to build one!

)

I'm sure this isn't the only way, or even the best way, but mine has lasted through all 7 control surfaces with the riveted trailing edges. All of mine are pretty much arrow straight with no warp or waviness.

mikehoover · #7 08-28-2006, 11:52 PM

Vans manual mentions building a 2' x 8' "flat box". It is easy to build out of 2x4's and some 1" MDF board or plywood. Build the frame first on the 4x8 sheet of MDF or particle board while it is lying flat, then cut and add the 2'x8' sheets to the frame with countersunk sheetrock screws or whatever. The real trick I think with the flaps is with weight you need to use when building them. I used 3 bags of 25# lead shot (total 75 lbs.) from the local sporting goods store and broke them up into some six and 12 pounds bags I made of muslin cloth. A little extra work but it is nice to have them to spread along the entire length of the flap while working with it.

Steve · #8 08-29-2006, 09:20 AM

I built an 8 foot long frame with straight(?) 2x4s. I sized the frame to fit the precut MDF shelves from Ho Depot. Screw the box together. Here's the critical part: buy a package of shims while at HD. Put the shims as required under the top surface of the box to make it dead flat. I bought a large bag of potting soil and a package of Hefty freezer bags to make my hold-down weights.
Steve

captainron · #9 08-29-2006, 09:48 AM

Pool table?

rv9aviator · #10 08-29-2006, 09:59 AM

I used 3/4 MDF and made the top 24 X 96. I ripped 6 inch peices to make the sides of the table out of the same MDF. I glued and srewed the thing together and it is dead flat. I used a forstner bit and drilled 3/4 holes in the top for the clecos to pass through and my flaps are as straight as can be. I think there are some pictures of the table in the wing section on my " www.jimsairplanes.com " site. in the wing section. Just cklick the link below. Hope this helps some. The same table has been great for everything else too.