Bagging wings - A resource thread
http://www.rcuniverse.com/forum/images/icons/icon6.png Bagging wings - A resource thread
I am not a builder but I have a lot of respect for those who like to build.
Good idea ED! WattFlyer could use more tech info on this subject. It goes beyond just building super hi-tech wings for uber gliders too. These techniques can come in handy in building almost any model.
Im currently working on an e-Supra.
I made my own pod using the lost foam technique then attached it to a pre-made boom. It came in pretty light too about 105 gms. It still needs the pylon made and mounted so will likely end up closer to 150 gms when complete.
I have completed the horizontal stab and the fin/rudder. They came in at 34 and 37 gms.
Both wing tip panels are out of the bags but still need some trimming and sanding. They should end up around 130 gms - 140 gms or so.
Im using a refrigerator compressor salvaged from a dead refer. Cost = zero and it pulls a good 25" of vac. I use cheap plastic drop cloths and cheap caulk for the bagging.
You can build vacuum bagged wings surprisingly inexpensively if your careful :)
Its certainly a daunting task if you've never tried it before, but Phills and Curtis' videos help it all makes sense.
Ive made about every mistake possible on this build and it's till turning out halfway decent, so if I can do it, anyone can :)
That is great Larry. Do you have a build thread going somewhere? Please post a link.
Yes, this can be for building any kind of aircraft, not just gliders.
How well does that work for applying wood skin to a foam core?
I'm planning to mix up a few techniques in one build.
Bagging wings with wood skins is a common method. For example, www.arthobby.com sells a full line of wood skinned bagged wing gliders.
It works much better than the alternatives I have tried - bags of lead shot or rice or hoping tape will make a good bond everywhere.
For bonding balsa to foam you probably dont need the full vacuum pressure Im using, but the more you have the better it will work.
You need the shucks the cores were cut from as a bed to hold the cores straight and maintain the proper washout, etc while the glue dries.
What I do is leave the shucks outside the vacuum bag. Put the first shuck down on the bench, then lay a piece of cheap plastic drop cloth over it thats large enough to fold over the core. Leave enough room around the edges to run a bead of caulk.
Next lay down some paper towels on top of the plastic drop cloth to act like a breather. Thats so the air can get evenly drawn down on all parts of the core.
You need a layer of breather the full length of the core on top and bottom. If you dont want your breather getting glued to the core when excess glue gets sucked out, you need to run a layer of wax paper or more dropcloth plastic between the breather and the core. Make the breather larger than the wax paper.
So you have this set up:
plastic drop cloth "bag"
wax paper or more plastic
drop cloth plastic "bag"
Then I place bricks or some other weights all along the top shuck to weight everything down so it cant move when the vacuum starts to suck things down.
The above layers would be if you were using balsa and some wood glue to attach it to the foam.
In my case, where Im using epoxy resin and fiberglass/carbon fiber, I use plastic mylars .014" thick instead of the wax paper inside the bag.
The mylars are cut to the exact shape of the parts. You can pre-paint the mylars and that paint gets transferred to the parts when the epoxy cures - if you have prepped the mylars correctly! :)
I sometimes also use something called peel-ply between the mylar and breather. The peel-ply is just 100% polyster or nylon dress liner fabric. It helps 'peel' any excess epoxy from the outside of the mylars.
Once the core and bag is all set, I stick the vacuum hose in the bed of caulk so that it sticks into the bag far enough to lap onto the breather material. If you dont do that, the plastic drop cloth will just seal the end of the tube.
below are a couple of pics of some parts bagging.
In the fist one the vacuum hose is off to the left.
In this one you can see the vacuum hose. Notice lots of weights to keep things stable and flat.
You need lots of weight to keep things stable and flat when you pull the vacuum. 17 psi can add up to a huge amount of force trying to bend your parts where you dont want them to go :)
By the way, its important to keep the plastic sheets, breather, wax paper, etc all nice and smooth and with a uniform thickness everywhere on the core. If you end up with folds or creases in the bag or breather or un-even layers, those creases can be imprinted into the final part.
I also only use Viva paper towels as they have no texture to them. At hi vacuum levels, some paper towels have enough texture to show up on the final part! Little impressions of flowers or butterflies dont go well with most glider wings :D
My plan currently is to:
Cut retract wells, servo wire runs and servo bays
Inset the wood for the hinge lines, and sand to match the cores
Inset retract mounts and other "hard points" in the foam cores
Join the shucks for the wing and lay on bagging plastic set on a solid core door blank. (8 ft X 4 ft... its a big wing and will overhang a couple inches each end!)
Join the wing cores and affix CF TOW on high stress lines (I want continuous CF TOW from tip to tip)
Add the LE and TE wood stock, shaping the wood so the skins can overlap them
Join the wing skin panels ( this could be "fun" ) I plan to double-stick tape the skins to the shucks so they can't shift.
Packing tape applied to shucks where epoxy might try to glue the wing to the shucks.
Lay in the wing skin (1/64 ply upper, 1/32 ply lower) wing core and slow cure epoxy and lay the opposing shucks on top... cover with bagging plastic and weights... draw the vacuum.
I think I'll need to skin top then when that cures do the bottom.
I need some REALLY slow cure epoxy... Its a big wing and I want it to act as one piece.
Is this a giant flying wing? Cool project!
I think you will find you get better - more uniform - compression on the cores if you leave the shucks outside the bag.
If you try to rely on the shucks putting pressure evenly on the core, it doesnt work so well. Where ever you have a difference in thickness on the shuck or at corners edges, etc, the pressure varies as the foam compresses unevenly. This is mostly a problem around the leading edge where you have more curvature.
If the vacuum bag is directly against the core, you get the full vacuum pressure on every sq inch. Leave the shucks on the outside and use them to hold things straight and weigh them down a bunch while its setting up.
My favorite epoxy is MGS. Using the slow hardener you will have maybe 5-6 hours of pot life for a 1mm thickness of epoxy. Mix up smaller batches at a time and you should be fine. You're still going to have to work fast though!
The down side to MGS is its cost.
One question - why the thicker skin layer on the bottom? I would have thought you would treat the skins like a spar cap and want the thicker section on the top?
One other thing I forgot to mention. I both of those pics above, I used .7mm drop cloth plastic. Thats too thin and it tears too easily. You may be able to see that I used multiple layers of the plastic to form the "bag". I much prefer to use heavier gauge plastic around 1.5mm -2.0 mm.
On the other hand, if you're careful, the .7mm is good if you have some part with large compound curves. It will stretch and form better to the core shape without wrinkles.
One more thought I had. For a project your size, paper towels wont work very well for a breather. Id try using some of that dress liner fabric or something similar as long as you canget it in a roll wide enough for your cord distance.
The 100% polyester will act as a breather and peel-ply in one go.
Wing is for a 107 in span Me-262. Kit specified built-up structure and a pair of .91 glow ducted fans. I'm sticking in a pair of 5000 watt EDFs and didn't trust the wing structure so I had cores cut.
Upper surface is thinner for the curvature of the top surface, lower is thicker for more strength. The more curved upper surface will gain some stiffness by the "coke can effect" but I'll have the CF TOW backing up the wing skins for the loads expected.
CF TOW strips and light fiberglass will be on the inside (foam side) of the skins at all skin joints. Trying for minimal work to get a smooth finish.
Final expected RTF weight between 30 and 35 lbs. (kit specifies 30 to 32 lbs RTF with glow fans)
Been delaying the project because I needed to learn to handle something as fast as I expect this to go...
EDF model in the avatar has one of the fans that will go in the Me-262. Its overweight and overpowered on purpose.
Might call it an "Extreme" E-pylon racer... expecting 200+ mph.
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