[Courtesy of Scobie Puchtler scobie "at" seanet.com, November 2000]
You simply do NOT need an electric pump of any kind in order to get professional vacuum bagging results. You can vac a wing using nothing more than a $40 hand pump (sold in auto parts stores for testing vac systems in modern automobiles, either molded nylon or cast aluminum, and looks like a little pistol grip kinda thing (the pumping motion is like those grip-strengthener excersize things)). As long as you are going to do one glider at a time and aren't trying for production output, this will be radically simpler, WAY cheaper, MUCH easier to set up, and just as effective as a more expensive rig. Oh, and it's totally peaceful and quiet, with no motor noise. Once you have the hand pump, bag material, sealant strip, and something to act as breather.... that's it, you're ready to bag. (of course you'll need stuff to put IN the bag, like wing cores and glass and epoxy and mylar, but that's the other half of the story)
The hand pump has a nice little vac gauge built right into it and is capable of sucking MORE than 20 inches hg. It is important to get good fresh bag material and seal it up well, with just one outlet tube for attaching the pump (you can use the tubing that comes with the pump, or some actual vac tube from an automotive supply. AGAIN, Do NOT get fooled into thinking you need ANY fancy hose-to-bag fittings, valves etc. These are expensive, hard to find, and just another place for vacuum leaks to occur. The sticky bag sealant strip works perfectly well as a seal around your one vacuum tube (you just put the tube somewhere convenient along the sealed seam and seal it in with some extra sealant strip), and a few strips of paper towel near the end of the tube inside the bag assure that the tube won't sqeeze itself shut)
Once your bag and tube are sealed, suck out any large airmass remaining in the bag with your shop vac or whatever (you can do this by just holding the vac tube up to the hose of your shop vac and forming an incredibly crude seal with your hands) then, when the bulk of air is removed, plug the hand pump onto your tube, and in a minute of pumping or so, you'll be achieving whatever vac you want. You simply leave the handpump attached during the whole bagtime. If the bag is well sealed, the vac will only drop an inch or two of vac per hour, so a few visits to your bag during the process to pump a few times each visit will keep the entire thing sucked down just fine. I've left my setup while sleeping and come back after six hours to find I'd gone from 22in to 16in... not really a problem, and probably more vac than was needed for the job even at 16in. (from the research I did, it seems that most folks use high vac numbers because they've invested in systems that can produce that amount of vac, not because the job actually requires it, but even so, you can go as high as you want with the hand pump) Even some of the more sophisticated electric pump systems still cycle through a few inches as they turn on and off, so I'd be impressed if someone could prove that their $300-plus system actually created a better wing. Vac is vac, after all.
If you truly can't afford to visit your bag job every 2-4 hours and give the pump a couple sqeezes, then I guess you'll need a self regulating powered system, but it'll cost ya, either time or money, probably both.
I don't know why this cheapo method isn't more publicized. It may be that it's just too tempting to invest in a complex system and then come to believe that it is the only way to achieve the desired results. I've seen this problem over and over in my manufacturing experience.
To be fair, it would be easy to set up a hand pump system that was disappointing. You DO have to build a good bag with no leaks, but the funny thing is, the simpler your bag system is, the less likely it is to leak! I do use the cast Nylon bagging material that is 'officially' the best stuff, though more pedestrian plastic sheeting might work just as well, I just haven't tried it yet. Additionally, I think that you cannot get away with using the clip style bag sealers and such, as they are just not hermetic enough for anything but a continuous pump. You have to use the sticky adhesive strip sealant for all the open edges and for sealing in a single tube to hook the pump to, but it's just not that hard to get a no leak bag, and a great habit to acquire early in the game.
Most of my bags are a single piece of the cast nylon, folded in half and sealed on the three open sides. I usually put down all the sealant strip on one side of the plastic, then peel off enough of the protective paper tape to seal about one and a half edges prior to putting in the wet wing, so that I still have a generous opening to insert the work. The tube is sealed into one corner of the bag right at the fold in the nylon bag material. Once the wing is in the bag, I seal the remainder, massage all the sealed edges one final time to bed the sealant well, and then start the actual vac process. The cast Nylon is nice and clear, so it's easy to judge the quality of the seal because you can see the contact with the sealant so easily. This would be much harder with opaque, or even partially opaque bagging material.
A nice advantage of the hand pump at this stage is the ability to work wrinkles out of the parts of the bag that actually cover the wing bit by bit as you gradually pump down to your final vac setting. Once you have a good bag made and closed, the only other source of potential leak is the valve in the pump itself. I suppose some hand pumps may have faulty valves, but my cast aluminum hand pump seals awfully well, as evidenced by my remarkably slow leak rates. When it's time to remove the wing, I usually end up just cutting one edge off the bag. A new sealant strip can be added to that edge the next time you use the bag.
There is still alot to learn about the composites, mylars, waxing, layups, epoxy, heat curing, etc, but the less time you spend overbuilding your vac system, the more you'll have to dedicate to learning really good wing-making skills.
There is another local builder here in Seattle producing beautiful wings with hand vac. People love his planes, and he is selling them in small quantities from time to time. I don't think anyone knows that he bags this way. I'm sure they all assume he has some hyper-sophisticated system. He's the one who got me into bagging this way, and I've never regretted it for a second. Probably never would have bagged wings at all if I'd had to get a fully electric system up and running.<<<<
Lift, Scobie in Seattle