Hollowing out balsa

On the hollowing out business, the rule of thumb is that if you don't go through in two or three spots, you haven't hollowed out enough. Get the outside shape as close as possible before doing any hollowing inside. Matthew has gone to molded top decks of late, which turn out quite strong and are a uniform 3/32 inch thickness. I would say that is a good target to shoot for. Weigh the block before you start, and then weigh it periodically as you go. (You have a good gram scale, don't you?) That way you can see the progress you make and it gives you incentive to go a little farther as you near the end. Just remember, you can put carbon fiber over the outside if it gets too spongy, and you can, also, add a former inside if you need it. Some leave spaces like formers as they carve, but adding a former is both lighter and stronger (The grain goes the right way.)

Anyway, to do the carving you need a nice long X-Acto blade and some of those nice gouging blades to go along with it. Experiment. See what is best for you.

The most important thing you need to know about any structure is that the middle of the part doesn't matter - it's just dead weight. Bill Werewage once postulated that the best way to get a good strength/weight ratio would be to build the airplane out of solid wood, then get trained termites to eat out everything but the outer 1/16"! In other words, shells are good, solid parts are bad, only slightly stronger but a lot heavier.

The way I hollow blocks is to hog out most of it with a sharpened brass tube. I use a piece of 1/2" K&S tubing. Jut take core sample at shallow angles and chip it out. When most of it is removed, I use a Dremel sanding drum with a coarse drum, to smooth out the tool marks. At the end, I reach in there with a piece of sandpaper and smooth it out more. You can do the entire job witht eh sanding drum, but be prepared for an incredible amount of dust. Chips are easier to clean up, and also harder to breath.

> How
>thin do they need to
>be?

Depends on how strong the wood is, and what it's going to do. For fuselage blocks, I usually stop at around 3/32 on average, but if its heavier I would go thinner.

In the original MAN article, Dee Rice advises to pick out the lightest balsa you can find for the fuse top. One reason is that it isn't hollowed out. I think it was to keep the building "easy" as compared to the Nober for example (-sort of a quote from the MAN article). He also claims that the toal weight should come out around 40-42 oz without hollowing. I also recall a PAMPA article which says not to hollow the fuse (column from the NW rep I think).

It may be that the Brodak wood is a little too heavy (maybe not). I also agree with the statement that the outer layer of the wood is carrying the structural strength. However the thickness of this layer depends a lot on the type of wood. A harder balsa can be thinner than a very light (read weak!) piece. The thickness may also depend a little on those "unscheduled" landings. Covering the fuse with silkspan will go a long way to making the monocoque fuse stiffer.

I would, also, agree totally with Brett's comment about sanding to final contour on the inside. It is not only easier getting to the final shape, but getting the inside somewhat level and smooth to match the outside countour goes a long way in relieving stress areas which adds to strength. The molded pieces are going to be stronger--especially if built of of several layers of 1/32 balsa--but a carved block will be ample strength in and of itself.

I always like Windy's observation of doing an autopsy of the remains after a crash. "If it didn't break, it was built too strong. Take weight out of that area next time." (However, the nose section and the controls still need to be made "bullet proof."

I get a little frustrated at advice like this. Maybe it is ok when you are at the expert level, but for those of us at the beginner/intermediate level, we need a plane which can survive now and then. I don't say build a brick, but don't build a fragile plane either.

And after 2g here, and 2 g there, you save 4 grams. Now stick on your favorite heavy 4 stroke (+2 oz or more!). Go figure!

Sorry for the rant. I think the Oriental builds quite nicely if you follow the plans.

I think it might be noted that a lot of planes in the "early days" were not made as light as they could have been. Blockes were not hollowed out as well. The lightest wood was not always used. I think the first Nobler was something like 52 ounces in weight. The extra weight it carried, plus the lack of power in the engines of those days is the reason we only have one hourglass in the pattern. The planes weren't capable of doing two consecutive hourglass figures. They could barely do one.

One thing I keep preaching to beginners is don't attempt to fly 5 foot bottoms. It is the attempt to fly the pattern as written that causes the most problem for the person who is still learning. Fly ten foot or fifteen foot bottoms until you get the shapes down, and get them down consistently. We don't learn by crashing. We learn by flying.

Once we progress beyond the basics, most crashes are preventable. And then we can start to build our planes to survive a thousand flights, not a thousand crashes, and begin to move up the ladder a whole lot quicker.

The Dremel method is quick ( and useful to folk like me who want to build everything in sight) but here are a few tips:

Wear a dust mask...I aggravated a cold and had to have several days off work

Try to do the dremmeling outdoors and stand upwind of the dust.

Look 'through' the shell and aim for a uniform penetration of light, darker patches indicate thicker balsa.

I take consolation from Leonard's observations that a few 'penetrations' indicate that one is getting near to optimal thickness

I patched my holes with balsa shavings and PVA from using a razor plane to roughtshape the outside of the turtle deck. The patches are nearly invisible.

Brodak balsa is light it's nearly all six pound stock, I wieghed most of the pieces. I save 15 grammes (1/2 oz) by hollowing the turtle deck but am actively considering moulding future turtle decks. The Top Flight Nobler seems to use a version of moulded turtle decks if I recall correctly.

I am a little anxious about the strength of the plane. I am aware of the original's weight and am aiming for the 40oz region for my Fox .35 powered Nobler.

Some attempt at hollowing is better than no hollowing at all. The laws of physics indicate that a lighter object be it a plane, car, boat turns tighter with less effort and accelerates faster than a heavier one so any contribution to weight loss will be helpful to tighter turns and less stressful (to plane and pilot) transitions.

Get a shop vac. Tape the nozzle to your dremel when you do that stuff! Also, wear a mask! A respirator is even better!

When I was a lad of 10 in 1952, I built my first CL airplane, a Howard Ike powered by an OK Cub .049. Everything had to be hollowed out. At that age, my tool kit was Dad's old razor blades and Mom's kitchen knives. I finally got the darned thing built, but it was pure torture.

At age 11, I took a vow that I would never hollow out balsa again, and that there was always some alternative construction technique. Everything else is easier and lighter and stronger. I have kept that vow, and I am sticking with it.

I've been doing a lot of hollowing these days, with the 1/2A Scientific scratch builts, and cowls for stunt ships such as the Nobler. I use a mask, but they can get sweaty in a hurry. One simple expedient, especially when you haven't gotten down to the 400 grit sanding yet, is to hold the object of your hollowing desire quite low relative to your face. Then the dust and scrap stuff doesn't get up your nose.

When you restore planes as I do, it is interesting to see the pains (or abject lack of same) that some people take to hollow parts out. Wafer thin, diaphanous stuff that would make a negligee look like depleted uranium (well, almost...). Or, you can read the lawyerish 2 pica type font disclaimer on the side of a Tylenol bottle through the wood (well, almost...). And, at the other extreme, some of the nose blocks and other parts remain as solid as the were in the kit!

Nice reading (!), but No Thanks. Look at George's Nobler on the front cover of "Stunt News", Then look at "Boogie Woogie" on the back cover. I will probably build a Brodak "Nobler" this year, and I will toss the Brodak turtledeck into the scrap balsa bin and build a "Boogie Woogie" flavored turtledeck. Cover it with silk, then cover the whole fuselage with silkspan, and we are probably lighter and stronger than hollowed out balsa. And prettier, at least to my eyes.

Like a monk, once I have taken a vow, I stick by it. I don't do hollowing out, and you can talk all you want of better tools, but it falls on deaf ears.

I just hollowed out the top fuselage block on my Brodak Oriental tonight, and I found a technique/tool that you may find helpful.

After the first phase of gouging with a piece of 1/2" brass tubing as Brett suggested, I switched to a replacement sleeve for a drum sander that was 1" in diameter, 4" long, and had a #80 grit. I just used it by hand, going back and forth crosswise with a 'rocking' motion, and also lengthwise in long strokes. It worked great, and removed the high spots from the 'grouging' step very nicely.

I bought the drum sanding sleeves at Home Depot. There are various sizes available, but I caution you that they are not cheap. They are expensive, but well worth it, and with our light duty requirements (balsa), they will last a long time.

Cheers,

If you think I am joking, well it was when I was a kid and hadn't bought my first dremel too.