I've been building for awhile, and never had a problem with weight for my finished airplanes, but as I start to get more interested in 3D, I'm wondering how to truly "try" to build an airplane light. I don't use excessive epoxy, or glue. I think CA all non-gapped balsa etc...but what do you guys do, or what steps do you take to try to build an airplane "light"?
Thanks!
Reid

From what I've built in the past (as well as what I've read), the density of balsa is usually responsible for making a plane light or heavy. I'm sure alot of us have replaced some wood parts that came with a kit due to poor quality either in the form of warpage, too light or too heavy. I know I have! Often times, prior to gluing parts together some of the light ply formers and fuselage doublers can be lightened by way of removing some wood in low stress areas. When building, about halfway through I step back and take a general over-all view of the airframe and try to look for areas where I can save some weight. For example, on a recently built Pica Jungmeister, before covering I got out my 3/4" hole saw and cut lightening holes in the aft portion of the fuselage crutch and also between the riblets on the tail surfaces. Interesting to pickup all the saved balsa donuts (from the lightening process) and see how much weight was actually removed. Prior planning and well thought-out installation of the radio gear is essential by way of having to add little or no nose/tail weight to attain proper CG balance. You might try using Elmer's Pro Bond for areas that may require epoxy but isn't overly necessary like doublers and fillets. Just a little Pro Bond expands (foams) as it cures (overnight) which surrounds and penetrates the joint and then hardens to a sandable consistancy.

You asked for some alternate options - those are some that I use! I'm sure there's others.

Ted

I build for electric and need to keep things as light as I can. I build 40-60 sized electrics from kits and from plans. Light servos and RX help the most. I also use lightning holes and substitute balsa for ply aft of the wing. I build up the tail feathers instead of glueing those huge planks way out on the end of the plane. I build wingtips with formed laminated basswood and with electrics we never balance by adding weight we shift the power pack to get the proper balance. In fact I put RX and servos aft of trailing edge usually. I don't use a rudder much so I omit that servo too.

I think the parts in most kits are designed from first, an ease of manufacturing and 2 an ease of assembly point of view. Weight saving is probably third. Building light means selecting the right wood for the job. You can't use all 4-6 balsa unless you design for it, nor can you use all 12 lb and get a light kit. Each has it's place. Where appropriate, I like to substitute built up for sheets, like the tail and wing tips, etc. Also, if the plane may be heavy in one direction, plan ahead to locate servos and battery to minimize the need for extra weight. If you are fanatic, such things as subbing 1/16 balsa 2-ply crossgrain sheet for 1/8 light ply, or using balsa sheet laminated with a little fiberglass between to save a few grams. I like to use multiple1/16 strips soaked in water and then wrapped around the outline of a wing tip, rudder, or elevator to substitute for some built up or plank designs. It can save weight in a place where it will do much good in the balancing of the model, too. Choice of finishing, and practices in finishing is important, too. When you make a joint, don't fill an existing gap with ca. Make the joint so no gap is present and use the lightest touch of glue needed to bond. Lots of ca or epoxy gaps are heavy.
Be careful what you change. You can make something light enough to fail.
I think the biggest mistake people make in kit mods is when they 'spruce up' some piece. You can make something more rigid so the stress is moved to another part not designed to handle it. Don't expect to make great strides all at once. The very best way to learn about lightening came from talking to the 'old farts' who have been in the hobby for over 25 years. Back when, the power to weight ratios of model engines was much lower than today, so they had to scrimp on weight. You can see that in some old plans from the mags. A plane designed in the 60's for a .60 sized engine may be almost overpowered with a .46 today. And those guys are a vanishing resource. Also, read the mags about electric, indoor, rubber and cmpetition flying. Especially competition. Any competition. Those guys spend time and effort as well as $ to get light and strong, because it gives them the edge. A pilot who owns a world record in pylon is in my club. In a brute power sport, he sweats the weight because at the speeds and g's his models endure every extra ounce becomes almost two pounds of force the airframe must absorb.
It all comes with experience, and attection to detail.
Good luck,
Mike

Most important: I've got sandpaper and I'm not affraid to use it.

When you're looking to save weight, remember that every ounce removed from the tail will allow the nose to be a few ounces lighter.

I hardly ever build a balsa sheet tail surface. I usually build a lightweight frame with most of the same thichness and sheet it (saves about half the weight) or build two thin frames and attach those to either side of a thin center sheet which then gets lightening holes (saves 2/3 or sometimes more.)

Pull-pull systems make control more precise AND remove most of the weight of pushrods. And , again, a lot of the pushrod weight is in the tail, so you get a bonus multiplier for all of that part.

The best covering available (in terms of strength and arguably in terms of appearance, thogh definitely not in terms of ease of use) is also the lightest by far... that would be micafilm. Very easy to save a couple of ounces in the tail of a 3-D plane with big tailfeathers.

Wanna get extreme? What are your wing ribs made of? There's an awful lot of strength in a T-section that supports the covering over a well designed center spar (nice warren truss for instance with some extra webbing where needed)... To take a much simpler approach, cutting holes in wing ribs doesn't hurt, but if you add up how much weight it can do away with, it's not the most helpful thing either.

A lot to be said about material...

Yes balsa density is import but, to get really light, you have to get creative. For example I cut ribs from foam. Simply cut a core and slice them off. You have to go a little thicker (how thick depends on density of foam used) than the original balsa rib but, still a nice weight saving.

Less glue (I use thinned aliphatic quite often), tight joints (less glue) and, as already mentioned, lots of sanding. :thumbup:

Composites, honeycomb structures balsa/foam hybrid construction etc. add a whole new realm of possibilities, the key is experimentation and innovation.

my 2cents.

Ive built several 3D aircraft in the past, mostly Cardens and I would recommend building the aircraft as it comes out of the box.Build as close to stock as possible when weight comes into mind. The only thing I do is some lightening holes in light ply with a circle cutter to lighten that up but thats it.

Heres why........Ive flown a lot of IMA and 3D airplanes, and the ones that fly the best are the ones that have the proper area for a particular weight. Manufactuers usually figure out wing loading and design kits around that. If an airplane is too heavy we all know it flys like a brick, but it also does not fly so well if it is too light. The aircraft wont penetrate well if its too light it wont snap crisply and it just doesnt handle as well. For instance I have a 40% Carden Extra 330 that is 38lbs. That is the best weight for that airplane. Ive seen and flown ones that where in the 37lb range and it just didnt fly as well. Just be careful with how much glue you use and watch out on the sanding and have fun..

My two cents
Pete