All these "Reduce your weight by 19%!!!" email spams reminded me of a challenge I have with a couple of my overbuilt old planes:
How should I reduce a weight of a fully built, but uncovered plane? I am afraid that drilling holes in the wing ribs, the aileron, the fuselage, erc., will weaken the structure. Any rules-of-thumb here?
Any other means of cheap weight reduction?

It is a lot easier to do this before building the plane.

Provided you do not over do it, cutting holes in the wing ribs helps, as well as cutting holes in the vertical stab. Several carefully place holes in a solid vertical stab can save a couple of ounces since you will not require some nose weight. The same with the horizontal stab if it is solid. Depending on the size of the plane, I would probably not mess with the ailerons and the elevator is a toss up.

The fuselage can also be lightened, especially if it is of the box style. A few elongated holes in the underside of the fuselage in back of the wing, same with the sides and top. Be careful with the fuselage. It is easy to over do it and weaken the fuselage. Take a look at a 4* fuselage and you will get a general idea of what is OK .

Another place to lighten is with the fuselage formers. Many of the older planes had formers that extended into the plane quite a bit. Part of this extension is not needed and can be removed.

If you are painting the plane, consider using latex with a clear coat over it to fuel proof it. On a 60 size plane you could save about 3 oz (or more) over a conventional "dope" paint job.

Remember, if you can remove one ounce from the tail, you will save 2 ounces (or more) in the nose.

FWIW, I recently scratch built a Bingo (old 4* like plane). The plans were extremely overbuilt. They had wing ribs of 1/8" balsa with no lightening holes and 1/4 x 1/2 spruce main spars on the top AND bottom of the wing AND 1/4" square spruce trailing (sub ) spars also. I used 3/32" balsa for the ribs WITH lightening holes, 1/4 x 3/8 Basswood for main spars and 3/16" basswod for the sub spars. Did a bunch of other lightening also. The plans state a weight of 7 - 7 1/4 lbs with a KB 65. I have 6 lbs with a Saito 72.


Hope this helps.

Campy makes a great point - If your airplane is tail heavy, the more weight removed there pays back dividends proportional to the tail length....

Along with that, since the flight loads imposed on the structure by the tail become larger moving forward from the tail, the stringers and formers can be made progressively thinner and lighter the further aft you go... I expect you could make the aftmost formers out of much thinner stock than those forward. Likewise, the stringers could be tapered in thickness as you go aft if you have the patience... the biggest challenge becomes the ability to handle the airplane in and out of the car and storage...

I think you could make an airplane that resists flight loads OK, but could not handle an inadvertent collision with your field box

Cheers!

Jim

Try a power belt sander. Take her down to right where you are going to break through!

Just kidding!

If you're not using lightweight wheels, get some. They save a ton of weight. Tail weight is critical. Don't use a heavy tailwheel, get one of the foam ones. An ounce saved at the tail = 4-5 oz at the nose.

Thanks guys - I think I canuse all your advises.
I will lighten the plane like crazy (without overdoing it). The most overbuilt part is the center section of the wing where the main landing gear is attached. I was probably on CA fumes when I covered the center section with two layers of thin Fiberglass...
The plane was supposed to be ~4 lbs, and now I would estimate it to finally be over 5. It can probably lose half of the gain.
If I choose my LA 25 it will be still underpowered. If I use my FX46, I will still lighten it, but move two servos to the tail. The extra 3.5 oz of the servos and their brackets a foot and a half behind the CG will more than compentate for the extra engine weight. I will NEVER add balancing weights.
As for the wheels, I noticed that 2.75" foam wheels and MUCH lighter than 2" old hard plastic ones.

Be careful when lightening the center section. Since the LG attaches there it is going to take quite a bit of stress/pounding. You should be able to lighten it, just don't weaken it.

I presume you will also be resheeting this area. Prep the wood in this area with ONE coat of sanding sealer. After sanding it, I suggest using one layer of 2 oz fiberglass cloth applied with a waterbase polyurethane undiluted. Any weave can be filled with a coat of the poly mixed with micro balloons. This has excellent strength and will be a lot lighter than epoxy resin.

A very good builder at my club showed me one of his scratch built wings. It had 1/2 the formers as any wing I've ever seen. He then told me his secret... cap strips. Basically, turning the formers into I-beams causes the wing to be stronger with much less wood in it. He did make the point that when you add the strips, do it from inside out, and one former at a time (top and bottom). Do not cap one side then the other or you will have a warped wing when you are through.

I ran across the following web site a few years ago that got me thinking light.

[link=http://webpages.charter.net/bobad/]http://webpages.charter.net/bobad/[/link]

Here is a direct link to the article

[link=http://webpages.charter.net/bobad/light.htm]http://webpages.charter.net/bobad/light.htm[/link]

I used the techniques described in this article and on the site to build a 3 lb 12 ounce Burrito.

[link=http://www.rcuniverse.com/forum/m_1676792/anchors_1857391/mpage_5/key_/anchor/tm.htm#1857391]Burrito Showcase[/link]

Note that I could save more weight by going to a lighter .32 sized engine, and smaller servos.