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Lightening holes in profile fuse - worth it?

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Dennis Toth:
Rebuilding a profile fuse and looking to save some weight. This will be full electric so no full length hardwood motor mounts  but it will still have plywood doubler on the nose. I am thinking about using a 3/8" core and laminating 1/16" cross grain on each side to get to 1/2" and have some stiffness. I am considering cutting lightening holes in the core and wonder if this really saves a significant amount of weight? The core is medium balsa and weights around 1oz. What has been the experience of the group?

Update: I just weighted the piece that I cut out for the wing, it weights 3.3gm (just under 1/8 oz.), the whole core weights 25gm (0.88 oz.). Not sure it is worth it but it might save the weight of the laminating glue?

Best,     DennisT

Avaiojet:
Dennis,

If you think you have a heavy model have at it. A hole weighs nothing. What was in the hole has weight.

The more holes you make the more weight is removed and the model gets lighter. "Science."

Best kept secrete in the Forum is this Build. And boy, did I ever make some holes! ;D

You've seen the finished model, comes in around 54 oz, tank, engine and finish included.

Here's the build,   

https://stunthanger.com/smf/cfc-graphics/'mig-3-reconnaissance-aircraft-and-warbird!'/

Holes start around page 3 or 4.

Trostle:
Not really related to weight savings for holes in the nose of a profile fuselage, but I sometimes wonder if lightening holes are worth the trouble.

Next time you assemble built up wing with cut out made in the ribs.  Weigh the rib before and after the holes are made.  The material removed from the holes does not weigh very much.  Yes, a gram saved here and a gram saved there adds up, but those cutouts do not make much difference.  More weight can be saved by simply using lighter wood.

Keith

Dan McEntee:
   Making holes in balsa wood after the fact doesn't really safe a whole lot and you might be surprised at how little it saves. If a hollow structure is designed to be that way and has proper structure it can still be light and quite strong and resist twisting. Like Keith points out, better wood selection is a better option if possible, but if you are using a kit supplied piece of wood and have no other option, keep the wing/engine section more solid and sand the aft section a little thinner. Drilling and cutting holes interrupts the grain and that is most of the strength. If you cut a bunch of holes then you have to cover them with something else and you are just putting weight back on with that wood and the required glue.  You build in lightness right from the start. It's almost impossible to take away significant weight as an after thought. You are better off trying to save some weight with the finish by keeping it simple and not so busy.
  Type at you later,
   Dan McEntee

Dave Hull:
If you cut large holes in the tail area, consider:

--You will need ribs or truss structure to support any external skins. They need to be sized to handle buckling loads (compression induced bowing) Gluing to the overlying skins prevents buckling until the glue line fails or the wood crushes. Thinner and harder internal struts might be considered better if well glued to the skins

--If you choose to leave out the wood skins, then be aware that the covering is now supplying much of the stiffness. Or, the stiffness removed is not actually replaced if the modulus of the finished covering is poor. However, I've seen some pretty "holy airplanes" that performed way better than I expected them too. Around here, Tim Meeks was a proponent of this kind on construction, I believe.

--If you leave out the wood skins, also be aware that people handling your plane will have to be advised, repeatedly, where they can hold it. Or you will get holes. An after the fact, unintended holy airplane. Trust me on this one....

--Orienting balsa skins with the grain pure fore/aft direction may improve the lateral stiffness and help avoid tail whip, but it is not as good in torsion. If your plane handles corners funny, ask someone to watch from behind as you pull out of a square corner. Does it have pronounced stab tilt? How much is it interfering with your trim? If memory serves, when you twist the fuse you will get some max stress aligned at 45 degrees to the longitudinal axis. (Balsa is a non-isotropic material. Stronger and stiffer along the grain. In composites, they layer uni materials to take advantage of that. You can also do it with wood. That's why plywood is so versatile.)

--The best way to figure out a building style is to keep weighing everything and keep records. Build it one way, but compare what you likely would have gotten the other way. I've done some studies like this and as Keith pointed out, you can get pretty fancy and not save much. Worse, you can get fooled into using more zoot, like carbon fiber pushrods and such when they are not needed and simply get heavier. Weight is a touchy subject, especially during the design of full scale aircraft where careers are at stake. Just ask any Mass Props guy....

The Divot

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