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chalker7 wrote: In AMA events we regularly make helicopters that weigh less than 0.5 g and are larger than SO helicopters. It is all about wood selection, size and specialized construction techniques.

Woah, tell me more. I've got an idea for a chinook style helicopter, but my test model turned out WAY over 3.5 grams, and it's not to max dimensions either.

Much info about building light in last years archive, both Helicopters, Wright Stuff and Bridges/Towers but here's a summary.

Build with a plan. You should know how much your copter will weigh BEFORE you build it. The weight plan is something engineers call a Bill of Materials. It lists every part of an assembly (don't forget the glue!) and many details about the parts, including weight! Add up the part weights, you have the assembly weight. Too heavy? Redesign the parts. For balsa assemblies, this generally means changing the cross section or density.

Select wood carefully. Balsa is a natural material that varies greatly. Grain can be straight or wavy, there can be defects from bugs or processing. The density is ALL over the place. When you buy wood, don't buy sticks, buy sheets. Its much easier to check the quality of a sheet and make your own sticks, cheaper too. Hold the sheet up to the light to examine the grain. It should be straight and even. No bright cross grain spots (a defect called a shake, already pre-broken there!). Lots of holes from insects, again breaks already started. Select for density and stiffness. WEIGH the piece before you buy it and calculate the density. It should match what your Bill of Materials is calling for. Compare similar density good sheets for stiffness, buy the stiff stuff, put the floppy stuff back.

Special assembly techniques. Look around the web for indoor free flight. http://www.indoorduration.com/ has a lot of the techniques described. A couple of quick examples.
- See the thread on this board on glue. Glue is a necessary evil to hold parts together, but you should use ONLY what's needed and not a micro gram more. Glue adds weight FAST.
- Build with a scale next to you and use it. Keep records. If you don't know what your pieces weigh or how you got there, how can you control weight?
- Take advantage of material properties and mechanics. Why do you think bikes are made of hollow tubes instead of solid wires? It's HARD to make a good tube. BUT, lots stiffer for the same amount of material. Stiffness is OFTEN more important than strength.
- Take advantage of special materials. Boron was specifically called out in the rules, have you asked yourself why? Because it is a wonder material that used correctly could improve your helicopters for very little weight. Problem is, there are significant safety hazards if not handled correctly and we didn't want students injured. BUT, we left a lot of materials that are almost as good, kevlar, carbon fiber, tungsten wires, etc.

Hope that's a helpful start.

Jeff Anderson
Livonia, MI

Carbon fiber is so ridiculously expensive.

Ol3f34 wrote:Carbon fiber is so ridiculously expensive.

Not sure what constitutes ridiculously expensive, but see: https://www.a2zcorp.us/store/Category.a ... Pultrusion

If link doesn't work, try http://www.a2zcorp.us and search for carbon pultrusions.

Also look for kite parts. Or generally search for carbon fiber pultrusions.

Anyway, I found several appropriate carbon fiber shapes. Example, 4 mm by .6 mm rectangle, 1meter length, 3.33, enough for a least one, possibly two motor sticks. Or 3mm by .12mm by 10 m strips, $24 enough for a BUNCH of copters. Use by laminating two sides of your much smaller motor stick to make it unreal stiff.

Used sparingly on your rotors, the 1 mm by .12mm strip could be useful and increase damage resistance. The 1.5 mm by 0.7 mm tubes might work for rotor spars, a little pricy at $5.38 per meter, but if you never break another spar?? 1.0mm by .5 mm by $5.59/meter might even work and save considerable weight.

Or how about the hollow tubes (square or round), around $10/meter, enough for three copters, no laminating, possibly a little heavy, but not if you get the right size, about 4 mm should do.

Overall, yes, more expensive than balsa, new building skills to learn, have to be VERY aware of weight as usual, but ridiculously expensive, not so much. Offsetting that is less time spent finding good balsa, and probably less time repairing your copters.

Here's one approach, build prototypes in balsa to get a good basic design, final versions (as long as weight doesn't increase) with carbon pultrusions for contest durability.

Jeff Anderson
Livonia, MI

jander14indoor wrote:

Ol3f34 wrote:Carbon fiber is so ridiculously expensive.

Not sure what constitutes ridiculously expensive, but see: https://www.a2zcorp.us/store/Category.a ... Pultrusion

If link doesn't work, try http://www.a2zcorp.us and search for carbon pultrusions.

Also look for kite parts. Or generally search for carbon fiber pultrusions.

Anyway, I found several appropriate carbon fiber shapes. Example, 4 mm by .6 mm rectangle, 1meter length, 3.33, enough for a least one, possibly two motor sticks. Or 3mm by .12mm by 10 m strips, $24 enough for a BUNCH of copters. Use by laminating two sides of your much smaller motor stick to make it unreal stiff.

Used sparingly on your rotors, the 1 mm by .12mm strip could be useful and increase damage resistance. The 1.5 mm by 0.7 mm tubes might work for rotor spars, a little pricy at $5.38 per meter, but if you never break another spar?? 1.0mm by .5 mm by $5.59/meter might even work and save considerable weight.

Or how about the hollow tubes (square or round), around $10/meter, enough for three copters, no laminating, possibly a little heavy, but not if you get the right size, about 4 mm should do.

Overall, yes, more expensive than balsa, new building skills to learn, have to be VERY aware of weight as usual, but ridiculously expensive, not so much. Offsetting that is less time spent finding good balsa, and probably less time repairing your copters.

Here's one approach, build prototypes in balsa to get a good basic design, final versions (as long as weight doesn't increase) with carbon pultrusions for contest durability.

Jeff Anderson
Livonia, MI

I agree with everything Jeff said above, but also would like to point out that carbon fiber is not necessary to either get down to 3.5g. It is entirely possible to build a 3.5g helicopter with just hobby shop balsa wood and (light) tissue paper. It's all in the wood selection and careful construction techniques. In fact, I would say using carbon fiber is actually a detriment to most helicopters. It's actually quite dense (at least, compared to balsa) and difficult to work with, so weight can add up quickly. The primary benefit carbon fiber will have over balsa is durability, but I'm not entirely convinced that will be worth it considering the amount of effort it will take to build a 3.5g carbon fiber helicopter.

chalker7 wrote: .... It is entirely possible to build a 3.5g helicopter with just hobby shop balsa wood and (light) tissue paper. It's all in the wood selection and careful construction techniques.

Good advice.

Our helicopters last year were all less than 3.5 grams with larger rotors and solid motorsticks. We actually had to add weight to bring them up to 4.0g. The only specialized material we used was film covering rather than tissue paper. We have carbon fiber available but it really is more trouble than it's worth.

Also, since this is the duration thread, has anyone tried their helicopter this year (with 3.5g and unlimited motor size)? What kids of times are you getting?

I haven't begun experimenting yet, but I'm curious to find out how the unlimited rubber thing will work out. It's yet another variable we have to balance; winds vs. mass. Each copter will enforce its own rubber mass limit

lucwilder42 wrote:I haven't begun experimenting yet, but I'm curious to find out how the unlimited rubber thing will work out. It's yet another variable we have to balance; winds vs. mass. Each copter will enforce its own rubber mass limit

Yep, in fact I'm not entirely convinced carrying 2 grams last year was ideal. I heard rumors of teams doing quite well with 1.5g or less (with the specific intention of cutting mass and unused winds).

4 minute flight

Is that time with a helicopter with this year's specs???