Designs

[jander14indoor]

Actually you can get good answers in helicopters too, structural design and stiffness are integral elements of a light helicopter. Rules were just written so it wasn't the main focus for coax designs so it hasn't been discussed much.

If your issue is with a Chinook, you need to give us more info, I'm not sure how the torque will cause problems unless you are trying to lighten your motor sticks to allow more structure to link the two rotors. If that's the case, first solution I'd try would be rolled motor tubes. Think of a balsa straw. Lots of info on the web on how to build one, search the archives for Wright Stuff, I know we've discussed it there.

If your problem is between the rotors, you need to describe your current approach more as that drives what might work.

Jeff Anderson
Livonia, MI

[LKN]

Thanks for the advice Mrsteven, SLM, and Jeff, I'll probably take this to the Helicopters page. I'm not too sure on the details but I'm trying to help a teammate out with building a helicopter for the national competition. The problem is that NC's state competition is always the last weekend in April (the 28th this year) so we are pretty rushed as builders on our team.

So, applied to Boomilever next year, it seems that square cross section legs (at lower density but greater cross section?) are the way to go for next year.

[SLM]

So, applied to Boomilever next year, it seems that square cross section legs (at lower density but greater cross section?) are the way to go for next year.

For boomilever, tension, compression and bending probably play a more important role than torsion. If the structure is relatively narrow in width, then torsional forces may be negligible. Given what we know about towers and elevated bridges, I would say buckling due to compression will be a major concern for boomilevers as well. This means a square section is probably not the best choice, especially for the compression member(s) that are adequately braced.

[LKN]

Possibly bring back the tube shape concept for the section of the boom under compression? That would be interesting... I might give it a shot this summer. The challenge would be attaching the tension members efficiently to the cylindrical body and creating an area to support the loading block.

[SLM]

Possibly bring back the tube shape concept for the section of the boom under compression? That would be interesting... I might give it a shot this summer. The challenge would be attaching the tension members efficiently to the cylindrical body and creating an area to support the loading block.

Interesting idea, certainly worth exploring. A tubular section has a high resistance to buckling as well as torsion. You certainly would need less (and even maybe no) bracing with this kind of member than others. Visualize a triangular boom consisting of a single horizontal tubular member and two inclined tension members with no bracings. Whether it would be more efficient than a truss-like structure remains to be seen. And as you said, figuring out a way to support the load block could also pose a challenge here.

[jander14indoor]

From past discussions, Boom has three big challenges.

Efficient design of the compression members on the bottom to prevent buckling.
VERY strong joints of the small tension members at the top AND tip to the wall mounting plate to hold that high load without creating stress risers. I think the difficulty of joining to the lower compression members at the tips have prevented anyone from using tube compression members effectively. Not saying its impossible, but it is a challenge.
VERY light back plate to mount to wall while supporting that tension member load.

Less important, but not to be forgotten, don't let the compression legs slide down the wall.
Oh, and sway when loading is BAD.

What I was always impressed by was that you could SEE the incipient buckling on the lightest designs as they approached failure. The compression members took on beautiful S curves as maximum load approached, left side mirroring the right.

Jeff Anderson
Livonia, MI

PS, NC isn't alone, the Michigan tournament was the same day.

[LKN]

The tension members already sound like trouble, but the the tension bearing block that has a nut through it sounds like a nightmare. I have never competed in boom before, and I have a feeling I am definitely not alone on this. Any ball park idea what weight that tension bearing block should weigh? I would guess less than a gram, but under 15kg of tension at some angle I have no idea. I have been looking back and it seems some of Aia's designs might have even used glue that looks like gorilla glue for the tension members to hold at the joints (yikes!). I think the tension members connected to the cylindrical compression member will be an issue, but could be solved. I don't think it would ultimately be a better design than two-stick and braced compression members when it comes to high-levels of competition, time will tell if any teams can pull it off.

On that PS Jeff: Thanks, it is nice to know at least there is another state who have teams going to nats with builders under the same stress. Not sure why NC kept Storm the Castle for Division C through States this year, even though now its not a national event. Two and a half week turn around time from trebuchets to learn how to build a robot arm, let's go!