Designs

T-B
Member
Member
Posts: 31
Joined: September 11th, 2009, 9:02 am
Division: C
State: NC
Has thanked: 0
Been thanked: 0

Re: Designs

Post by T-B »

Our team doesn't have the benefit of coaches who are knowledgeable about engineering, so we try to learn what we can on our own. We've done pretty well so far, but to take it to the next level, we need to understand more. SLM, Balsa Man, and others,do you have a textbook (college or whatever) that you could recommend for us to read up on this stuff?
User avatar
mrsteven
Exalted Member
Exalted Member
Posts: 815
Joined: March 13th, 2011, 5:40 pm
Division: C
State: IL
Has thanked: 0
Been thanked: 0

Re: Designs

Post by mrsteven »

T-B wrote:Our team doesn't have the benefit of coaches who are knowledgeable about engineering, so we try to learn what we can on our own. We've done pretty well so far, but to take it to the next level, we need to understand more. SLM, Balsa Man, and others,do you have a textbook (college or whatever) that you could recommend for us to read up on this stuff?
I'm in the same boat you are, but I have learned extraordinary amounts of information from this site. Go through the posts from towers, boomilever, and bridges in the archives, you'll find a ton that will help (especially from those listed above). I'd venture to say that the posts on the engineering forums here are the best information you can get (with some minor supplements that you get from google searches)
Bookwise? Try some structural engineering type books but I think you'd be set if you read everything this site has in terms of building. I did this for towers and helicopters this year, trust me, its worth it
2011 Helicopters State Runner-up
2012 Helicopters State Champion
2013 Robot Arm State Champion
SLM
Member
Member
Posts: 195
Joined: January 31st, 2009, 2:24 pm
Division: Grad
Has thanked: 0
Been thanked: 0

Re: Designs

Post by SLM »

T-B wrote:Our team doesn't have the benefit of coaches who are knowledgeable about engineering, so we try to learn what we can on our own. We've done pretty well so far, but to take it to the next level, we need to understand more. SLM, Balsa Man, and others,do you have a textbook (college or whatever) that you could recommend for us to read up on this stuff?
I doubt you can find any structural engineering textbook that would help you learn how to design towers or boomilevers. Structural analysis textbooks generally deal with methods and techniques for determining forces and displacements in various types of structures. There are also textbooks on structural design which focus on the design of real-life structures (generally made of steel and/or concrete.) But, even if you have the necessary math and physics background, you will quickly get confused trying to learn from such books on your own. You definitely need a knowledgeable person to put the content in context for you. That is what engineering faculty do (or are supposed to do) for engineering students.

Look for practicing engineers or engineering faculty in your community, they might be willing to help. If that is not an option for you for next year, I would be more than happy to help by explaining the relevant engineering topics, answering your specific technical questions, giving feedback on your design(s), and providing general guidance for this event. If this option works for your team, contact me via PM at the beginning of next season for devising a work plan and schedule.

mrsteven is correct, this forum contains many useful posts. The challenge, especially for first timers, is to find the relevant ones and then stitch them together to build an effective understanding of the underlying knowledge for designing and constructing competitive bridges/towers/boomilevers. It would have been great to have all the necessary and relevant design and construction knowledge in one place (like a book), organized in a manner that makes the content readily accessible to novice and seasoned competitors alike.
Balsa Man
Coach
Coach
Posts: 1318
Joined: November 13th, 2008, 3:01 am
Division: C
State: CO
Has thanked: 0
Been thanked: 3 times

Re: Designs

Post by Balsa Man »

Let me echo what both SLM and mrsteven said.
To that, I'd add a couple things:
Doing the up-front research to understand what the forces are, and how they work, is the key first step to a successful design.
Once you understand that, individual component (as opposed to full structure) testing can be VERY helpful in getting to wood selection that will work. Tension testing is easy- you just need a way to hold one end of a piece, and hang a weight bucket on the other. Compression testing is harder- you need a way to get the test piece vertical, and the loadvertical along it's axis, but it can be done. The nice thing is, with euler's equation, when you get a column strength for one length, you can calculate for any other length.
A combination of careful reading of the posts here, and some web research will get you a long way. Finding an engineer you can talk to can be a major help.

If, as it sounds like, its back to booms next year, some very different design challenges. The basics are the same- some pieces have to work in tension, some in compression; angles change the forces. The magnitude of forces, both in compression and tension, are a lot more than you have to deal with in towers.

Like SLM, I'll be glad to pass on what we've learned- this will be the third time for me going through booms, so have picked up some understanding and tricks. One of the biggest challenges is how to attach the tension members in a way that does not add a lot of weight.
Len Joeris
Fort Collins, CO
T-B
Member
Member
Posts: 31
Joined: September 11th, 2009, 9:02 am
Division: C
State: NC
Has thanked: 0
Been thanked: 0

Re: Designs

Post by T-B »

Thanks, SLM and Balsa Man. Reading this forum is exactly how we've gotten as far as we have -- 2nd at state in '12, 13th with catastrophic failure in '11, 2nd in '10, and 3rd '09). We read this forum almost every day, but we really want to take it to the limit this next year. We will build over the summer and practice some new skills -- laminating, maybe building i-beams -- and then in the fall, we will redouble our efforts. We live in a very competitive city; five of the top six teams in our state are from here, so it is sometimes difficult to find experts who aren't already aligned with another team. But we will try that first. If that doesn't work out, SLM, we'll ask you questions privately or on the forum.

Thanks
User avatar
LKN
Member
Member
Posts: 69
Joined: March 14th, 2011, 7:32 pm
Division: C
State: NC
Has thanked: 0
Been thanked: 0

Re: Designs

Post by LKN »

I have a teammate who is working on the Helicopters event this year. I know this isn't the helicopter forum, but I feel that there is more knowledge here about his problem.

What shape/structure of balsa wood is the most resistant to twisting? The body of his helicopter slightly twists when wound very tight, according to him hundreds of rotations of a rubber band. Therefore, something high in strength to a good amount of compression (not sure how much, but it wars a 1/2 by 1/8 stick of dense balsa) while remaining as light as possible, but over 3 grams. He cannot just use a very dense small stick of bass/balsa.

Now, how this ties in to this event.
I am pretty sure boomilever presents a very similar challenge: build a structure that is very strong in compression and also very resistant to twisting.

Do T sections or I-beams possibly have a place here in resisting torque?
- LKN
NCSSM '13
User avatar
mrsteven
Exalted Member
Exalted Member
Posts: 815
Joined: March 13th, 2011, 5:40 pm
Division: C
State: IL
Has thanked: 0
Been thanked: 0

Re: Designs

Post by mrsteven »

Howdy,
Helicopter is a bit of a dependent event with what kind of rubber, standard design or a chinook set up? affect how I'd go about this alot.

You're saying the body is twisting, not the rotors correct? A little increase of pitch from a high torque never hurt anyone (in fact in many cases is beneficial) from the rotors initially

With the body, if its a generic helicopter (no chinook bonus, standard coaxel) if twisting use a rectangular piece in respect of the helicopter motor on the shorter face. This should stop any bending inwards and I've never seen one of those 'twist.' last year when I was on the standard design, I used a thicker low density balsa piece for this purpose. I think that the thin high density piece might be the issue. Its just too thin at 1/8. I believe my motorstick was in the range of 1/4 X 1/2 but low density. Under 3 grams shouldnt be an issue (you want slightly under so you can add balast to lower CG for extra stability)
However this is in terms of the coaxel, this might not fix an issue on the chinook... but in all honesty I've never noticed a slight twisting to ever be an issue affecting time. So if the twisting is truly causing a large diminishing in time, I'm fascinated in what you're doing to get that. Shouldn't matter a whole lot

My thoughts... don't take as an all-knowing response.

Boom: I have no clue...
2011 Helicopters State Runner-up
2012 Helicopters State Champion
2013 Robot Arm State Champion
User avatar
LKN
Member
Member
Posts: 69
Joined: March 14th, 2011, 7:32 pm
Division: C
State: NC
Has thanked: 0
Been thanked: 0

Re: Designs

Post by LKN »

Thanks Mr. Steven. He is actually using a chinook design though... the body/supports are twisting against each other.
- LKN
NCSSM '13
User avatar
mrsteven
Exalted Member
Exalted Member
Posts: 815
Joined: March 13th, 2011, 5:40 pm
Division: C
State: IL
Has thanked: 0
Been thanked: 0

Re: Designs

Post by mrsteven »

LKN wrote:Thanks Mr. Steven. He is actually using a chinook design though... the body/supports are twisting against each other.
mhm. well with my chinook I never encountered the issue where twisting caused problems (well, or much notable twisting itself). Understandably you probably don't want to get much more specific on general forums with nats coming up. But ya, upping the dimensions and lowering the density subsequently to get same mass (or even less if you dare O.o) is how I'd go about that without really knowing much more than a vague-ish question
2011 Helicopters State Runner-up
2012 Helicopters State Champion
2013 Robot Arm State Champion
SLM
Member
Member
Posts: 195
Joined: January 31st, 2009, 2:24 pm
Division: Grad
Has thanked: 0
Been thanked: 0

Re: Designs

Post by SLM »

LKN wrote:... What shape/structure of balsa wood is the most resistant to twisting? The body of his helicopter slightly twists when wound very tight, according to him hundreds of rotations of a rubber band. Therefore, something high in strength to a good amount of compression (not sure how much, but it wars a 1/2 by 1/8 stick of dense balsa) while remaining as light as possible, but over 3 grams. He cannot just use a very dense small stick of bass/balsa.

Do T sections or I-beams possibly have a place here in resisting torque?
With torsion, the primary concern is not the twisting of the member, rather it is the (shear) stress formed on the outer surface of the member. It is this stress that causes failure.

Generally speaking, closed cross-sections (like rectangle, square or circle) better resist torsion than open sections (like T, L or I). Furthermore, among the closed sections, the rectangular shape is the weakest against torsion whereas the circular section offers the most torsional strength. Here is an example showing the relative strengths of the most common closed sections.

Suppose we have a member having a 1/2" x 1/8" rectangular section under torsion. Let’s denote the torque being applied to the member as T. Then, shear stress caused by T on the outer surface of the member can be shown to be approximately equal to 450 T. If, however, we use a 1/4” x 1/4” square cross-section (which has the same area as the rectangular section), then the shear stress reduces to 308 T. This is a stress reduction of about 32%. That is, by changing the shape of the cross-section from rectangular to square, we reduce the stress in the member by about 32%. If we took the same cross-sectional area (1/16 in^2) and mold it into a circular shape, then shear stress reduced even further, it reduces to 227 T. This is a reduction of about 50%, compared to the rectangular section.

Conclusion: Use a member with a circular cross-section. If that is not an option, then use a square section, instead of a rectangular one.
Locked

Return to “Towers B/C”

Who is online

Users browsing this forum: No registered users and 5 guests