Author Topic: designing a space-frame structure - which comes first - frame or body  (Read 5991 times)

Offline RipSlider

  • Jr. Member
  • **
  • Posts: 25
Hello all.

I'm thinking, more for practise working with metal more than anything else, making a model boat with a space-frame "fuselage" with the body sort of hung off around the frame. The space-frame would be entirely load bearing - i.e, motor, rudders etc would act on the frame, and the body would just move as a function of the frame moving.

This may sound overly complex, when most people just whack a load of glass-fibre on a mould, but I keep niggling away at designing and prototyping a racing wave piercer, and this has enormous pressure placed upon it from all directions as it goes stright through waves rather than over then. A 2 metre model punching stright through a wave at 50mph needs to be pretty tough.

Also, as I say, it makes for good metal work practise.


It would seem the question has gone missing when I re-read this, so here it is: Which comes first at design time - frame or body. I.e, do I build a hull, and then fit a frame inside it - which may mean slight wonkyness - or build the frame dead true, and then build the hull around that.

Thanks

Steve

Offline SPiN Racing

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 505
From the race car stuff I do.. the reason for a body on a race car is to keep the air out.
Sooo using that thought the body for a boat is to keep the water out.

I would lay out the compionents you need to make the boat go.. engine, running gear, radios.. etc. THen lay them out in a manner consistant with where they may go in the final design on a sheet of paper in the rough dimensions of the boat.

Two basic designs really when it comes to cars.. Monocoque and tube chassis.

Monocoque uses a frame design that is the basic design of the car/boat/airplane and then they apply the skin to it, and the skin itself is a structural member. Think of an aluminum john boat. The hull itself is a structural member attached to the frame sections inside it. OR.. think of am aircraft... they have thin ribs and formers inside, with the skin riveted to it, to add structural integrity.

Now.. tube shassis.. the structure is complete.. in a race car.. and will drive happily around completely intact, WITHOUT the skin. The vechicle is complete without any bodywork.  PRE World war one aircraft were for the most part this design. They were a wooden structure that was complete, and stressed, and ready to go.. WITHOUT the fabric covering.
As time progressed they learned that Doping the wings, and fuselage, causing the fabric to become very stiff, and strong actually increased the structural integrity, not just improving aerodynamics.


SO thinking along those lines...
Do you build a super strong internal structure, with a fiberglass shell pinned to it? (or aluminum)
Or do you build a basic frame structure that accounts for the skin being bonded, riveted, or welded to it.. and the skin itself is taut, stiff, and or stressed?

If I was dsigning it... I would draw up/layout all the inner formers, and or internal structure, with keel the works. Aluminum is fine. Make a nice structure that is strong. BUT.. make it in such a way so that the external dimensions are exactly that of the INSIDE of the outer hull panels. Then bond them to the structure so that each panel is attached to each frame section securely. That way the internal structure that you design is solid.. but has the added overkill regidity of the skin to reinforce it.

Is that overkill? I would think.  BUt if it works.. and it meets your needs.. no problem.

I am trying to picture a tube chassis car modified so that the outer tubes match the cars contours.. and then bonding/riveting the entire outer skin to be a stressed surface. Of course using steel or aluminum for the skin.  The car would be significantly heavier. And would be bomb proof.


Scott
SPiN Racing

Offline Brass_Machine

  • Administrator
  • Hero Member
  • *****
  • Posts: 5203
  • Country: us
Hey Steve,

Scott has some great advice. My take is just a little different. Now me not being an engineer, take what you will. One of the main differences I see with a race car and a race boat are the bodies. Granted with a car the body helps keep out air, but it also plays a key role in aerodynamics. If you look at say the differences between a NASCAR racer and a ROLEX DP racer... the NASCAR bit is essentially a flying brick. While it is fast, it is not same as a DP racer or F1. In boats, the hull plays a major design in how fast it will go, how well it will ride the water and how well it will split the waves.

If it were me, I would spend some time on hull design. I believe the hull should be designed first (or with the frame). It would suck to design the frame then find out it doesn't work with the hull you have in mind.

Just my thoughts.

Eric
Science is fun.

We're all mad here. I'm mad. You're mad.

Offline SPiN Racing

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 505
QUITE True.  :thumbup:

I joke that it is to keep the wind out... and while this is true for cars like NASCURR. You are quite correct about the rest of the automotive racing world.

Below you can see a picture of one of the formula cars I raced, its called a Sidewinder, and has a Rotax 493 to the right of the driver, and a snowmobile clutch system driven off a jack shaft on the left side. The suspension is quite sophisticated for the rules saying you can only use a rubber disk 2" in diameter.. and implement it in any way you like. Thats a whole different thread.

A friend's sidewinder


Diffuser view.... AND YES>. it has been rearended quite a few times... and repaired.


The thing is... these things keep the air out... however they allow a bit under the front, as well as some in through a duct for cooling in the front... The thing is.. the rear of the car is a massive diffuser that is a bit over 4' long. These things corner in excess of 3G in some high speed places if the track is smooth etc. QUITE a rush.  Soo yes... they keep the air out. BUT. They do it in a manner as to get downforce while not increasing drag.

The hull design as Eric mentioned.. is critical to the functionality of the boat. How it planes.. IF it planes.. and of course.. how it slices a wave. IIRC from what my dad was telling me about them.. displacement? boats? Not sure on the term.. but it was the wave cutter types.. are usually long and very thin in cross section.. almost like a sub, but they stay above the water, and simply go through the waves, vs over them.  There was some formula for length vs width to make a proper cutter. Otherwise there was some tendency to go over the waves. I presumed you were already on that path, as that was the hull choice.

SPiN Racing

Offline Brass_Machine

  • Administrator
  • Hero Member
  • *****
  • Posts: 5203
  • Country: us
Hey Scott... now we are talking! Please, some threads about sidewinder... the suspension all that jazz would really be appreciated. Definitely more pictures! Would also like to hear about your experiences racing that thing.

Back to Steve's topic... Have you considered buying a pre-made hull? I was thinking along those lines for when I eventually (one of these years) build one. I figured I would take a mold off of it so i could make my own replacements in glass or CF.

Eric

Science is fun.

We're all mad here. I'm mad. You're mad.

Offline RipSlider

  • Jr. Member
  • **
  • Posts: 25
Thanks for the replies chaps.

I'll answer in backwards order to the questions:

Eric:
Carbon was my first choice for this - I'm pretty happy working in Carbon fibre, but I was put off for a few reasons. The first is that this boat will be designed to go as fast as I can possibly make it - 120mph+ is the end target, but will be based partly on a wave-piercing design ( with an Axe-bow and a 3 point hydroplane all thrown into the mix ). Because I'll be racing this at sea - it's almost impossible to use I.C or jet power on inland waters in the UK for boats - it could be hitting some fairly sizable waves at very high speed. I CAN just keep upping the carbon thickness, but I reckon I will end up with 1/2"+ thick of carbon all around the hull, and sadly Mrs Steve knows full well how much carbon is at the moment as she saw my last bill!

Also the boat will be between 5.5 foot and 6 foot long, and I have no access to an auto-clave that big without spending thousands to hire one ( and lets say 2x de-bulks and 1x bake x 3layers is at least 4 days that I would be hiring the clave out for. I reckon it would end up being 2,000 for the carbon, resins, waxs etc, and another 3,000 for the autoclave. - about $8,000 for a toy boat.)

My thinking is that a space frame will be good practise for working with metal, and will allow for a stressed Ali skin to go over the top of it. This should keep stregth up at a much lower cost. Weight I am less concerned about as it the boat is too light it will need a load of ballast in the nose anyway for the design to work.

Lastly, this may end up being a flash steam driven boat ( flash steam model boats were doing 150mph+ in the 1950's ) and I'm a bit wary about using carbon around lots of open flames from a flash steam plant.


Scott and Eric

I think the question I'm struggling with most is how I deal with the detailed design. It would be fairly simple to build an empty hull, and then fit a space frame into it - no real planning needed, just juggle the metal in based on a back of a fag packet sketch. That makes the design easy, but the build itself I would expect to be fairly tricky - how do you weld the frame inside the hull for example?

Designing the frame first, and then working a hull around it would seem to take a LOT more effort at the design stage, but I suspect it would make for a much easier build.

So I think that's my basic question - Do I go with "difficult design, simple build" or "simple design, complex build"?



Many thanks for all the responses so far.

Steve

Offline Brass_Machine

  • Administrator
  • Hero Member
  • *****
  • Posts: 5203
  • Country: us
...
Designing the frame first, and then working a hull around it would seem to take a LOT more effort at the design stage, but I suspect it would make for a much easier build.
...

I like that option. The design process would be more difficult... but not certainly impossible. If you know the basic shape of the hull, then I imagine it wouldn't be that difficult.

I would like to see how this works out. Please, do a build (and/or design) thread when it comes time.

Eric
Science is fun.

We're all mad here. I'm mad. You're mad.

Offline Brass_Machine

  • Administrator
  • Hero Member
  • *****
  • Posts: 5203
  • Country: us
...
Also the boat will be between 5.5 foot and 6 foot long, and I have no access to an auto-clave that big without spending thousands to hire one ( and lets say 2x de-bulks and 1x bake x 3layers is at least 4 days that I would be hiring the clave out for. I reckon it would end up being 2,000 for the carbon, resins, waxs etc, and another 3,000 for the autoclave. - about $8,000 for a toy boat.)
...

Steve,

That is much larger than anything I thought of building! That would be awesome to see running!

Eric
Science is fun.

We're all mad here. I'm mad. You're mad.

Offline SPiN Racing

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 505
I will dig through my archives of pictures.. and see what I have to compile a sidewinder post. I will also drag my tush down to the shop and snap some pics if the car is out of the trailer.  :thumbup:

I think an option you might want to think of.. going down the road John mentioned with the CF strips internally used for the hull design... is building the external hull from CF or fiberglass, vacum bag it. Peel ply.. the works. We make the CF panels at the shop like that. Once you make the external hull, make yourself some layers out of say.... Balsa wood, that match the internal contours of the hull. That way uoi make say.. 20 layers of balsawood with a 1/4 inch gap between them. Then you will have a internal dimension of the hull.
From there.. you can make your frame to match the contours of the sides of the balsa layers.. and if you make sure they are able to be seperated.. you can lay them out one at a time to make each box shape, and make sure the metal matches the lines of the exterior.
Once the cage is made, you should be able to simply set it into the hull, and glass/CF it into it.

OR.. if you go with thin aluminum for the hull, and need to weld/tig it in.. you would have minimal gaps for the length of the frame.
SPiN Racing

Offline RipSlider

  • Jr. Member
  • **
  • Posts: 25
I'm not sure I will be able to vac bag - there are some very acute internal angles in the design, and those are an absolutely swine to Vac-bag.

from the point of view of "learning new skills" a stressed Ali skin would be good - and also light on the wallet ( cos I know a chap who can do wonders with a TIG welder ) but carbon is still feeling like the easy route forwards, although Mrs Steve will go absolutely metal if I buy in a couple of hundred feet of high-M tow.

Have had another crack at drawing out what I want tonight, but no luck. I think it might be easier to set to with a large lump of blue foam and make the hull as a prototype. From their it's a pretty simple jump to actually building the hull. And that sort of makes my decision for me - build the hull and somehow bodge in a space frame afterwards.

Steve

Offline SPiN Racing

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 505
Re: designing a space-frame structure - which comes first - frame or body
« Reply #10 on: June 25, 2009, 03:16:58 AM »
Here is a clip from youtube of a guy building a car body for a car of his own design. Its timelapse.. there are a handful of videos.
The sad thing to me... the car was coming along nicely.. he was doing well... and then he got kicked out of the shop he was in cause his landlords went out of business or something.. then he ran into financial issues, and had to speed things up.. and he just slathered glass over the foam.. and it got REAL rough. So sadly A TON of work now to get it back to something remotely usable for making a plug IMHO.

Anyhow.. heres the link to the first clip.
http://www.youtube.com/watch?v=BJFSXIP6Vw8
SPiN Racing