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designing a space-frame structure - which comes first - frame or body

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RipSlider:
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

SPiN Racing:
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

Brass_Machine:
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

SPiN Racing:
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.

Brass_Machine:
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

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