Space frames for main beams

But why would F1 use a spaceframe when they can get the strength in a skin that they need to have for another purpose anyway? They probably could build an F1 lighter with nothing but a carbon space frame but there is no point !

1. F1 has min weights.
2. They need a stiff skin for aero, no point having a light strong car in the aero is up the spout.

Therefore monocoques rule F1, makes sense, no?.

Go back to the beginning of the thread an look at the big Shuttleworth cat using a space frame. This application makes sense to me, I can't see how monocoque construction would negate the need for internal strengthening for the rig that this beast sports. Spaceframe is an ideal solution for the problem.

As one previous poster said there is no one truth here... it all depends.

BTW military transports are all about load space, where do you get this stuff? I am not an expert on military aircraft but this FA18 looks like its chok full of frame work to me.

 

Also alot of aircraft are semi-monocoque...

Read http://en.wikipedia.org/wiki/Monocoque#Aircraft

Its never black or white, tis alway more complicated than that.

 

It makes perfect sense to build an F1 car as light as possible, even though there is a minimum weight limit. You can use ballast in the floor to bring the car up to specification if needed, and the result is a lower CG = better handling = wins.

It's true, it's never black and white, but that's exactly what Petros is claiming. The fact is, in almost every application where strength/stiffness and light weight are demanded, monocoque construction has almost entirely superceded spaceframes.

 

I don't know enough to know if that is correct or not, I suspect its not the case for all applications but I am not going to argue it.

I didnt get that from Petros post, to me he was saying that it can have certain advantages, especially cost for the one off home builder who has more time than money. I think its great that using an alternate method he can rival exotics in building kayaks. I'm not sure how far the concept can be taken... I'm sure it has limitations. Anyway in the context of the OPs question, the big Shuttleworth, I think that a space frame would be very hard to top for that application.

With F1 yes but my point is that is the combination of requirements that make monocoque the best choice. If you could some how discount the aero requirement a carbon space frame could conceivably produce a lighter car for the same strength. However you can't, so monocoque it is!

 

Polite pontification?, masalai interrupts again, I like the subject & I would like to hear of more on the application of space-frames as may be relevant to yacht design (particularly cats and in cross members). Is overall stiffness an advantage in the total construction or a little flexing between hulls?

 

Sebastian Schmidt used something like space frames on Alinghi, the catamaran:

http://www.sebschmidt.ch/portfolio/99143/article/99143_article_Seahorse_Aug2000.pdf

 

Actually, I REALLY don't think you can build a pure monocoque OR spaceframe design today, and be at all competitive. The fact is that in almost ALL applications you'll find where either option was selected primarialy, there is also an element (or several elements) of the other incorporated. Even in SOF construction (by my understanding) the overall structure gains stiffness from the tension on the skin.

In that light I'm going to say that you're all right (and, at least somewhat, wrong). Both technologies are the best, when blended in whatever ratio works best for your application. Even the monocoque racing bicycles used in the olympics several years ago were shaped similarly to a trellis, the advantages are just too lucrative to avoid combining the techs!

Also, Petros:
According to the research I've seen (sorry, can't find it a.t.m. to give a reference) lifting foils are actually a bit MORE efficient than planing surfaces, for every speed...just that you have to size the foil right for the application. In the research, they found that an optimized lifting foil obtained a lift:drag ratio of 22:1 while an optimized planing surface only managed 17:1. That said, you have to optimize the foil for the speed, or else you're providing too much lift, and creating a planing surface anywise (or, if it's too small to support planing, you're bobbing like a buoy in a storm-tossed sea). The end result of this thinking (so far) is embodied in the "ladder foil"design, which is used in l'Hydroptere; with multiple, small foils mounted at different heights along a trunk, you get a good balance of lift/speed. Once you reach a high enough speed that your foils are producing to much lift, one or more foil sections get flown out of the water, and the rest below them are now providing just the right amount of lift. I'm not saying that ladder-foils are perfect, though, just trying to illustrate the technology as it stands today; there is ALWAYS room for visionaries to advance the science!

 

Take a 2D truss structure, use the more & smaller elements as you describe above to the very extreme, and the result is I-beam with +-45 deg fibres everywhere, the amount of them reducing on flanges AND 0-degree fibres on flanges. In that case each element is made of just one yarn of fibre and there are certainly a lot of them.

Take a 3D space-frame and use the more & smaller elements as you describe above to the very extreme, and the result is obviously a monocoque.

The difference in spaceframe vs. monocoque is quantitive rather than qualitive, but you treat it like it would be otherwise. Both are the just the same by the principle quoted above in bold type from theoretical point of view.

Therefore according to your own words, the results (monocoque & I-beam) are even more weight efficient in both 2D & 3D cases. In reality that all holds true when size of the I-beam or monocoque can be decided on structural reasons only. In a sea cayack case that means it's so small you can't fit in it, and stability is inadequate.
In some cruising cat the same problem is even greater, hull shell dimensions are not optimum for structural perspective, and some core would be beneficial as a result. This unoptimal sizing is the reason that allows thin skinned construction over spaceframe being competitive in weight alone, but that's certainly a very bad choice in economical perspective.

Also If materials used have not equal strength in all directions like wood or composites do (but metals usually don't), the joins in spaceframe are not only labour extensive, but will be much less optimal in weight / strength (or stiffness) than in metals. Any change of direction of the load from one member to the next in wood construction causes always some significant off-axis loading which is critical and the primary cause for the extra weight needed in joins compared to what would be there by assuming same weight / strength. Even in metal spaceframes that same negative effect in join areas has consequencies in fatigue perspective, if external loads vary like they will in boat construction.

Trusses & spaceframes also work only if external loads are being applyed in predetermined places, as point loads to the join areas. I-beams & monocoques allow distributed loads much beter, but need extra reinforcements on point load areas. That should be the primarily difference between those in practical point of view. Are the loads distributed or point loads.

 

A lifting foil in water close to the surface has maximum pressure difference limited by cavitation at any given speed. When speeds get really high, that means foil performance is significantly limited from what it could be if used on submarine while very deep under the surface. A planing surface doesn't have that limitation, thus the L/D it can have does not drop with speed. Overall it means planing surface will be more efficient than a foil if speed is fast enough. Using supercavitating foils don't change that result.
When high pressure under a planing surface exceeds 3 atm, it still has less than 0.5 times the wetted area of the foil with same planform area. If a foil were to produce the same lift, it can't be 0.5 times the planform area to compensate as that would require negative absolute pressures on top surface which is physically impossible. That means the foil has to have more wetted drag than planing surface in those conditions resulting worse L/D for the foil. There is no way around that fact once speeds are high enough.
The foil still has the significant advantage of allowing smoother ride in waves.

 

sailor2:

I AGREE with you that foils will cavitate & lose significant efficiency at high speeds when they're too near the surface. HOWEVER, a well-designed ladder foil will still have foil sections well beneath the surface that are still providing about 22:1 L. ... that's why ladder foils are better than single inverted "T" foils for variable-speed applications.

Beyond that point, however, I couldn't disagree with you more. A foil CAN be designed with 0.5 times the planform area and provide MORE lift than the planing surface, at ANY realistic speed (i.e. less than 1/4-1/3 the provisional speed of sound through water @ that temperature). The advantage of the hydrofoil is that it develops more lift through AOA AND bernouli's principle; by combining the benefits of the two, you gain. Also, the AOA of a foil can be made more/less agressive as efficiency demands, whereas it would be EXTREMELY difficult to vary the geometry of your planing hullform.
So I guess, yes, you're right in that "There is no way around that fact once speeds are high enough." BUT, you're being extremely unrealistic if you think you're going to be powering a boat fast enough that the water over the "top" surface of the foil will be vacuum-boiling. (at which point any non-jet, water-propulsion system would cavitate for the same reason, and I'd bet on the jet system blowing itself apart under that pressure/flow rate...air propulsion systems [fanjet] may be able to move a boat that fast, but there we're getting WAY off-point for this discussion)

I mean you no disrespect by this post, but I do definitely think your logic was a bit flawed there.

 

Dynamic pressure is defined by 0.5*rho*v^2
take rho as 1020 kg/m^3 for seawater and v = 20m/s ; pressure=204kPa = 2.08 atm. As a comparison 1 atm = 98kPa, so you need to go at least 10 meters deep to get 196 kPa absolute pressure to allow pressure difference to get same level as dynamic pressure at 20 m/s speed by having top surface of the foil at total vacuum and lower surface at 2.08 atm absolute pressure at average. Just notice that max peak pressures are much higher than average pressures and you see that peak pressure at the lower surface will be more than 3 atm absolute.

I don't consider 20 m/s speed out of the question for a boat, therefore I do consider cavitation for those speeds as a realistic issue to be considered and effecting usable foilshape limiting using high Cl values already. Compare those values for max speeds already acheaved by 500m records by sailcrafts. The average speeds are already well above those, not even mentioning peak speeds. At 50 knots even moderate Cl already can cause cavitation, if foil sections are not suitable to avoid it. Speed of sound in water are more like 1500 m/s out from memory.

 

I think you're missing the increased lower-surface pressure on the foil due to deflection (i.e. Angle of Attack) forcing the water beneath the foil down. Hydrofoils don't ONLY generate force through Bernouli's principle, they also develop force through deflection, similarly to the planing hull. Therefore, as long as the foil is deep enough in the water that no air is trailing down to it (tilting the foil's spar can greatly improve this), and enigneered/adjusted so that it isn't vac-boiling the water (thus creating its own cavitation & stalling), I still think they can develop more lift/area AND more lift/drag than a planing hull.

One caveat: I realize that I'm describing an idealized foil here, and that there ARE some flows very near the tips that sill be slightly less efficient, but we're also describing idealized planing surface here, are we not?

Question:
If, by your explanation above, all foils at <10m depth will experience cavitation issues at or near 20m/s, how then do you explain l'Hydroptere's ability to use VERY shallow (less than 2m when flying) foils to maintain lift/stability while traveling at 60+ knots (39+ m/s)? I don't know about the "efficiency" of a planing surface at those speeds, but I am quite sure that a planing surface with as little surface area as those foils would be too unstable to manage. (and one with 1/2 the wetted surface would be nothing short of suicidal)

 

You may wish to consider the wetted surface realities of these boats, which run at considerably more speed (in the neighborhood of 220 mph) than does Hydrop, yet they have a pretty good safety record in spite of their suicidal drivers.

 

Chris, those high powered boats (cats/hydroplanes?) are really wing in ground effect designs (WIIG) and they are also riding on their propellers; you can hardly call them planing designs, more like aircraft with a cushion of air between hull platform and water surface.

 

Hmmm...reverse airfoils to increase downforce, modified aviation FANJET engines (on most, but not all of them now), trimaran with 2 keels & 2 rudders... Yep, in all but one of those pictures there is NO wetted planing surface, as the craft is mid-bounce & completely off the water (though it still has keels & rudders wetted). I don't think this is going to help your increased efficiency argument much though, as they have VERY HIGH (relative to a hydrofoil optimized for such speeds) wetted areas when they're actually touching the water.

I will admit, though, that hydrofoils most likely would not be a good solution for turbine-powered top-fuel tris, as the higher CG from "flying" it above the surface (further than how far they usually float when "planing") would raise their CG and it's already too high for those speeds. As far as useable stability goes, how often have you seen a race day with these things end without at least ONE of them upside down, with pieces broken off? Yes, the driver almost always lives, but the boat more often does NOT...good thing they spent the extra $$$ and weight to reinforce the cockpit, eh?

Last dig: You also have to figure in DURABILITY when considering these racing-"boats"...and, unfortunately, a planing hull will beat a hydrofoil for durability every time (at least with the materials accessible to us today). So, since breaking off a foil would instantly send the racer end-over-end, planing it has to be.