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Discussion Starter · #1 ·
okay, getting ready to start my hood layup (making my own)
and nobody has ever been able to answer why all the CF makers recommend hoodpins and why the CF aftermarket hoods are so heavy
i say heavy cause the lightest one i've seen posted was somewhere in the 13 - 15 lb range
(i have a working background in CF, ie we mfgr some products using it, mainly tube structures and i've worked with CF building a couple experimental category airplanes).
on the subject of the wgt of the CF hoods being offered, it's not just my opinion about they're seeming to be unnecessarily heavy - the composites engineer we consult with agreed with me, we would have expected a strong CF hood to be in the 4-5 lb range without a clearcoat or painted, add 2 -3 lbs for a heavy epoxy clearcoat and maybe little less for a painted hood
i've looked (google'd, etc) for incidents of CF hood failures on the web and only found two, the most recent being a S4 where the hood (installed without hoodpins) apparently opened at speed and slapped back into the windshield
there were cracks about 6-7" to the rear of the front edge of the hood
but don't know if those occurred prior to the hood releasing from the hood latch or after it hit the windshield
the engineer i consult with "suspects" the hoods out there are being offered by folks with no composite engineering background and laying CF up like you would fiberglass and most folks layup fiberglass like you would for a boat - boat layup is really crude (and heavy) and done that way as it's an economical (laborwise) approach to building a large structure out of fiberglass
but back to point - where weight + strength is the focus, as in aviation applications, CF is used. Properly designed, that hood bare of any metal fittings (hinge and latch) should not weigh more than 5 lbs maybe 6 if you overbuild. In aviation, for fuselage structures where you have similiar loads and load paths, they'll sandwich CF or even fiberglass onto 1/4" thick rigid foam and that gives the structure rigidity and flex or distortion resistance - something to do with spacing 2 layers of CF skins 1/4" apart gives it more structural rigidity than if the two layers were laid up without the 1/4" spacing
not sure of the proper engineering term, but CF has very little lateral strength, ie a sheet of CF laid up will springboard side to side if unsupported same as a thin sheet of stiff plastic would - in canoes they refer to the canoe's hull "oil canning" - that's the movement or strength i'm referring to
This method also would seem to address the hoodpin issue - or rather help eliminate the need for them
if those cracks in the S4 CF hood i mentioned earlier, occurred before the hood released and were the cause of the hood releasing - if they did, i can only speculate they were from "oil canning" from pressure from under the hood, ie as speeds build, increased pressure from speed would cause varying amounts of flex as pressure increased and possibly the existence of buffeting from external air flow, could cause fatigue cracking at the flex points
given that spacing two layers of CF 1/4" apart would eliminate the "oil canning", those cracks wouldn't occur.
then designing in and laying in some ribbing (again foam and CF) on the underside would give you a hood that was rock solid
but i'm speculating, not having seen a hood in person that failed
any input from folks that have seen a cracked CF hood would be appreciated
and btw, i'm going to make a mold for the hood but only making one hood - the mold will be available for sale if anyone's interested
my hood will have a centered hood scoop similiar the new Ferrarie superamerica but the bulges for the hood scoop, both on the top surface and undersurface, will be removeable foam inserts so mold will be useable for OEM hood shape lay up use
my main reason for doing my own CF hood is one that is as light as possible, under 8-9 lbs painted


Modified by larryccf1 at 12:28 PM 9-10-2005
 

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Re: CF hoods - questions for a RKI (larryccf1)

Larry, will you be using a vacuum bagging process for this part or will it be cured in an autoclave? You are correct that traditional composite layup (air dry in a mold) is quite crude as it oversaturates the composite with resin, which accounts for the majority of the weight. Vacuum bagging provides positive pressure over the entire surface of the mold to squeeze out excess resin creating a more efficient assembly (I know you know this but other readers may not). As you mentioned the aerospace industry, which lead the way in composite use, takes this to an additional level by the addition of heat AND constant pressure making the most efficient composite assembly. It would be interesting to find out if other commercially available CF hoods are cured in an autoclave. I can't imagine that they are AT LEAST using a vacuum process. Also I wonder if they are using some type of finite element analysis to strengthen the assembley where necessary and lighten up on the materials where not required.
As you know, regarding the core material you suggested, seperating the skins of the CF works the same way as the flange in an I beam and strengthens the assembly tremendously with little weight increase from the core by further displacing the surfaces that are experiencing compression and tension from one another. This is all done without appreciably increasing weight. A great idea http://****************.com/smile/emthup.gif
 

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Discussion Starter · #4 ·
Re: CF hoods - questions for a RKI (GTItraveler)

on the vacuum bagging, nope - don't a vacuum pump that would handle that size mold
but as another project, i have a "fred flintstone" approach that works as well as vacuum bagging
you might get a kick out of this
i'm making 4X8' CF panels for a trade show display stand
in the pix below, have laid up on the curved table a 1/8" pc of 4X8' masonite, and laid up a single layer of CF, then layer of peel ply on top of that, some bleeder ply (same as used in vacuum bagging), then layer of perforated plastic, then a pc of 4X8' sheet of 1/4" masonite that has a 1/2" thick pc of foam bonded on the bottom side to press against the underlying layers
then over that a sheet of dacron cloth (4' wide strip) that is anchored at the "hinge" end and at the other end a ratcheting wench with a tube welded on for a take up spool
i can pull that cloth pretty tight and evenly press that foam down to about 1/4" thickness pretty evenly the full area of that board
saves me time in setting up for vacuum bagging and the bagging materials, tear down time etc
that whole affair is laid up inside garage (i love working in A/C environment) but put wheels on one end to roll out into driveway
covered tightly with a clear plastic tent i've measured 175 - 180 degrees under the tent just from sunlight & greenhouse effect of the plastic tent
pix of press table with a panel being laid up - ou can see the ratchet wench on far side of the near end

and a pix after layup has cured and covers pulled off

for the hood i'd use a different epoxy to be able to post cure up to 250 - 275
but back to your point - composites outfit offered a seminar (Hexcel) and basically the nut of it, while 43% wgt ratio (epoxy to cloth wgt) is optimal strength factor, even vacuum bagging you're lucky to pull enough epoxy out to get it down into 55-60% range
even if they aren't vacuum bagging, that doesn't explain the wgt they're running - and suspect their at least smart enough to be using pre-impregnated cloth
for those who didn't follow the above every epoxy resin has it's own set of tables for saturation ie, on carbon fiber, resin "xxxx" achieves full strength at 43% ratio, ie if cloth is 9 0z per sq yard wgt CF, then it only takes 43% of 9 ounces of epoxy XXXX per square yard for maximum strength
problem is if you just poured that amt of epoxy onto a square yd of 9 oz cloth, you'd never get it properly saturated evenly - so you have to mix and pour out 8 - 10 ounces of epoxy per sq yard, work it into the cloth and then squeege out what you can, lay some peel ply on top - the peel ply acts like a blotter to pull some more out of the cloth,
the bleeder ply on top of that further acts as a blotter or absorbent to epoxy wicking thru the peel ply and the epoxy wicking thru is being pulled or squeezed out by the pressure from a vacuum pump or from the pressure of the foam press thingee pixed above
 

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Re: CF hoods - questions for a RKI (larryccf1)

here is a pic... not the best one of a kid that had his CF Hood fly open..
not at the most recent WF but the yr before before

I guess he was doing like 100 MPH... No Hood pins...
the hood flew back... cracked the windshield.... and put a good dent in the roof line above
the windshield...
but the hood held together pretty good...
Get Hood pins !!!
 

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Discussion Starter · #7 ·
Re: (FiftyPence)

fiftypence
nope, we actually have running water and indoors WCs here
that's a trailer i use for storage
needs a god washing & a coat of wax
vdubbin0 - if hood didn't crack before letting go or lifting up, suspect hood latch wasn't properly aligned or adjusted
got a feeling 50% of the reason the hood mfgrs recommend hoodpins is to prevent liability and liability argunments from incidents involving hood latches not properly adjusted


Modified by larryccf1 at 3:00 PM 9-10-2005
 
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