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
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
