If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Gelcoat Crazing and Structural Integrity
Hi,
My first glass glider was a cheap, but well-crazed, 15 meter ship that I flew for 5 years before trading up to a newer model. Other then potential performance issues (if not sanded smooth) ... do we have any cases where moderate/heavy crazing led to damage to the underlying glass ... which grounded the ship at annual - or which caused an in flight failure. If you have crazing, but sand and wax the ship occasionally - does this provide sufficient protection - say for 5 years - until you got around to refinishing. thanks, KK |
#2
|
|||
|
|||
Gelcoat Crazing and Structural Integrity
On Mar 5, 2:21*pm, "Ken Kochanski (KK)"
wrote: Hi, My first glass glider was a cheap, but well-crazed, 15 meter ship that I flew for 5 years before trading up to a newer model. Other then potential performance issues (if not sanded smooth) ... do we have any cases where moderate/heavy crazing led to damage to the underlying glass ... which grounded the ship at annual - *or which caused an in flight failure. Hi Ken, I have been grinding out gelcoat cracks for 35 years and never found any of them that didn't stop at the fiferglass structure. On ships with thick gelcoat (DG's) the 'working' back & forth on each side of a large crack can leave an etched mark in the fiberglass structure, but examination with a magnification glass shows no broken fibers, just resin that has flaked away in the emmediate area adjacent to the crack. I have found that painting on a fresh coat of resin will make this 'etched' make disappear. I know of no inflight failuers caused from simple gelcoat cracks. I believe it's a good idea to have your local glider repair guy take a look to be sure your crack isn't a tell- tail indicator of underlying structure that has moved (failed). If you have crazing, but sand and wax the ship occasionally - does this provide sufficient protection - say for 5 years - until you got Yes around to refinishing. thanks, KK |
#3
|
|||
|
|||
Gelcoat Crazing and Structural Integrity
JJ Sinclair wrote:
On Mar 5, 2:21 pm, "Ken Kochanski (KK)" wrote: Hi, My first glass glider was a cheap, but well-crazed, 15 meter ship that I flew for 5 years before trading up to a newer model. Other then potential performance issues (if not sanded smooth) ... do we have any cases where moderate/heavy crazing led to damage to the underlying glass ... which grounded the ship at annual - or which caused an in flight failure. Hi Ken, I have been grinding out gelcoat cracks for 35 years and never found any of them that didn't stop at the fiferglass structure. On ships with thick gelcoat (DG's) the 'working' back & forth on each side of a large crack can leave an etched mark in the fiberglass structure, but examination with a magnification glass shows no broken fibers, just resin that has flaked away in the emmediate area adjacent to the crack. I have found that painting on a fresh coat of resin will make this 'etched' make disappear. I know of no inflight failuers caused from simple gelcoat cracks. KK's question is an interesting one indeed (to me, anyway!). And JJ's considerable anecdotal experience matches what my own engineering background (aerospace - essentially ME with some extra finite element and fluid dynamics thrown in) leads me to conclude...at least about pre-carbon-reinforced FRP gliders. In essence, the FRP structure is a relatively flexible, load-carrying, substrate 'prettily covered' by relatively inflexible, weak, gel coat. (Imagine climbing rope covered in chocolate.) Which material is more likely to crack? What mechanism exists to propagate cracks from the stiff/weak material across the boundary into the flexible/strong material? Logic aside, massive amounts of FUD (Fear, Uncertainty, Doubt) permeate the glider pilot and the A&P/IA communities. Happily (as owner of an experimental, 1st-generation FRP ship, who has put all but 30 hours on the airframe), I haven't been forced to actively deal with FUDmeisters. OTOH, I/my club *has*. We had (~10 years ago) a G-103 fail annual for: 1) cracks radiating from the corners of spoiler boxes; 2) spanwise cracks along the sharp-radiused leading edge bits; and 3) chordwise cracks along some portions of the lower wing surfaces. The larger portions by far of both wings were uncracked. The A&P/shop were 'relatively structurally unfamiliar' with glass gliders; they provided a $12k bid to remove/repair-as-necessary *only* the cracked regions. Implicit was any *real* structural cracks would result in considerably higher costs. And, being a club ship, the possibility someone(s) had actually overstressed the glass could not be dismissed out of hand. Spurred by cheapness and theoretically-based suspicion, after obtaining commitments of technical guidance from a German-trained glider repair guy and a 'signoff-IA' familiar with his talents/work, I helped convince the club/members to provide grunt labor ourselves. About 4 months later (overwinter), we had 100% de-cracked, urethaned wings and a re-annualled G-103; total cost ~$3000. Not a single crack had propagated into glass...IOW, add one anecdotal experience to JJ's. I believe it's a good idea to have your local glider repair guy take a look to be sure your crack isn't a tell- tail indicator of underlying structure that has moved (failed). At the risk of putting words into JJ's mouth, experienced glider repair people tend to be comfortable doing such precautionary checks using the non-invasive 'tap method' assisted in some cases with selective gel coat removal. Point being, that the mere presence of gel coat cracks on 1st-generation FRP ships is NOT sufficient reason to panic/ground-the-ship/throw-massive-sums-of-money-and-labor-at-it/etc. The reason for the above's '1st-generation FRP' proviso is carbon-reinforced/all-carbon ships are stiffer, reducing the relative stiffness mismatch between the structure and gel-coat. They're also newer, with less history to draw upon. If you own and/or know the history of such a crazed bird, and have any issues (your own doubts, or someone else's FUD input) with them, and your finances aren't unlimited, take it to 'someone JJ-like' for assessment be*fore* assuming the worst. (I am not a professional glider repairer, have never played one on TV, nor have I ever met/worked-with/spoken-with one named 'JJ'. It simply goes against my grain to spend glider-bux in the manner of Jimmy Buffet...i.e. foolishly! That noted, I thank JJ for having the courage to publicly state what too often gets the 'skeleton in the family closet' treatment. FUD forever!!!) Regards, Bob - Flame Suit On - W. |
#4
|
|||
|
|||
Gelcoat Crazing and Structural Integrity
JJ Sinclair wrote:
On Mar 5, 2:21 pm, "Ken Kochanski (KK)" wrote: Hi, My first glass glider was a cheap, but well-crazed, 15 meter ship that I flew for 5 years before trading up to a newer model. Other then potential performance issues (if not sanded smooth) ... do we have any cases where moderate/heavy crazing led to damage to the underlying glass ... which grounded the ship at annual - or which caused an in flight failure. Hi Ken, I have been grinding out gelcoat cracks for 35 years and never found any of them that didn't stop at the fiferglass structure. On ships with thick gelcoat (DG's) the 'working' back & forth on each side of a large crack can leave an etched mark in the fiberglass structure, but examination with a magnification glass shows no broken fibers, just resin that has flaked away in the emmediate area adjacent to the crack. I have found that painting on a fresh coat of resin will make this 'etched' make disappear. I know of no inflight failuers caused from simple gelcoat cracks. I believe it's a good idea to have your local glider repair guy take a look to be sure your crack isn't a tell- tail indicator of underlying structure that has moved (failed). Schleicher has a "Maintenance Instructions" for paint cracks he http://www.alexander-schleicher.de/t...ackrisse_E.pdf Some highlights: * all cracks should be inspected regularly * paint cracks, even deep ones going into the resin top skin, may not be an immediate problem * steps must be taken to prevent further damage from moisture and UV * paint cracking is a problem in the USA and Australia, but not in Europe, and the repair experience accumulated in those countries should be considered. It seemed to similar to what JJ was saying, but from the manufacturer's viewpoint and experience. Perhaps Schmepp-Hirth and DG have similar instructions, and these might be useful to show an inspector balking at approving an annual inspection. -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4 * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org |
#5
|
|||
|
|||
Gelcoat Crazing and Structural Integrity
Eric Greenwell schrieb:
Schleicher has a "Maintenance Instructions" for paint cracks he http://www.alexander-schleicher.de/t...ackrisse_E.pdf Some highlights: * all cracks should be inspected regularly * paint cracks, even deep ones going into the resin top skin, may not be an immediate problem * steps must be taken to prevent further damage from moisture and UV * paint cracking is a problem in the USA and Australia, but not in Europe, and the repair experience accumulated in those countries should be considered. LOL - sorry for the "lol", but when i hear that paint cracking is no problem in Europe... then Germany must be on the moon. Most older ASK-21s (ours are from 198x and 1990) are showing bad cracks. The first got already a new gelcoat and the 1990 one is due any year now. Not for cosmetic reasons but for protection against moisture - here in Germany it sometimes rains..... :-) The old Mini-Nimbus we have (one of the last ones build) did get a new gel-coat in 2000. And that was REALLY neccessary. You were nearly able to hurt yourself on the cracks. So Schleicher is telling total BS there. just my 2c... had to get that out.... CU Markus |
#6
|
|||
|
|||
Gelcoat Crazing and Structural Integrity
Schleicher has a "Maintenance Instructions" for paint cracks he
http://www.alexander-schleicher.de/t...ackrisse_E.pdf another interesting tidbit: " we suggest to spray the sanded surfaces first with polyester fillers, to sand then again, and to respray them finally with a white paint system on a polyurethane basis.." i.e. ditch the gel-coat. John Cochrane |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Cessna 152 spin integrity | Ricky | Piloting | 48 | January 28th 08 10:27 PM |
New Technology Questions The Integrity Of Current Composite Construction | Larry Dighera | Piloting | 1 | October 11th 07 04:35 PM |
Cambridge 302A Data integrity. | Bob | Soaring | 4 | June 6th 07 02:04 PM |
Looking for a DER/Structural | mhorowit | Home Built | 3 | June 25th 06 04:33 AM |