Taping wing joint with fiberglass and epoxy
10-09-2002, 09:14 PM
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Taping wing joint with fiberglass and epoxy
I've just finished the step in building a Flyin'King which calls for fiberglass tape and epoxy (or CA) wrapped around the wing joints. At first glance, it seems like it strengthens the joint.
But since the tape is being glued to balsa sheeting, how is the joint strengthened? Any stresses on the wing will simply tear the sheeting, won't they?
But since the tape is being glued to balsa sheeting, how is the joint strengthened? Any stresses on the wing will simply tear the sheeting, won't they?
10-10-2002, 10:13 PM
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Taping wing joint with fiberglass and epoxy
It's just tying the two wing halfs together as though it were one long sheet of balsa.Glass it to just outside the wing saddle and it will add a lot of strength.
10-11-2002, 10:43 AM
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Taping wing joint with fiberglass and epoxy
The glass tape spreads the loads over a lot of area, and converts the loads to a pull-push on the balsa with the grain, which is the strongest direction.
Yes, if it were acting perpendicular to the grain, the balsa would shatter fairly easilly, but directly with the grain it'll take a lot of stress.
Usually, the fiberglass tape that's just 2 inches wide is thicker than it really needs to be... And the sheeting thickness is more desghned to keep you from punching through with your fingers while handling the plane than for the loads it'll take in flight. For an average .40 size trainer, 1/32 center sheeting would be plenty for flight loads, but you'd break it trying to put the wing on.
Yes, if it were acting perpendicular to the grain, the balsa would shatter fairly easilly, but directly with the grain it'll take a lot of stress.
Usually, the fiberglass tape that's just 2 inches wide is thicker than it really needs to be... And the sheeting thickness is more desghned to keep you from punching through with your fingers while handling the plane than for the loads it'll take in flight. For an average .40 size trainer, 1/32 center sheeting would be plenty for flight loads, but you'd break it trying to put the wing on.
10-11-2002, 12:03 PM
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Taping wing joint with fiberglass and epoxy
I should have added more detail in my original question.
The Flyin'King has an 88" wing. It is built in 3 sections. The center section is about 40" and is centered on the top of the fuselage. The two outer sections have about 4 degrees of dihedral.
The wing joints are planked from the leading to trailing edges. The wood grain of the planking is parallel to the tape. There are plywood joiners and ribs at the joints and they are epoxied together.
Instructions call for a 1" strip of fiberglass tape afixed to the joint. To soak the tape I thinned some epoxy in lieu of using CA.
It seems that an event causing the epoxied joiners/ribs/spars to fail would almost certainly also cause the taped balsa sheeting to fail. So it appears that the epoxied fiberglass tape has another purpose, but I'm not sufficiently experienced to figure it out.
The Flyin'King has an 88" wing. It is built in 3 sections. The center section is about 40" and is centered on the top of the fuselage. The two outer sections have about 4 degrees of dihedral.
The wing joints are planked from the leading to trailing edges. The wood grain of the planking is parallel to the tape. There are plywood joiners and ribs at the joints and they are epoxied together.
Instructions call for a 1" strip of fiberglass tape afixed to the joint. To soak the tape I thinned some epoxy in lieu of using CA.
It seems that an event causing the epoxied joiners/ribs/spars to fail would almost certainly also cause the taped balsa sheeting to fail. So it appears that the epoxied fiberglass tape has another purpose, but I'm not sufficiently experienced to figure it out.
10-11-2002, 11:19 PM
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Taping wing joint with fiberglass and epoxy
Havn't Seen a plane yet with the wood grain parallel with the wing chord, not good engineering practice, although maybe I misunderstood. Does the wing have balsa sheeting from the leading edge to the mainspar. Does it specifically call for the sheeting to be parallel to the chord? Finishing epoxy works better for fibreglassing as its thinner and doesn't leave that sticky residue after it cures. Call back as we want yo help you be sucessful.
10-12-2002, 02:15 AM
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Taping wing joint with fiberglass and epoxy
If you are really worried...
Forget the fiberglass, and shift to silk and yellow glue. 2 layers of light silk equals 2 oz fiberglass for joint reinforcement, and its lighter, and has a finer weave for less filling (Monokote hides it with no fill). The correct weight of silk is "10 mm Habotai." Which is about average scarf weight. (its amazingly strong stuff... 10 times the strength of the same weight of steel. Appx 3 to 4 times the strength of the same weight fiberglass cloth.)
First layer of silk is 1.5 inches on each side of the joint line. You can cover the full bay width each side of the joint line with the second layer if you want, and then the silk reinforces ALL of that balsa. Mix the yellow carpenter's glue with water 50-50, and paint on just enough to make the silk stay in place. The silk will shrink as the glue dries, but you don't want to leave any wrinkles if you can help it.
I've replaced the fiberglass on a lot of models this way. Most noteable is my TameCat .46-.60 size trainer which had 2 mid-airs, lost 6 inches of one wing and 4 inches of the other in R/C combat. The wing center joint didn't budge. My 4*40 done this way flew through a reinforced Gremlin R/C combat wing. You can believe the silk will hold.
Forget the fiberglass, and shift to silk and yellow glue. 2 layers of light silk equals 2 oz fiberglass for joint reinforcement, and its lighter, and has a finer weave for less filling (Monokote hides it with no fill). The correct weight of silk is "10 mm Habotai." Which is about average scarf weight. (its amazingly strong stuff... 10 times the strength of the same weight of steel. Appx 3 to 4 times the strength of the same weight fiberglass cloth.)
First layer of silk is 1.5 inches on each side of the joint line. You can cover the full bay width each side of the joint line with the second layer if you want, and then the silk reinforces ALL of that balsa. Mix the yellow carpenter's glue with water 50-50, and paint on just enough to make the silk stay in place. The silk will shrink as the glue dries, but you don't want to leave any wrinkles if you can help it.
I've replaced the fiberglass on a lot of models this way. Most noteable is my TameCat .46-.60 size trainer which had 2 mid-airs, lost 6 inches of one wing and 4 inches of the other in R/C combat. The wing center joint didn't budge. My 4*40 done this way flew through a reinforced Gremlin R/C combat wing. You can believe the silk will hold.
10-12-2002, 04:05 PM
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Taping wing joint with fiberglass and epoxy
The grain of the planking is parallel to the ribs. The planking covers only the rib bays on either side of the wing joint. The tape is wrapped on the joint from trailing edge to leading edge and back, also parallel to the ribs.
I don't doubt that the epoxied tape is strong. It is the balsa to which it is afixed that I'm questioning.
I see a better use for fiberglas (or silk) tape if on the wing's leading edge. For my beginner flying experiences, this is where I, for one, need strength.
Another good application for the tape is on my Venture60 across the middle of the wing where it attaches to the fuselage. In addition to being the wing joint, this is also where the wing dowels are located. I'd prefer the dowels to rip out the fuselage than to rip out of the wing.
But, again, my experience is insufficient to understand how/why a 1" wide strip of epoxied tape, afixed to balsa sheeting at outer wing joints, adds materiallyl to what already is an internally strengthened wing joint.
I don't doubt that the epoxied tape is strong. It is the balsa to which it is afixed that I'm questioning.
I see a better use for fiberglas (or silk) tape if on the wing's leading edge. For my beginner flying experiences, this is where I, for one, need strength.
Another good application for the tape is on my Venture60 across the middle of the wing where it attaches to the fuselage. In addition to being the wing joint, this is also where the wing dowels are located. I'd prefer the dowels to rip out the fuselage than to rip out of the wing.
But, again, my experience is insufficient to understand how/why a 1" wide strip of epoxied tape, afixed to balsa sheeting at outer wing joints, adds materiallyl to what already is an internally strengthened wing joint.
10-12-2002, 04:26 PM
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Taping wing joint with fiberglass and epoxy
Leverage. The further the structural member is from the "pivot point" the force is acting on, the less material is needed to prevent the structure from breaking.
The fiberglass tape, especially the lower surface portion is well below the effective pivot line which would be jus a bit below the top surface of the high point of the airfoil when exposed to upright flight loads. Fiberglass and other cloth type reinforcements are strongest under tension, and so the load goes to trying to pull the tape apart "the hard way." Since the plane can go inverted, the load line could be along the lower surface of the airfoil, and then the tape on the upper suface would be under tension... and be giving more strength than the lower surface. (whichever is being "compressed" won't do a lot of good in this manner of taking the loads.)
There's also the effect of the tape joning the balsa sheet, which has a good compression load factor.
The sheet grain running parallel to the wing ribs... Did you correctly follow the plans? Its really unusual for the grain not to be parallel to the main spar. Take a couple of scraps of the sheeting. Try pullng straight along the grain and it will be VERY hard to break without bendig the wood to crack it. Try pulling cross-grain and it will shear with a very light load. Same thing happens when you try compression, except its harder to avoid accidentally bending the wood as you push, so it fels like it lts go at a much lower load than it would if the wood remained straight. If you've got a big enough scrap, roll it into a tube with the grain running parallel to the centerline, and its EXTREMELY hard to break in compression or tension along th grain. (you get a bit of this effect in the leading edge section of the sheet, as it makes the curve)
I'd re-read the building instructions and if the sheeting is on 90 deg to the way its supposed to be... rip it off and re-sheet it. The sheeting takes more load than the ply dihedral brace.
The fiberglass tape, especially the lower surface portion is well below the effective pivot line which would be jus a bit below the top surface of the high point of the airfoil when exposed to upright flight loads. Fiberglass and other cloth type reinforcements are strongest under tension, and so the load goes to trying to pull the tape apart "the hard way." Since the plane can go inverted, the load line could be along the lower surface of the airfoil, and then the tape on the upper suface would be under tension... and be giving more strength than the lower surface. (whichever is being "compressed" won't do a lot of good in this manner of taking the loads.)
There's also the effect of the tape joning the balsa sheet, which has a good compression load factor.
The sheet grain running parallel to the wing ribs... Did you correctly follow the plans? Its really unusual for the grain not to be parallel to the main spar. Take a couple of scraps of the sheeting. Try pullng straight along the grain and it will be VERY hard to break without bendig the wood to crack it. Try pulling cross-grain and it will shear with a very light load. Same thing happens when you try compression, except its harder to avoid accidentally bending the wood as you push, so it fels like it lts go at a much lower load than it would if the wood remained straight. If you've got a big enough scrap, roll it into a tube with the grain running parallel to the centerline, and its EXTREMELY hard to break in compression or tension along th grain. (you get a bit of this effect in the leading edge section of the sheet, as it makes the curve)
I'd re-read the building instructions and if the sheeting is on 90 deg to the way its supposed to be... rip it off and re-sheet it. The sheeting takes more load than the ply dihedral brace.
11-11-2002, 06:32 PM
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Taping wing joint with fiberglass and epoxy
The Flying King and Venture have been around for a while, and Bruce Thorp is an excelent designer ( and a real nice guy too) so I'd go with what is on the plans/instructins.
The fiber glass strip is VERY important...case in point
The Sig Kougar has sheeted foam wings and is simply but glued together. There is a 1" glass strip at the center section. My buddy and I dive these at the field full throttle, and do a snap roll right on the deck right in front of us. Mine lasted through about 10-11 years of this untill the elevator pushrod gave way. Eventhough the plane was a mangled mess, the wing center section was perfectly intact.
On the otherhand a buddy bought a Kougar built by someone else. The 2nd or 3rd time he did this diving snap, it sounded like a bomb went off in the plane. The fuse went almost a 1/3 of a mile (litterly), and the wings landed right infront of us. The center senction was glued real well, but there was NO glass cloth.
I don't understand the mechanics of how it helps, but I am a firm believer that even this small 1" strip works very well as it was so graphically demonstrated.
The fiber glass strip is VERY important...case in point
The Sig Kougar has sheeted foam wings and is simply but glued together. There is a 1" glass strip at the center section. My buddy and I dive these at the field full throttle, and do a snap roll right on the deck right in front of us. Mine lasted through about 10-11 years of this untill the elevator pushrod gave way. Eventhough the plane was a mangled mess, the wing center section was perfectly intact.
On the otherhand a buddy bought a Kougar built by someone else. The 2nd or 3rd time he did this diving snap, it sounded like a bomb went off in the plane. The fuse went almost a 1/3 of a mile (litterly), and the wings landed right infront of us. The center senction was glued real well, but there was NO glass cloth.
I don't understand the mechanics of how it helps, but I am a firm believer that even this small 1" strip works very well as it was so graphically demonstrated.
11-12-2002, 01:51 AM
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Taping wing joint with fiberglass and epoxy
The best thinner for epoxy is DON'T thin it.
Rather than thinning 30 min epoy, get 2 hour finishing epoxy. Adding alcohol to 30 min epoxy can slow the cure rate to more than 4 hours anyway.
When you add alcohol, you WILL be adding water to the epoxy. (alcohol absorbs water from the air... you can't stop it.) Depending on how much water get in... the epoxy can get porous and "gummy." The result can get to the point of stuff that seems to never completely set up and is so porous that there's no fuelproofing value. Long before its that bad, the epoxy is worthless for structural purposes.
Rather than thinning 30 min epoy, get 2 hour finishing epoxy. Adding alcohol to 30 min epoxy can slow the cure rate to more than 4 hours anyway.
When you add alcohol, you WILL be adding water to the epoxy. (alcohol absorbs water from the air... you can't stop it.) Depending on how much water get in... the epoxy can get porous and "gummy." The result can get to the point of stuff that seems to never completely set up and is so porous that there's no fuelproofing value. Long before its that bad, the epoxy is worthless for structural purposes.
11-12-2002, 09:14 AM
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Taping wing joint with fiberglass and epoxy
I like to use denatured aocohel to thin epoxy, cheaper and smells better, also, some rubbing aocohel contains a small amount of oil.
Jerry
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11-12-2002, 12:48 PM
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Taping wing joint with fiberglass and epoxy
IF you intend to thin it, denatured alcohol / methyl hydrate is what I use. And yo only need a few drops per ounce. Don't thin it too much, as you will weaken it.
Heating it is the best way to 'thin' epoxy, but it will cut the cutre time in half. either heat it up wile mixing, or heat it with a heat gun / hair dryer while applying. Just don't burn yourself!
tack the glass in place with a shot of spray adhesive. add the resin / epoxy, and scrape away the extra, and to leave it smooth.
New West Systems make an epoxy that flows like water! You can find it at better boating shops, as it is mainly a boat repair compund. Good stuff though! But... Ca-Ching!!! $$$$$$$$
There's an article in this months RCM magazine on exactly this subject.
Heating it is the best way to 'thin' epoxy, but it will cut the cutre time in half. either heat it up wile mixing, or heat it with a heat gun / hair dryer while applying. Just don't burn yourself!
tack the glass in place with a shot of spray adhesive. add the resin / epoxy, and scrape away the extra, and to leave it smooth.
New West Systems make an epoxy that flows like water! You can find it at better boating shops, as it is mainly a boat repair compund. Good stuff though! But... Ca-Ching!!! $$$$$$$$
There's an article in this months RCM magazine on exactly this subject.
