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Building Tips and technical articles. => Building techniques => Topic started by: Motorman on July 30, 2023, 09:19:43 PM
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I see allot of kits that have spars made from balsa sticks. When I had my RSM Tundergazer kit, the plans showed 2 balsa sticks with CF laminate glued in the middle. It said "don't skip this step". So, I developed a way to do this with a glass table, parchment paper, aluminum straight edges and thin CA.
Two Balsa sticks 3/32 x 1/4 with Dave brown .005" CF laminate cut and sanded in the middle. Just clamp it together and soak the edge with CA. Comes out very light, straight and strong glued into the ribs with CF vertical. Used it on 2 Warren truss wings with great results.
Just wondering if it's common to go to all that trouble or, do most just use plain balsa sticks?
MM 8)
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I see allot of kits that have spars made from balsa sticks. When I had my RSM Tundergazer kit, the plans showed 2 balsa sticks with CF laminate glued in the middle. It said "don't skip this step". So, I developed a way to do this with a glass table, parchment paper, aluminum straight edges and thin CA.
Two Balsa sticks 3/32 x 1/4 with Dave brown .005" CF laminate cut and sanded in the middle. Just clamp it together and soak the edge with CA. Comes out very light, straight and strong glued into the ribs with CF vertical. Used it on 2 Warren truss wings with great results.
Just wondering if it's common to go to all that trouble or, do most just use plain balsa sticks?
MM 8)
I can only speak for myself, but I do not use CF to reinforce more than about 12" from the CL. In a different lifetime we could get light straight grained spruce which made wonderful spars. I have used your method in the top spar of unlimited sailplanes with the CF horizontal, and it is effective, but we were dealing with stress far in excess of what we experience in one direction only. I am not sure if having both spars CF reinforced might make the wing too stiff or, for that matter, if too stiff is even a bad thing.
Ken
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I never have. But I've never built a plane where the plans called for it. The ground breaks my planes before any structural failure in flight. Never had one break up in flight and I've never had one not hit the ground if I fly it enough. Don't think the CF reinforcement would help my particular skill set.
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I just use plain balsa sticks, but then, I don't win the Nationals.
The only real way to know is to build some wings then test them to destruction. Then, if it's a proper engineering environment, have a never-ending discussion about how well the tests reflect the actual flight loads.
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I just use plain balsa sticks, but then, I don't win the Nationals.
The only real way to know is to build some wings then test them to destruction. Then, if it's a proper engineering environment, have a never-ending discussion about how well the tests reflect the actual flight loads.
Fortunately, in this case, David and others have done the work for you, you need the spar as it is designed.
This particular design was a Ted Fancher idea, and was arrived at sitting at lunch one day at Harry O’Shortals, a (mercifully) closed greasy spoon in Napa.
Brett
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Fortunately, in this case, David and others have done the work for you, you need the spar as it is designed.
Yes, I should have put something in there about just following the plans as given if you didn't design the thing -- or understand that if you do, it's now your design and your blame if anything goes wrong.
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Just to explain it to the slow-learning armchair structures nerds, can someone please explain to me the science behind this?
I understand that having the carbon web vertically in the spar makes sense, section modulus and all that sort of thing, but adding balsa to both sides seems a bit redundant. Surely sticking the web to one side of the balsa spar would achieve the same result? Is having balsa on both sides to aid in adhesion to the rest of the structure?
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Just to explain it to the slow-learning armchair structures nerds, can someone please explain to me the science behind this?
I understand that having the carbon web vertically in the spar makes sense, section modulus and all that sort of thing, but adding balsa to both sides seems a bit redundant. Surely sticking the web to one side of the balsa spar would achieve the same result? Is having balsa on both sides to aid in adhesion to the rest of the structure?
You need something to keep it from buckling under compression. It may or may not be the best possible solution, but its pretty simple and requires no other real changes, note that Motorman could just replace the spar with something else and not make any significant structural changes. That would be a mistake, of course, because all the simple options are either much heavier (spruce) or much weaker (plain balsa).
Very few people can appreciate how much load you get on these wings, both from cornering much, much harder, and from being able to fly them in almost absurd conditions. This biggest problem with designing these airplanes is not how to make them lighter, or how to get slightly better cornering, it is keeping them together for any length of time.
Brett
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Carbon lamination info copied from the Thunder Gazer building manual: https://www.rsmdistribution.com/guides/tgmanual.pdf
Stab/Elevator lamination instructions:
- Laminate ST1, ST2 and E1 using medium CA.
- Laminate Carbon Fiber .007” strip to LE and TE as follow: cut stock to length with some spare, cut CF strip the same, and scuff both sides of the CF with 80 grit sandpaper (be very careful with CF splinter).
- Laminate CF to balsa wood with medium CA holding the balsa against a long metal ruler to avoid warps. I wrap some masking tape on the finger and run over the CF to adhere firmly on the balsa, and then laminate the second balsa layer.
- These laminations are required for leading edge and trailing edge of stabilizer (check the plan stab cross section detail) and DO NOT SKIP this step!
Wing Spar lamination instructions:
- Laminate the .007” CF strip to the 3/32 x 3/16 laser cut balsa spars as you have done with the stab.
- Be sure to scuff the CF with 80 grit sandpaper, stretch the CF along a metal ruler and with medium CA glue the spar following the metal edge to avoid warps.
- Turn it over and finish the lamination to the other side. You will end up with a 3/16 square spar with a CF strip laminated within the balsa, running vertically.
- Sand the surplus CF strip to have a perfect straight spar, be really picky here, this will pay off when finishing the wing.
- Now slip the spar top and bottom to the ribs and be sure to use it vertically (when looking over the wing you will see the 3 laminations lines.
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You need something to keep it from buckling under compression. It may or may not be the best possible solution, but its pretty simple and requires no other real changes, note that Motorman could just replace the spar with something else and not make any significant structural changes. That would be a mistake, of course, because all the simple options are either much heavier (spruce) or much weaker (plain balsa).
Very few people can appreciate how much load you get on these wings, both from cornering much, much harder, and from being able to fly them in almost absurd conditions. This biggest problem with designing these airplanes is not how to make them lighter, or how to get slightly better cornering, it is keeping them together for any length of time.
Brett
Having a think about it, laminating both sides does sort of lend it's to a belt and braces approach, as we know having the carbon delaminate from the balsa will allow it to buckle in very short order, I guess having two gluing surfaces may mitigate a buckling failure through delamination of one side.
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Carbon lamination info copied from the Thunder Gazer building manual: https://www.rsmdistribution.com/guides/tgmanual.pdf
Stab/Elevator lamination instructions:
- Laminate ST1, ST2 and E1 using medium CA.
- Laminate Carbon Fiber .007” strip to LE and TE as follow: cut stock to length with some spare, cut CF strip the same, and scuff both sides of the CF with 80 grit sandpaper (be very careful with CF splinter).
- Laminate CF to balsa wood with medium CA holding the balsa against a long metal ruler to avoid warps. I wrap some masking tape on the finger and run over the CF to adhere firmly on the balsa, and then laminate the second balsa layer.
- These laminations are required for leading edge and trailing edge of stabilizer (check the plan stab cross section detail) and DO NOT SKIP this step!
Wing Spar lamination instructions:
- Laminate the .007” CF strip to the 3/32 x 3/16 laser cut balsa spars as you have done with the stab.
- Be sure to scuff the CF with 80 grit sandpaper, stretch the CF along a metal ruler and with medium CA glue the spar following the metal edge to avoid warps.
- Turn it over and finish the lamination to the other side. You will end up with a 3/16 square spar with a CF strip laminated within the balsa, running vertically.
- Sand the surplus CF strip to have a perfect straight spar, be really picky here, this will pay off when finishing the wing.
- Now slip the spar top and bottom to the ribs and be sure to use it vertically (when looking over the wing you will see the 3 laminations lines.
i have built this TG about 7 years ago ( and written this manual back then) the plane is still in good service and i think the lamination design is great and light. A bit expensive but worth the extra work. I hardly stray from a well thought design, and every RSM design is a class of new and amazing ideias, Eric Rule "Rules". I came back to C/L building after a 10 years hiatus building Jets and 1/3 scale r/c models and the TG was an eye opener on design, materials and lightness. So glad i returned with such fantastic machine. For a r/c builder , carbon fiber and innovative ideas are common ground, so i had no second thoughts on that