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#21
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Lift pins
On Saturday, July 27, 2019 at 6:13:06 AM UTC-6, wrote:
On Saturday, July 27, 2019 at 1:02:23 AM UTC-4, Sky Surfer wrote: Interesting discussion.Â* Another question for Bob K. and other experts:Â* If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like?Â* Mostly foam surrounded by many layers of fiberglass?Â* About how much glass versus foam?Â* Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator. Two very strong beams(caps) at the top and bottom to handle tension and compression. Fillers(spacers?) between that handle the shear loads between the caps and also provide for strong pins that handle bending loads on many ships like Schleicher. Others(the "Glasflugel method) use pins on the ends of the spars that plug into sockets on the root rib of the opposite wing to handle bending. In all cases there are many plies of cloth to tie the 2 webs together and handle shear loads. The "spacers" are commonly plywood or solid fiberglass for portions of high(pin)loads, and foam for the balance. Shear layers are most commonly glass. UH 28 layers of glass/resin bind the spar caps and shear web of my Kestrel 19. Frank Whiteley |
#22
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Lift pins
I will state, I am not an expert on internals of a FRP spar.....
My knowledge.....mostly wood laminations (not including balsa at the root for spar) with glass or CF for ultimate strength. Further out, yes, maybe balsa for reasonable strength and lighter weight. Engineering basic is...."the more you make 3 dimensional, the better...", thus Origamy works with thin paper. There is always diminishing returns...eventually you get past one engineering segment and start to lose on others.... I will let real engineers (ME or aeronautical) to dispute my comments. |
#23
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Lift pins
On Friday, July 26, 2019 at 10:02:23 PM UTC-7, Sky Surfer wrote:
Interesting discussion. Another question for Bob K. and other experts: If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like? Mostly foam surrounded by many layers of fiberglass? About how much glass versus foam? Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator. Spanwise fibers for tension and compression in the spar caps at the top and bottom to react accumulated tensile and compression forces due to bending. Bias fibers in between to react shear. Localized reinforcements to react discrete loads at main pins and root rib. Some manufacturers use some wood, but not me. The shear web between the upper and lower spar caps is usually a composite sandwich similar to the wing skin; for most of the span it's 6mm to 10mm PVC foam with one to three plies of bias cut (+/-45) fiberglass or carbon on each side. The unidirectional fibers in the caps have in the past mostly been fiberglass or carbon fiber tapes or raw tows. These days carbon fiber spars are usually made of prepreg carbon and autoclaved. In my shop we use strips of a pultruded unidirectional carbon fiber product called Graphlite. Composites usually have lower compression strengths than tensile strengths, so sometimes the spar caps are asymmetrical in depth, with a deeper section for the upper spar cap than for the lower. At the spar stub, the spar is wrapped with several plies of fiberglass or carbon fiber to react shear due to the moment applied by the force couple between the attachments between the two wing spars. The wing spar tapers in depth (top to bottom), to fit inside the tapering wing. The spar caps are also usually tapered in one or both dimensions to tailor their strength and stiffness to bending loads that diminish rapidly as you go from root to tip. In my gliders the spar caps are about 0.37" deep from root to tip, and taper in width (front to back) from 1.76" at the root rib to about nothing at the end of the 18m tip extension. Out there the loads are so low that the wing skin itself has enough strength to handle bending loads. If you want to see this stuff in action, come by our workshop in Arnold CA. We are just now dressing out a pair of spars to go into the next wingset. Thanks, Bob K. |
#24
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Lift pins
OK....stupid question....most FRP works better in tension (from what I know) than compression.
Sooooo.....why is top layer of spar thicker than bottom (I will assume.....yes, I know what assume can mean....) since I will assume bottom of spar has more tension than top (which I will assume is in compression). Not picking, just asking based on your post.... |
#25
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Lift pins
On Monday, July 29, 2019 at 6:27:07 PM UTC-4, Charlie M. (UH & 002 owner/pilot) wrote:
OK....stupid question....most FRP works better in tension (from what I know) than compression. Sooooo.....why is top layer of spar thicker than bottom (I will assume.....yes, I know what assume can mean....) since I will assume bottom of spar has more tension than top (which I will assume is in compression). Not picking, just asking based on your post.... More cross section is required to resist buckling. UH |
#26
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Lift pins
On Monday, July 29, 2019 at 3:27:07 PM UTC-7, Charlie M. (UH & 002 owner/pilot) wrote:
OK....stupid question....most FRP works better in tension (from what I know) than compression. Sooooo.....why is top layer of spar thicker than bottom... As Uncle Hank says, the upper spar cap is generally in compression, and the lower cap is in tension. The compression force in the upper spar cap is about the same as the tensile force in the lower cap. But since composites are generally weaker in compression, you might want to give the upper spar cap more cross-sectional area so that the stress (force per unit area) is lower. Or, another way of looking at it is that you are leaving some unnecessary material out of the lower spar cap since you don't need as much there to react the tensile forces. The result, of course, is a set of wings that can support a greater load factor in normal flight than while inverted. But since most gliders are neither intended nor used for high-g inverted flight, it's a reasonable compromise, especially when you're dealing with relatively low strength-to-weight materials like wood or fiberglass. With my gliders, I use the same amount of carbon in both the upper and lower spar caps. It makes it easier to fabricate the wing spars, and with the high-strength Graphlite material the weight penalty is negligible. Thanks, Bob K. |
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