Jarno (AutoReply) has talked about this enough that I finally got moved to do some analysis. He is a pretty smart guy, but the published analysis he shared was opaque enough that I wondered what liberties they took, and how they did things. So, I did some analysis. First the background...
Standard wisdom on wing design is that increasing aspect ratio while maintaining wing area is positive for performance, but that it results in a heavier airplane. The performance improvements happen because the higher aspect ratio and greater span have lower span loadings and less induced drag. The more weight is because an increased portion of the wing is skinned (only the part in the fuselage is not) and because the bending loads in the spar are bigger, heavier spar results. Both skin and spar heavier is a heavier wing...
Along comes Jarno with all of his experience and analytical approach, and he says that there really is no weight penalty in increasing span. Hmm, if you continue to build with the same cloth on the skin, weight should go up slightly, and the spar will get heavier, but a massive core will get lighter. I decided to run some numbers for an airplane of approximately my numbers.
Specifics: 1700 pounds supported by the wing mount, 100 ft^2 of wing area, designing to 6 g and FOS of 2.0, untapered, aspect ratios of 6, 8, 10, and 12. Aspect ration of 12 puts span at 34.68 feet, about as big as you can fit in a standard T hangar. Wing layout is two pieces connecting through the fuselage like a sailplane with a pair of pins through the main spars and drag spars each connected to the fuselage with one pin. Construction is hot wired flotation billet (2lb/ft^3), skin and drag spar is 22 oz/yard TRIAX vacuum bagged on, and the spar is tailored spanwise with skin bending stiffness included (for tailoring) until nearly at the fuselage where all of the bending has to be carried by only the spar. Trailing edge is 100% flaps and ailerons, and I did not count weight back there. Additionally, I did no checks for wing flutter modes. Anybody who can help on that part? I want to talk to you if you do...
Analysis methods. I use one program to get shear and moment and torsion distributions along the wing, 15 stations, copy that into my composites solver, where I analyze and tailor the caps and shear web by station, then calculate the total spar weight, smoothing from station to station. Yeah, I did not work out the exact number of plies and where each stops, this was a lot of effort as it was, I was not going to go to all of that fuss on each one for an academic exercise…
First I did spars like on my bird. Spar caps are UNI Tape 3” wide and 0.025” thick, number of plies tailored along the span, open wet layup. Rest of the spar is a 1” PU foam shear web core, 2 ply 7 oz UNI shear web wrapping the outside of the spar, the rest of the shear web is inside the channel, 7 oz UNI lapping 2” of the caps, shear webs are open wet layups. Conventional wisdom holds, as AR goes up, foam got lower, skin weights got a little higher, spar grew in weight more than the rest of that stuff got lighter. Wing weight was 187, 200, 216, 241 lbs. Going to an AR of 12 from 8 costs you 41 pounds. Not a good trade… Interestingly, the outer third of the span has the spar down to minimum gages, I tapered the caps to 2" through there, so a tapered planform should save some more weight.
Second iteration was doing the spar caps from Graphlite rectangular rods (0.06 x 0.18). Both the caps and webs are tailored spanwise. Minimum gages are also present in the outer third of the span. Hmm, dead heat. Growing span does increase spar weight but about as much as the core and skin combined got lower. Wing weights were 182, 181, 182, and 183 lbs. Hmm, no weight penalty worth talking about to go from AR of 8 to 12, and even at AR of 6, the Graphlite spar system is lighter that the glass.
Jarno, somebody else thinks so too. With carbon structures and massive foam cores, you do not save weight by going low aspect ratio. The benefits of increasing aspect ratio come at virtually no increase in weight with massive core wings, so you might as well get better climb, more speed and altitude capability and better glide.
Studies to follow? Do not hold your breath. I am in the midst of annual inspection (airplane is apart and three cylinders are out for overhaul!), doing some gunsmithing and training, still building the homebuilt, return to service flight and breaking in the new cylinders, and hosting a party to celebrate 2000 hours of flight time. Maybe after Sun-n-Fun…
Billski
Standard wisdom on wing design is that increasing aspect ratio while maintaining wing area is positive for performance, but that it results in a heavier airplane. The performance improvements happen because the higher aspect ratio and greater span have lower span loadings and less induced drag. The more weight is because an increased portion of the wing is skinned (only the part in the fuselage is not) and because the bending loads in the spar are bigger, heavier spar results. Both skin and spar heavier is a heavier wing...
Along comes Jarno with all of his experience and analytical approach, and he says that there really is no weight penalty in increasing span. Hmm, if you continue to build with the same cloth on the skin, weight should go up slightly, and the spar will get heavier, but a massive core will get lighter. I decided to run some numbers for an airplane of approximately my numbers.
Specifics: 1700 pounds supported by the wing mount, 100 ft^2 of wing area, designing to 6 g and FOS of 2.0, untapered, aspect ratios of 6, 8, 10, and 12. Aspect ration of 12 puts span at 34.68 feet, about as big as you can fit in a standard T hangar. Wing layout is two pieces connecting through the fuselage like a sailplane with a pair of pins through the main spars and drag spars each connected to the fuselage with one pin. Construction is hot wired flotation billet (2lb/ft^3), skin and drag spar is 22 oz/yard TRIAX vacuum bagged on, and the spar is tailored spanwise with skin bending stiffness included (for tailoring) until nearly at the fuselage where all of the bending has to be carried by only the spar. Trailing edge is 100% flaps and ailerons, and I did not count weight back there. Additionally, I did no checks for wing flutter modes. Anybody who can help on that part? I want to talk to you if you do...
Analysis methods. I use one program to get shear and moment and torsion distributions along the wing, 15 stations, copy that into my composites solver, where I analyze and tailor the caps and shear web by station, then calculate the total spar weight, smoothing from station to station. Yeah, I did not work out the exact number of plies and where each stops, this was a lot of effort as it was, I was not going to go to all of that fuss on each one for an academic exercise…
First I did spars like on my bird. Spar caps are UNI Tape 3” wide and 0.025” thick, number of plies tailored along the span, open wet layup. Rest of the spar is a 1” PU foam shear web core, 2 ply 7 oz UNI shear web wrapping the outside of the spar, the rest of the shear web is inside the channel, 7 oz UNI lapping 2” of the caps, shear webs are open wet layups. Conventional wisdom holds, as AR goes up, foam got lower, skin weights got a little higher, spar grew in weight more than the rest of that stuff got lighter. Wing weight was 187, 200, 216, 241 lbs. Going to an AR of 12 from 8 costs you 41 pounds. Not a good trade… Interestingly, the outer third of the span has the spar down to minimum gages, I tapered the caps to 2" through there, so a tapered planform should save some more weight.
Second iteration was doing the spar caps from Graphlite rectangular rods (0.06 x 0.18). Both the caps and webs are tailored spanwise. Minimum gages are also present in the outer third of the span. Hmm, dead heat. Growing span does increase spar weight but about as much as the core and skin combined got lower. Wing weights were 182, 181, 182, and 183 lbs. Hmm, no weight penalty worth talking about to go from AR of 8 to 12, and even at AR of 6, the Graphlite spar system is lighter that the glass.
Jarno, somebody else thinks so too. With carbon structures and massive foam cores, you do not save weight by going low aspect ratio. The benefits of increasing aspect ratio come at virtually no increase in weight with massive core wings, so you might as well get better climb, more speed and altitude capability and better glide.
Studies to follow? Do not hold your breath. I am in the midst of annual inspection (airplane is apart and three cylinders are out for overhaul!), doing some gunsmithing and training, still building the homebuilt, return to service flight and breaking in the new cylinders, and hosting a party to celebrate 2000 hours of flight time. Maybe after Sun-n-Fun…
Billski
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