Billski has ardently (but patiently ) extolled the merits of solid wing cores many times (see here, many others), observing that they are often lighter and easier to build than a comparablysized wing with sandwich skins, ribs, flanges, etc. I'm a believer.
And yet, because Weight is the Enemy (tm), and noting that the foam in a typical GA wing might weigh as much as the facings, I'm sure I'm not the first to wonder if we need all of it, or if there is extra (lazy!) foam that can be removed while maintaining the integrity of the wing.
If this is old territory, please just put me straight. I know I've seen the idea mentioned, but I don't recall that it was put out of its misery.
For consideration: Below is a drawing a NACA 2413 core of 48" cord (each square is 1"). The area is about 204in2. If the core is made of 2.1 lb/ft3 foam, then each linear foot of wing core span weighs 2.97 lbs.
If we cut the lightening cores shown below, the core weight is reduced by 36%. Each footspan of foam core then weighs 1.9 lbs, a reduction of just over 1 lb per foot of span.
Considerations:
How to do the cutting: In a Peter Shipol video, his cores were in 4 foot (spanwise) sections. His wing core templates on either end had the wing profile (for hotwiring, as normal) as well as the lightening channels. He heated a length of pipe (maybe EMT conduit?) with a torch and melted a hole from one end to the other of each lightening channel. Then he dropped his nichrome/stainless wire through the hole, hooked it to his transformer and bow, and cut out that channel. It wasn't highprecision work (like a wing outline), it was a oneperson job and went fast. The video is here, see 1:20 through 2:05.
Will the foam be overstressed? I couldn't locate detailed specs for DuPont (formerly Dow) Styrofoam Bouyancy Billet (the foam typically used for massive wing cores), but Billski has previously posted that it has a compressive strength of 70 psi. Another Dow XPS product (“Highload 60”) has a compressive strength of 60 psi and a listed compressive modulus of 2200 psi and a listed tensile strength of 85 psi. No tensile modulus was listed, but a similar XPS foam by BASF has a listed tensile modulus of 4.35ksi. So, I’ll use those numbers as rough standins for the DuPont 70psi buoyancy foam.
Backoftheenvelope inflight loads: Our plane has a typical lightplane level flight wing loading of 15 lbs/ft2. If we are designing for 9G ultimate, that will be 135 lbs/ft2 of lift force. Let’s make the (overly simplistic?) assumption that this lift is produced entirely by reduced pressure on top of the wing and nearambient pressure on the wing’s lower skin. So, if the inside of our wing/voids are at ambient pressure and the area outside the top skin of the wing is 135 lb/ft2 (about 1 psi) below ambient pressure (to produce the needed lift), then the foam core would primarily be in tension from the top laminate to the bottom laminate). If I’ve got that right . . .
Looking at the wing with the lightening channels, the cores are about 1.5” apart at the closest. There are 7 toptobottom “beams” from the nose to the flap hinge, and the sum of their thinnest portions is 30% of the wing chord (without the flap). If our total wing core toptobottom is under 1psi tension (again, oversimplified) , then the concentrated tension through these thin points is 3.3psi. This is well under the 85 psi tensile strength of the foam, and under this load the foam would be expected to elongate just .004” (this is a very conservative estimate, as I assumed the “thin” columns were 6” long. The thin portion is actually relatively short). If we envision the foam will instead be in compression, the loads are similarly quite low compared to the foam’s compressive strength and compressive modulus.
If we vacuum bag the wing during construction, the forces on the foam will be higher than the aerodynamic forces in flight. Assuming 20”Hg vacuum (about 10 psi), the thin column sections will be under about 33 psi of pressure, which is well under the foam’s 70 psi listed compressive strength (which is typically quoted at either 5% or 10% compression, depending on the standard used). If the foam’s compressive modulus is 2.2ksi, then a 6” column that is under a 33 psi load can be expected to compress by 0.09”. That ain’t zero, but it’s not a lot. And, again, the “thin” portions aren’t actually the whole 6” height of the wing at its highest point, but are a much shorter length. So actual compression would likely be much less.
Alright, that’s the suggestion in a (long) nutshell. Is cutting the lightening channels in the core likely to compromise the wing? Or is it too much trouble to save 27 lbs on a 25’ span wing? Thanks for any input and for any unvarnished corrections to my "logic" or my calculations
Mark
And yet, because Weight is the Enemy (tm), and noting that the foam in a typical GA wing might weigh as much as the facings, I'm sure I'm not the first to wonder if we need all of it, or if there is extra (lazy!) foam that can be removed while maintaining the integrity of the wing.
If this is old territory, please just put me straight. I know I've seen the idea mentioned, but I don't recall that it was put out of its misery.
For consideration: Below is a drawing a NACA 2413 core of 48" cord (each square is 1"). The area is about 204in2. If the core is made of 2.1 lb/ft3 foam, then each linear foot of wing core span weighs 2.97 lbs.
If we cut the lightening cores shown below, the core weight is reduced by 36%. Each footspan of foam core then weighs 1.9 lbs, a reduction of just over 1 lb per foot of span.
Considerations:
How to do the cutting: In a Peter Shipol video, his cores were in 4 foot (spanwise) sections. His wing core templates on either end had the wing profile (for hotwiring, as normal) as well as the lightening channels. He heated a length of pipe (maybe EMT conduit?) with a torch and melted a hole from one end to the other of each lightening channel. Then he dropped his nichrome/stainless wire through the hole, hooked it to his transformer and bow, and cut out that channel. It wasn't highprecision work (like a wing outline), it was a oneperson job and went fast. The video is here, see 1:20 through 2:05.
Will the foam be overstressed? I couldn't locate detailed specs for DuPont (formerly Dow) Styrofoam Bouyancy Billet (the foam typically used for massive wing cores), but Billski has previously posted that it has a compressive strength of 70 psi. Another Dow XPS product (“Highload 60”) has a compressive strength of 60 psi and a listed compressive modulus of 2200 psi and a listed tensile strength of 85 psi. No tensile modulus was listed, but a similar XPS foam by BASF has a listed tensile modulus of 4.35ksi. So, I’ll use those numbers as rough standins for the DuPont 70psi buoyancy foam.
Backoftheenvelope inflight loads: Our plane has a typical lightplane level flight wing loading of 15 lbs/ft2. If we are designing for 9G ultimate, that will be 135 lbs/ft2 of lift force. Let’s make the (overly simplistic?) assumption that this lift is produced entirely by reduced pressure on top of the wing and nearambient pressure on the wing’s lower skin. So, if the inside of our wing/voids are at ambient pressure and the area outside the top skin of the wing is 135 lb/ft2 (about 1 psi) below ambient pressure (to produce the needed lift), then the foam core would primarily be in tension from the top laminate to the bottom laminate). If I’ve got that right . . .
Looking at the wing with the lightening channels, the cores are about 1.5” apart at the closest. There are 7 toptobottom “beams” from the nose to the flap hinge, and the sum of their thinnest portions is 30% of the wing chord (without the flap). If our total wing core toptobottom is under 1psi tension (again, oversimplified) , then the concentrated tension through these thin points is 3.3psi. This is well under the 85 psi tensile strength of the foam, and under this load the foam would be expected to elongate just .004” (this is a very conservative estimate, as I assumed the “thin” columns were 6” long. The thin portion is actually relatively short). If we envision the foam will instead be in compression, the loads are similarly quite low compared to the foam’s compressive strength and compressive modulus.
If we vacuum bag the wing during construction, the forces on the foam will be higher than the aerodynamic forces in flight. Assuming 20”Hg vacuum (about 10 psi), the thin column sections will be under about 33 psi of pressure, which is well under the foam’s 70 psi listed compressive strength (which is typically quoted at either 5% or 10% compression, depending on the standard used). If the foam’s compressive modulus is 2.2ksi, then a 6” column that is under a 33 psi load can be expected to compress by 0.09”. That ain’t zero, but it’s not a lot. And, again, the “thin” portions aren’t actually the whole 6” height of the wing at its highest point, but are a much shorter length. So actual compression would likely be much less.
Alright, that’s the suggestion in a (long) nutshell. Is cutting the lightening channels in the core likely to compromise the wing? Or is it too much trouble to save 27 lbs on a 25’ span wing? Thanks for any input and for any unvarnished corrections to my "logic" or my calculations
Mark
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