From: To: Subject: KRnet Digest, Vol 95, Issue 1 Date: Sunday, July 13, 2003 12:01 PM Send KRnet mailing list submissions to krnet@mylist.net To subscribe or unsubscribe via the World Wide Web, visit http://mylist.net/listinfo/krnet or, via email, send a message with subject or body 'help' to krnet-request@mylist.net You can reach the person managing the list at krnet-owner@mylist.net When replying, please edit your Subject line so it is more specific than "Re: Contents of KRnet digest..." Today's Topics: 1. Plywood vs composite skins (Traveler) 2. Vents (John and Janet Martindale) 3. Pin holes (Robert Stone) ---------------------------------------------------------------------- Date: Sat, 12 Jul 2003 13:56:31 -0700 From: "Traveler" To: "KR builders and pilots" Subject: KR>Plywood vs composite skins Message-ID: <001901c348b8$12d90260$2e35e044@Alien1> Content-Type: text/plain; charset="iso-8859-1" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: list Message: 1 On the pros and cons of using plywood vs. composite skins here's a = couple of interesting posts. The first is an excerpt by Ron Scott in a = piece in the Winter 2000 SAA magazine. ( www.sportaviation.org ) Of course prices have gone up since = the article was written.=20 ALTERNATE MATERIALS Ron Scott, SAA #67 "Have you priced plywood lately? A well known aircraft supply house = lists a 4'x8'x1/16 " of 45=B0 mahogany plywood at $161.00 plus freight. = This would be the variety one would skin a Tailwind wing with. Then the = plywood is normally covered with polyester fabric to pre-vent cracking = of the final paint job on the wing...a further cost.=20 Fiberglass sheet might be an alternate material for this application. = Looking at today's prices for glass cloth (10 oz.) and vinyl ester = resins, the cost of a 4'x8x(approximately)1/16" sheet would be somewhere = in the neighborhood of $40.00. This assumes a 2 ply lay-up on a smooth = waxed table. Once set up, one could lay-up a 4'x8' skin in about 15 = minutes. After the skin has cured, it is bonded to the wing structure = using an epoxy adhesive. All bonding areas must be cleaned and scuffed = before bonding. Ol' Ironsides is now 31 years old and still going strong = using the above process on the whole airframe in a skin stressed = application. Since no additional fabric is required on top of the = plywood, the glass skins are just sanded and painted...another cost = reduction.=20 Unless you are blessed with a mattress full of greenbacks, you may want = to check further into alternate materials and "beat the system" on the = ever increasing high cost of construction materials.=20 If you would like further information on fiberglass covering and = fiberglass landing gear fabrication, see November, 1971, January, 1972 = and February, 1972 in Sport Aviation." A different viewpoint (in part) is put forward by Bob Waldmiller = concerning wing skins. He is building a highly modified aircraft called = "Excalibur" based on the Corby Starlet for One Design aerobatic = competition. This is a short excerpt from a very interesting article. = Some of his ideas could be used by KR builders. The extended article = and pictures are at www.eaa1000.av.org . Click on "Aircraft our members = are building" and navigate down to Excalibur. =20 The heaviest thing about a plywood covered wing is the skin. = Unfortunately, even though a wing skin is lightly stressed, the skin = must be stiff enough to resist buckling. I needed the plywood skin to = carry wing torsional loads so going back to a fabric covered wing was = completely out of the question. How about high tech composites? Most = people don't realize just how heavy a high tech material such as carbon = fiber/epoxy really is; it's roughly three times heavier than birch = plywood. So why don't I just make the carbon wing skins 1/3 the = thickness of the plywood skins? Well, when you do that, the carbon skins = are only 25% as stiff as plywood for buckling resistance. The bottom = line is a solid carbon fiber skin, designed to the same buckling = criteria as a solid wood skin, will be heavier. So next time someone = tells you that they wouldn't consider flying a wooden airplane, you can = tell them that wood is, pound for pound, more efficient than carbon = fiber when used for wing skins...and it's cheaper!=20 The wing spar is a totally different animal, however. A wood wing spar = is quite heavy when compared to a carbon fiber equivalent. I did the = math and there was no way to justify a wood spar in Excalibur. Carbon = fiber has material properties (strength and modulus of elasticity) that = are an order of magnitude higher than Douglas Fir and even though the = density of carbon fiber is 3 times higher, you only need 1/10th the = amount of material. Unlike wing skins, a spar doesn't generally have = limits placed on it for buckling problems. Its shape takes care of that. = The result was I could build a carbon fiber spar that would carry my 750 = pound airplane to =B110 g's and it would weigh only 19 pounds! That's = probably half the weight of the Douglas Fir equivalent! All in all, the = lighter but stronger spar made up for some of the weight of the plywood = wing skins. Thomas Dalby Boulder City, NV KR-1.5=20 ------------------------------ Date: Sun, 13 Jul 2003 17:46:10 +1000 From: "John and Janet Martindale" To: "KRnet" Subject: KR>Vents Message-ID: <004001c34912$d63bfb40$fe6ecacb@amipentium200> Content-Type: text/plain; charset="iso-8859-1" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: list Message: 2 Thanks for the ideas fellas (Todd, Colin, Larry). First up I'm going to slip a short piece of tube square cut over the 45 = chamfer on the vents to obtain a more static pressure and confirm the = nature of the beast then options are either, fly on one tank only (ie. = prevent cross flow thru the tap), one-way valves in the lines, or seal = the vents up and use one way vented caps....if none of that works I'll = go into the wing....failing that I'll fly upside down like all you = people in the northern hemisphere with a wonky compass dip...couldn't = resist :-) John and Janet Martindale 29 Jane Circuit TOORMINA NSW 2452 AUSTRALIA ph: 61 2 6658 4767 email: johnjane@chc.net.auFrom kstolls@juno.com Sun Jul 13 08:45:50 2003 Received: from outbound-20.nyc.untd.com ([64.136.20.100] helo=webmail05.nyc.untd.com) by lizard.esosoft.net with smtp (Exim 3.36 #1)id 19bj2s-00012t-00 for krnet@mylist.net; Sun, 13 Jul 2003 08:45:50 -0700 Received: from cookie.untd.com by cookie.untd.com for <"P+4qF/GRNm1VMj781TClBHvMICqAWRNVloKYIiBJ7lXho4afXWMOng==">; Sun, 13 Jul 2003 11:45:19 EDT Received: (from kstolls@juno.com) by webmail05.nyc.untd.com (jqueuemail) id H4YUF5HB; Sun, 13 Jul 2003 11:45:19 EDT Received: from [67.195.23.219] by webmail05.nyc.untd.com with HTTP: Sun, 13 Jul 2003 15:45:17 GMT X-Originating-IP: [67.195.23.219] Mime-Version: 1.0 X-Original-From: kstolls@juno.com Date: Sun, 13 Jul 2003 15:45:17 GMT To: krnet@mylist.net X-Mailer: WebMail Version 2.0 Content-Type: text/plain From: kstolls@juno.com Message-Id: <20030713.114519.15272.229451@webmail05.nyc.untd.com> Subject: KR>Plywood vs composite skins X-BeenThere: krnet@mylist.net X-Mailman-Version: 2.1b3 Precedence: list Reply-To: KR builders and pilots List-Id: KR builders and pilots List-Post: List-Subscribe: , List-Unsubscribe: , List-Archive: List-Help: Re: > I dont feel that plywood is a good choice. Hi everyone... from Mattoon, IL. I am a newbee to the KRNet as of a couple of days ago. This will be my first post. I would really be afraid to use sandwiched foam for plywood. If you think about it, when you hang 150+ lbs on the firewall, the fuselage sides between the engine and the landing gear take a lot of stress. The top longerons are being stretched (tension), and the bottom ones are trying to buckle (compression). The plywood is crucial in sharing this load. It serves as a shear web for the trussed fuselage and insures that the sides maintain their shape in load. The outer skin of a sandwich would do very little in sharing that shear load because it is only attached to the fuselage by the core foam. Sandwiched structure is great for other types of loads, but I think that the outer skin of the sandwich in that application would only keep the wind out like Mark said. I agree with Mark in that you cannot beat plywood. I’ve been filling and sanding my wings for a few weeks now and will finally just give up and take defeat from some of the inperfections. That all said, I think it IS neat to brainstorm ideas and the ‘what ifs’. I’m not hanging my life on a plywood structure substitute, but IF I were on a desert island that had plenty of spruce, foam, glass and epoxy… but no plywood, I think I would sandwich the spruce members between fiberglass layers on the fuselage. In other words, I would put foam between the spruce members… sand it to the thickness of the side frames, and then glass both sides (inside and out & biased to the longerons) with a couple of layers of glass, being sure to bond the glass to the spruce everywhere. I think that would make a much better structure than gluing sandwiched foam to the outside of the fuselage. You would need to glass AFTER the fuselage is assembled because I doubt that you could bend the sides after the glass was applied to both sides of the side frames. Even if you could, the inside skin would go into compression and do less good. You would also have lots of fun trying to sand the foam on the inside surfaces… and would need to sand perpendicular to the longerons on the outside to maintain the fuselage curvature. After the boat stage were finished, I think I would probably get homesick and try using the structure to float home rather than spending 20 more years on the island building something airworthy! (I’ve been building mine since ‘81). Good luck in whatever you decide, but just remember that neat ideas may get you into some real trouble some day if you aren’t careful! I’m sure I will get a flashback of every deviation from plans that I have made when I get that dude up to 500 feet for the first time! Take care, Kerwyn Stoll Mattoon, IL KR2 N40102 (60%) ------------------------------ Date: Sun, 13 Jul 2003 11:04:58 -0500 From: "Robert Stone" To: "KR builders and pilots" Subject: KR>Pin holes Message-ID: <001501c34958$82910e40$ba21f218@hot.rr.com> Content-Type: text/plain; charset="Windows-1252" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: list Message: 3 Netters, I don't understand all the talk about pin holes being such a = problem. When I built the KR-1 and KR-2 years ago (l977) after laying = up fiberglass and resin we covered the entire surface with a piece of = cordite sprayed with mold release. This is kind of like cling wrap only = thicker. When it was dry and the cling wrap removed the surface was so = smooth that very little sanding was required. After minimal sanding a = coat of feather fill was applied and sanded. Feather fill sands out = very easy to a surface smooth as a shirt button. There were a few pin = holes of course but there was also a product like bondo that came in a = tube like tooth paste. each pin hole got a finger dab of this paste and = then final sanding ready for sealer and paint. The finish was so good = on our KR-2, we won the best composite prize at the Chino Airport EAA = Fly-in in 1977. One of the judges told me that their final decision was = biased on the beautiful finish. There are much better products on the = market today than Feather fill and bondo so I just don't see pin holes = being that big a problem. Bob Stone, Harker Heights, TX rstone4@hot.rr.comFrom jmw116@socal.rr.com Sun Jul 13 10:30:07 2003 Received: from orngca-mls02.socal.rr.com ([66.75.160.17]) by lizard.esosoft.net with esmtp (Exim 3.36 #1) id 19bkfm-0001or-00 for krnet@mylist.net; Sun, 13 Jul 2003 10:30:06 -0700 Received: from computer (cpe-24-24-217-97.socal.rr.com [24.24.217.97]) by orngca-mls02.socal.rr.com (8.11.4/8.11.3) with SMTP id h6DHPwk05662 for ; Sun, 13 Jul 2003 10:25:58 -0700 (PDT) Message-ID: <000901c349b8$24126020$61d91818@computer> From: "Justin" To: "KR builders and pilots" References: <20030713.114519.15272.229451@webmail05.nyc.untd.com> Subject: Re: KR>Plywood vs composite skins Date: Sun, 13 Jul 2003 22:29:32 -0500 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2720.3000 X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2727.1300 X-BeenThere: krnet@mylist.net X-Mailman-Version: 2.1b3 Precedence: list Reply-To: KR builders and pilots List-Id: KR builders and pilots List-Post: List-Subscribe: , List-Unsubscribe: , List-Archive: List-Help: You are incorrect by you saying it is hanging on by the foam. I am making it to where the wood and fiberglass are having full contact the entire fuselage which it will be doing the same work as the plywood. A few ways of doing the load is either put the glass on at a 45 or put one going along with the longerons and the other vertical. I will still have to do my design and annalists on this with it compared to plywood (AS&S). Justin ----- Original Message ----- From: To: Sent: Sunday, July 13, 2003 10:45 AM Subject: KR>Plywood vs composite skins > > Re: > I dont feel that plywood is a good choice. > > Hi everyone... from Mattoon, IL. I am a newbee to the KRNet as of a couple of days ago. This will be my first post. > > I would really be afraid to use sandwiched foam for plywood. If you think about it, when you hang 150+ lbs on the firewall, the fuselage sides between the engine and the landing gear take a lot of stress. The top longerons are being stretched (tension), and the bottom ones are trying to buckle (compression). The plywood is crucial in sharing this load. It serves as a shear web for the trussed fuselage and insures that the sides maintain their shape in load. The outer skin of a sandwich would do very little in sharing that shear load because it is only attached to the fuselage by the core foam. Sandwiched structure is great for other types of loads, but I think that the outer skin of the sandwich in that application would only keep the wind out like Mark said. > > I agree with Mark in that you cannot beat plywood. I've been filling and sanding my wings for a few weeks now and will finally just give up and take defeat from some of the inperfections. > > That all said, I think it IS neat to brainstorm ideas and the 'what ifs'. I'm not hanging my life on a plywood structure substitute, but IF I were on a desert island that had plenty of spruce, foam, glass and epoxy. but no plywood, I think I would sandwich the spruce members between fiberglass layers on the fuselage. In other words, I would put foam between the spruce members. sand it to the thickness of the side frames, and then glass both sides (inside and out & biased to the longerons) with a couple of layers of glass, being sure to bond the glass to the spruce everywhere. I think that would make a much better structure than gluing sandwiched foam to the outside of the fuselage. You would need to glass AFTER the fuselage is assembled because I doubt that you could bend the sides after the glass was applied to both sides of the side frames. Even if you could, the inside skin would go into compression and do less good. You would also have lots of fun trying to sand ! > the foam on the inside surfaces. and would need to sand perpendicular to the longerons on the outside to maintain the fuselage curvature. After the boat stage were finished, I think I would probably get homesick and try using the structure to float home rather than spending 20 more years on the island building something airworthy! (I've been building mine since '81). > > Good luck in whatever you decide, but just remember that neat ideas may get you into some real trouble some day if you aren't careful! I'm sure I will get a flashback of every deviation from plans that I have made when I get that dude up to 500 feet for the first time! > > Take care, > > Kerwyn Stoll > Mattoon, IL > KR2 N40102 (60%) > > > _______________________________________________ > see KRnet list details at http://www.krnet.org/instructions.html > ------------------------------ _______________________________________________ See KRnet list details at http://www.krnet.org/instructions.html End of KRnet Digest, Vol 95, Issue 1 ************************************