chasingmars

Funny you should say that. It's written in OpenOffice, exported by the build in PDF feature. I'm visiting family this weekend, being away from home, I can't build, so I'll probably write, and get up to date again by Sunday (maybe)... thanks for the kind words, I'm glad you found it worth reading.

Nope, compared to glass done acceptably. A level of workmanship adequate for safe construction of glass, generally, would seem to be inadequate for carbon. While I haven't worked with carbon, so this is based on research and speaking with those who do, the bubble size limitations, fibre alignment limits, etc for glass would all have to be tightened for carbon due to it's brittle fracture, stress riser sensitivity and such. It may well be easier to get good alignment with a carbon tape, but what about you uni/bid replacements? either, you mix materials, which requires reengineering to look at a carbon spar cap in a glass wing to avoid undesirable load transfer, or you sacrifice drapability to manage ease of fibre alignment, or you wrestle with fibre alignment. And this is really the heart of the contention I have with how this stuff is presented. Carbon is so expensive, there is a genuine risk that people just "throw a little in" on the mistaken impression it will add strength. A laminate over a foam core is (essentially) constrained to uniform strain in each skin (an approximate that I ask the engineers to excuse). Carbon being so much stiffer reaches it's failure strain before the glass has taken up appreciable load, so the when the panel is deflected, the Carbon initially takes all the load with the glass doing little until the carbon fails, leaving the glass in a damaged panel to now take all the load without the carbon's help. A little carbon here and there for added stiffness, if you don't engineer it properly, is one of the easiest ways to make your plane unsafe. TMann, you're use of carbon, taking a major subassembly proven elsewhere and adopting it, ensuring the load paths are through structures that are either to one design or the other and meet at a interface that is common to the design is quite reasonable. throwing in some carbon for stiffness however, is not.

Heh, you wouldn't be the first to mistake a very direct manner for condescention or arrogance... While I've never met Marc, I've come to the opinion that it's simply a strong case of the (un)common hazard of being an engineer. We tend be misperceived like that sometime... God knows why?

This is part of what sometimes I think forms a seed of the irritation some of the *ahem* older (happy belated bday Marc! ) more experienced posters here (which I'm not one) feel sometimes I think. Mostly because it presumes the ten minutes you would need to spend using a search engine to find the answer to your question is more worthwhile than the ten minutes it takes for someone to answer your question. Of course, value there depends on perspective, and I've sometimes also been guilty of posting a repeatedly discussed question without first searching. I try not to though. anyhow, I'm a vacuum bagger, starting chapter seven, my opinion's been posted, and to be honest, it's not worth it. that said, I'll disagree with Marc on cosmetics. it's easier to get a nice looking part vacuum bagging than hand laid as I never seem to get even small bubbles that would be acceptable. I also use proper materials from composites suppliers and bag at high vacuum where the underlying foam warrents, and have had generally good results, at an expense that really just isn't worth it. But it's fun, and once a part is in the bag, it keeps me from my fatal flaw of poking at it just to try to make it a bit more right until it's ruined I agree with Marc on the structural issue. These planes are plenty strong for their mission goals, and changing the mission goals gives you more issues than just weight reduction solves.

http://www.maddyhome.com/canardpages/pages/chasingmars/index.html I'm not quite up to date, I have some I haven't posted yet, and some I still need to write. I'm glad you found it interesting.

(Regarding sensitivity to fibre orientation Unfortunately Jon, here you believe wrong. I've read some academic papers on this - carbon is substantially more sensitive to perfectly straight fibre runs when compression loaded than glass, especially when it comes to impact toughness, but also generally. Essentially, carbon is weak in shear so when the fibres aren't well aligned, you get shear loading introduced that causes premature compression failure, as I recall. I'll see if I can't dig up the reference for a more accurate recollection.

How much do you want to spend per pound of weight savings. I always try to figure out how much a given savings will cost in time and money before I decide to try it. Weight not in the plane is worth something, and that value will be different for everyone, but really, that value is not infinate. Let me suggest a little thought experiment. Carbon vs plans 7725. Plans 7725 is listed at 300x300 lb/in strength, 8.8 oz fabric weight. Carbon 282 is the same 300x300 according to ACS, 5.8 oz fabric weight 38" 7725 is $5.90/yd at ACS today, 39" 282 is $45.50/yd. density for glass is about 2.6, for carbon about 1.75, and for MGS about 1.2. With these starting assumptions for a 50/50 v/v fibre:resin ratio (Some math happens ) 1 sq yard of 7725 E-glass laminate weighs 12.9 oz (68% glass w/w) 1 sq yard of 282 E-glass laminate weighs 9.8 oz (59% carbon w/w) The resin used is nearly the same, by the way, so no real savings there. Savings is 3.1 oz at a cost of 39.60, or 205 $/lb. This assumes that you can substitute layers 1:1, which you can't by the way, and that you have absolutely no waste, so cost goes up from there for wherever you need to adjust the laminate schedule. Assuming that about 1/2-2/3 of the cloth you cut remains on the plane (wastage in cut plan, trimming, etc) and you're looking at a cost of 300-400 $/lb saved. i.e. if you can't afford it for a savings of a few ounces, you can't afford it to save a few pounds. For me, I cost weight savings at $100/lb and 10 hours work/lbs. for what it's worth (or not worth), EDIT : THE ABOVE SHOULDN'T BE TAKEN TO MEAN 282 CAN BE SUBSTITUTED FOR BID WILY-NILY. It's just an example. In reality, you do *need* to account for the differnt properties of carbon compared to glass (in part, that carbon's not well suited for shear loads).

My house is a small bungalow, a bit under 1000 sq ft. I build in the basement, and sometimes, the living room (piling all my furniture into the kitchen temporarily) when I need extra space. It's much less than ideal, and I will only be able to go so far before I need to work something else out, but you can get an awful lot done in a small space, and that's all stuff you don't need to do later on. Waiting for an ideal space is a sure way to never start.

This question recently came up on the Cozy List, check the archives there for a few opinions on the matter. As someone who vacuum bags essentially everything (so far) except my tapes (and I considered it till I realized it was silly), my take is this: (reposted from the Cozy List)

http://www.maddyhome.com/canardpages/pages/chasingmars/Chapter04/2007-04-20.pdf (Again, untested, not recommended, think at your own risk!) You don't.

Does this forum have a wiki section? This is exactly the sort of post that would make for good wiki-conversion material once it's gets exhaustive/exhausted/exhausting Anyhow, with the big *UNPROVEN* flag, I'll add my passenger side "ski tunnel" mod.

Given the formulation changes to 3M 77 it's looking like this is no longer feasible, as I gather there now a component (acetone?) that is a solvent for the wing foam.

You haven't been watching TMann check off chapters... he might not make quite 2 years, but he and I started at the same time about a year ago and he's checked of about 3 times as many building chapters in that time. I'm trying to figure out how he does it!

To be fair, in that pic, the recip engine is stripped to the last pin, while your rotor is sitting there all pretty and assembled, tip, corner seals, backing springs and all...

On re-reading my post, it makes it sound like I'm saying it varies proportionally with these factors, while that's true for the square of the lift coefficient, it's not for span loading. Lower span loading (and by extension, higher aspect ratio all else equal) gives you less induced drag. Don't remember exactly, but I don't think it's a proportional to the inverse relationship.

Again, not quite. Induced drag varies with the square of the coefficient of lift and the span loading of the aircraft. So the original poster is correct if wing area is held constant (except it's more relevant at slower speeds than cruise, see below). It's also not true that the faster you fly the more lift the wings produce. It's true that the faster you fly the more lift they produce for a given coeffient of lift, but in a given airplace, flying faster means more forward stick (or forward trim) to reduce the wing's angle of attack so that for level flight weight balances lift. The consequence of this is that as you get going faster, the drag due to lift (induced drag) becomes small in comparison to parasitic drag. That's where the real speed advantage of smaller, more highly loaded wings comes in, lower wetted area makes for less parasitic drag at high speed.

I've given some thought to doing something similar, on the idea that I'd like the precision of CNC... one of the things that I was thinking about though, I'm curious how you approach: When you cut the cores by hand, you're following a template that's on the surface of the foam, whereas with the CNC machine, the "template" or, rather, tool path, is in the plane of the machines degrees of freedom. Where the ends have the same profile path shape, size and stations, this is irrelevent, but where, as with our wings, the templates are different, this changes the shape of the wing. Do you calculate out from some sort of imaginary surface position where the templates would be and then generate your tool path from that extrapolation (and if so, how about cases, if any, where the templates aren't in parallel planes?), or do you use the CNC machine to guide your wire over a physical template on the foam the same as by hand? Some other way? I was thinking about trying to do the extrapolation, it should just be a bit of vector mathmatics, but wondering if anyone has tried it already? Craig.

Not sure that's all of it, we had a dry spell here too, so part of the issue seems to be also getting it over to North America from Germany (just inference)... Anyhow, my supplier (Composites Canada, they have a web site) did mention when I was chatting with them about how you folks down south don't seem to be getting any that they didn't see why they couldn't ship there... might be an option, not sure.

For what it's worth, my supplier got his shipment in some weeks back. Got six gallons of 285/287 about two weeks ago... This is up in (Toronto) Canada, and since the North American distributor (Airheart) is up here in Hamilton, that's a stop on the way from Germany for the stuff, so maybe soon you'll have it again.

Tons of supplies... my supplier is a five hour drive away and by yard is about twice the price of by roll, and I'm a jump in the deep end with both feet kind of guy sometimes... So yeah, $100 bucks for a homemade resevoir system, $400 for a nice compressor pump that I justified half as "I can use a big compressor" and half as "I can draw vacuum with this", a thousand feet of 15 mil plastic that I use for just about anything needing plastic cause in bulk it was "only" a buck a yard, a 100 yards of bleeder breather, a hundred yards of coated peel ply (in addition to the 10 yards uncoated that'll make the tapes called for in the plans), 40x 25 foot rolls of vac bag tape (like chewing gum) which when I first tried thought "why didn't I just use tape" but now love cause it works so well .... list goes on...yep, a good $2000... vac bagging is expensive if you buy in bulk, and more expensive if you don't. I'm a perfectionist, so I'll take the time to get this working, but knowing what I know now, I can't honestly say I wouldn't rather just be hand laying. But I am having fun with it, and that's more important to me (and better I think for completion) than just doing the hand layups and wondering if I'd be having more fun the other way... and of course there's one more reason I'm bagging, also part of being a perfectionist... I can't leave well enough alone... if I hand layed, I know I would be messing with it until it had cooked and probably ruin things for overworking something that should have been left well enough alone. With the bag, once the vacuum is on, it *has* to be left alone, and you can't mess with it, it's done, for better or worse, and you'll find out which when you tear of the peel ply and plastic.. oh yeah, that's the real reason right there... it's like christmas every time, unwrapping the parts when they are done, even just the test panels I've done... but is it rationally worthwhile? No. Oh yes, that's true, but as you say, baseline target, of the sort it's the most I'm willing to commit to at any point in time, so now that I'm 6K in, and 40K to go, I'm ok on budget

Might be tough getting landing speed down to 45 kts necessary for LSA compliance with no flaps (pretty much a given on a canard)... at 1320 lbs gross, that would need (2 W = V^2 x A x rho x Clmax) ... about 120 sq feet of wing area... More than the much heavier cozy... then again, you're only going to be going 120 kts, so maybe that's fine.

Jon's got some good advice above... Advice I should have taken... Don't get me wrong, I'm still happy and having fun getting my vacuum setup going, but I've spent about $2000 I probably needn't have on good vac bagging supplies and such. I've spent a good long time trying to get it to work, made a bunch of test panels and no parts yet save for lots of carved foam for when the setup works right. I've done precisely one hand layup, hit wieght near enough to exactly what I'd like to build to, and had I just done this and skipped vac bagging, I'd probably be mostly through chapter four by now. Flip side, resin and glass, while pricy, aren't the predominant cost driver on the aircraft and an extra $2000 is only about 5% if it's worth it. It has yet to be so, but that will hopefully change once I fix my vacuum regulation issues... we shall see... I may have a bunch of vac bagging stuff to sell you Anyhow, as for infusion, that's a whole different level of complexity. The vacuum needed for it to work well may be too much for some of our foams, especially wings, and you might need complex modeling software or a dozen trial runs to get the flow pattern right to produce good results. infusion is not really a one-off production technique suitable for aircraft... looked at it in some detail anyhow, and that was what I came to. For what it's worth... Craig. Plans #1457