Why composites
dash said:
I think that the new Cessna looks great. I wish they had've used more composite as this is a relatively low risk venture for them.
Oh gosh why? What would MORE composites do for you? I think composites would make it heavier and more expensive.
Composites are OK in the right place but there are trade offs. Metal structure is very light, strong and very crash worthy. Composite structure tends to be heavier when used for primary structure. Why? Well because of the variations in properties due to the manufacturing process, so engineers overkill the margins of safety, in part because the FAA makes them. Imperfections and sub-par properties are harder to see and inspect in composites.
Cost? Well the panacea of low cost composite construction has been the holy grail of manufactures. Instead of 1000's of parts with a metal plane (counting all the little rivets) you have one big molded part, which sounds good to bean counters. However in the real world, real high end composites, not hot tub / boat composites, is a very expensive and tedious deal. Lay ups, bagging, autoclaves and than sub assemblies bounded or bolted together, UGH,
Than if there is an error you scrap the whole thing. Remember the Beech Starship, all composite corporate pusher. It was slower, carried less and cost more than a Beech King Air, which is still in production. The Starship has gone the way of the dodo bird.
Hey I don't want to be a target of haters out there. I have design and analysis experience since I worked with composites at Boeing years ago. We called it black aluminum. I know the theory of composites and appreciate the proper application. In a Lancair it's fine and well done, no doubt. In fact Lance Neibauer first Lancair O-200 was what got me excited about experimental aircraft. What is there not to love about sexy curves. However for a LIGHT SPORT PLANE aluminum is ideal. Look they have people with PhD's in aerospace structure that know what they are doing. For Cessna, with metal working skills makes engineering sense for them. Could they have made it out of composites? Sure they could but in my opinion it would have cost a little more and may be weighed more. A home built Lancair is labor intensive, not to mention the kit cost a bunch, also an indication of cost or materials and labor to produce them. They are just not going to push the technological envelope for a LSA. Now a F-117 stealth fighter or B2 stealth bomber, sure, but they cost billions. A Lancair, sure, but people are not going to pay top dollar for the most technological airframe to go 135 mph.
Not sure why composites would make you happy, but if Cessna made their LSA out of composite it would weight more and cost more. What goodness would you get. Some think composites last for ever? Really, well in engineering school we learned a thing called "entropy". Everything in the universe strives for more disorder and materials want to break down back into their basic elements. I would feel better flying a 60 year old metal plane that has been inspected for cracks and corrosion than a 60 year old composite plane. What are all those surface cracks in a composites surface? Now why do you need to keep them out of sun and heat? Micro surface cracks or something more serious? Delaminations? Inspection methods are more complex and require very special equipment and special operators to find damage and defects in composites. That means EXPENSIVE.
Last, if you are not going to build the WHOLE plane out of composites in ONE BIG layup you will have production joints. Trying to bolt composites together is troublesome. Unlike metal which is basiclly isotropic (material properties the same in all directions, which it's not but close), composites are very directional (Orthotropic ) properties. Composites have poor edge bearing strength. When you drill a hole in a material, say metal, put a bolt in it, the bearing strength in the base material is KEY in that shear joint. The point is bolting composites together or making "HARD POINTS" for landing gear, mounts, transitions to metal material structure, is a work around and inefficent. They do it by making composite overkill. They incorporate pad up areas (very thick), bushings and bearing plates, but in the end bolted joints due to edge bearing is kind of ugly in composites. If you could it would all be bonded. So again the holy grail is not to have any bolted joints but everything bounded together in one big composite structure. However to do that, you need to "CO-CURE" the whole plane in one huge autoclave (a big pressurized oven with vacume lines and probes). It is a big complex machine. To get a whole wing or fuselage much less a whole plane cured at one time is at the very high end of technology. I don't know Cessna's technology but guess they don't have huge autoclaves to cure a whole plane. Most of the new Bizz jets are made with metal, primary structure wise.
The exception is Raytheon Aircraft Primer entry level Bizz jet. I think they use a one piece carbon wound fuselage, way beyond what we do. ONE PIECE. One big part. They have made missiles like this for decades. The idea is to make it automated and not have people taking individual plies and put them on a form, which is labor intensive. The little LSA is not suited for carbon wound fuselage, but ideal for a round tube like a missile or even a small round fuselage of a bizz jet. Again the idea is to make it chaper and quicker not necessarily better from a performance stand point. Raytheon hopes they get their billions in development cost out of it by winding out lots of planes, because the start up is way more expensive. The LSA market is much smaller and profit margins not so great. Cessna use to sell C-152's at cost to get people into their product line.
RV's are metal and kick everything else in the kit plane world's behind. Check kit cost, performance and value. Also in 60 years a well built RV will be around. A Lancair with cracks coming out of the composites near the engine mount or gear will be prohibitively expensive to repair or replace. Just one man's opinion, based on my engineering and homebuilding background (and I have helped on a Lancair and Long-EZ). If you like sanding and dust, compoisites are the way to go.
