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Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

That`s very interesting Djcross.
Thanks for your reply.

Load demands on fast millitary jets would be higher than in a commercial aircraft, for example G loading, obviously. High loading/stress areas are still handled by traditional metal structures in commercials, like wing spar boxes.

Are you saying that composite skinned wings with a plastic honeycomb, say on a wide bodied airliner actually weigh as much as the same size/ wing type made from aluminium with all fittings, control surfaces, actuators, fuel tanks, etc..
I was led to believe this was not the case. Published figures say these wings are cheaper to process and give increases in range, payload-because of the reduced weight ;lower maitenence costs during operator lifetime, etc...etc...

By the way, the very large machine that fashioned the front fuselage in the picture can machine to an accuracy of 6 microns, a tolerance usually associated with much smaller scale equipment.

Thanks for your reply.

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Enthusiast Joined: Nov 10, 2005 Posts: 39 Status: Offline

Complex, highly integrated aircraft structures lose their weight advantage because of the reinforcements needed to carry point loads.

Let’s take a theoretical example of an all-composite airliner wing (B787). Each wing has leading and trailing edge lift and control devices (slats flaps, ailerons and spoilers), also attachments for actuators, landing gear, fuselage mate, hoisting, jacking, and equipment mounting for electrical/hydraulic and fuel components (harnesses, plumbing, pumps, motors, fuel probes, valves, Etc.). Any maintenance access panels/doors require a doubler around the periphery to carry loads around the cutout and provide attachment for door fasteners. This results in many hundreds of point loads.

Point loads are very complex and can result in compression, tension, torsion, bending, shock and vibratory loads in the same flight. A composite laminate cannot handle these types of complex loads and will fail, usually by delamination. To handle point loads an insert (usually metallic) is needed to distribute/spread that point load over a large area so the composite will survive for the lifetime of the airplane. The “spreaders” or “load buster” fittings can be co-cured (elegant solution), secondary bonded or mechanically fastened (ugly, brute force solution) to the composite structure. Use of hundreds of these fittings is where the composite weight advantage is lost.

The key selling point for complex composite built using VARTM or other advanced processes is reduced touch labor. Metallic airplanes are a million rivets and other parts flying in formation. Each of those rivets/parts requires touch labor that adds up to a huge assembly touch labor cost. For composites, major subassemblies can be molded in a few operations. Manufacturing engineers develop molds fixtures for wing skins, spars and ribs. Graphite cloth and woven 3D preforms and fittings are placed in the mold at the leisure of the fabrication crew. When the graphite material is in place, the mold tool is wrapped in a plastic bag, vacuum is applied and resin is injected. Once cured, the bag is opened and finished skin/spar/rib is removed. The lower skin is placed in a tool and spars/ribs are bonded to it. At this stage, the wing box resembles a complex bowl. That “bowl” is sealed and leak checked because it will hold thousands of pounds of jet fuel. Next, plumbing, and wing internal components are added. The last step is to bond the upper wing skin, completing the wing box.

The wing box is moved to the fuselage mate station, then flaps, spoilers, ailerons, landing gear and doors are added in traditional assembly fashion.

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

I think it is only a matter of time before industry develops new composite materials that can carry point loads. The difficulty is finding the right combination of materials and manufacturing techniques like vacuum-assisted resin transfer molding as you have stated. Metal composites show the best promise.
The fear is that it is still unproved technology, remember the Airbus rudder failures, which blamed the pilot. BS! Testing of these materials, like checking adhesive bonds takes a long time although Boeing has developed a laser pulse testing rig that can do that very well.

I guess in the case of JSF, it would come down to COST and the still unproven durability/performance of these materials. The Eurofighter Typhoon is 40 percent composites but if you notice from the picture it is not used in any areas you wouldn`t expect composites.

Elite 1K Joined: May 26, 2005 Posts: 1152

The Airbus crash was a combination of things (they always seem to be).

The copilot who was flying did something really stupid, even after in previous flights he'd been told to stop. When they encountered wake turbulence from a 747 he threw the rudder hard to the stop one way, then back to the other stop repeatedly which is what he claimed he was taught, but he learned the wrong lesson because the training program was on a sim with a bad procedure (ignoring rudder input in the sim until a certain bank angle was reached), which apparently he took to mean he had to throw the rudder hard over when he didn't. Be that as it may, he was told by a previous pilot to _never_ do that again.

Then you have the Airbus' FCS which allowed the rudder to exert enough force to rip the v-stab right off the aircraft, and the actual rudder pedals which had a reduced amount of force required for full rudder from previous models. And that particular model had had rudders damaged from excessive inputs prior to the crash.

So originally the first suspect was a failure of the composite structure, but really it wasn't the culprit at all.

Enthusiast Joined: Nov 10, 2005 Posts: 39 Status: Offline

snypa777 wrote:

The Eurofighter Typhoon is 40 percent composites but if you notice from the picture it is not used in any areas you wouldn`t expect composites.

The use of composites on Eurofighter mirrors the "black aluminum" school of composiite design used on F-22, F-35 and other western airplanes (probably 1 step better than the WECSOAD - Wiley E. Coyote School of Aircraft Design). Skins built using fiber placement techniques are mechanically fastened (sometimes bonded if the forces are very low). Those skins have metallic or carbon inserts to prevent fastener pull-through. It is very labor intensive, fragile to impact damage, difficult to inspect, and only marginally lighter than aluminum.

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

DJCROSS wrote:

So originally the first suspect was a failure of the composite structure, but really it wasn't the culprit at all.

Would we have seen the same failure if the rudder was made of traditional components?
Rudder failure has happened on the A310, 300. Airbus use a rudder travel limit unit which is supposed to stop a pilot exceeding the design stress limits of the structure. On flight 587, many people believe this equipment was not working correctly, some people whom have examined data recordings in detail disagree! The theory from the NTSB was based on manufacturers data. The NTSB say that an 11.5 degree rudder movement caused the failure. the rudder pedal data recorder says it was at 4 degrees when failure occured.
No one has really got to the bottom of that one. The official report was just too nice and neat. If you want to avoid liability you just deny everytjing and hope nobody points the finger in your direction!

Elite 1K Joined: May 26, 2005 Posts: 1152

Part of the problem is the frequency of the measured rudder inputs, part of it is as you said "CYA" attitudes. But also the co-pilot, that is the part that angers me about that whole incident. Who in God's name thinks slamming the rudder hard over back and forth is a good wake turbulence avoidance manuver?!? That IS what the copilot did on a prior flight in wake turb, and it IS what appears to have happened on the flight that crashed. It's just maddening to think about.

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

Ok, even if the co-pilot made the error, made me sick thinking about it, there was equipment on board to prevent it becoming catastrophic. It may not have worked. My point is that the rudder has fallen off the A-300 several times...Seems to be a problem there. Not heard of many 747s losing theirs.....

Elite 1K Joined: May 26, 2005 Posts: 1152

I agree the A300 and 310 probably had a design flaw, but it is not the composite structure. It didn't fail until loads significantly greater than the design spec were placed on the v-stab. If the FCS had not allowed that much rudder authority, or if Airbus had maybe reconsidered the design after the first couple of rudders came loose or broke off, or... if, if, if. Noone knows for sure what happened, all we can really do is leave it up to experts to try and piece it together. But I think you're right, Airbus probably got off lightly based on the facts.

Enthusiast Joined: Jan 30, 2006 Posts: 46 Status: Offline

I've worked with carbon fiber products in the past, especially on one particular power-plant. I know for a fact that as far as weight/stenght go it's the only way to go. If NASA has anything to do with the F-35's, an I hope they do, then some sort of carbon fiber composite would more than likely be a given. I can't imagine why there would be 2nd. thoughts, but then again with technology to-day, who knows, maybe carbon fiber now compared to what may be being offered would be like copairing hamburg to filet mignon.

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