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Metal vs Composite aircraft constuction


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The following is a discussion on Metal vs Composite aircraft constuction;


I don't run down composite structures, I just debate how appropriate they are when the full life-cycle of an aircraft structure is considered. There are some direct operating costs that can be saved by producing aircraft structures with composites, and the first owner will probably realize these benefits.



My world revolves around the airframes that are still flying after 30 years (40, 50, 60, 70, even) and they've changed hands dozens of times. It's a very different perspective.




The argument that a well controlled factory can produce a superior aircraft is not in question. I'd agree with that. It's the continued airworthiness costs that are my concern. A de-lamination zone in a CF spar - scrap the wing. Find corrosion on the web of an aluminum spar - clean/replace/reinforce and it's on its way.




Once the composite airframe design becomes the norm, then aircraft will not live as long as they did with metallic airframes. The utility, economy, and accessibility of general aviation is something that we take for granted today. Making airframes cost 5x more and last 1/2 as long will be detrimental to the industry as a whole.





""A de-lamination zone in a CF spar - scrap the wing. ""




Then why would you have a repair schedule from some manufacturers for a 100 to 1 scarf joint to take out the damaged area.




Your statement does not fly.






blakmax (Aeronautics)


30 Nov 11 4:02







I am not so sure that composites are so susceptible to being scrapped for minor damage as suggested by Steven. Many repair methods are available for composite structures including adhesive bonding and mechanical fastening. My main concern is that many OEMs would use resin injection to "repair' a delamination. I have yet to see realistic tests which demonstrate a restoration of strength and restoration of fatigue properties after resin injection. I HAVE seen(old)data which showed that injection repair of impact damage reduced the fatigue life of a 24 ply composite by 2/3rds compared to leaving the damage alone.



However I am also not a fan of scarf repairs as suggested by BE, especially at a scarf ratio of 1/100. Suppose you have a structure which is 0.25 inches thick. If a scarf of 1/100 is applied then the scarf will be 25 inches long, on both sides of the defect, giving a 50 inch repair. I can't see any DER approving a process to remove 50 inches of good material.




Now my expertise is mainly in metal bonding, and if Steven's metal aircraft were bonded using appropriate design methods and valid processes, they would last even longer than the alternate mechanically fastened method.










SparWeb (Aerospace)


2 Dec 11 15:24







What can I say? The backrooms of numerous repair shops are stocked with plastic airplane wings they can't fix. They need OEM engineering support for repairs because the scope of damage exceeds the MM limits. They wait and wait because the queue is very long. The work could be done, but the approved procedure, drawings, materials, processes all need detailed evaluation and documentation before the wing can be airworthy again.



I have been whacked by FAR 25/23/27/29.613 enough times to be wary of composites. Maybe I'm just unlucky, but the airframe types I've been dealing with don't use materials in Mil-Hdbk-17. No data, then no proof, so no approval.




Since I am not the OEM, I don't have the proprietary data on hand to back up analysis of a repair. To get it would cost as much as a new wing. To develop the data would cost more than a new wing. The client buys a new wing.




I'm not talking about dropped screwdrivers, either. Everybody reading this thread knows you can find that kind of repair in the Maintenance Manuals of any aircraft. I'm talking about hangar rash that cracks the LE. Birds nests in the landing gear well. Fuel seepage. Disposition after a lightning strike. All the fun stuff that happens to airplanes in their lifetimes. That doesn't get written up in the MM because every case will be different.




Nobody can cope with these repairs unless they have access to coupon strength/humidity/temperature test data. As I said, that belongs to the people who developed it for the type certificate.




To make statements that "it's easy" means you either you work for an OEM, have a deal with one to license the data, or acquired the data by "other" means... and I'll leave it at that.


Steven Fahey, CET


berkshire (Aeronautics)


2 Dec 11 22:44









I agree with you.


When I shut down my repair station I had 11 plastic wings from various manufacturers that were unrepairable because the repairs needed were outside the scope and detail of the maintenance manual (We burned them.), and it was cheaper to buy a new wing than engineer a repair. Another thing driving this was the customer who very often felt that a new wing would just be better than one that was fixed.


Right now in the USA there is one manufacturer of plastic single engine land aircraft, who is charging far more than cost for engineering expertise for relativly simple repairs that are outside the somewhat limited scope of his repair manual, just to ensure that damaged aircraft are totaled out instead of being repaired.




The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor


thruthefence (Aerospace)


3 Dec 11 13:29







"Designing repairs" has become a profit center for the oem's, "plastic or otherwise.



Had a "plastic" aircraft attempt to taxi with the tail tie down still attached, damaging the attach point. The repair (and associated paperwork) cost thousands of dollars to acquire.




At the time of the incident, the type was in the field for years. You can't convince me that that this hasn't happened before. If so, and the data was approved, why wasn't the repair scheme added to the maintenance manual?




Because all you have to do is double click a file, and send an invoice for $1500.00, and the end user has to pony up the money.






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Hidden damage is the thing from ground contact or "hangar rash" etc. I'm so old fashioned in that I still believe it steel tube for the fuse and wood for the spar ( like a Cub or Auster). You can keep them going for ever, especially if you put oil or nitrogen inside the tubes. We could go all modern and have an Al spar.


If you are going for the ultimate in performance and can afford the cost and risk the carbon fibre is so far ahead, especially if we insist on building to a low weight limit, where you are virtually forced in to going down that path. Too expensive and complicated for the average builder or maintainer. Nev



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