Composite fuselages

[No. 2]

For those that are involved in the manufacturing of composites, could you explain to me what the main challenges are in manufacturing a composite fuselage that has to withstand in-flight pressurisation? How does this compare with manufacturing a traditional metal-alloy fuselage? :confused:

Thanks

[used2flyboeing]

THe Raytheon Hawker - wound like a baseball all composite fuselage is stronger & lighter than aluminum - a similarly constructed Premier ran off the runway in flight test - hit an immovable object - tore the wings off - went up in a fireball - but all those inside survived ..

[Volume]

I´ll try to highlight some aspects, but for shure there is lots more to know...

Main load on a fuselage is the circumferal tension stress in the skin due to pressurisation. Therefor preferred orientation of the fibres is in this circumferal direction. If the composite fuselage shall be cheaper and lighter than the conventional aluminum design, you have to avoid any joints in this load direction, so you have to produce seamless tube sections, so to speak.
Unfortunately your outer surface is the one which have to be smooth and tight toleranced, so you have to produce it in a closed female mold. This raises a problem, because automatically winding fibres inside a circular mold is nearly impossible, any tension on the fibres needed to lay them straigt will lift it up from the mold again. And curing under pressure will not work, because it will not push the fibres onto the mold surface, but just tension it. Therefor fuselages like for the raytheon Premier I are wound around a male mold. This makes getting the fuselage from the mold a real hard job, because it is not so easy to split the mold and remove the part. And you have to account for local reeinforcements, which means more wallthickness, or (because of the smooth outer contour) a slightly smaller mold in this area. But fibre tension during layup will be less at these locations, so you may have build in voids or delaminations.

You can design and build excellent composite fuselages, using complicated layup, mix of different materials (pure fibres, unidirectional tapes, fabric of different styles like plain wave, satin etc, non crimped fabrics and many more) will allow you to place fibres in exactly the right orientation and wallthickness you need. This makes your fuselage an excellent lightwight design, and stronger and much more fatigue resistant than metal design. But you have to produce such design by handlayup with skilled people in sophisticated molds. This is the way gliders are produced. You will never build such a design at costs, that can compete with metal. And costs are the driving factor, the operators want to earn money with their fleet, they don´t care about having the most sophisticated, lightwight structure, it must do the job required at a competitative price, and composite is today far from this point.

While producing large continous structures (i.e. wing upper surface), composites are hard to beat. But when it comes to cutouts for doors, windows, manholes, systems etc. your design and the manufacturing process gets horribly complicated. It is of course much easier to mill a door frame from an aluminum block, than placing hundreds of different prepreg pieces in a complicated mold. If time and money plays no roll, you can produce integral door frames in one shot with the outer skin, reducing weight dramatically. But noone will ever pay for this.

The last aspect is repairability. If you damage a door frame on a metal fuselage, you cut out the damaged parts and rivet in some spare parts or add some doublers. This work is done fast, easy and cheap. If it comes to composite repair, it takes time, skilled personal and lots of money. This does include detection of the damage and its extent. You can design composite parts in a way, that cutting out the damaged part and riveting a doubler onto it is possible, but then you can´t optimize your fible placement, you have to build in additional fibres in unloaded directions, just to give the material enough strength for allowing riveted repair. But then your weight becomes equal to metal.

Maybe the higher costs of optimized design will someday succeed, because you get an airframe with no corrosion and fatigue problems, and a virtually unlimited service life. But you have to earn your money now, you won´t buy an expensive plane that eventually saves you lots of money in 20 or 30 years. The airlines have to survive the next few years, and even this seems to be a hard job. Noone can afford to think and invest in such a long term.

[No. 2]

Volume,

Thanks for that excellent post. Could you tell me your opinion on how Beoing will overcome these issues for the 7E7, on what will be the largest all-composite fuselage?

Thanks.

[Volume]

I have no idea how Boeing will do the job, I´m quite shure even Boeing doesn´t.

On the ´Dreamliner´-Homepage Boeing states :

" The airplane is super-efficient, using 15 to 20 percent less fuel on a per passenger basis than today's airplanes of this same passenger capacity. This makes the 7E7 as fuel efficient per passenger as today's most fuel-efficient wide body airplanes - the 777 and 747. This is really significant.

The efficiency improvements come from advances in four areas: engines, aerodynamics, materials and systems. The new engines being developed for this airplane offer about a 10 percent improvement in efficiency. Using super computers, we can design the airplane to minimize drag, making it even more smooth and streamlined, which provides another 3 to 4 percent improvement in efficiency. The wing area of the 7E7 is about 10 percent smaller than its much older competitor. The new shape of the wings and raked wingtips is streamlined and strong, yet flexible and light. More durable engines made from advanced and lighter materials provide another 3 to 4 percent improvement in efficiency. And the introduction of more efficient on-board systems provides the final 3 to 4 percent of improvement. It is the combination of these factors that give us our 15 to 20 percent improvement in efficiency. That also means this airplane costs about 10 percent less to operate than today's similarly sized airplanes. "


So Boeing seems to neglect the weight savings of the composite fuselage for their fuel economy improvement calculations. I´m quite shure negotiations with potential operators will quickly turn the fuselage to conventional metal, if it doesn´t offer a remarkable fuel saving.
I might be wrong ...

[Genghis the Engineer]

If I may, I'd like to add a couple of points to Volume's very informative post.

Fatigue is a big player in pressure vessels. The infamous Comet MK.1 failures back in the 1950s were due to fatigue caused by cyclic stressing of the pressure cabin. Due to that, and over the years since, much has been learned about the fatigue of metal pressure cabins - we'd all be in serious trouble if it hadn't. But, the level of knowledge of composite fatigue, which happens in very different ways to metal fatigue, is much lower. Because of this, and the fact that one thing we do understand is that composite fatigue involves gradual weakening of the whole material (rather than crack propogation in descrete spots) it's necessary to apply large safety factors to any composite primary structure, beyond that necessary for similar alumiunium alloy structure. This forces the weight up, almost certainly beyond what would be shown strictly necessary if we understood things a lot better.

The second is that in it's ability to create complex shapes in single parts, the designer is capable of massively reducing the number of parts in an aircraft - the logistics and manufacturing cost reduction due to this is very significant. I recall a conversation I had with the Quality Manager at a certain aircraft manufacturer who was enthusing that for a large door assembly he could reduce the number of discrete parts from about 14,000 to about 300. This is a big pay-off for the cost of tooling up to a completely different type of technology.


What specific problems does Boeing have to solve? Well in my opinion the biggie is in the science of predicting the strength of a composite structure at any point in its life. If they can get to a really precise prediction of that, then they can keep the initial weight (and materials cost, although that's a lesser player) down to an absolute minimum and start to get the gains they're claiming.

G

[ElectroVlasic]

From GlobalSecurity.Org (http://www.globalsecurity.org/military/systems/aircraft/b7e7.htm) :Composite materials got the nod over rival aluminium alloys as they provided greater durability, reduced maintenance, and increased potential for future development. Interestingly, there was little weight difference between the two materials and the cost of the fabricating composite components has become more competitive. It seems this agrees with the discussion in this thread. A lot of the discussions of composites on the 7e7 talk about weight savings, but the more expert analysis that I read says there will be no weight savings. Also I have read that most of the layup work for the 7e7 will be done by robots, thus reducing labor cost. Add this to the fact mentioned above that composite structures lower the part count, the goal of minimizing labor cost is clearly a focus.

Can anyone comment on how composites will increase the potential for future development, as mentioned in the quote above?

Also can someone comment on the risk factor: what if this doesn't go to plan? I saw the TV documentary on JSF and it showed a failed use of a one-piece wing surface, and I've read how a big part of the failure of the A12 attack bomber was due to the use of composite structures that never met their design goals.

Thanks!

[John Farley]

Thanks Volume. Very good stuff.

The only point you made that I would dare to question is the one about the outside shape is more important than the inside. I am not sure that always applies. Certainly the more accurate shape of the AV-8B wing is the inside. The skin had to mate with various internal components and these joints needed to be fuel tight at a significant pressure as well as accurately mate with internal components for strength considerations. So far as an airliner fuse is concerned there may well be cosmetic reasons for controlling the external shape but the boundary layer being what it is I would not worry about the aero aspects. It is all too easy to over engineer the external profile of the rear parts of aeroplanes.

When MDC did the graphite epoxy wing for the AV8-B back in the late 70s (!) I had dinner with John Hart-Smith who as I am sure you know did the structure for the all composite Lear Fan and was ‘borrowed’ at weekends by Sandy Mac for this then revolutionary fighter wing structure (because Sandy clearly felt John was as good as they came in those days). A quarter of a century ago John was remarkably conservative in his views about the merits of going from aluminium to plastic for primary structure. I seem to recall that the weight saving by going composite for the B wing (230 sq ft) was about 230 lb. However, unless you fully design and certificate the same strength and shape in metal I remain a tad dubious about the accuracy of such comparisons.

[ElectroVlasic]

An article (http://www.heraldnet.com/stories/04/12/05/100loc_7e7001.cfm) came out recently on this topic. Some points: By using composites, lower maintenance costs will save more money than will the lower fuel burn
Since composites do not degrade due to corrosion parts do not need to be replaced as often, which saves both the replacement cost of the part as well as the cost of the downtime of the aircraft
Aluminum planes go for C-checks every 24 months, the 7e7 will go every 30 months, with a goal of making that 36 months
Extending the maintenance cycle by six months would give airlines an extra 169 flights between C-checks
And for some good-old A vs. B talk: But simply hanging 7E7 engines on the A330 won't work, he said. The A330 already is heavier than the 7E7, and the 7E7 engines will be bigger, meaning more drag. That would make the A350 a less efficient airplane even if Airbus uses lighter composites, Bair said.Seems to me that the A350 will need to get an increase of ~30% in range over the A330 to compete with the 7e7, and that sounds like quite a tough goal to meet without starting over from scratch. It'll be very interesting to see what transpires over the next 4-5 years.

--ev--