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Fly-by-Wireless Plane Takes to the Sky
galactic_grub writes to tell us that engineers in Portugal have built and flown a plane with no wires or mechanical connections between the major systems, only a wireless network. From the article: "Tests flights carried out in Portugal have shown that the system works well. Cristina Santos, at Minho University in Portugal, who developed the plane, says the aim is primarily to reduce weight and power requirements. 'Also, if you do not have the cables then the system is much more flexible to changes,' she says."
Pretty darn resistant to lightning, actually. A lot of designing goes into making sure that critical systems remain functional and that nothing carries an excessive current in the event of a lightning strike, which happens a lot more frequently than most passengers probably realize. During the 80's NASA did a very extensive investigation into the effects of lightning on airplanes. Some of the test pilots involved had their planes hit hundreds of times while deliberately flying through the most active parts of the storms. A source I just googled up says the average passenger plane gets hit once a year. According to another source the last commercial airline accident attributed to lightning was in 1967, which was due to a fuel tank explosion, not a control outage.
Old style plane controls were based on either cables (not suitable for larger aircraft) run from the pilot's controls (yoke, pedals, throttle) to the control surface or else on hydraulics. In the latter, there are hydraulic valves actuated by the pilot, and the pressure is transferred via hose from the pump to the valves to hydraulic cylinders or motors that move the control surfaces. Anyone who is familiar with hydraulics knows how heavy those components are. Fly-by-wire eliminates the direct link, allowing much shorter hydraulic routing, replacing hoses with pumps at the point of use, or even replacing hydraulics with electrical actuators. All the components are surge protected and wiring is typically triple redundant.
I believe there are three dangers presented to airplanes by lightning: interference, stray currents, and energy dissipation. Interference can be dealt with by minimizing the opportunity to pick up signals (the 777 for example uses fiber optics instead of wires) and signal processing. Stray currents, which can damage componenets, are handled by isolating the electrical systems from the structure and using surge protectors. By energy dissipation I mean resistive heating of the airframe. This normally isn't a problem with aluminum airframes/skins, because the bolt passes straight through the plane with little trouble. With composite fuselages like on the A380, there is typically a safe path designed into the system for the same purpose. Otherwise a bolt might find a relatively small current path and overwhelm it, heating it so fast it could actually vaporize violently (a somewhat more technical way of saying it explodes).
This is simply incorrect for a couple of reasons. Whether or not composites are strong or stiff depends on the material -- composites like carbon fiber are both very strong and stiff (compared to say aluminum or steel) while composites like kevlar are less stiff but still quite strong. But a composite is just a heterogeneous material, usually a fibers laid in a matrix, so it can have almost any set of properties.
In fact, a composites are generally, anisotropic meaning that their strength and stiffness vary with direction. Think of it this way, if you pull on a strip of filament tape along the strip, its hard to break, but if you pull across the strip, it tears easily. Filament tape and duct tape are fiber composites -- like the carbon fiber in the Boeing 787 Dreamliner. Aluminum, by comparison, would be equally strong (and stiff) either way. Of course, carbon fiber is much stronger and stiffer than duct tape.
Stiffness and strength should be explained. Stiffness is a material's resistence to deformation under loads. Flexibility is the opposite of stiffness. Most aerospace materials are modeled to act alot like springs -- increasing the load results in proportional change in length. Stiffness in tension and compression (pulling and pushing) is measured using Young's Modulus, E. E is a constant, single scalar for a given alloy (temper, etc) of metal, but changes depending on the orientation of a composite structure. For composites, its described using 0th, 1st or 2nd rank tensors -- depending on how hard my professor wants to make the problem. There's also shear stiffness measured by the shear modulus, G. Both moduli, E and G have units of Pascals.
Strength is the stress -- load per area, given in Pascals-- at which a material fails. There are different definitions of failure, and so different values of strength for a given material -- but one of the most popular ways of looking at it is "when does the material stop acting like a spring. How much force can be applied before it won't return to its original shape?" That's the yield strength of the material and it works for our purposes.
Also note that the density of the material plays are part. Steel is stronger and stiffer than aluminum, but aircraft are made out of aluminum because they must be light. Aluminum has a higher strength to weight ratio than steel. So, pound for pound, its stronger -- but its yield strength, measured in Pascals, is lower.
As it turns out, carbon fiber -- pretty much the definitive composite material in aircraft -- is lighter, stiffer and stronger than aluminum -- the definitive metal. E for carbon fiber (the fiber without a resin matrix) > 200 GPa. E for aluminum (7075 T65) = 72 GPa. Yield strengths: Carbon fiber >3 GPa. Aluminum ~= 500 MPa Aluminum has a density of about 2.7 g/cc while carbon fiber is more like 2 g/cc. Note that the choice of matrix (the resin that holds it together) and layup of fibers affects the strength and stiffness of the fibers, but these numbers are a good start on raw material properties
Clearly, composites are not necessarily flexible -- in fact, if there's a distinctive property of carbon fiber, its that its very, very stiff. In fact, that is the property my composites professor emphasized in class time and time again -- possibly because its such a pain in the ass to do failure analysis on carbon fiber laminates. Composites are complicated materials.
One last note: flexibility is not necessarily a bad thing. But I'll save you the lecture... check out the Active Aeroelastic Wing F/A-18.
--sabre86