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Fri, Sep 13, 2002

Pull Dents, On the Plane

Bird Strike, Hail Damage -- Dents Come Out, Electronically

Click to EnlargeOriginally, the process was designed to fatigue-test metal, by electro-magnetically pounding down on it. As some McDonnell-Douglas engineers were testing the equipment, though, one noticed that, rather than pounding the surface down, it was actually pulling it up.

"Magnetism," was the quick explanation -- but this was aluminum. After refining the idea, and determining exactly what was happeneing, the new process, Electromagnetic Dent Removal, was available to industry.

Any smooth dent -- typical in bird strikes and hailstorms -- is a candidate for this process. Large (palm-sized and larger) dents can be worked, as well. Some dents -- such as those on the exact front edge of air inlet nacelles -- can sometimes be removed in this way; but the only way to know for sure in some difficult cases, is to try it.

The process looks simple: the dent is located and its edges are defined, and a clear mylar mask is placed over the damaged area. The EDR tech then hooks up one side of the electric field to the panel; and then he sets the other half of the working end of the machine on the dent, and pulls the trigger. BANG! The underside of the dented metal is negatively charged, and the hand piece localizes a massive positive charge, literally pulling the metal up, electro-magnetically.

The power unit of the system is constructed very much like a high-powered photo studio strobe light. OK -- make that, "two strobe lights." Two banks of capacitors charge up, and the first bank discharges into the airplane's panel. This loads the panel with a negative charge. Just after that (about 2 microseconds), a second charge is sent through the control head, which has, as its working surface, a flux-concentrated coil. The head, though, is charged with a positive charge.

What happens is that the huge load of electrons in the aircraft panel are magnetically dragged to the head. They go so fast, and are propelled by such a strong charge, that they act to move the metal itself. The panel gets sucked up flush to the coil, and the stress in the panel is relieved. The dent disappears.

How much power is invloved? The unit we saw at NBAA, though it plugs into standard 110BZC, stores a lot of juice in its capacitor banks. A maximum charge (to straighten nearly 1/8" thick plate) may be about 500 volts, and 28,000 amps. Folks who use pacemakers don't operate such equipment!

The process has been used by McDonnell Douglas, and then by Boeing, since 1986; but it's only recently that Boeing has decided to allow its use on non-airliner machines.

We saw this demonstrated at the NBAA convention in Orlando, and it's really remarkable. As long as there are no creases (it's remarkable, not magic!), the process looked like it could handle most of the common dents we all hate to see in our aircraft.

The dents come out, the paint stays on, and nobody but you will ever know they were there. Nothing had to be taken apart; no parts had to be ordered; there is no new corrosion exposure -- and the airplane is back in the air, just like that, looking good, and performing properly again.

Cost and Logistics

The hand-held control head about the size of a portable typewriter, and weighs about eight pounds. It hooks into a big ol' box that's about the size of a TIG welder, though (without the tanks). Although the control head is convenient to use (it plugs into standard 110 VAC), it's a big box. Therefore, the service (performed at your site) involves a non-trivial expense. Most of the cost, in fact, is concerned with the transportation of the equipment. However, if that bird strike on your G-V is really bothering your, or you and thirty of your friends' planes were all caught out in the same hail storm, EDR may be just the ticket.

The company is based in Garden Grove (CA), so a day of making your dents go away can set you back anywhere from $12,000 to $15,000. However, as a shared cost, it's not bad; furthermore, there's no AOG time to worry about, and you don't have to disassemble anything, or repaint anything. Insurance companies know about this process, though its use is generally limited to big-buck airframes. If enough small-buck airframes all were together, though, it might not be a bad idea for owners to pool resources, and put the two-man team to work. The work is scheduled through Boeing's RAMS (Recovery and Modification Services).

Can't I just send my parts to them?

Because the actual moment of dent-relief is so frought with pure power, parts that are not anchored to the aircraft may assume unintended shapes or sizes. That's not to say your leading edge will wrap itself into a ball; it's just that, once you replace it on the wing, it may not line up. Likewise, control surfaces, if removed from the airframe for repair, may sometimes bind in their mating parts. Yes, it can sometimes be done; no, it's not recommended.

Pretty cool.

FMI: dennis.j.pinto@boeing.com

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