Thanks, CJ, for usng that cheezy picture of me in my work clothes

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I have both the helicopter helmet in the pic (a Gentex HGU-56 for you helmet fans), and the helmet I wore during my military days - a Gentex HGU-55 designed for fixed-wing airplanes with ejection seats. The HGU-55 is the one you typically see on USAF aircrew. As far as quality, Gentex seems to be the standard.
As you might expect, a helicopter helmet provides much better crash protection. It has additional webbing and pads that allow the helmet shell to absorb some impacts without transmitting them to your skull. The down side is that it's bulky - especially the one I wear. I feel like Darth Vader when it's strapped on!
The fixed wing helmet is designed for good visibility, noise attenuation and some impact protection - but not as much as a helo helmet. It's lighter, so that fighter guys don't have to strain their neck muscles as much in 9G turns. It's also a much closer fit to help ensure that it stays on during an ejection.
Helmets do provide some additional noise attenuation, but I've found that my noise-cancelling earphones work better than a helmet for cutting down noise. Most of the people I work with use foam earplugs when wearing helmets.
I plan on wearing the fixed-wing helmet when flight-testing my RV because of the relatively close head-to-canopy clearance - I don't think the helo helmet will fit. Except for aerobatics, I doubt I'd wear one after flight testing is done since I don't think the risk warrants it. But it would be interesting to find out how people get hurt in flip-overs, since the roll bar is supposed to provide protection. Would a helmet help? Who knows.
By the way...if the previous 'guest' poster is the gentleman in BC with the beautiful RV-6 'Imitutor', I hope your ship is back in the air!
Dave