I don't know if this has been discussed here on the forum before or not, but Santa is looking for something to give me this year that is bike related. I'm wanting to make the trip to Sebring, Florida in February for the 24 hours of Sebring race. Do you guys n gals think an aero helmet would be a benefit on a recumbent, or would the tail be pointing down and cause more drag than a regular helmet? Anybody heard of any wind tunnel testing of helmets for recumbents? I wouldn't think there would be too much of an advantage for anything under 100 miles, but 24 hours??? It might get me a half mile or two....
Oh yes, and if anyone has come across a good one (that isn't too pricey), what kind is it--brand name????

The standard DF time trial helmet points down in back due to your recline...  I think Greg Gross tried one at the TTTT one year, not sure how well it worked, or what he had to do to it. Obviously, you're looking to have a helmet that minimizes the "open clamshell" profile to the wind.
 
I like my Lazer 02 - I can angle it down somewhat, and it doesn't have the adjustment knob down low in the back, hitting the headrest.  However, I don't know of any "recumbent specific" helmets out there.  Looking at photos of distance racers like Maria or others, I don't think they try to do much special to minimize helmet angle.
 
Here's Lynn on her Musashi, with the Air Attack helmet Doug mentioned above:

 
Here's me at HHH, on the Xstream, alongside Doug - note that Doug sits somewhat more upright in the Velo:
 

 
By contrast, here's Kent Polk from a couple years ago, laid out at about 10 degrees...  that reclined, I don't think there's much you can do about helmet angle!
 

 
Here's Maria Parker, in a marketing photo, for a bit of bike cheesecake    
 

 
 

Standard setup helmets work fine so long as it won't rub the tire. But honestly the giro shield is the way to go. It's what I use now.  Unless you want zero air flow, in which case the expensive casco is the best.

I'm overseas and have been riding with a demo Casco Speedaero RS.   I think its actually got better airflow than the Giro .. but its also stupid expensive at about $450 shipped into the US
the Photocromatic visor is nice but I don't think it would be dark enough for our texas summers.  
 
Quote from Action Lad on Dec 18^th, 2016, 8:47pm:

Quote from MrWizard on Dec 19^th, 2016, 8:24am:
Now that looks like something I can wear.  My head, whether on the bike or trike, is tilted back a bit to keep my breathing airway as clear as possible.  This is the first shield-type helmet glass that doesn't seem to be open at the bottom: https://www.youtube.com/watch?v=MlDzBt4Vw10&t=62s 
Then there is the cost thing.  Won't be getting one soon.  

Quote from MrWizard on Dec 19^th, 2016, 9:11pm:

I love my giro attack helmet.  I don't know about it's aerodynamic advantage except I have many memories of the wind catching my helmet's visor and blowing it back. The Giro never does that.

I'm such a cheapskate......I settled for a $40 Giro Foray. It looks halfway aero and it's bright yellow. I may have to alter it to accommodate my laid back head position, but EVERYTHING gets altered in my world.