NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Dalton E6B Dead Reckoning Computer
From: Paul Hirose
Date: 2008 Aug 02, 10:12 -0700
From: Paul Hirose
Date: 2008 Aug 02, 10:12 -0700
01 0609 Renee Mattie wrote: > I saw an old Weems and Plath Dalton E6B at Bacon's and snapped it up. > Imagine my surprise to see that they are still making these things, > and offering them for sale at http://www.mypilotstore.com/MyPilotStore/secp/22. > > On the slide-rule side, the grommet seems to be off-center. So if I > set the dial for 60 miles/hour, I'll apparantly go almost 18.1 miles > in 18 minutes, though almost exactly 9 miles in 9 minutes. > > Have I got a good one or a bad one? A not so good one, I guess. The centering error on mine is barely detectable, about the width of a line. I also have a Jeppesen CR-2 flight computer. It's much more compact than an E-6B because the effect of wind is calculated by a different principle. With the side of the device that's pictured at the web site, you separate wind into headwind and crosswind components. Crosswind and true airspeed are set on a sine scale (around the outside in the picture) to obtain wind correction angle. Headwind is mentally subtracted from true airspeed to obtain groundspeed. By contrast, on an E-6B you construct the wind triangle and read wind correction angle and groundspeed directly. The solution takes more space, but workload is less. Either computer can solve for drift and set in marine navigation. Just mentally apply a convenient scaling factor of 10, 20, etc. to the values on the wind and airspeed scales. I solved some of the old Silicon Sea problems that way. For time / speed / distance computations both devices have the same facilities. As usual with slide rules, the user is responsible for placing the decimal point, so ship speeds are as easy to handle as airplane speeds. Or car speeds -- I used my E-6B a few days ago to check speedometer error. When compressibility is significant (say, above .4 Mach) the CR computers are superior at converting calibrated airspeed to true airspeed. E-6Bs assume incompressible flow, so they're inaccurate at jet cruise speeds. But airspeed computation is simpler, and the error means nothing at typical light plane speeds. Deluxe E-6Bs are made of sheet aluminum, while CRs are plastic and can warp if left in a hot car. -- I block messages that contain attachments or HTML. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---