>>>The depth of knowledge and experience at DC-3 Airways never ceases to astound me...<<

Ken and Mark,Yep, I agree. Now all we need to know is the (simulated) focal plane and focal distance of the Bitzer/Beaumont driftmeter, version 7. Mark? Dave?Best,Gary

Now all we need to know is the (simulated)>focal plane and focal distance of the Bitzer/Beaumont>driftmeter, version 7. Mark? Dave?Dave comments:Fp/Fd = 0.44True air speed (Knots) = 0.3 times the altitude in feet, divided by the time in seconds to move from top to bottom of the eyepiece.Approximately!MarkMark "Dark Moment" BeaumontVP Fleet, DC-3 AirwaysTeam Member, MAAM-SIM[a href=http://www.swiremariners.com/cathayhk.html" target="_blank]http://www.paxship.com/maamlogo2.jpg[/a]

To All,Thanks, Alex, for the fine info, and to Mark and Dave for following up. I just finished an initial test, and results were little short of amazing.I set weather to calm (no wind), and flew along the Texas and Louisiana coastlines where I could pick up ground landmarks at essentially zero altitude. I used two baselines for groundspeed: (1) IAS corrected for temperature and altitude, via the E6-B; and (2) flying directly away from, or toward, a VOR, at least 20 miles distant, and using the co-pilot side groundspeed gauge. I tested it at various altitudes from 18,000 down to 3,000 feet. The two baselines were always in very close agreement (max difference was 2-3 knots), so I felt confident about the actual groundspeed.I made one tweak . . . I used 0.28 times altitude divided by seconds, instead of the 0.3 in the last post on the subject. With this constant, my driftmeter calculated groundspeed was no more than 1 or 2 knots off the baseline from 18,000 down to 5,000 feet. At 3,000 feet the time it takes an object to get through the driftmeter is swift, and my timer doesn't have 10ths of a second so I made several runs and when half came up 6 seconds and half 7 seconds, figured on 6.5 :-). Even with the approximation, it was only off 3 knots. With a good stopwatch, it'll be just fine.I learned some tricks for timing -- it's difficult to pick up an object coming out of the driftmeter at the top -- and I'm sure I'll find more. It's not too important at high altitude, but at 3,000-5,000 feet it is. The simplest one is to pick up a road, rail line, anything straight, that's moving at an angle toward the middle of the driftmeter . . . then you have a bead on it and can start the timer the instant it hits the top of the meter. Many thanks again to all for converting a vague reference in an article into a useful tool. What a great bunch of guys :-).Best,Gary