On Sun, 12 Mar 2006 13:31:49 -0800, Andy wrote in message <[EMAIL PROTECTED]>:
> Jon S. Berndt wrote: > > David Megginson wrote: > > > I agree with Melchoir on this point, and, in fact, I have an > > > unbiased reference point. I took my 13-year-old daughter and her > > > friend for a one-hour flight yesterday in beautiful, sunny > > > late-winter weather. While we were taxiing at 1000 rpm, the > > > propeller was -- just barely -- visible as a blur in the air. > > > When I increased to 2000 rpm for runup, my daughter's friend > > > immediately shouted out, unprompted "I can't see the propeller any > > > more!" > > > > I wonder if there is a physical or physiological explanation for > > this. > > Here's a shot: > > Physiologically, your retinal cells act as accumulators. As the prop > speed approaches infinity, the likelihood of any given photon from the > propeller disk having bounced off of a blade first is just the area of > the propeller blades divided by the area of the disk. So all your eye > cells can report is the average color, which is obviously almost > entirely transparent (the back of my envelope tells me it should have > an alpha of about 0x04 -- pretty much completely transparent). > > But as the blades slow down, they spend more time being focused on > specific cells, which accumulate the stimulii and pass on a color to > the brain that is an average of the past N milliseconds (I think N is > on the order of 30 or thereabouts -- it's definitely more than 20, ..say 25Hz, for PAL, I think 24 or 30 for the 60Hz Americas. Movies in cinemas 24Hz, so they either show every 24'th frame twice or run the movie at 24/25 speed on TV and vice versa. Early movies used 16Hz, so here you can see the flickering. ..was 60Hz power a factor when they chose 24Hz for movies? > which is the framerate of a PAL television). So the brain sees a > signal that it knows is different for different directions, even > though those directions are "flickering" rapidly. > > And here's where psychology comes in: our brains just don't interpret > flickering signals from the eyes the same way as averaged signals. We > see "something there" in the flickering case, where the nearly > transparent case gets "interpreted" as empty space, even though it > very slightly different from the empty space outside the propeller > disk. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ------------------------------------------------------- This SF.Net email is sponsored by xPML, a groundbreaking scripting language that extends applications into web and mobile media. Attend the live webcast and join the prime developer group breaking into this new coding territory! http://sel.as-us.falkag.net/sel?cmd=lnk&kid=110944&bid=241720&dat=121642 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
