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, 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. Andy ------------------------------------------------------- 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 [email protected] https://lists.sourceforge.net/lists/listinfo/flightgear-devel
