[simpits-tech] Hey Wayne, I got a GeneScreen question.....

[wledzian1 at comcast.net]

'exactly twice the size' is not so much a hard and fast rule as it is an optical constraint. Essentially what I'd do is plug the screen parameters into my calculator, then play with the eyepoint and mirror radius to get the calculated geometry to match the actual geometry as closely as possible. Bear in mind that an error as little as half an inch in the screen position is the difference between an image that converges only a few feet away and an image that's beyond infinity (negative convergence). A quick trial with the numbers below does indicate a very nearly 20-foot diameter mirror. To get more precise results, I'd need some more detailed screen measurements. I can tell you with confidence now, however, that reducing the mirror size to 15 ft diameter is NOT feasible with that screen. 

-Wayne 

----- Original Message -----
From: "Matt Bailey" <dabigboy at cox.net> 
To: simpits-tech at simpits.org 
Sent: Sunday, October 13, 2013 12:42:15 PM 
Subject: [simpits-tech] Hey Wayne, I got a GeneScreen question..... 

I am in the midst of designing a shop for the Sabreliner. I'm trying to 
nail down exactly how large my screen is going to be if I build 
something like what Wayne and Gene have. I already have that BP screen 
from FlightSafety.....the screen will hang above the cockpit, with its 
bottom bow starting at about 8ft off the ground. 

Here are some quick specs on the screen: 

Top bow diameter (corner to corner): 133" 
Bottom bow: 124" 
FOV: 180* 
Height: 48" 

Here are a couple pics of the beast: 

http://www.flickr.com/photos/alfamatt/9416907015/ 

http://www.flickr.com/photos/alfamatt/9419673508/ (That weird 
reflection on the ground is from the optics of the screen) 

Gene indicated that it was a pretty hard and fast rule that my Mylar 
frame would have to be exactly twice the size of the BP screen, and 
that the screen would not mimic the shape of the BP screen, but would 
be a spherical section. Is there any way to make it slightly smaller 
(say, 15ft diameter instead of 20ft+++) and still retain the correct 
optics? Also, do I base this sizing off the top bow, the bottom bow, or 
an average of the two? 

I am also not sure how high the Mylar screen needs to be. From pics on 
Gene's site, it looks like it would be placed with its center at the 
same Y position as the pilot, but curved in such a way that it 
represents a section of a sphere whose center would be the same center 
as the BP screen...no? 

Matt 
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