Download the X-Plane 10 Manual - X-Plane.com
Download the X-Plane 10 Manual - X-Plane.com
Download the X-Plane 10 Manual - X-Plane.com
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128 APPENDIX B. TROUBLESHOOTING X-PLANE<br />
B.7 Airplanes Flutter and Crash in <strong>the</strong> Simulator<br />
The tendency for some aircraft to flutter and crash is a known limitation. Just as a car can only go<br />
a certain speed with a given horsepower, <strong>the</strong> X-<strong>Plane</strong> simulator can only accurately model flight at<br />
a certain speed with a given frame rate.<br />
If <strong>the</strong> frame rate gets too low for <strong>the</strong> flight model to handle, <strong>the</strong>n <strong>the</strong> plane is likely to start<br />
oscillating quickly back and forth (referred to as “simulator flutter,” often occurring with autopilot<br />
on) as <strong>the</strong> flight model tries unsuccessfully to predict what <strong>the</strong> plane will do next. At this point,<br />
<strong>the</strong> <strong>com</strong>puter is running too slowly to take small enough steps in <strong>the</strong> flight model to see what <strong>the</strong><br />
plane will really do at each moment. Smaller and more maneuverable planes will accelerate more<br />
quickly, and greater accelerations require a higher frame rate to simulate.<br />
This occurs due to <strong>the</strong> way that X-<strong>Plane</strong> moves aircraft within <strong>the</strong> simulation. X-<strong>Plane</strong> calculates<br />
<strong>the</strong> acceleration of <strong>the</strong> craft for each frame, <strong>the</strong>n adds up <strong>the</strong> acceleration between frames to move<br />
<strong>the</strong> plane. This works fine if <strong>the</strong> frame rate is reasonably high and <strong>the</strong> accelerations are reasonable<br />
low. In fact, for any reasonably normal aircraft that has reasonably normal accelerations, a frame<br />
rate of 20 fps or more is fine.<br />
Problems occur, though, when you have very light aircraft with very large wings going very<br />
fast, or sitting on <strong>the</strong> ground with landing gear spread very far out from <strong>the</strong> center of gravity.<br />
All of <strong>the</strong>se things add up to <strong>the</strong> same result—high acceleration. A light aircraft gives high<br />
acceleration because <strong>the</strong>re is little mass, and <strong>the</strong>refore little inertia. Big wings give high acceleration<br />
because <strong>the</strong>y put out lots of force. High speeds give high acceleration because <strong>the</strong>re are high forces<br />
under all that air pressure. A widely spaced landing gear gives high acceleration because it has a<br />
huge lever arm on <strong>the</strong> center of gravity.<br />
X-<strong>Plane</strong>, of course, can handle <strong>the</strong>se high accelerations, but it needs a high frame rate to do it.<br />
For <strong>the</strong> flight model to work, <strong>the</strong>re can only be a certain amount of velocity change per frame of<br />
<strong>the</strong> simulation. If <strong>the</strong> accelerations are high, <strong>the</strong>n <strong>the</strong> frame rate better be high so that <strong>the</strong>re is a<br />
reasonable velocity change (i.e., acceleration) per frame.<br />
To determine how high a frame rate is enough to handle a given acceleration, just find <strong>the</strong> frame<br />
rate at which <strong>the</strong>re is no flutter.<br />
For example, imagine a Boeing 747 at approach speed. It slowly lumbers along, hardly accelerating<br />
at all. One frame per second could track that flight accurately. Now imagine holding a paper<br />
airplane out <strong>the</strong> window of a car at 80 miles per hour and letting go. The plane doesn’t smoothly,<br />
gradually, accelerate up to speed, it disintegrates in a thousandth of a second! To simulate that<br />
may require a simulator to run at one thousand frames per second!<br />
So, while a simple 20 frames per second works fine for most any aircraft, when small, light,<br />
big-winged craft with widely spaced landing gear designs start flying fast, <strong>the</strong> accelerations <strong>com</strong>e<br />
up enough that in extreme cases, <strong>10</strong>0 fps might be needed to model accurately.<br />
This is more of a problem with planes that:<br />
• are small because <strong>the</strong>y maneuver much more quickly than big planes,<br />
• are light because <strong>the</strong>y have less inertia and react faster,<br />
• have long wings because <strong>the</strong>y have more leverage on <strong>the</strong> center of gravity, thus reacting faster,<br />
• have big wings because <strong>the</strong>y get more lift, thus reacting faster, or<br />
• have widely spaced landing gear because <strong>the</strong> gear has more leverage on <strong>the</strong> craft, causing it<br />
to torque <strong>the</strong> plane faster.