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发表于 2020-11-1 23:51:33
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摘自X-Plane说明书中关于实时气动和帧率关系的一小部分内容：
Airplanes Flutter and Crash in the Simulator
The tendency for some aircraft to flutter and crash is a known limitation. Just as a car can only go a certain speed with a given horsepower, the X‑Plane simulator can only accurately model flight at a certain speed with a given frame rate.
If the frame rate gets too low for the flight model to handle, then the plane is likely to start oscillating quickly back and forth (referred to as “simulator flutter,” often occurring with autopilot on) as the flight model tries unsuccessfully to predict what the plane will do next. At this point, the computer is running too slowly to take small enough steps in the flight model to see what the plane will really do at each moment. Smaller and more maneuverable planes will accelerate more quickly, and greater accelerations require a higher frame rate to simulate.
This occurs due to the way that X‑Plane moves aircraft within the simulation. X‑Plane calculates the acceleration of the craft for each frame, then adds up the acceleration between frames to move the plane. This works fine if the frame rate is reasonably high and the accelerations are reasonable low. In fact, for any reasonably normal aircraft that has reasonably normal accelerations, a frame rate of 20 fps or more is fine.
Problems occur, though, when you have very light aircraft with very large wings going very fast, or sitting on the ground with landing gear spread very far out from the center of gravity. All of these things add up to the same result—high acceleration.
X-Plane, of course, can handle these high accelerations, but it needs a high frame rate to do it. For the flight model to work, there can only be a certain amount of velocity change per frame of the simulation. If the accelerations are high, then the frame rate better be high so that there is a reasonable velocity change (i.e., acceleration) per frame.
To determine how high a frame rate is enough to handle a given acceleration, just find the frame rate at which there is no flutter.
For example, imagine a Boeing 747 at approach speed. It slowly lumbers along, hardly accelerating at all. One frame per second could track that flight accurately. Now imagine holding a paper airplane out the window of a car at 80 miles per hour and letting go. The plane doesn’t smoothly, gradually, accelerate up to speed, it disintegrates in a thousandth of a second! To simulate that may require a simulator to run at one thousand frames per second! |
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