The goals in an external display system for a flight simulator is to have the maximum possible field of view, minimal parallax, and an accurate representation of the external world view that is in proper perspective to what a pilot would see while seated in the flight deck of the real aircraft.
Choosing larger LCD displays will maximize field of view. Placing LCD displays as close as possible to the flight deck will also maximize the field of view. However, closer placement to the flight deck worsens parallax. This can be fully corrected in software settings for the eye-point of one pilot. Therefore, in a home simulator setting which is usually operated by a single pilot, the disadvantage of parallax is less important than the advantage of a larger field of view.
Field of View
Once a decision is made on LCD display placement, the flight simulator software needs be provided with accurate field of view measurements for each of the external displays. This is critical for the external world view to accurately reflect what a pilot would see from the viewpoint of sitting in the flight deck of a real B-737 aircraft.
Determining field of view for an LCD display requires making measurements and use of trigonometry but is not that complicated. Field of view is calculated by right triangle trigonometry. Width of one leg of the triangle is measured from the exact center of the display to the lateral edge of the display on one side. The other leg of the triangle is eye-point distance to the center of the display. This is measured from the display surface to the pilot’s eye-point while seated in the flight deck seat.
An angle is calculated by taking the arctan of the half the display width divided by the eye-point distance to the display. This calculated angle is then multiplied by two so that it is equal to the total horizontal field of view from one edge of the display to the other edge of the display. This same approach is used for calculating the vertical field of view, but using half the monitor screen height instead of width.
Prepar3D allows for configuration of field of view for multiple displays. For each display, the field of view measurements are entered into a view frustum. Rotation offsets are entered for the left and right display so that the external world graphics would reflect what would be seen for those viewpoints. For example, if the field of view for each display was 54 degrees wide, then the left display would be given a -54 degree rotation offset, and the right display would be given a + 54 degree rotation offset. Prepar3D also allows entry of an adjustment factor for the display bezel width. The process of entering this data into Prepar3D’s view groups is not very intuitive and the documentation is sometimes confusing. The Prepar3D website has a Learning Center that has a section dedicated to setting up View Groups.
In this flight simulator, the pilot’s eye-point is located 55.7 inches from the center of each of the three external LCD displays. The displays are 57.3 inches wide and 32.9 inches tall, so the horizontal field of view is 54.4 degrees for each monitor, or 163.3 degrees for all three monitors. Vertical field of view is 32.9 degrees.
Parallax
Parallax is caused by the pilot’s seats being offset approximately 20 inches from the flight deck centerline – to the left for the captain, and to the right for the first officer. In the real world, most objects are at a substantial distance away. This results in the angular difference between the captain’s eyepoint and first officer’s eyepoint to be minimized. In practical terms, when the aircraft is lined up on a runway, both pilots would perceive the aircraft to be aligned straight.
In a simulator, when the external world view is created on a display and it is placed close to the pilot’s eyepoint, then it will appear to the pilot that the aircraft is rotated or crabbed if the pilot moves away from the flight deck centerline, which is where the pilot seats are located in a B-737 flight deck. This crabbed visual perspective is unrealistic and especially disconcerting while taxing, taking off, or landing.
Parallax can be minimized to negligible amounts for both the captain and first officer if they visual display is placed a a substantial distance forward. In a Level D flight simulator, this is accomplished by creating increased distance using extremely large and expensive collimated mirror systems that creates an image at optical infinity. This is not practical for a home flight simulator.
For a home flight simulator, parallax can be completely corrected for one pilot’s position, either the captain or first officer. But the consequence of doing this will double the parallax effect for the other pilot seat on the opposite side of the flight deck centerline. Although a realistic home flight simulator will have a flight deck to accommodate two pilots, operation of the aircraft for taxi, takeoff, and landing is typically done by one pilot. Therefore, correction of parallax for one pilot is a reasonable solution. It is easy to set up multiple view groups in Prepar3D so the parallax can be corrected for the captain or first officer, depending upon which view group is selected for use in the Prepar3D menu at simulator startup.
The parallax correction is done by entering a rotation offset correction for each of the three displays. The degrees of correction are calculated by the arctan of the lateral displacement divided by the eye-point distance to the display. For this simulator the pilot’s eye-point lateral displacement is 20 inches from midline. This is the lateral displacement from midline that is present in the real aircraft B-737 flight deck. The distance to the display is 55.7 inches. This results in a calculated parallax value of 19.75 degrees. Correction for parallax is done by adding this amount to each of the rotation offsets for each of the three displays.
Here is an Excel spreadsheet which can illustrated the calculations needed for configuring View Groups in Prepar3D, taking into both field of view and parallax corrections. Here is a PDF description of the configuration procedure for View Groups with Prepar3D.
