[Courtesy of Helmut Lelke, hlelke "at" worldnet.att.net, February 2002]
Active Pitch Stabilizer Concept
What are the three basic control system elements in the actively stabilized flying wing plank? The concept is illustrated in Figure 7.
1. Servo motor driven elevons provide the means to control airfoil pitch
moments by controlling the airfoil moment coefficient (cm).
The AOA sensor body is connected to and moves with the main part of the wing. In order to avoid wing wake, it is mounted behind and above the wing atop the vertical fin. The AOA sensor consists of a simple pivoted vane constructed of 1/32 in sheet balsa with a couple of tiny magnets attached to it forward of the pivot. A small linear hall-effect component senses the magnet rotations resulting from vane movements and transmits that angle information to the single chip computer. The computer processes this and other pitch control information and drives the RC servo in a way to return the flying wing plank to the design trim attitude. Other pitch controls inputs for the Albatross II example include elevator and electrostatic autopilot control.
The diagrams illustrate system responses for three situations. From top to bottom, the first diagram shows the plank at its design trim angle-of-attack - system reacts with zero elevon deflection. In the second diagram the wing angle-of-attack is negative relative to trim angle - system drives elevon up. The third diagram covers the positive angle-of-attack deviation from design trim angle - the system reacts by driving the elevon down.
How much elevon deflection is correct? Part of that answer is shown in the plot of Figure 8a. The plot indicates the minimum angle-of-attack to elevon gain required to make the flying wing plank neutral stable, like that of a symmetrical airfoil. This means that a gain of a somewhat greater magnitude than that indicated will be necessary for the flying wing to become stable and to fly like a conventional tailed airplane. A good rule of thumb is to use a gain factor neutral value plus 1. For example, with a CG located at the 30% point, the preferred AOA sensor gain should be adjusted to 1.7. See Figure 8b for definition of AOA sensor gain.
The gain plot was derived from empirical data. Results are based on a 20%
elevon width and typical airfoil deflection to cm responses. It turns out that
the gain is not a strong function of flap width and moderate deviations from the
20% have little effect on the final results.