This airplane is expected to be safe from destruction by flutter instabilities.

The proposed APEX high-altitude aerodynamical-research sailplane (see Figure 1) has been predicted to be free of flutter instabilities within its flight envelope. Designed to fly under remote control at altitudes of up to 100,000 ft (30.5 km), the APEX airplane would feature a stiff boron composite structure, the vibration-mode characteristics of which would be such that they should enable the airplane to fly at relatively high subsonic mach numbers without risk of destruction by flutter.

Figure 1. The APEX Sailplane, shown here as rendered by an artist, would be a unique, remotely piloted research airplane that would fly at high altitudes.

This prediction is the product of a flutter analysis that included a modal analysis based on a mathematical model of the dynamics of the airplane structure. Modal analysis is an essential part of flutter analysis; it is also needed in analysis of results of ground vibration tests and in the development of control laws. The flutter analysis was performed in lieu of flight tests to provide assurance of flutter stability, which tests are beyond the scope of the APEX project.

Figure 2. The APEX Flight Envelope does not enclose any flutter instability, according to flutter analysis. (Vne = Velocity never exceed; Vd = Velocity in dive.)

In preparation for the flutter analysis, the Advanced Soaring Concepts mathematical model of the structural dynamics was converted from a format denoted "COSMOS" to a format denoted "STARS" and validated. Detailed and accurate mass and stiffness distributions were included in the model.

The results of modal analyses were examined and plotted, and deflections were interpolated. Final flutter solutions were computed by use of a matched point, so that the flutter-stability calculations could be confirmed by recalculating them with flow parameters at the predicted stability boundary. The updated results of modal analysis were found to follow reasonable patterns. Flutter instabilities were found to lie well outside the flight envelope (see Figure 2).

This work was done by Roger Truax of Dryden Flight Research Center.