Ten years of stall testing

Abstract

This paper discusses the author's experience during four flight test programs to improve the characteristics of business jet aircraft. In each case, an experimental prototype was used to evaluate aerodynamic configurations for the first time. This was invaluable because in some cases aerodynamic techniques, used to make wind tunnel models behave more like the full scale aircraft, were used for the first time to improve aircraft handling characteristics. The characteristics problems solved by simple aerodynamic modifications included wing drop (roll ofo, stable deep stall and high angle of attack divergent wing rock. The effects on high angle of attack handling of the following aerodynamic devices are discussed: strips, wing fences, vortex generators, vortilons and Delta fins. It has been my privilege, over the last several years to be involved in several characteristics development programs on business jet aircraft. These programs have been very rewarding for the following reasons; the flight test aircraft was used as a true experimental vehicle, where new ideas were tested for the first time, the test pilot was an integral part of the engineering development team, the programs led to significant improvements in aircraft handling and, therefore, safety and in each case, the end result was a simple, inexpensive aerodynamic change to the aircraft. It is not unusual for there to be differences between the characteristics predicted by wind tunnel tests and those seen during flight tests. When this occurs, some developmental flight testing is required. General aviation seems to be the only flight test area where a true experimental flight test program is economically feasible and I believe that this was critical to the success of these programs. In several cases, the devices used to modify aircraft characteristics were the same as are used to reduce scale effects to make windtunnel models behave more like the aircraft. The characteristics problems solved by simple aerodynamic modifications included wing drop (roll off), stable deep stall and high angle of attack (AOA) divergent wing rock. Several hundred aerodynamic configurations and many thousands of stalls were flown during these very intense flight test programs. As many as four configurations were flown per day. Even though these were hazardous tests, no incidents or aircraft damage occurred during the tests. The effects on high AOA handling of the following aerodynamic devices was evaluated; strips, wing fences, vortex gencrators, vortilons and Delta fins.