Advanced Trainer Flight Test Support
The
development of an advanced trainer configuration required the capability
to demonstrate a safe spin and recovery. The safety of flight issues
that arise in the evaluation of a new configuration's post stall characteristics
required the development of a high angle of attack simulation that would
support and guide the exploration of the new vehicle's characteristics.
Because of Bihrle's background in the earlier wind tunnel testing and
simulation development for this vehicle Bihrle was tasked to update
the simulation of the aircraft to support this test requirement as well
as to provide flight test analysis and support as the test program progressed.
Following the development of the simulation flight model, incorporating a complete non-linear model of both the static and dynamic data collected during earlier wind tunnel tests initial simulation runs indicated that standard aggravated control inputs would result in oscillatory
departure and spin behavior, undesirable for the training application. The initial spin tests confirmed this behavior (see the comparison between simulation and flight in figure
1) with highly oscillatory spins at steep to moderate angles of attack, depending on the control inputs. This behavior became particularly acute during the recoveries, where the normal recovery procedure used by the test pilots, releasing the controls, periodically led to a violent roll rate spike approaching 300/sec.
Because of safety concerns heightened by an earlier incident in the normal flight regime, the flight test program was suspended for fear that the elevated roll rates would cause structural damage to the wing fuel cells and the wing structure itself. Up to this point, the simulation
had not demonstrated this behavior, and upon closer inspection the difference arose from the recovery procedure used during the simulation sessions versus flight. As shown in the roll spike recovery time history, the simultaneous release of aft stick and the rudder input can result in
the rapid reduction in the angle of attack in the presence of one of the large sideslip excursions seen during the oscillatory spin. The configuration exhibits significant lateral stability below stall, and in combination with the high sideslip angle, a large rolling moment results. The
fact that this behavior was not evident in earlier simulation runs was because the simulation pilot was recovering from the spins by releasing pro-spin rudder input first, and once the yaw rate (and the oscillations) subsided, the longitudinal stick was released for the ultimate
recovery. This is the recovery technique used in the earlier time histories shown above, and following Bihrle's recommendation and demonstration to the flight test team, this recovery procedure was tested in flight and verified. At this point the flight suspension was lifted and the test
program was able to proceed, with this recovery procedure recommended for the remainder of the test.
Bihrle's participation in the success of the high angle of attack flight test program extended beyond simulation support, flight data review and test recommendation. The oscillatory nature of the spin was deemed undesirable for the training application, and a program was undertaken
to modify the spin. Rather than taking a more traditional approach to modifying the flight behavior that would have involved making small empirically directed configuration changes and flight testing to evaluate their effect, the customer opted to conduct engineering studies first.
Bihrle was tasked to review the flight test data and following some experience driven empirical parametric changes to the flight model, it was found that the key aerodynamic effect was an unstable (uncoordinated) roll due to yaw rate characteristic that occurred on the baseline vehicle post stall. By making this value coordinating in the spin region, much less oscillatory behavior resulted.
A short dynamic test entry was undertaken to identify a possible configuration modification that would improve the roll due to yaw rate behavior. Fuselage strakes were quickly identified to have a beneficial effect, however the effect was highly dependant on the location and the
length of the strakes, with shorter strakes quickly losing the positive influence.
A simulation of the spin with the fuselage strakes displayed significant beneficial effects in minimizing the oscillatory spin. Nevertheless, the simulation results also showed that the shorter strakes were not sufficient to reduce the spin oscillations.
Subsequent flight tests proceeded with the shortest strakes first, because of the desire to minimize the nose up effect and drag from the addition of the strake. As predicted by the simulation, the spin oscillations did not subside until the largest strake was added.
The high angle of attack flight test program was conducted in a highly compressed schedule and with very acute safety concerns. Bihrle's participation in the planning and execution of the test program allowed the test program to proceed safely, even after problems were encountered.
In addition, the use of an analytic approach to understanding and modifying the spin behavior ultimately resulted in a significant savings in flight test time and money as well as overall program time.
Other Flight Test Support Success Stories
X-31
Departure Flight Test Analysis and Support
F-16
MCID Configuration Development Flight Test Support
