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Mathijs Kok

Bronco advance flight mechanics: Low speed flying characteristics

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The Bronco has some of the most detailed flight mechanics ever done for FSX. A lot of time was spend on getting the less obvious characteristics right. But that means that sim pilots can encounter things they have never felt before. We like to explain these in some detail. Feel free to ask us more about it here. John, who did these amazing files is on hand to assist.

In the beta tests we seen a few times that pilots had problems in the lower speed regime. You will not encounter this much if you stay over 100 knots but if you like a more detailed explanation of the effects read the text below. The flight mechanics do mimic this behavior rather accurately.The text comes from a NASA publication.

Even the dreaded Dutch roll behavior that the real aircraft shown a few times was detected by beta testers. Actually that's rather cool!

Roll Characteristics

Lateral control input in rate and magnitude varied considerably in roll characteristic tests. Full aileron input was not employed due to spiral instability. These data are summarized in Figures 37 and 38. To minimize the differences in control input the summary parameters of bank angle rate and peak-to-peak sideslip angle were normalized to aileron deflection. The effect of the differential propeller beta system on roll response appears to be negligible as seen in Figure 37. This could be due to the fact that small control inputs were used. Full scale airplane tunnel data (Ames test 388) indicate a 25 percent increase in roll control power with differential beta on. The effect of the differential propeller beta system on sideslip angle occurring in rolls is apparent in the data of Figure 38b. The pilot also indicated that the differential propeller beta system improved the adverse yaw characteristics.

Turn Characteristics

Turn characteristics for 20° banked turns are shown in Figures 39 and 40. The rudder fixed turns in Figure 39 indicate larger yaw rate, sideslip angle and bank angle deviations. Due to the high rudder control power and/or weak directional stability, very small rudder inputs were used in the coordinated turn data of Figure 40. The calculated steady yaw rate for a 20° banked turn is 5.8°/sec at the airspeed employed in the flight tests.

Dynamic Lateral-Directional Stability

Lateral-directional dynamic stability tests were difficult to perform at low speeds. The maneuvers were initiated with rudders and due to airplane spiral instability and the low dutch roll natural frequency, the maneuvers were not held for a long enough time to record a sufficient number of cycles. The available data are shown in Figure 41.

Lower frequencies and less damping is evident than estimated from static stability derivatives obtained from Ames test 388 and estimated rate derivatives. The destabilizing cylinder gyroscopic effects were included in the estimates and would not be expected to cause the difference from flight test results. The data scatter is believed due to inadvertent aileron and rudder inputs while the airplane motions were decaying after the disturbance input. Figures 41b and 41d show the bank to sideslip amplitude ratios from the dynamic stability tests. Due to the low frequencies and relatively short duration of the test maneuvers considerable scatter occurred.

Although no tests were performed to evaluate the degree of spiral instability, it was apparent to the pilots that spiral instability existed. The estimated dynamic stability calculations also predicted the instability and indicated that the divergence rates would be rapid, i.e., time to double amplitude of only several seconds.

Pilot reports indicated a coupling of lateral-directional dynamic motions with longitudinal modes. The coupling on pitch angle, angle of attack and airspeed can be seen in Figure 42. The coupling may be initiated by the pitching moment due to sideslip angle. Inadvertent elevator and lateral control inputs are also evident in the data of Figure 42 which may be contributing to the longitudinal coupling.

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