Abstract
The longitudinal flying qualities of airplanes is going to worse in lower airspeed region, as shown in the criteria for short-period mode in MIL-F-8785C. However, Background Information and User Guide for MIL-F-8785B does not give the detailed explanation on this degradation. Our study pays attention to the deviation in lower airspeed region from short-period mode approximation, which is valid in higher airspeed region. The main parameter of this deviation is Phugoid-mode natural angular frequency ω_<n ph>, which cannot be neglected in lower airspeed region because its oscillation period is going to be shorter in that region. Also, we investigated the relations between short-period mode natural angular frequency ω_<n ph>, and ω_<n ph>. The results of flight simulator experiment show that the flying qualities evaluation based on Short-period mode approximation is improper in lower airspeed region and then upper limit of ω_<n ph> should be added into the criteria for longitudinal flying qualities.
