Research on the Neal&Smith Criterion Application on Aircraft Flight Control System Handling Qualities Assessment

This paper utilized Neal&Smith criterion to quantify handling qualities of some aircraft augmented with Control and Stability Augmentation System (CSAS) over the flight envelop. Some aircraft mathematical model which augmented by longitudinal pitch Rate Command-Attitude Hold(RCAH) controller are employed. The aircraft handling qualities are described in terms of transfer functions of the pilot mathematical model, which includes a pilot’s neuromuscular delay, phase and gain compensation terms. If desirable closed loop dynamic performance can be obtained by the pilot mathematical model described by transfer function characteristics, then the satisfactory closed loop dynamic performance will also be achievable for the human pilot with decent workload. Consequently, the assessment results can be used to identify CSAS controller’s satisfactory effect domain, and furthermore, in aid of CSAS gain scheduling strategy design.


Introduction
Aircraft handling qualities tend to be quantitatively expressed rather than in terms of pilot's opinion during the aircraft Control and Stability Augmentation System (CSAS) design and test process. Neal&Smith criterion is one of the existing aircraft handling qualities assessment criterion, which is developed in aid of the design of an airplane flight control system's dynamic characteristics. Usually, several criterion, such as Gibson&Bandwidth criteria, * C and Control Anticipation Parameter (CAP), can also be applied simultaneously for ensuring satisfactory dynamic response characteristics [1] [2]. Neal&Smith criteria estimates aircraft handling qualities on the basis of pilot mathematical model compensation [3]. Some certain pilot flight mission which related with different flight phases, need more stringent requirements to complete the tasks successfully. The specification requirements are stated in respect of airplane classification, flight phase categories and handling qualities levels. Usually, those missions requiring similar handling qualities are grouped into three flight phase categories (e.g. flight phase Category A, B, and C) [4] [5]. This paper applied Neal&Smith criterion to quantify handling qualities of some aircraft augmented with a CSAS controller over the flight envelop ( F ) in category B. The assessment results can be used to identify CSAS controller's satisfactory effect domain ( 1 F ), and furthermore, in aid of CSAS gain scheduling strategy design.

Neal&Smith Criterion
Neal&Smith criterion shows a critical measure of pilot's handling perception for the pitch tracking task, which is assumed that pilot model trims the gain and phase characteristics to minimize aircraft closed loop resonance and the low frequency droop [3]. If desirable closed loop dynamic performance can be obtained by the pilot mathematical model described by transfer function characteristics, then the satisfactory closed loop dynamic performance will also be achievable for the human pilot with decent workload [4].

Transfer function
In order to apply Neal&Smith criterion to assess the handling qualities of some aircraft augmented with CSAS, an aircraft and pilot closed-loop system models equation (1) is conducted in figure 1. The aircraft handling qualities are described in terms of transfer functions of gain and phase compensations in this situation. The pilot model transfer function is described as equation (2), which includes a pilot's neuromuscular-delay, phase and gain compensation terms (1) (2), is the pilot mathematical model.

Pilot model compensated aircraft
In the basis of the transfer function equation (1) and (2)

Neal&Smith Criterion Check
As the full-compensation pilot model conducted above, the aircraft handling qualities can be assessed against the criterion chart. From gain and phase characteristics assessed in Nichols chart , the parameters can be obtained as follows: • Resonance peak (closed loop)=10.4 dB • Pilot phase compensation at  Figure 3 shows the change in Neal&Smith parameters along with variation in altitude and airspeed, which can be analyzed in two cases.
• Case 1: Varying altitude at constant airspeed It can be observed that aircraft augmented with 1 C is more sluggish at the higher altitude, and the stronger Pilot Induced Oscillation (PIO) tendency introduced. Accordingly, pilot compensation and resonance peak amplitude increased as altitude rising, which lead to a handling qualities degraded trend.
• Case 2: Varying airspeed at constant altitude Increasing airspeed at constant altitude is less sluggish, due to the resonance peak drops with bigger damping ratio in pitch oscillations. Accordingly,pilot compensation and resonance peak amplitude decreased as airspeed increasing, which lead to a satisfactory handling qualities trend. Consequently

Conclusions
This paper applied Neal&Smith criterion to quantify handling qualities of some aircraft augmented with 1 C over F in category B. With varying flight condition parameters (altitude and airspeed), the basic aircraft handling qualities are much different from the augmented one. Instead of representing natural response of the basic one, the augmented aircraft handling qualities assessed by Neal&Smith criterion more depend on the equilibrium point which the flight controller is designed upon. For the future research direction, the second equilibrium point in the uncovered area of F could be chosen to design the second local controller. Furthermore, the gain scheduling strategy can be designed motivated by this research for the purpose of F full coverage.