CONCEPT OF
AFCS Yaw axis Handling Design for Low Speed Condition"
FIELD
This present invention relates with the concept of AFCS yaw axis handling design for low speed condition of a 4-axis limited authority duplex autopilot system of a conventional helicopter with hinge less main rotor and bearing less tail rotor system.
BACKGROUND
Helicopters are inherently unstable and 100% stabilization cannot be achieved by any kind of aerodynamic design. Inspite of this the helicopter can be flown in all the flight regimes though it might require a higher pilot work load. To reduce this work load usually a system is integrated in the helicopter which provides stability augmentation which is achieved by means of state feedback. Generally, this augmentation system is a limited authority Automatic Flight Control System (AFCS). Alongwith Stability Augmentation System (SAS), AFCS incorporates Control Augmentation System (CAS) to distinguish between disturbance and pilot input and also to meet handling qualities response as per the defined standards.
For CAS function, the pilot input on the concerned axis is processed through a mathematical model and required attitude/ attitude rates are computed. The mathematical model should closely equate natural helicopter response as desired to aid pilot i.e. the response to pilot input with AFCS engaged should conform to handling qualities. The computed CAS attitude and attitude rate form the new reference values for SAS to maintain. It is observed that the helicopter response to. rudder input in the low speed regime (indicated airspeed < 50 kmph) is different when compared to response in the high speed regime i.e. at hover, a step rudder input produces a steady yaw rate whereas at cruise speeds it only produces a yaw attitude change. Hence, CAS should model the output (attitude/ attitude rate reference) according to speed in yaw axis. Similarly, the resumption to SAS state on removal of pilot input should differ between low speed and high speed condition.
This invention relates with the methodology of implementing response to pilot input during low speed condition in yaw axis. The concept is evolved based on flight test experience of 4-axis limited authority duplex autopilot system of a conventional helicopter with hinge less main rotor and tail rotor system. The same will be proved through flight testing subsequently.
DESCRIPTION
To illustrate the concept of designing yaw CAS for low speed, following inferences should be considered for yaw axis operation:
i. A step rudder input produces a steady yaw rate.
ii. The output effect (helicopter yaw rate) differs for same rudder input in right and left direction.

iii. As the helicopter flying condition or configuration changes (altitude of operation, wind or All Up Weight (AUW)), the default rudder for each flying regime is changed. This further result in uneven margin for left and right rudder travel for corresponding flight regime.
iv. Low speed regimes are generally flown in feet on condition. So, momentary occasion of feet-off needs processing to detect as valid feet-off or not.
v. The low speed maneuvers are majorly landing, take-offs, hover, spot-turns and sideward flights. Out of these, spot-turn maneuver relates to handling of high yaw rate through AFCS, which can be as high or more than 60 deg/sec. And.recovery from such high rate maneuver, normally.requires appreciable rudder input.
vi. Output system travel, of AFCS should not act opposite to pilot input.
Based on the points above, following are arrived for AFCS- yaw CAS design for low speed condition as part of claim 1:
1. The CAS design involves implementation of mathematical models termed as rate model and attitude model each one to generate steady yaw rate and the yaw attitude references in effect to rudder input respectively.
2. The CAS design should be able to identify different low speed maneuvers and should have logical computation to generate attitude rate and attitude references accordingly.
3. The rate model should be adaptive, capable of providing yaw attitude rate reference, while catering for direction, magnitude and acceleration of rudder input.
4. For a basic model AFCS (attitude control), availability of inputs to estimate correct wind vector and reading of AUW is redundant, nevertheless altitude information is generally available for usage. Hence, the generated yaw attitude rate reference or attitude reference should be synchronized with current helicopter state. Doing so, removes a requirement of having gains scheduled on altitude/AUW/wind direction.
5. As low speed regimes are flown in feet-on/ hands-on condition, so it becomes necessary to implement proper state referencing system for transition between CAS and SAS which can crucially play a role for momentary or appreciable feet on/ off condition. The concept for same is presented below as part of claim 2:
5.1 As indicated, the step rudder input in low speed condition generates a steady yaw rate, hence the generated attitude rate reference should be memorized while switching between CAS and SAS to implement a proper state referencing similar to natural helicopter response. That is, once the pilot feet are back on rudder and if there is no change in rudder input subsequently, the attitude rate reference should hold the resultant attitude rate reference value. This resultant attitude rate reference should be output of logical computation over current generated and memorized attitude rate reference.

5.2 Utilization of memorized attitude rate reference is governed by following two cases:
5.2.1 If the feet-on is appreciable, then the direction/ magnitude of AFCS output system travel should be compared with direction/ magnitude of rudder input to avoid commanding of AFCS output system opposite to pilot. Based on the direction/ magnitude difference, the memorized attitude rate reference should be accordingly negated while being in CAS.
5.2.2 If the feet-off is appreciable, then depending on type of low speed maneuver SAS should have provision to accordingly negate or hold the memorized attitude rate reference.
6. As an alternate to compensate for correct adaptive rate control or synchronization of references with helicopter state, a different set of control gains (acting on the error w.r.t the desired attitude rate and attitude references) from SAS can be invoked while in CAS to aid pilot input.

Claims:
1. The concept of designing yaw axis Control Augmentation System (CAS) based on inferences of helicopter response to pilot input for low speed condition is the claim 1.
2. The concept of transfer of control between Control Augmentation System (CAS) and Stability Augmentation System (SAS) on removal of pilot input in yaw axis for low speed condition is the claim 2.