Title: DEVELOPMENT OF CONTROL SATURATION WARNING SYSTEM TO ALERT HELICOPTER PILOT FOR POSSIBLE ROTOR STALL CONDITIONS DURING TURNING MAOEUVRE

FIELD

The present invention relates to the concept of control saturation prediction logic to provide an on-board Control Saturation Warning System based on certain flight parameters to alert helicopter pilot of an impending safety hazard so that he can apply appropriate controls to recover.

BACKGROUND

Advanced Light Helicopter (ALH) is fitted with a hinge-less rotor system. Therefore, its response is much faster compared to a helicopter with articulated rotor system. When flying a high 'g' left turn (Clockwise rotating Rotor) it has been observed that the cyclic control travel limit to the right side may be too short to allow this turn to be stopped by simply moving the stick. When cyclic saturation is reached there is an abrupt loss of available lift to counter the left turn.

DESCRIPTION

AGILITY OF HINGE-LESS ROTORS

The Helicopter is a flying machine which uses rotating wings (i.e., rotors) to provide lift, propulsive forces and control moments to enable it to fly. Helicopters with a single, main rotor system require a separate rotor to overcome torque. This is accomplished usually through a variable pitch, anti-torque rotor or a tail rotor. The tail rotor also provides directional control.

Among the entire helicopter, mechanical systems/ components (i.e., Rotor system, Engine, Transmission etc.), the main rotor head is one of the most important component mainly due to the fact that it has to undertake many functions. The main function of the main rotor head is to transfer rotor thrust and rotor Moments to the Fuselage. The rotor systems of the older generation helicopters were of articulated type with mechanical hinges to reduce the excessive moments from being transferred to the fuselage. However these hinges are prone to wear and tear and also make the rotor head very complex.

To reduce the complexity of rotor head as they were the cause of high production and maintenance cost, blade hinges were replaced by flexible elements with the help of composite materials (Fibre glass reinforced plastic), which were able to absorb high forces by means of elastic deformation. In case of hinge-less rotors, flapping and lead-lag hinges are removed and their tasks are now being performed by the elastic deflection of the component. The omission of the hinges leads to mechanical simplification and a better aerodynamic shape of the rotor head. At the same time hinge-less rotor provides the required agility of helicopter.

The dynamic behaviour of the helicopter depends to a large extent on the moment that is transferred from the blades to the fuselage through the rotor head. Articulated rotors have in general a small flapping hinge offset where by only small moments can be transferred as a result of the short moment arm. However in the hinge less rotors flapping hinge is further away from the rotor centre and can therefore transfer larger moments because of the longer moment arm.

The control of the Helicopter occurs mainly by the tilting of the rotor thrust vector which produces the required moment at the rotor head. The hinge-less rotor can deliver greater control power if designed with larger flapping hinge offset. The direct consequence is a reduction of the helicopter time constant. This time constant is defined as the time which is needed to achieve 63 % of the angular velocity at a defined control input. Due to a smaller time constant, hinge-less rotors have quicker response as compared to articulated rotors.

TURNING MANOEUVRE

In forward flight, the rotor disc is tilted forward, which also tilts the total lift (thrust) force of the rotor disc forward. When the helicopter is banked, the rotor disc is tilted sideward resulting in lift not being vertical. As the angle of bank increases, the total lift force is tilted away from the vertical, thus decreasing the vertical lift component. To compensate for this decreased vertical lift the rotor lift has to be increased. This is achieved by increasing the angle of attack of the rotor blades by moving the collective stick up.

Compared to fixed wing aircraft the helicopter is not symmetric in its dynamic behaviour. This is due to dissymmetry of the lift caused by the advancing and retreating rotor blades. When the helicoptef^rnravfes through the air. the relative airflow through the main rotor disc is different on the advancing blade (blade which moves towards nose of the helicopter) than on the retreating blade (blade which moves towards the tail of the helicopter). The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is reduced by the helicopter's forward airspeed. Therefore, the advancing blade side of the rotor disc produces more lift than the retreating blade side. This situation is defined as dissymmetry of lift.

In any helicopter to commence a turn, the cyclic stick is moved in the direction of the desired turn (moye left for left turn) and thereafter neutralized to stop the roll rate to maintain the desired bank angle. Due to this asymmetry the mechanism of the turn to the left and right of a helicopter are different.

CONTROL SATURATION PHENOMENON

Advanced Light Helicopter (ALH) is fitted with a hinge-less rotor system. Based on its mechanical response characteristics it is equivalent to an articulated rotor system with a hinge offset of more than 12% of rotor radius which increases the moments generated at the rotor hub. Therefore, the helicopter response is fast.

When flying a high 'g' left turn (Clockwise rotating Rotor) it has been observed that the cyclic control travel limit to the right side may be too short to allow this turn to be stopped by simply moving the stick. When cyclic saturation is reached there is an abrupt loss of available lift to counter the left turn. This phenomenon is called "control saturation".

In ALH with its hinge-less rotor system and clockwise direction of rotation, more care in handling of controls needs to be exercised by the pilot in making a left turn as compared to a right turn, especially close to the ground. If this care is not exercised in the use of the collective, lateral cyclic and longitudinal cyclic, a large area of the rotor disc may be aerodynamically stalled and the rotor will not generate the required rotor aerodynamic moments to recover from the left turn.

It has been analysed that the factors which adversely affect the recovery from a left turn are :

High collective pitch.

'g' loads.

Roll rate to the left.

High left roll angle.

Fuselage nose up pitch rate.

Combination of these factors could lead to a situation where a large part of rotor disc stalls making the right lateral cyclic ineffective. In such a situation, the cyclic may move to its extreme right limit while.the helicopter may not respond to it and still be banked to the left.

RECOVERY FROM LATERAL CYCLIC CONTROL SATURATION:

Any one or more of the following is/are needed to recover from such a situation: Use of opposite rudder - right rudder to recover from left turn.

Lowering of collective - to recover the rotor from stall.

Movement of cyclic stick forward - to reduce'g'.

This recovery is critical close to the ground as the helicopter may lose height during the recovery process.

Claim

1. Concept of control saturation prediction logic to provide an on-board Control Saturation Warning System based on certain flight parameters to alert helicopter pilot of an impending safety hazard so that he can apply appropriate controls to recover. The flight parameters used for the desired logic to flash warning are explained below:

a. Sum of lateral & collective cyclic angles greater than a specified value

b. Helicopter roll angle less than a specified value ensuring that it is in left bank

c. Helicopter positive pitch rate greater than a specified value based on flight test data

d. Roll rate less than a specified value

The specified values have been arrived at based on extensive flight test data available in various flight conditions. The prescribed logic has been validated by flight testing on the helicopter as well and found to be effective.