Control & Guidance : A primer

The major lift producing component of an aerospace vehicle is the wing surface and the control deflection is provided by the control surfaces.
When the control moment and the aerodynamic moment are balanced, this state is called the trim state of the vehicle.
For a statically stable vehicle(C.G. ahead of C.P.), the control moment generated by the actuator deflection produces the required AoA, and the moment produced is stabilizing the vehicle.

The case is different for a statically unstable vehicle (C.G. behind C.P.), where this creates a destabilizing moment.

The flying vehicle with a stable configuration can be designed without an autopilot theoretically. However, for precise control of lateral acceleration, autopilot is an integral element in the design procedure.
A trim-state is achieved when the aerodynamic moment generated due to AoA is balanced by the control moment generated due to fin deflection, i.e,

Aerodynamic Moment = Control Moment

Requirement of a trim state

 

 

 

The vehicle’s static stability is directly proportional to the distance between the C.G. and C.P., more the distance, the more is the control effort required to trim the vehicle.

For a high manoeuvring vehicle and where control effort needs to be optimized, it is desired to keep this distance as minimum as possible.

SR-71 Configuration

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