35.Types of control surfaces. The geometrical characteristics of control surfaces.
Primary controls
The main control surfaces of a fixed-wing aircraft are attached to the airframe on hinges or tracks so they may move and thus deflect the air stream passing over them. This redirection of the air stream generates an unbalanced force to rotate the plane about the associated axis.
A ilerons Elevator Rudder
Aileron surface
Secondary controls
In addition to the primary flight controls for roll, pitch, and yaw, there are often secondary controls available to give the pilot finer control over flight or to ease the workload. The most commonly available control is a wheel or other device to control elevator trim, so that the pilot does not have to maintain constant backward or forward pressure to hold a specific pitch attitude[4] (other types of trim, for rudder and ailerons, are common on larger aircraft but may also appear on smaller ones). Many aircraft have wing flaps, controlled by a switch or a mechanical lever or in some cases are fully automatic by computer control, which alter the shape of the wing for improved control at the slower speeds used for takeoff and landing. Other secondary flight control systems may be available, including slats, spoilers, air brakes and variable-sweep wings.
36.The aerodynamic characteristics of stabilizing and control surfaces.
Stability is the tendency of a system to return to its equilibrium condition after being disturbed from that point. Two types of stability or instability are important.
A static instability:
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A dynamic instability:
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An airplane must be a stable system with acceptable time constants. To assure this, a careful analysis of the dynamic response and controllability is required, but here we look only at the simplest case: static longitudinal stability and trim. This will tell us something about the aerodynamic design of the surfaces -- the load they must carry, the effect of airfoil properties, and the drag associated with the surfaces.
If we displace the wing or airplane from its equilibrium flight condition to a higher angle of attack and higher lift coefficient:
we
would like it to return to the lower lift coefficient. This requires
that the pitching moment about the rotation point*, Cm,
become negative as we increase CL:
At the rear of the fuselage of most aircraft one finds a horizontal stabilizer and an elevator. The stabilizer is a fixed wing section whose job is to provide stability for the aircraft, to keep it flying straight. The horizontal stabilizer prevents up-and-down, or pitching, motion of the aircraft nose. The elevator is the small moving section at the rear of the stabilizer that is attached to the fixed sections by hinges. The elevator is used to control the position of the nose of the aircraft and the angle of attack of the wing.At the rear of the fuselage of most aircraft one finds a vertical stabilizer or fin and a rudder. The stabilizer is a fixed wing section whose job is to provide stability for the aircraft, to keep it flying straight. The vertical stabilizer prevents side-to-side, or yawing, motion of the aircraft nose.
37.Concept about hinge moments of control surfaces. Aerodynamic balance.
A means of reducing the hinge movement and thereby the physical effort needed to control an aircraft. If control surfaces were hinged at their leading edge and allowed to trail from this position in flight, the forces required to change the angle on all except light and slow aircraft would be prohibitive. Some form of aerodynamic balancing is required to assist the pilot to move controls easily in the absence of power-assisted controls. The most common forms of aerodynamic balancing are inset hinges, horn balances, internal balances, and tab balances.
