Both the size and arm of the VTP determine the directional stability of the aircraft. Wing sweep and wing dihedral also contribute, but small compared to the VTP, to the aircraft’s directional stability. For conventional empennage designs and any other empennage designs where the VTP is located behind the airplane’s C.G., the aircraft will exhibit directional stable behaviour. This is illustrated in the diagram below. This is the reason for VTP are always found to be located towards the rear of the fuselage. Tailless aircraft substitutes the VTP with the use of drag rudders. Drag rudders are also always found behind the airplane’s C.G. The further the VTP is behind the C.G., the more static directional stability the aircraft will have. M E 5 7 8 A i r c r a f t D e s i g n 20
Directional Stability II M E 5 7 8 A i r c r a f t D e s i g n 21 Directional stability due to wing sweep Side slip on swept back wing produces a differential in lift and subsequent differential in induced drag. The result is that the aircraft will exhibit the tendency to yaw and align itself to the airflow direction. Directional stability due to wing Dihedral Side slip on wing with dihedral produces: (1) a side pressure force on the aircraft. (2) an equivalent increase in AoA on the wing on the side slip, and an equivalent decrease in AoA on the wing opposite to the side slip. The aircraft will exhibit a tendency to roll in the direction opposing the side slip as a result, and indirectly realigning itself to the airflow direction, giving the aircraft additional directional stability.
M E 5 7 8 A i r c r a f t D e s i g n 22 What does it mean if the aircraft is not stable? The pilot is required to take hold of the pitch and roll controls all the time and fighting for stability and levelling the aircraft. This is obviously a dangerous situation, and it may be very stressful and tiring. For VFR flying, it may still be manageable. An IFR unstable aircraft is dangerous because the pilot's workload becomes too much when we have to concentrate on the instruments as well as keeping the speed steady. Safe control and stability of the airplane must be achieved by employing active computerized flight control systems. To design an aircraft, full understanding of aircraft stability and control is of paramount importance to the safe operation and success.
Aircraft Major Components M E 5 7 8 A i r c r a f t D e s i g n 23
Wing M E 5 7 8 A i r c r a f t D e s i g n 24 The wing is the most important component of an aircraft’s structure. After all, from the most basic mechanics of flight, the wing is what makes an airplane an airplane, what enables an airplane to fly. An airplane is basically designed about the wing.
Wing Configuration Wings are designed based upon what the aircraft is designed to do.
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- Spring '19
- Aerodynamics, Wing design, Aircraft Design