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Old 16th Jul 2006, 15:39
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Mad (Flt) Scientist
 
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Originally Posted by Basil
Straight wing aircraft do not suffer from dutch roll - unless you intentionally cause it by continually gently applying left and right rudder when you will get a yaw induced oscillatory rolling motion.
Sorry, but that is completely nonsense. As long as an aircraft does not have some kind of control augmentation system (i.e. FBW) or a configuration grossly different from the standard aircraft - wing.fuselage,tail - it WILL have a Dutch Roll mode. The mode may be well damped, the mode may have a frequency different to the normally expected range, or the roll/yaw ratio may be at one extreme (so you may get a mode which is mainly yaw, or mainly roll) but it will be present.

2. Left wing sweep relative to airflow is reduced therefore:
a) Airspeed normal to leading edge is increased so V increases.
b) Camber to airflow is increased so CL increases. (remember L= CL*1/2*p*V2*S)
c) Lift on left wing increases so left wing rises (slightly assisted by sideways lift on fin).
That's just the dihedral effect - Cl-beta - and is present regardless of sweep or no sweep.
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