Originally Posted by
gestrich19
If the relative wind and heading change simultaneously (one does not cause the other), then why does the aircraft change course? What is the cause directly preceding the change in direction of flight? I thought it was the slight change in relative wind which caused the aircraft to align with it due to the force of air on the vertical stabilizer and fuselage. Although it may happen in an instant, the relative wind changes first and the longitudinal axis then aligns with the new relative wind. Do I misunderstand?
I'm not sure I understand what it is you're asking, but I'll try to start from scratch and explain as best as I can...
We know that... during a steady, coordinated turn that lift is inclined to produce a horizontal component of lift equal to the centrifugal force in the turn. ... a steady turn is achieved by producing a vertical component of lift equal to the weight of the airplane. If either one of these values do not equal the respective value (i.e. horizontal lift does not equal centifugal force, etc) the airplane will be out of symmetry and some type of acceleration (either positive or negative) will occur (i.e. uncoordination, etc).
I think what you're trying to get at is static stability and dynamic stability. If it is please let us know... that's a whole 'nother topic...but I'll try to give a brief intro:
There is equilibrium... when the sum of all forces and moments equal zero. In this condition there is no acceleration (i.e coordinated flight). If you put a control deflection in, you'll unbalance the moment of force and some type of acceleration will result (i.e. positive or negative).
Static stability: The tendency of an aircraft to return to equilibrium. Postive static stability is the tendency is return to equilibrium if disturbed. Negative static stability is the tendency to continue in the direction of the disturbance. Neutral static stability is somewhere in the middle (remain in a displaced condition). When dealing with static stability, the resulting motion is not a factor.
Dynamic stability: same as above, but deals with motion in time.
The Rudder helps to overcome adverse forces which try to bring the aircraft out of equilibrium during a turn. A yaw string attached to the nose shows the relative wind... in a coordinated turn the relative wind comes from straight ahead.