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 Aerodynamic Center Overall Stability  Most aircraft in the world have both a main wing and a stabilizer. Both these wings contribute to the overall longitudinal stability of the aircraft. Both may be stable. However, it is also possible that the main wing is unstable. To assess the stability, we usually plot a graph of pitching moment vs. angle of attack. |
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The graph to the right
shows a typical plot of the CM vs. CL
for a stable wing.
The graph tells us that as angle of attack increases (CL increases along the x-axis.) a negative (nose down) pitching moment is formed. Conversely if CL decreases (i.e. the pilot pushes forward) a nose up pitching moment is formed (positive Cm) We can conclude that a graph of CM vs. CL should always have negative slope, as shown to the right, if the aircraft is to be stable. |
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In reality both the tail
and the main wing have separate plots of CM vs.
CL.
The c of g is always well ahead of the stabilizer (on a conventional aircraft) and therefore, the tail is very stable. That is shown in the diagram to the right by the very negative slope to the green line. The c of g however, is sometimes behind the ac of the main wing. If this is the case the wing will be unstable. That is indicated in the diagram to the right by the upward slope of the red line. |
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The total stability is simply the sum of the tail and main wing stabilities. In the example above the red and green lines are added to get the blue line.
As long as the total aircraft CM vs. CL graph has a negative slope the aircraft is stable.
A conventional aircraft may have an aft c of g limit which is behind the ac of the main wing (but still many feet in front of the tail's ac.) However, the pilot should remember that if the c of g is allowed to shift back too far the main wing will become so unstable that the positive contribution from the tail will not be enough to overcome it. At that point the whole aircraft will become unstable, and impossible to fly.
Velocity or Angle of Attack for Trim?
One of the most critical things for a pilot is to be able
to trim the aircraft.
In simple terms trimming relieves the pressures on
the control column so that the pilot does not have to hold the controls
in a certain position.
Often pilots talk about trimming for the climb attitude, or cruise attitude, etc. Some, more sophisticated souls realize that the aircraft is not trimmed to maintain a certain attitude. Instead the idea of trimming for a certain speed is suggested. This is closer to the truth, but still not quite right.
The movie below shows a main wing and stabilizer in equilibrium. Equilibrium means that the nose down moment created by the wing is exactly balanced by an equal nose up moment created by the stabilizer. As a result the pitching moment is zero. It is important to remember that the pitching moment must be zero at almost all times during flight. A positive or negative pitching moment is only required for a short moment to start the aircraft pitching, or stop it from pitching.
In the movie you can increase the airspeed. No matter how much you increase or decrease the speed however the force on the main wing and tail will change in proportion. For example if the lift from the wing doubled, the lift from the tail would also double. Therefore, the pitching moment from each would also double. But, they would still be equal put opposite. Therefore, we can conclude that changes in speed will have no direct effect on pitching moment.
Developed by Geistware of Indiana© ., 1999.
Updated January 1, 2003