CG and Elevator Responsiveness

[chris]

Hey guys,

I aksed this Q on the flightinfo forum, and now I will transfer it here. I hope some people can add to the discussion.

I realize that the elevators become more responsive as the CG moves aft. (i.e. lighter force required to generate a response).

However... if the rudder is more effective with a more forward CG (i.e. with multi engine a/c, Vmc is lower with a forward CG because the arm between CG and the rudder is longer), then why doesn't this principle apply to the elevators?

It seems to me that all movements occur through the CG, as that is where the weight is centered. With a forward CG, the arm between the elevators and the point from which the aircraft pitches is longer, so more moment is produced here. But, this analogy is obviously wrong.

Am I omitting the Centre of Pressure, or should this not be a factor for pitch authority?

Thanks in advance.

[SUSPilot]

The elevator is more effective if the CG moves forward. This is because there is a bigger lever arm that the control surface is working on. If the CG moves AFT you will LOSE controllability.

[PhotoPilot]

It's true that the elevators have more authority as the CG moves forward due to the longer arm, but, depending on the airplane design, they may become less effective.

Remember that the horizontal stabilizer is counteracting the nose down tendency of the plane. As the CG moves forward, that nose down tendency increases, requiring more tail down force to counter it. Larger elevator control movements may be required to accomplish the same pitch change as that larger nose down force is countered. The controls might feel sluggish and unresponsive, the added tail down force increases the effective weight of the plane and adds drag, stall speed increases, cruise speed decreases, and rotating for take off or flaring for landing could be difficult.

If the CG moves aft, it begins to counter act the nose down force, taking some of the pressure off the elevators and requiring less tail down counter force. That means that, though the elevators have less distance over which to act, they are more effective because the airplane is in a more neutrally balanced state. Small inputs can make a large difference in pitch because there is more excess force available from the elevators and less nose down to fight. The controls will be very sensitive and over controlling is a strong possibility. The lower tail down force decreases drag, decreases stall speed, increases cruise speed, but makes stall recovery more difficult and presents handling problems due to the control sensitivity.

Of course, if you move the CG so far aft that you turn the nose down tendency into a tail down tendency, then you've got a new set of problems . . .

[seagull]

I think this question reflects a confusion between control effectiveness and stability, which are not actually related in the way the questioner seems to think. An oversimplification might be that the stability going up leads to the controls being heavier at a faster rate than the increase in moment arm contributes to effectiveness. The increase in stability is actually making the aircraft more resistant to change in pitch.