Elevator/stabilizer effectiveness
 

Hello,

I am studying the JAA ATPL exam Principles of flight and I found a question with a strange answer (Fly Around database).

The question is:

- The center of gravity moving aft will :

Answer :

- Increase the elevator effectiveness.

Well, I thought that the greater the distance between a control surface and the CG, the greater the control surface's effectiveness. However if the CG is moving aft, the distance between the stabilizer and the CG is reduced. Thus the stabilizer effectiveness should be decreased too.

Did I miss something ?

Thank you very much for your help.
Have all a good day,

Emmanuel Cordier.

CG is quite close to, but in front of, the centre of lift. Moving CG aft moves it closer to the centre of lift, so that the tailplane has to do less work to match the pitching moment effects of CG and CofL not being in the same place.

So, it becomes more effective, since any elevator movement will be more powerful in terms of pitch response.

G

Thanks
 

Thanks alot for your fast answer Genghis, it makes sense.

Emmanuel Cordier.

Its really a poorly worded question and a true answer cannot be provided. Control effectiveness address whether the control can do what it is designed to do. Stabilizer for stability and elevator to change pitch attitude.
- The question does not address the CG's starting location, and its distance to the center of pressure/lift (only that we know its moving "closer").
- If CG moves aft of CG limits - aircraft is trending toward unstable, and controls are becoming less effective. Less nose down moment, does elevator maintain effectivenss to lower nose? If CG moves aft of CL then lift acts through the CG for a nose up moment (not good)
- If CG moves forward of CG limits, aircraft is trending towards stability to the point that the controls may be ineffective. More nose down moment, does elevator maintain effectivness to raise nose?

- If the aircraft is operated within the its CG limits, control effectiveness will be within design limits.

Are any of the above A, B, C or D?