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This question is from CFI written exam. (6824)

“Under which condition is a forward CG most critical?”

The answer is

“Forward CG is most critical during landing. If it is too far forward, the elevators may not have enough power to get the tail down for a proper approach and flare.”

I don’t know why if CG is too forward, the elevators may be less effective. In my mind, if something has longer arm, that one becomes more effective like teeter-totter.

That would be great if someone could help me!

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One intuitive way to think about this is that the pitching motion of a rigid body occurs at its CG. At forward CG, the elevator, which pitches the aircraft up, indeed has a longer moment arm. On the other hand, however, the wing lift vector, which tends to pitch the aircraft down, also has a longer moment arm. The proportion of change of the wing lift moment arm is quite a bit more than that of the elevator, for the same change in CG. In the end, the wing lift, which is also quite a bit larger, wins this race. Hence, forward CG is more critical for maneuvering.

Strange that it says landing, which I assume is the flare portion, is the most critical condition. I would've thought takeoff rotation would be equally bad, especially if you have a mistrim.

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  • $\begingroup$ I guess the latter comes from the implicit assumption that the landing flaps configuration creates more pitch-down moment. In practice, this is true for most aircraft. $\endgroup$ – Zeus Aug 8 at 8:20
  • $\begingroup$ @zeus Very true, but you'd think that the pilot would've trimmed the aircraft on approach prior to flare. $\endgroup$ – JZYL Aug 8 at 12:45
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The elevator is producing downward lift. It is you pushing down at one end of the teeter totter. Your kid is sitting on the other end of the teeter totter, and the teeter totter's center of gravity is somewhere between the pivot and your kid. If you stop pushing down, he drops. If your kid moves farther out, say by leaning back, you may not have the strength to keep pushing down and down he goes. Having the CG too far forward is like your kid moved farther out than you can handle.

This is a problem when the horizontal tail is working at the limit of its strength you might say, at low speed and with the elevator at maximum deflection, without any slipstream blowing on it. When the CG is too far forward, it can't push down hard enough to hold the nose up at the normal landing speed, and has to go faster to do so.

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  • $\begingroup$ Your teeter totter analogy is flawed. A plane doesn’t have a fulcrum. Rotational motion is about the CG. So forward CG makes a long moment arm. $\endgroup$ – TomMcW Aug 8 at 2:54
  • $\begingroup$ Obviously it's an imperfect analogy, it being in fluid. But the neutral point is effectively the fulcrum for the purpose of thinking about the moments and forces involved, and the analogy does the job for someone trying to mentally imagine what is happening. $\endgroup$ – John K Aug 8 at 3:00
  • $\begingroup$ @TomMcW this center of gravity thing will earn you points here, but in reality putting the fulcrum at the center of lift makes the static stability model work quite well. The CG torque arm is constant, the tail downforce varies with airspeed. As the plane slows down to land, aerodynamic down force can reach a limit if CG is too far forward. It is indeed a teeter totter, with weight on one end and and elevator trim blowing in the wind on the other. $\endgroup$ – Robert DiGiovanni Aug 8 at 20:14
  • $\begingroup$ @RobertDiGiovanni I see what you guys are saying. Makes sense. $\endgroup$ – TomMcW Aug 9 at 18:18
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By too far do you mean outside the fwd limit? If yes then is a question of the elevator having insufficient 'arm' to counter the increased 'arm' of the (too fwd)CG.

If the CG is within limits or even 'on' the fwd limit then the airplane should have no problems with elevator effectiveness. Airlines usually add a bit onto the manufacturer's limit as the manufacturer limits are based on static conditions (no movement of load) but airlines usually have people and equipment(galley carts) going thru the cabin in flight so the airlines will add some buffer.

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