C of G and pitch stability

[guclu]

Let me try to explain,


First of all you must understand what a moment is.

Moment = F X a (Force times arm.)

For an aircraft to be steady in pitch, the total moments acting through CG must be zero in the neutral plane. That is to say the moment of Lift from wings and moment of Lift from the tailplane (elevator) must be zero.

For the wings, lift is much greater than the lift of the tailplane (which is actually a lift in the direction of the earth for the tailplane) but the arm (the distance to cg) is much smaller.

Now, try to think that cg is at the most limit forward position. The arm of the tailplane is getting so much that its affect is much greater than before. So that is to say, for example, when your pitch is moved only small amount upwards, the lift of the wing and tailplane will increase. But consider the arms. If it is say 1 for the wings it is 100 for the tailplane. So even a small increase in angle of attack will affect the moment very much for the tailplane. In the most forward limit, the tailplane is very much stabilising. (Stabilising = Wanting to return the airplane to its original, not disturbed position). That is any change in pitch will be strongly resisted by the tailplane, because of the long arm.

The other scenario is the most limited rearword position of CG where the arm of the tailplane is at lowest value. At this position, the tailplane doesn't have that much stabilising affect and every movement you make to the elevator will not be stabilised by the tailplane. (Neutral tendency). This is aircraft is very sensitive to elevator control.

I hope it helped.

Guclu