Keel effect
<P>In aeronautics, <B>keel effect</B> is the result of the sideforce-generating surfaces being above (or below) the <A title="Center of mass" href="http://en.wikipedia.org/wiki/Center_of_mass"><FONT color=#0000ff>center of mass</FONT></A> (which coincides with the <A class=mw-redirect title="Center of gravity (aircraft)" href="http://en.wikipedia.org/wiki/Center_of_gravity_(aircraft)"><FONT color=#0000ff>center of gravity</FONT></A>) in <I>any</I> aircraft. Examples of such surfaces are the vertical stabilizer, rudder, and parts of the fuselage. When an aircraft is in a <A class=mw-redirect title="Angle of sideslip" href="http://en.wikipedia.org/wiki/Angle_of_sideslip"><FONT color=#0000ff>sideslip</FONT></A>, these surfaces generate sidewards <A title="Lift (force)" href="http://en.wikipedia.org/wiki/Lift_(force)"><FONT color=#0000ff>lift</FONT></A> forces. If the surface is above or below the <A class=mw-redirect title="Center of gravity (aircraft)" href="http://en.wikipedia.org/wiki/Center_of_gravity_(aircraft)"><FONT color=#0000ff>center of gravity</FONT></A>, the sidewards lift forces generate a rolling moment. This "rolling moment caused by sideslip" is "<A title="Dihedral (aircraft)" href="http://en.wikipedia.org/wiki/Dihedral_(aircraft)"><FONT color=#0000ff>dihedral effect</FONT></A>". Keel effect is the contribution of these side forces to rolling moment (as sideslip increases), i.e. keel effect is the contribution of the side forces to <A title="Dihedral (aircraft)" href="http://en.wikipedia.org/wiki/Dihedral_(aircraft)"><FONT color=#0000ff>dihedral effect</FONT></A>. Sideforce producing surfaces <I>above</I> the center of gravity will <I>increase</I> dihedral effect, while sideforce producing surfaces below the center of gravity will decrease dihedral effect.</P><P>Increased dihedral effect (helped or hindered by keel effect) results in a greater tendency for the aircraft to return to level flight when the aircraft is put into a bank. Or, reduces the tendency to diverge to a greater bank angle when the aircraft starts wings-level.</P>
<P>Keel effect is also called "Pendulum Effect" because a lower <A class=mw-redirect title="Center of gravity (aircraft)" href="http://en.wikipedia.org/wiki/Center_of_gravity_(aircraft)"><FONT color=#0000ff>center of gravity</FONT></A> increases the effect of sideways forces (above the center of gravity) in producing a rolling moment. This is because the <I>moment arm is longer</I>, <I>not</I> because of gravitational forces. A low center of gravity is like a pendulum (which has a <I>very</I> low center of gravity).</P>
<P>References: Illman, Paul; The Pilot's Handbook of Aeronautical Knowledge; Fig 2.34</P>
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