Motor Thrust Angles--Down and to the Right---Why?

 1) Right-thrust angle:  The right-thrust angle is required for the *primary* reason of countering a LEFT yaw due to P-factor.  P-factor, or Propeller-factor, is a relatively simple concept when understood, which states that for a clockwise-rotating propeller (when viewed from the cockpit), as airplane angle of attack increases, a LEFT yaw is produced due to a thrust differential the propeller creates.  Imagine looking at the airplane from the left side (looking directly down the left wing), with the motor to your left when viewing the plane this way.  With the prop spinning, and for a positive angle of attack, it would be slanted slightly like this: /.  Since incoming air is moving directly from the left side of the page, to the right side of the page, however, you can see that as the blade swings down on the right side of the plane it is swinging also slightly "into" the wind.  Also, the downward-swinging blade will have a higher blade angle of attack (than the upward-swinging blade) due to its geometry and orientation to the incoming air.  This creates extra thrust on the right side of the plane.  As the blade swings up on the left side of the plane it is swinging slightly "with" the wind.  This creates less thrust on the left side of the plane.  The result is a LEFT YAW due to P-factor.  The right thrust angle counters this left yaw due to P-factor, for an upright, positive angle of attack.   Note: A Right-thrust angle would make the P-factor yaw *worse* not better if the plane were flying upside-down.

2) Down-thrust angle:  The down-thrust angle is required for three primary reasons that come to mind: 1) to offset the pitch up response due to a "pendulum" type effect, 2) to counter the climb response due to higher-speed airflow over the wings, and 3) to reduce yaw due to P-factor.  1) Pendulum effect (due to the Center of Pressure [CP] along the vertical axis being higher than the Center of Gravity [CG]): As throttle is increased, assuming a high-wing trainer-type airplane, the motor/fuselage tries to swing forwards since the wings above create a relatively large portion of the plane's total drag, thereby shifting the airplane's CP to be above the CG.  Imagine standing in front an airplane suspended in the air, manually pulling on the plane's nose while pushing back on the main wing situated above the fuselage. This is what the air is doing to the plane as throttle is applied.  The plane will pitch up.  2) Climb response due to higher-speed airflow over the main wings.  Airspeed over the wings due to prop-wash alone tries to almost instantaneously make the airplane climb.  Down-angle counters this "instantaneous" climb for increased throttle.  Also, the aircraft will speed up over time, causing additional lift trying to make the plane climb.  Again, down-angle counters this resultant climb effect.  3) The down-angle of the motor also reduces the "slant" of the propeller to incoming air (when viewed from the left side of the plane again), making it more upright (|), and less slanted (/), thereby also reducing the yaw due to P-factor simply by removing some of the slant (angle of attack), which causes P-factor!  Therefore, the down-thrust angle counters all three of these things: the pitch up, the climbing effect, and some of the yaw due to P-factor!

Conclusion:

Torque is *not* why motors are often angled slightly down and to the right.  Rather, the thrust angles are to counter the tendency to pitch up with increased throttle (for a high-wing airplane), to climb with increased throttle, and to yaw left due to P-factor being induced at positive angles of attack.  

For more information on P-factor read: http://en.wikipedia.org/wiki/P-factor.