As experienced modelers know, many aircraft fly better with the motor angled a few degrees down, and a few degrees to the right. (This is for conventional aircraft, with puller, not pusher designs, and for right-hand props [clockwise-rotating when viewed from the cockpit]). However, after hearing or reading many people inaccurately attribute the *reasons* for this, I decided to write a bit on the matter here.
First off, the torque of the motor is *not* the reason for angling the thrust vector down and to the right. Torque roll from the motor for a standard prop (clockwise rotation as viewed from the cockpit) would try to *roll* the plane left. Angling the motor right will try to *yaw* the plane right. These are two different axes of rotation. Torque roll is primarily countered by trimming the ailerons with some right trim, or adding a little extra weight to the right side of the plane, *not* by angling the motor to the right. I will admit that angling the motor to the right will have a *small,* but *very minor* effect on the roll due to several yaw-to-roll coupling factors (including dihedral-sideslip effects, fuselage-wing "shadowing" created by side-slip angles, and wing lift differential due to yaw rates--I could explain these more in detail--if you'd like me to just comment below), but these effects are minor enough that it is incorrect to say that the motor angles are needed due to motor torque trying to induce a torque roll. After quite a bit of research, and using my educational background and experience, I will give the following relatively simple explanations of the *primary* factors requiring the right and down motor angles:
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, to the right, 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 primarily to offset the climb response of higher-speed airflow over the wings. As throttle is increased, airspeed over the wings due to prop-wash alone tries to almost instantaneously make the airplane climb. Down-angle counters this climb for increased throttle. Also, the aircraft will actually speed up, causing additional lift trying to make the plane climb. Again, down-angle counters this climb effect. Secondarily, 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 both 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 climb with increased throttle, as well as the tendency 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.