There are a lot of people out there explaining why model airplane engines are angled down and to the right. Unfortunately most of them are not correct.
If you’re wondering why you’re told to point your engine to the right, the explanation most commonly offered on the internet is P factor.
P factor is short for propeller factor. You can get a pretty good description of this phenomenon from Wikipedia, so I’ll give you the short version. Your plane is traveling forward. You pull up on the elevator, and now your plane is traveling forward with an increased angle of attack. As a result, the downward traveling blade (right) is able to produce more thrust than the upward (left), thus causing the plane to yaw to the left. Therefore we point the engine to the right to counteract this force.
This sounds good at first, but it’s not correct. If right thrust counteracts the left yaw caused by P factor when you yank on the elevator, then the plane would turn right when you level off, because the P factor goes away but the engine is still pointing to the right. You’re supposed to counteract P factor by using your rudder.
The second most commonly misattributed reason for right thrust is engine torque causing the plane to roll to the left. Torque does indeed cause left roll, but it is corrected by aileron trim and you therefore don’t have to think about it. It’s not much of a factor because there’s more torque at high throttle, which is also when there is more airspeed, which causes a simultaneous increase in the correcting effect of the wings and tail.
So, why is right thrust needed? It’s very simple. The propeller creates a spiral slipstream that surrounds the fuselage. Most planes have the vertical stabilizer on top of the fuselage. The slipstream hits the fin on the left side, pushing it to the right, which causes the plane to yaw left.
The engine is angled to the right to counteract this effect. Both the problem and the solution are proportional to the throttle setting, so they balance each other nicely.
But why do different planes have differing amounts of right thrust? The two main factors are the location of the tail fin and the ratio of airspeed to slipstream strength. High airspeed causes a straightening of the slipstream and thus less yaw, whereas low airspeed combined with lots of engine effort will have more side forces in the slipstream and create more yaw. For examples of the two extremes, imagine a pylon racer vs a big, draggy biplane with a long, low pitch propeller. This explains why biplane designs sometimes call for as much as 5 or 6 degrees of right thrust, whereas faster planes typically have very little or even none.
It’s tempting to think that RC planes have down thrust simply because when the throttle is advanced the nose comes up and the plane climbs too much, just because this is one of the effects of applying power. A properly adjusted plane will maintain a level attitude at all throttle settings, or at least come close to this ideal, and proper adjustment is achieved by pointing the engine up, down, or level as required by the rest of the airframe. Airfoil selection and wing location relative to the engine are the key factors determining the proper vertical orientation of the thrust line.
For example, a typical balsa trainer needs a lot of down thrust. This plane has a high lift airfoil, which translates to high drag, and it’s on top of the fuselage, so the center of drag is very high in relation to the center of thrust. This causes a rotational tendency with the center of drag as the axis. The result is that the airplane pitches upward when the throttle is advanced, as if you were pushing forward on an object hanging from the ceiling by a string. The high angle of attack slows the plane down, and the plane will stall or at least wallow around, or if the engine is very powerful the plane will go slow and climb like crazy. Pitching up and climbing can be mitigated by trimming the elevator down, but then you have a twitchy plane with a bunch of lift in the rear, and the nose drops when you cut the throttle and try to land. To maintain pitch stability at all throttle settings, the ideal thrust line theoretically should pass through the center of drag (although this ideal is not achievable in most designs). The engine should be pointed down, so the imaginary line along the propeller shaft will project aftwards pretty close to the center of drag.
I built a 20 size trainer a long time ago and soon discovered that I hadn’t built it with enough down thrust. The plane nosed up at high throttle but didn’t go fast because of the high angle of attack. If I trimmed the elevator down for speed it would drop like a rock when the throttle was lowered. When I added floats the problem was fixed and the plane flew perfectly with zero pitch change from idle to full throttle, and top speed increased in spite of the extra drag from the floats. The reason is because the floats added drag on the bottom, which brought the center of drag down closer to the thrust line projected from the back of the engine.
Another interesting case is the 049-assisted two meter glider with an engine pod installed on top of the wing and held in place by the wing rubber bands passing over a thin plywood flange at the base of the pod. Because the engine is a few inches above the center of drag, it would cause the plane to pitch down if it were installed straight ahead at zero degrees. Engines should be mounted on this kind of pod with several degrees of up thrust. Because the center of thrust is almost directly above the center of drag, the thrust line can’t go through the center of drag. But it’s close enough for government work, and an engine pod equipped plane will also have only a slight difference in trim between power-off operation and power-on.
Finding the correct vertical thrust line can be somewhat tricky because of variations in airfoil, wing incidence, wing placement, and airplane dimensions. Different airfoils behave differently as the angle of attack changes. For instance, the center of lift of a flat bottom wing moves as speed and angle of attack change. So if you are tempted to think that a low wing plane needs the engine straight ahead or even pointing up slightly, this isn’t necessarily the case because a flat bottom wing on the bottom of the plane might still pitch up as the throttle is increased. Undercambered airfoils, depending on the exact profile, can be more affected by angle of attack and airspeed than flat bottom airfoils, but they also tend to produce less drag in general. Symmetrical airfoils maintain the same center of lift at different angles of attack and therefore do not react much to changes in trim or throttle. The main factor affecting a symmetrical airfoil is the thickness, which determines the amount of drag produced.
Determining the correct vertical thrust line is a tricky proposition, dependent upon a lot of variables, and difficult to do on paper. It’s best accomplished via flight testing, which is covered in another article on this site. I just thought it would be nice to write this article to foster a better understanding of the cause and effect of center of drag and engine thrust angle, so if you design a plane or modify an existing one you’ll know where to start.