A bit of a note here as well, the Spiralling slipstream has a lot to do with that "left Yaw", possibly moreso than "P" factor or gyroscopic forces from the prop. It's my understanding that "P" factor comes into play during pull-ups, as the engines torque and airflow off the prop make the aircraft tend to yaw left during Pitch changes. During normal straight-ahead flight, spiralling slipstream washes against the vertical stabilizer, and since the slipstream is rotating L-R as it hits the stabilizer, it tends to push the tail to the right, making the aircraft yaw to the left. The more slipstream or thrust off the prop, and the higher the rpm, the more pronounced this is, thus an engine with right, and down, thrust (depending on what is required to "balance" things under different power settings) will exhibit less pitch and yaw changes. Why "pitch"? The stabilizer/elevator on the right side of the plane is "shadowed" from the slipstream, so the air spiralling into the fin on the left side puts a positive pressure on the top of the stabilizer as well, causing the plane to pitch upwards in some cases.

In Full Scale planes, the pilot automatically trims and compensates with the controls, and "Cruise Trim" of the controls is set up for hands-off straight and level at normal cruise power. Change power settings, though, and the plane will change it's flight characteristics, with more power initiationg a left yaw and possible pitch-up depending on the design of the plane, and less power yawing right as the props effect ont he airframe is reduced. On a Twin, however, there is no slipstream off the props hitting against the Vertical stabilizer and rudder, so any torque reaction or slipstream affects that would make the plane yaw either left or right woudl have no control surface to directly counteract them, so the yaw tendency would be extremely high if the engiens did not rotate in opposite directions. On that note, hoever, please note that not all Twins have engines rotting in opposite directions. The P-38 and Mosquito had it, but most twin-engine bombers or heavier aircraft did not.

As for why full-scale aircraft do not have "thrust" built into the airframe? Some do!! All one has to do is examine a T-28 Trojan to see the pronounced Right and Down thrust cranked into the engine so that at full power settings the plane tracks as straight as possible. This was done because the T-28 was meant to simulate Jet aircraft as a primary trainer role. I'm sure if you look at ANY high-performance single-engine planes out there you will see some thrust dialed into the engine.

Best bet is to dial in the engines thrust with shims until it flies straight, with no yaw or pitch, no matter what the power setting is. As to how it behaves on pull-ups or on lines other than straight and level. . .that's what those 2 little sticks on the transmitter are for.