G'day Mate,
In actual fact, no right thrust &/or no down thrust, & the plane will still fly, get that straight in your head. IT WON'T CRASH. Ok the reason for adding right thrust to an engine is to help YOU fly the plane straighter through manouvers, like loops, stall turns & so on, I'll keep this simple, without too much technical crap OK. As the nose is pulled up in a loop, the torque effect of the prop, will make the plane YAW to the left, which takes it offline during the loop, that is the effect of too little right thrust, so some is added usually around 2 or 3 degrees, & this depends on lots of variables, eg. prop pitch, speed & so on. A good way to find the right amount of right thrust needed, is to pull vertical at full power, & adjust rudder trim to the right until the plane flies straight up, this may take a few tests to get it right, then land, & measure the angle of deflection of the rudder, note that angle, & increase the right thrust of the motor, exactly HALF that rudder angle. & check this with the radio on of course, So if the rudder trim moved the rudder, say 4 degrees, then 2 degrees, of right thrust is needed. More flight tests may be needed to confirm this setting, of course.
Ok now for down thrust, this is used to counteract the lifting forces of high lift wings, Eg, flat bottom wings, as on a trainer. The motor will pull the wing through the air much faster than it needs, to overcome the weight of the aircraft, & so it climbs, this is when down TRIM is applied, fine, but this is only effective at that particular throttle setting, any slower & the nose will drop, any faster & it will climb again.
So down thrust is added to the motor to reduce the effect of unwanted climbing under power, & also the plane will not need as much retrimming during the flight.

Sorry to ramble on, I have just come home from flying, & I had to maiden 3 new planes, 1 of which is a SIG Kadet, 25 size. It needed some down thrust, to help it fly better.