I'm building an small RC model basically from scratch using a set of CL plans as a starting point. My question is , how do I determine if/how much built in angle to the engine thrust is needed?

Specs:
.061 two stroke engine, 6x4 prop
length 23"
span 34"
mid wing, symetrical airfoil, no dihedral
tubular fuse, taildragger
240 sq in wing area
vert stab 15 sq in
horz stab 40 sq in (as plans, may cut down)
target weight 18oz.

Thanks Guys

Assuming zero incidence, & the thrust line on, or close to the chord line, I suggest that you don't add any down thrust with the mid-wing configuration. I would initially have zero right thrust, & be prepared to add a bit, if necessary.

There was a good article by George Hicks in a recent magazine, 3D Flyer maybe, I forget about the need or not for right thrust. The airplane has a natural left yaw because of the spiral slipstream and as a compromise moment balance the right thrust is used. The vertical heights of the aft fuselage and kind and area location of the vertical tail will give different yaw effects and so need different amounts of right thrust.

You can start with zero but be sure and keep the speed up and be ready to correct any yaw or roll that would be a problem (as if you would be sound asleep on a first test flight!) It is hard to go too bad with a couple of degrees. On most of my normal configuration models I have to admit I just eyeball in a touch of right thrust, mainly to be sure it isn't left thrust. On a high quality aerobatic machine a more precision approach will be needed.

Good information Ben and Britbrat, thank you.

The vertical stab/rudder is 60% above the fuselage and about 40% behind. I am also considering continuing the rudder below the fuse for added area. I would surmise that this would additionally reduce the amount of right thrust needed due to spiral slipstream.

The thrust line is indeed very close to (but not directly on) the chord line being only aprox. 1cm higher. Gear and wheelpants will be added drag below.

Given your info, I will probably build with only a degree or so, if any. I will need to test fly and make any adjustments before final construction of the cowl which lines up very closely with the spinner.

Thanks again!

I just love subrudders and subfins, since they remove the need for any sidethrust. Trainers normally need a bit of downthrust to remove excess climb under full power, since they are set up rather nose-heavy to have good pitch stability.

Right and down thrust is basically a carryover from free flight days where an aircraft had to transition from a high power climb to a floating glide without any control input. The right thrust enabled the model to climb to the right and glide to the left. The trim was set for an efficient glide, which tended to cause excessive pitch up under power, even with a pretty far aft cg.

A radio controlled model with both control and trim on three axes really doesn’t “need” either. It is usually adequate to simply trim out any turning, or climbing tendency. As Rotaryphile said, a little down thrust is appropriate (though not essential) for trainers due to their forward cg and good pitch stability. When building a kit it’s good to put in the thrust offsets called for on the plans because experience with that particular design indicates that that requires minimum trim input. The only way to know for sure is to fly it and see. I have two original designs that I have built recently and both flew right off the board with no side or down thrust offset.

(Any thrust offset is only good at one speed and power setting. You still have to trim or hold control at every other flight condition)

A thrustline offset is used because a permanent trim position of a flight surface changes effect as airspeed changes. A thrustline offset is used to compensate for how the engine pulls the airplane. With a lot of power, you get a lot of compensation, and with a little power, little compensation.

Even full-size aircraft have thrustline offsets. Witness the Aircoupe, with a lot of right and downthrust.

Downthrust is used to minimize pitch changes with throttle changes, and sidethrust (usually to the right) is used to minimize how much the engine pulls the airplane left or right as power changes.

Ideally, you want the aircraft to have the flight surfaces trimmed for neutral when in level flight at your desired "cruise" airspeed and throttle setting. For an aerobatic model with a fully-symmetrical airfoil, you'll of course have a bit of 'up' elevator in upright level flight, and need a bit of 'down' elevator in level inverted flight. Rudder and ailerons should be in their neutral positions, in trail with the main surface. Otherwise, the model will have a tendency to roll or yaw in varying amounts, depending upon the airspeed.

Some aerobatic models, because of their force layout actually have the engine positioned with some 'up' thrust in order for them to have the desired aerobatic characteristics.

It all comes down to handling. Adjusting of the thrustline is one of the many different things you may have to do to get your model to handle just they way you want it...just like you have to adjust wing and stabilizer angles with respect to the fuselage and each other, altering dihedral angles, and so on. Top-level fliers and designers work with their models, tweaking them until they perform just how they want them to perform.