If you fly a twin-motor glider that uses differential thrust for yaw control (or as an aid to steer without relying on the rudder), trimming the tail is part art, part engineering — and absolutely worth doing. A properly trimmed tail makes the airplane fly straight and hands-off in both powered and unpowered flight, reduces drag, and keeps thermal/soaring behaviour predictable. Below I’ll walk you through the thinking and the step-by-step work so you can get it dialled quickly and safely.
Differential thrust introduces yaw moments by changing the relative power between the two motors. That’s handy, but it also means the thrust lines, motor timing, and airframe rigging all interact with your tail surfaces. If the thrust lines aren’t neutral, the plane will crab, pull, or require constant correction — wasting energy and spoiling thermalling.
With props removed (safety), visually inspect each motor’s mounting. Both motors should point parallel to the fuselage axis in yaw (no toe-in or toe-out unless intentionally set). If the thrust lines differ, either shim a motor mount or adjust the mount so both point exactly forward. Small yaw offsets in the motor mounts are a common, often overlooked source of trim problems.
Next, make sure both motors have the same pitch and physical angle relative to the fuselage in the vertical plane. Any down- or up-thrust asymmetry will cause pitch trim differences between sides and will make differential thrust behave oddly.
With props back on and the aircraft restrained securely (held by a helper or strapped in a jig), do a low-power run-up. Do a low throttle and check whether the fuselage yaws or pitches. If it yaws toward one side under equal throttle, you have an imbalance — either in thrust or in small motor alignment. Try to correct the physical cause first. If the elcctronics allow left/right tuning, do it and see if can make things better.
With the mechanical baseline set, do a short low-altitude, low-speed flight (or a secure run-up check with props). Trim the elevator so the glider flies level at your chosen cruise speed and altitude with both motors at the same throttle (no differential commanded). If you have to apply differential command to hold straight or level flight at equal throttle, go back and re-check motor alignment and prop balance first since large trims are masking a physical problem.
Physical trim to get a model trimmed “hands-off” and flying level is mostly a sequence of mechanical checks and tiny geometry changes that remove built-in bias before you can even rely on radio mixes (low cost radio does not offer much for tuning anyway). The wing should sit at the factory or design incidence; the stabilizer is usually set slightly negative relative to the wing so the fuselage flies with a small positive angle of attack at cruise without continuous elevator. If the stabiliser angle is not appropriate for the configuration you’ll permanently need elevator trim. You may adjust the stabilizer by adding additional pieces or by using the adjustment slot if available. Most off the shelf gliders would provide you with two slots to play with.
If the glider is a DIY conversion, chance is that the position of the tail wing (horizontal stablizer) can be swapped. If the angle is not right, the glider will fly like a stunt plane for aerobatics (see the video). Proper physical trimming is what is needed to fix it. Some trial and error would be necessary to achieve proper level flight.