RE: Seagull Spacewalker II 120 Arf
Vagelis, do you have any report on trimming success of your SW?
What appears to me is that engine up thrust as well as elevator up trim are an effort to counter the down force of the plane. If the CG is proper, the down force is almost certainly negative wing incidence in relation to the stab.
The problem with making up engine trim and up elevator corrections, is that power on/off trim will likely not be the same. When power is off, the plane likely is not producing a good landing glide slope and needs down elevator when landing.
Did you check the relationship between stab and wing incidence? Very rarely up thrust of an engine is ever set. Down thrust is much more popular because it counters the lift changes at various speeds of an asymmetrical wing, ie the greater the speed, the greater the lift so more rpms will add down trim proportionately. Very often with symmetrical wings and all else being zero, no down thrust will be called for as it is a tool used with asymmetrical wings. Up thrust should not be used to counter a negative wing incidence... the incidence should be corrected.
That can be done by adjusting the relationship between stab and wing incidences... and can be done successfully at either the wing or stab, whichever is easier. With the low wing Spacewalker most likely the easiest would be to shim the back of the wing down to move the wing incidence in the positive direction. Do so in small increments of 1/64". Your goal should be to return the engine to its proper down thrust and obtain a neutral elevator/stab relationship. At that point, most likely power on and power off trim will be the same.
I've got one of the Seagull Spacewalkers for a future build and have made a note and taped to the box to be sure to check the incidences closely.
I'd really enjoy hearing back from you how the trim effort is going... thanks.