hi keith,
my tuppence worth on twins...
i've flown a few twins but no 3 or 4 eng models. the sound of 2 eng's 'on song' is, i have to say, a most beautiful sound indeed. nothing at all like a single 2-stroke screaming it's head off.
twins need the same care and attention when setting up the eng's as any model, and can be just as straight forward to fly. standard eng's and prop's are used. when running, the eng's must be tuned to produce a similar exhaust note, a musical ear is a big advantage here! because both prop's are turning anti-clock when viewed from the front, thrust lines are set up differently to counter torque and drag. the left hand eng could have 0-3 deg's right thrust, while the right hand eng could have 3-6 deg's right thrust.
the fun really starts when one eng quits, as often happens. if the l eng quits, the r eng has to counter its own torque+drag of fuse + l wing, hence large r thrust. if the r eng gives up, then l eng torque tends to cancel fuse drag+r wing drag, therefore less r thrust required on l eng.
i just realised keith, all you asked about were contra- rotating prop's and you end up with a mini lecture on twin eng thrust lines.
