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At slower speeds, the thrust line of each prop moves to the right.  This, of course, is because as the plane's angle of attack increases, the descending prop blade's angle of attack increases while the ascending prop blade's angle of attack decreases.  The descending blade produces more thrust and therefore the thrust axis moves to the right.  Just above the stall speed, you have the situation depicted below.  The thrust line of each prop has moved to the right approximately 12" and the thrust is now symmetrical about the CG.  In Burt's words, "Conventional twins are asymmetrical, the Boomerang is symmetrical".

So how well behaved is the Boomerang with an engine out?  Well, just above the stall speed, with one engine feathered and the other engine running at full power, with both feet off the rudders, a mere 1 1/2 degrees of aileron deflection will slip the plane sufficiently to counter the asymmetrical thrust from the working engine.