JG....Downthrust is just not a function of the centerline of crankshaft in relation to wing chord centerline...there are many other variables involved....most....but not all prop engines are typically mounted close to chord centerline and a small offset up or down makes little or no difference on effectively making a plane climb or descend....wing/stab incidence..... inherent/parasitic drag coefficients....airfoil design....fuselage design..only to name a few can influence undesired movement along any of the three Axis....Roll, Pitch or Yaw.....I've experimented on several large warbirds...I.E a Seafury and my Corsair with downthrust( Or rt thrust ) and my conclusion is theres not enough difference to warrant its use ...on the other hand on a twin its most appropriate to help during single engine operation....If one happens to lose an engine and slows to VMC(Speed at which eng. thrust overrides full rudder deflection...) no amount of down thrust will help....so each twin has its own engine out characteristics some better than others....Duelist happens to behave quite nicely under single engine ops....However B-25 loses an engine...other engine flys you to the crash site....Bill....