Expert: Mark Janus - 3/13/2008

Question
Hi Dr Janus, I'm aeronautical engineering student. I have some doubts about the common engine configuration for the Cessna 336,Dornier 335 and Rutan's Defiant.

1- Why the rear engine is not in the same axis that forward engine? What is the reason of this "displacement"?

2-Does the dimensions and power for rear and forward Propeller must be equal? why? or Why not?

3-Is there any book or web site that can you recommend me about this topic?

Answer
All engines are usually offset slightly to accommodate for engine torque effects...  I'm not 100% sure on this with regard to a pusher/puller config...but I'd say it's for the same reason (you don't have to run with both engines and hence have the same "problem" as single engine aircraft)

I would say for the props to be equal in size if they are close together (like a counter-rotating prop), but if they are apart like in a pusher/puller then the rear may be smaller (I say this because the slipstream behind the forward prop contracts) and hence the rear prop could be sized smaller to just work with the slipstream air of the forward prop...

disclaimer:  I don't know that much about pusher/puller configs...and don't really have any references that discuss this in depth...so you are getting more of my educated opinion than facts  

Sorry I may not have been much help here...