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Date: Mon, 23 May 2011 14:17:22 -0400
From: Ernest Christley <echristley@att.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Center line thrust
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Thomas Giddings wrote:
>  Thought you guys might like to see my engine compartment. My plan is to mount the Rotary on this mount with a 1/2 " 6061 T6
>  plate. The original design for the Questair Venture used a IO550 Continental bolted directly(via the oil pan) to this engine mount. Since I have no room below this mount because of the nose wheel retracting into it. I am using the mounting plate as a dry sump plate as well. Soooo. the reason I am asking about  raising the Center of thrust is when mounting the Rotary to the original motor mount shown in the Photo and using Marcotte PSRU which rises it another 1.5 in . It actually works out to be around 3 " after I factor in the height of the mount plate PSRU offset and the natural higher point of thrust with the 20B. I seemed to have missed this with all my backyard engineering. The reason i thought this would work better then LS1 I originally planned to install is how neatly the rotary bolted to the original mount without modification to the motor mount and nose wheel retract system. However if you guys think this a none factor . I will proceed . One positive about this I 
will have a Bigger prop then Normal and it will give about standard ground clearance. Any other thoughts or observations would be welcome.
> 

The line of thrust affects handling when moved in relation to the center of weight.  Those two have to be considered 
together.  If you move the thrust line above the CoW, adding power will push the nose down harder.  Below, and the power 
will push the nose up.  One thing to consider is that the line is in fact a LINE.  You can lift the engine higher and 
then change the angle to keep it pointing below the CoW.  In your case, that might add a significant down vector on the 
tail...which might be a GOOD thing if your rotary installation is lighter than the 540 the plane was set up for.

Did anyone happen to tell you that, "It depends."  The people that know what they're doing play with all this to get 
everything lined up just perfect for a specific flight regime.  You can't have it perfect for ALL flight regimes.  You 
should get a book on airplane rigging.  A few degree or inches here and there can significantly change how the airplane 
flies and how much drag it produces.