> Hello Anyone,
 >     I am currently thinking about starting a new project.. it's Going to 
 > a
 > twin Racing Bipe made from 2 scat cat type airframes and wings.. what I 
 > would
 > like to do is set the spacing between the motors as close as I can possib
 > get
 > them.. so if one engine quits it won't be so bad to land it. I would like
 > to
 > space it so the tips from each 8x6 just clear but I do not know if there
 > would
 > be any negative implications from that.. 
Neil,
There will indeed be potential problems with a set up as you describe.  
Think of it this way.  Imagine each prop blade as a high speed wing at 
several degrees of positive angle of attack.  Imagine now all the vortecies 
off the wing tips not to mention the airflow off the rest of it.  Where the 
props come the closest these vortecies would be banging against each other 
and actually negating some of the thrust from the props.  This would also be
happening near the tips where a larger share of the blades thrust comes 
from.  
One partial solution would be to have one engine counter rotating so the 
blades are going in the same relative direction as they pass each other.  
Better still would be a syncronizing system that keep both engines at the 
same exact RPM and goes so far as to track and keep the individual blades 
from the two engines in the same relative position.  Assuming two bladed 
props I would think you would want one to be verticle when the other is 
horizontle.  Obviously not an easy thing to do in a model.  I know there is 
a formula out there for what you want to do but I have no idea right now 
where to find it easily.
I suggest you build a test plane.  One that is stable and easy to fly yet 
has the provision on it where you can move the engines along the leading 
edge of the wing.  This way you can conduct test flights where all but 
engine position remains the same and see for yourself what changes occure.  
I think you'll begin to notice reduced performance once the blades come 
within a couple of inches of each other.  I would suggest too that you place
a flat, profile like wall between the two engines to help control airflow to
the props.  With 8 inch props I think it would only need to be 2-4 inches 
high and only as thick as need be for the required strength.  This will alow
you to bring the props as close as you dare.  It would be sorta like a 
partial duct for the props, just enough to keep the airflows separate.
But for a race machine, there are far easier ways to maintain one engine 
controlability than by placing the engines next to each other.
It does sound like an intersting project and I think you could discover a 
lot about props and prop wash with it.  Let us know what you do and what you
find.
TTYL
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 * QReader 1.10 * UNREGISTERED EVALUATION VERSION
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