Jerry wrote:
>    If we want to angle the engine off thrust, where is the pivot
> point?  Center of engine?   Prop flange face?  I have often  wondered
> about this.

Paul Lamar responded:
> Looks real good. Good idea with the laser.
> Prop flange center as that brings the flange out
> in the center of the cowl regardless of the off set angle or
> down thrust angle. That should be our reference point.

More precisely: in unaccelerated level flight(*), the thrust  vector is
perpendicular to the prop disc and passes through the center of  the prop
flange. So you can "pivot" the engine around any point you want, but
recognize that using a pivot point that is *not* the center of  the prop
flange will both rotate the thrust vector (the desired "offset")  *and*
translate the thrust vector (relative to the airframe).  Translating the
thrust vector will cause the airframe to fly in a different attitude
(center of drag will *always* be in line with the thrust vector).

Russell Kent

(*) things get funky in climb out, thus "P factor".

Russell,
Let me try to understand that in my terms, what I think you are  saying is
that if you want to offset the prop ( like a lot of designs call  for) you
move the prop flange left or right of center. Center being  longitude center
of thrust line of the aircraft.
 In this case the pivot point will be the vertical center of the
engine/gearbox/prop.

Or are you saying you pivot the prop on this thrust line center?
In this case the vertical center will be the prop center.
I think I'm confusing myself! but I've never come across this before
George (down under)

I believe what Paul is saying is:
The prop flange goes in the center, and you
angle the "back" end of the engine to achieve
the desired effect.
Archie

Close but not precisely. What I am saying is the designer of the
aircraft that comes with a premade cowl and specifies an
off set and a down thrust angle... those angles should be in
reference to the center of the cowl hole. This will put
the thrust line in the proper vertical and lateral
location back at the CG as pointed out by Russell. If there is no
premade cowl than we need drawings on where the thrust
line should go and or the point of reference. In that
case the prop flange may not come out in the center of
anything. The hole in the cowl may also not be in the
center of anything.

Paul Lamar ...No rotor no motor.

I wonder why the engine is off set anyway.  Seems like a major trade
off to improve handling for a  few seconds at the beginning of the
take off roll and then fly around all day with the thrust line not
lined up with the direction of flight.  Tailwinds which are  the
slickest birds flying in their class, set the engine straight ahead.
So do Cassutt Formula One racers.    If  you want to go fast, point
the engine where the airplane is going.   Save gas too.     Jerry


The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://www.rotaryeng.net
Copyright 1998-2006 All world wide rights reserved.