----- Original Message -----
From: kevin lane <n3773@comcast.net>
Date: Thursday, July 1, 2004 1:08 am
Subject: [FlyRotary] prop angle

> I am going to run the 2.85 drive with my 20B.  I
know that I need
> to angle
> the engine 1 to 1 1/2 degrees to the right.  I
read that some
> engines are
> also angled down apparently.  should I also take
this into
> consideration or
> can I simply mount the engine level?  is this
something pretty
> minor?  I
> haven't tried to measure yet how my fred breese
mount was built, and
> assuming I have burned all my bridges over
there....I can't ask the
> designer.
> Kevin Lane  Portland, OR
> e-mail-> n3773@comcast.net
> web-> http://home.comcast.net/~n3773
> (browse w/ internet explorer)
>

A flying airplane very rarely has it's fuselage's
centerline pointed directly into the wind.  In most
cases, that is where it would fly most efficiently,
but all sorts of things conspire to throw it off.
CG, speed, weight, a bug on the leading edge...they
all have some effect on the angle the plane makes
with the wind.  The designer takes this all into
account, and then may modify the engine placement to
account for some tendencies.  It is as much art as
science, and a lot of it depends on what you want to
get.

Changing the engine offsets changes the 'thrust
line', ie the direction is trying to pull/push the
plane.  The propeller gives a rotational velocity to
the air, which may cause one wing to produce more
lift than the other...which in turn gets translated
to a turning tendency.  Putting in a little offset
to the thrust line will counteract this tendency,
pull the aircraft back to where it should be.

When you cut power, will the nose tend to rise or
fall?  If it tends up and you prefer it to fall,
offset the thrust line to point up a little and move
the CG forward slightly.  The slight up vector will
give extra lift in the nose with the engine running.
 (Turn everything around for a pusher as usual).

All of this is playing in the margins, and is the
magic that good aerodynamicist use to tune a design
to get an airplane that 'feels' good and flies fast.
 The same tendencies can be created or counteracted
just as effectively with trim tabs, but at the cost
of a lot of extra drag.  Both methods will work, but
the former does is by adding power at the correct
point vs subtracting it, leaving a higher power
budget overall.