Paul,
I am doing final setup on thrust line and offset today in prep for
starting tubes will get back to you asap. Murphy suggests 0 to 1
degrees offset to the right for 180 HP Lycoming install. Prop rotation
is clockwise with ross drive.If my thinking is correct offset should
be
to the right. Murphy uses the firewall to prop flange for a reference
to
set offset , is this typical for all aircraft. What is your suggestion
on amount of offset.
Geir
Is that clockwise from the pilots view? Or is it clockwise standing
in front
of the plane looking back?
Are you going to use your current cowling? If so you must set it in
the corner
of a room and determine if the prop hole is symmetrical or off set to
one side or
the other? These two dimensions should be the same. Then the off set
angle
will start in the center of the prop hole. Very important when
designing
the motor mount. Its a mess if the PSRU does not come out in the
center
of the hole.
Paul Lamar ...No rotor no motor.
Most cowls are quite adjustable and can easily be trimmed to center
on the prop flange. The Tailwind uses the same cowl for the nose
gear version as it does for the Tail dragger, but the thrust line on
the nose gear version is one inch higher. I can't imagine taking a
cowl out of the box and having it fit perfectly without some
significant adjustment on the part of the builder. BTW, many aircraft
are built and flying fast and true with zero engine offset. I know
because I built the engine mounts. On the Tailwind, the leading edge
of the vertical stab is offset slightly to accomplish the same
thing. When I went to the 2:17 drive which is reversed, I moved the
stab leading edge to the opposite side to compensate. On Cassutt
racers, (my info is at least 10 years old) I believe the engines are
normally set on the thrust line and also they no longer off set the
vert stab. This in the quest for more speed. Think about it. If I
were building a new Tailwind today, I would zero the engine and the
vert stab. It seems to me that having the engine off set from the
thrust line would have to create some drag which would effect top
end and also fuel efficiency, particularly for fast planes with
small high reving props. Jerry
Yes but then you would have to apply rudder to keep flying straight.
The
slipstream off the prop will corkscrew around an under the fuselage and
hit
the vertical stab on one side (slipstream yaw). Some a/c have straight
motor
mounts but then shim the mount to the firewall.
John
John, this is another chance for me to display my ignorance, but I
have done that before with no lasting harm so here goes:
I think that when the airspeed reaches a certain point, the prop
wash away is swept away before it impacts the Vert Stab and rudder.
The result is the faster you go the less trim is needed until
eventually perhaps no trim at all is required, at least not to
counteract the cork screw effect. Jerry
Right you are Jerry, I just don't know how fast that would be and if you can
get that fast or the prop to turn slow enough. Also you would now be
designing for one very high airspeed and you would need to trim for any
other speed anyway. I tend to think in the 100-120 mph range and 500' and
below which I know isn't what most of the listers are thinking of.
John
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