Paul,

Remembering the trim situation at Reno and looking at Biz jet
   photos, I
noticed that the Biz jet engines that are mounted on the rear sides of
the
fuselage are tilted down at the rear. Is that optical illusion or
design to
offset pitch up at speed?

Dale Davies


I don't know the answer to that question.

Usually the thrust line goes through the CG.
Prevents pitch with sudden increase in thrust.

Paul Lamar


Paul,
   Look at a Cessna Caravan, when looked at from the side, the
   propellor angle in relation to the wing or fuse is very much angled down
   so much in fact I thought one parked at Charleville QLD., had a engine
   mount problem, until all the passengers climbed back on board and flew
   out to Lake Eyre.
  John Burey


Dale,  The thrust line is actually aligned with the angle of attack of
the main wing at cruise airspeed. If it is not  the top of the prop arc
and bottom arc AOA of the blades will be different (p-factor) and
efficiency will suffer. The thrust line may be close to going thru the
center of gravity because the engine being 1/3 the empty weight of the
plane will obviously be on the thrust line.  If the thrust line goes
thru the resolved center of the drag components then pitch changes will
be less. In reality prop ground clearance dictates engine height and
wing placement  by the designer. With a high wing the thrust tries to
pick up the nose. With a low wing the thrust tries to lower the nose.
With a mid wing such as a v-witt or cassut racer  or the canard pushers
pioneered by Burt Rutan, the thrust is lined up with the wing chord.
The ground appearance of a prop reflects the relative length of
the landing gear. A tail dragger has a pitched up prop dish. A business
jet needs  to get rid of lift at touchdown to get stopped and may have
the engines tilted down when parked, but the alignment with the wing AOA
at cruise  and drag center are the primary factors in the mounting.
    The AOA can be calculated from  lift(weight) =
roe X V squared X area X CsubL solving for CsubL and looking at thelift versus
angle of attack graph to read off the required AOA..  However these days
you can enter your plane in the X-plane simulator and read all sorts of
data to three decimal places. I am not sure how to calculate side thrust
but a thrust angle that requires zero aileron trim in X-plane is a good
starting point.  Most of the home builts can be flown in X-plane from
downloaded data files on their site.
george grimes