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



Thank you for the fairly detailed response. At the time of the Galoping Ghost
crash, the explanation was to fly in trim the trim tab had to be fully up to
push the elevator down a bit to allow level flight at speed. When the tab
failed the elevator would return to neutral and allow pitch up. This is at a
constant speed, not sudden acceleration. With a high wing, the wing drag is
above the thrust which should cause the nose to pitch up and a low wing the
opposite. I must be missing something. If I understand correctly, when the
plane is flying in cruise the prop should be 90 degrees to the line of travel
which would put the prop shaft in line with the direction of travel.

Dale Davies

Not the same thing Dale.
It all has to do with C sub l as a function of AOL.

Paul Lamar