effect of changed thrust line.

wrote:
Jim Logajan wrote:
wrote:
OK - got some more info.

The center of mass is something like 34 inches behind the firewall
and roughly 7 inches above the top engine mount point on the
firewall. so roughly speeking 13 inches above the prop centerline.

What is that the center of mass of, exactly? The empty airframe only
(i.e. no engine)? Or is it the center of mass of a complete airplane
(sans fuel, pilot, passenger, and baggage) with the engine mounted at
the original point?

I'm also curious as to how you determined the location in two
dimensions. I can think of a few ways of determining it, but they all
involve a bunch of effort.

Center of mass for the completed plane when built to specs. Provided
by the designer.

I don't suppose the designer was or is willing to provide general advice
or guidance on mounting your engine? Sure beats asking total strangers on
the net. :-)

If I were in your position these are a few of the things I'd keep in
mind:

* The center of mass (CoM) of an empty airplane and full airplane are in
different locations in three dimensions. In the two dimensions of
interest, depending on how fuel, passengers, and baggage are loaded, it
moves around in those two dimensions such that it is geometrically
impossible to move engine "down" and find a single angle that yields the
original moment arms for all load configurations. The one configuration I
don't think I would use to compute engine mounting angle would be the
empty configuration.

* Moving the engine down relative to the original design specs also moves
the empty airplane CoM down. Assuming the Corvair and O-200 are both the
same weight of about 200 lbs, and assuming the empty weight of the plane
is 815 lbs, then if the engine is moved D inches down, the CoM moves down
D*200/815 inches. If D = 4, CoM moves down about 1 inch.

* I don't know where the CoM of passengers and baggage would be, but they
probably move CoM down (and presumably fuel moves it up). As an example,
if all that usable load is about 600 lbs, then if that load's CoM is L
inches below the empty airplane's CoM, the CoM of the whole airplane
moves down L*600/(815+600). So if, for example, L = 5, then CoM moves
down about 2 inches. So between lowering the engine and throwing in
stuff, the CoM of the airplane in this configuration differs by about 3
inches from that used in the original empty airplane spec. But the load
probably moved the CoM forward or backward too, affecting another
variable in the equations. Fun, huh?

* How much is the horizontal tail in the propwash? That and a different
angle of incidence of the propwash on the fuselage might give me more
down pitch than I calculated by assuming the only factor affected by the
engine move and rotation was the thrust moment arm. I might also risk
losing thrust efficiency because of increasing fuselage interference in
the propwash.

* At some point I'd realize that the CoM is variable enough that for the
small amount I'd be moving the engine, it would probably be best to mount
the engine so that the engine thrust line is aligned with the most
natural fuselage longitudinal. Then make trim adjustments during the test
flights.