In article
,
Alan Baker wrote:

In article ,
Stealth Pilot wrote:

On Thu, 13 Nov 2008 22:20:25 -0500, wrote:



How does a person determine what the proper height of an engine should
be when building an airplane? If a particular engine design mandates
the prop is 4 inches, say, lower than where it would be with the
engine originally installed, what effect will it have on handling, and
what changes in downthrust might be advised?

We are building a Pegazair, and my Corvair engine would need to have
the cowl higher than ideal to keep the crank centerline at the same
hight as say, an O200. Weight wize, the engines are just about
identical as equipped Have not determined the center of gravity of the
engine yet, to determine the overall length of the mount.

For those unfamiliar with the plane it is a highwing STOL 2 placer
roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,
)

suck it and see.
your elevator should have enough authority to control the resulting
couple.
you should experience some upthrust but tweaking back the throttle
should control it.

your question in the first line....
take the centre of mass as the pivot point.
you have 4 force couples.

lift vs moment arm.
elevator down thrust vs moment arm
wing drag vs moment arm
thrust vs moment arm.

they will summate to zero in equilibrium but
you probably wont know any of the values o those forces.
alan baker will probably give a guestimate.
suck it and see.

I think you'll need the same side thrust but a little more down thrust
as the cessna 150..

Stealth Pilot

The math isn't all that hard.

Assuming the designed thrust line goes relatively close to the centre of
mass, then if you want to keep the torque created by thrust close to the
same, you need to change the angle of the engine by arctan(h/l), where h
is the amount you're moving the engine up or down and l is the distance
between the propellor and the centre of mass.

I suck at ASCII art, but:


-----T' (new thrust line)
^
|
h |
|
-----T--(old thrust line)--------------------------------C
l (CoM)

h/l is equal to the tangent of the angle TCT'

If the thrust line is not aligned with the CoM to begin with, then
situation isn't quite as simple, but for small misalignments the effect
is small and for larger misalignments the overall change in torques is
smaller in comparison.

Draw few diagrams of the situation and you'll see what I mean. I'm not
even going to try to draw that situation here. Basically, if the thrust
line was already above the CoM and you move it up, then the change is
smaller than the arctangent of h/l and if it was below the CoM the
change is a little greater than the arctangent.

Did a quick little check:

As an example, a Cessna 150 is about 25 feet long and from looking at
wikipedia's little jpeg, the centre of mass should be about 5 feet
behind the propellor disc.

So if you raise the thrust line 4 inches, you need to angle the engine
up an additional 3.8 degrees; arctan(4/60).

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg