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Engine thrust lines, was: Wings Level ?

 
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Beemer



Joined: 26 Aug 2006
Posts: 87
Location: Middle Georgia

PostPosted: Thu Sep 07, 2006 4:27 am    Post subject: Engine thrust lines, was: Wings Level ? Reply with quote

I used a laser level, a cheapy ($25) from the Home Depot Aviation Dept. The
bubble on it was wrong, but I calibrated it using a building level, and
marked new lines on the bubble with a Sharpie.

Here's what I did. Level the fuselage using the normal points. I elevated
the tail with sawhorses, and used wood shims to get it level. I deflated the
tires to level it side-to-side. I then hung two plumb-bobs on the motor, one
from the front prop hub, and one from the rear of the motor (crank pulley on
my Geo).

I then placed the laser level on the floor out in front of the plane about 4
feet, and put the light so that it gave a line on the floor, crossed the
bolt at the tailwheel, up through the bottom motor mount bolt, and through
the prop hub. I hung a third plumb-bob from the tail, just to verify the
laser vertical. This took some time, but it wasn't hard.

Now, I put masking tape on the floor, and using a straight edge, I drew a
line under the motor on the tape at the laser line. Using trig, I determined
that the rear of the motor needed to be .78 inches to the left of center, to
place the prop hub dead center of the cowl (airframe CL), and give me 2
degrees offset (right thrust). Go here:
http://www.gsal.org/tools/offset_calc.htm . This tool is for model
airplanes, but its basic trig, and works for anything.

Move the motor until the front plump line is dead center on the tape, and/or
laser beam is center on the hub, and the rear line is .78 to the left on the
tape. I then tacked up my mount tubing, re-checking the plumb lines about
1000x.

For the down thrust, I used a cheap angle tool I bought at Harbor Freight,
and placed it on a motor surface I found to be 90 degrees to the prop hub.
This happened to be my valve cover, but I could have used the prop hub
itself, as the tool has a magnet on it to hold it there.

For the vertical position, I used the laser strapped to my toolbox in front
of the plane, and leveled it horizontally. I had drawn a line on my firewall
where I needed the hub to be in relation to the cowl. When the laser is
level and matches the firewall line, the light is projected on the prop hub,
and I moved the motor up and down 'til satisfied. I then re-checked
everything another 1000x, and welded.

After I was done, I am totally confident my prop hub is where I wanted it.
This should work on any motor.

I'm not sure it's where Dan Denney intended it to be, but that's the info I
don't have, and couldn't get. If it's wrong from flight tests, I'll shim the
motor 'til its right.

Hope that helps,
Bradley

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_________________
Beemer
KF2 (and now an M3!)
Suzuki G10 three-banger
Middle Georgia
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