ok, so ive been told to make sure the engine is in the dead centre of the
firewall, but how do I do that? which is a small thunder tiger motor in a
balsa piper cub kit - the plans had were the holls should be, how ever I
didn't draw them on for some reason before building it.
Also whats the best way to check to see if everythings strait, as I fear
somethings may not be, should I sand them so they are strait I guess?
thanks in advanced
The engine may also need to be installed at a slight angle to the
right and also at a slight downward angle to provide the correct
thrust offset(s); scrutinize the plans carefully for that little
Presuming you still have the plans, make a copy of that portion of the
plans showing the firewall. Cut the copy to fit the firewall, fix it
with a glue stick or something similar, and drill it.
If you no longer have the plans, you have to locate the firewall
center lines the old-fashioned way.
To find the vertical centerline of the firewall, simply extend a line
from the vertical centerline of the fuse.
One way to establish the fuse vertical centerline is as follows :
Mount the wing and make sure it is precisely perpendicular to the
Using a measure of some sort (length of string, tape measure,
whatever) measure from the trailing edge of the wing at each wing tip
back to the point where the front of the vertical stabilizer contacts
the fuse. Measure from the other wing tip to the same point at the
vertical stab. Shift the wing right or left until repeated
measurements are the same for right and left wing halves. When the two
measurements are the same, the wing is centered on the fuse
Use your measuring thing to scribe a portion of a circle on the top of
the fuse where the radius of the circle extends from the wing tip. Do
that from both wing tips. Where the two semi-circles intersect is the
mid-point on the fuse at that radius. Use a length which places the
intersection near the tail end of the fuse.
Then use a shorter length to mark a second set of arcs on the top of
the fuse closer to the wing. Where those two arcs intersect is the
mid-point of the fuse at the shorter radius.
Now all you have to do is connect the two mid-points to establish the
fuse vertical center line, and the vertical center line of the
firewall is the same as the vertical center line of the fuse.
Note that the firewall itself may not be physically centered on the
front of the fuse, but you don't care about that except when it comes
to mounting the cowl. You're on your own there.
What you care about is establishing the firewall vertical centerline.
To establish the firewall _horizontal_ centerline use the same
process, except that you scribe intersecting arcs on the side of the
fuse based on radii extended from the wing trailing edge at the top of
the fuse and from an identical point at the bottom of the fuse.
Scribe two intersecting arcs from a long radius and two intersecting
arcs from a shorter radius. Connect the two sets of intersecting arcs
to establish the fuse horizontal centerline. Extend that horizontal
center line to the firewall, and draw the firewall horizontal center
line across the firewall exactly 90 degrees from the fuse horizontal
X marks the spot.
Now all you have to do is transfer the engine mount bolt pattern to
the firewall, making sure >it< it centered on the "cross-hairs" you
already made on the firewall.
Next time, drill the holes first.
the dash plumber at mindspring dot com
On Thu, 10 Jul 2003 18:37:47 -0400, Fred McClellan
However it is the prop shaft that should be on the centreline - NOT
the rear of the engine; so if the model has right-thrust & downthrust
then the engine should be mounted offcentre to allow for this.
To summarise, the position of the rear of the engine on the firewall
is relatively unimportant providing that the propshaft - and hence the
prop - is central, both horizontally and vertically.
The other thing to watch is that, even assuming your building is 100%
accurate, the offset engine plus silencer will probably upset the
lateral balance so you will probably have to add weight to the lighter
wingtip - a crude method I have used in the past is just to drill an
undersize hole then (very gently) hammer a nail into the wingtip and
cover the head with a small piece of film.
It is probably too late in your case, but it is usually obvious which
is going to be the "lighter" wingtip (opposite the cylinder in a
sidemounted engine) so I usually weigh the individual wing components
and use the heavier pieces of balsa in the lighter wing. If you are
lucky you can often get away with having to add no extra weight to the
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