mounting engine

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


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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 tidbit.

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 fuse.

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 left-to-right.

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 center line.

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.

Cheers, Fred McClellan the dash plumber at mindspring dot com

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Fred McClellan

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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 wingtip.



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