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Post by Ron Stuart on Feb 22, 2016 21:23:29 GMT -4
I'm building an AT-6 Texan/Harvard from a HOB kit and puzzled about both the plans and the building instructions with respect to the firewall. hopefully somebody can explain. 1. What is the horizontal line approximately 3/32” above and parallel to the thrust line on the detail of the firewall ? 2. What is the circle depicted on the firewall in the same drawing? I note that the centre of the circle is the intersection of the vertical centre line and the horizontal line referenced in #1 above, not the thrust line. 3. On the first page of the building instructions it says to mark the thrust line, the centre line, the top and draw the circle on the firewall. I understand the first three items, but I don’t understand what the circle is for?I do not see any further reference to this circle anywhere in the instructions. Because the side view of the fuselage clearly shows the engine being centered on the thrust line, I have drilled the firewall accordingly, which I hope is correct.
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glen
Instructor
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Post by glen on Feb 22, 2016 21:37:54 GMT -4
Ron does the kit call for downthrust built into the firewall ? If it does, that would explain the extra marks. As far as the circle goes, what motor mount comes with the kit ? if it's circular that would be my first guess.
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Post by Ron Stuart on Feb 22, 2016 21:50:05 GMT -4
No, Glen. No mention of downthrust anywhere in the instructions and no mention of it on the plans. Plans show the f/w exactly at right angles to the fuse and thrust line.
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daver
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Post by daver on Feb 22, 2016 22:15:59 GMT -4
Then you're expected to incorporate the down and right thrust via washers behind the mount. Engine is normally offset a bit to compensate, so that the prop hub ends up perfectly centered, even though the engine isn't. If you drilled the engine perfectly centered, and then incorporated down and right thrust (normally about 2 degrees ea), your prop will be off-centre a bit, really noticeable with a radial engine design.
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daver
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Post by daver on Feb 22, 2016 22:17:39 GMT -4
Also, the center of the firewall on the vertical, is going to be higher than the mount location, as you need to take into account for the thickness of the mounting rails of the engine. Lot of people center the mount, then bolt the engine on, and wonder why it's too high up.
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daver
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Post by daver on Feb 22, 2016 22:18:40 GMT -4
Most of the older mounts for small engines, are circular, I've still got a few lying around. I'd bet that's why the circle on the firewall.
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Post by Ron Stuart on Feb 22, 2016 23:09:29 GMT -4
Dave, these images are, hopefully, a little clearer than the ones I first posted. The circle on the plans is 2-1/4" diameter, while the mount that came with the engine is 1-9/16" diameter. The motor mount depicted on the side view of the aircraft is 1-1/2" diameter, essentially the same. You can see in the photo of the engine on the mount that the top of the mount rails appear to line exactly with the center of the shaft. Thanks for contributing. Ron
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Post by jollidude on Feb 23, 2016 3:33:49 GMT -4
Looks like they are expecting you to put in a bit of down thrust. The engine is mounted slightly higher than the centerline so that the prop will be centered when you mount the engine. I can't remember if I used down thrust when I built mine. Maybe Jimmy can check as he owns that plane now.
Wayne
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