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Different modifications, different leveling points, different CG results. Ultimately, that's why we do phase 1 testing.
 
Leveling, for sure. I agree that you can put the scales wherever is convenient as long as you have accurate measurements to where they are relative to the datum. Leveling, though, will make a difference in where the cg calcs out to be. Tail high and cg will move forward, tail low and it will move rearward. Fact is, you can level it any way you want also but it needs to be consistent with how they did it when they did the cg flight testing. My main problem is that the TC calls out the PK screw and center punch which I won't have on an experimental build and I don't know whether that is plumb to the structure or not. If I can't find out between now and in the long future when I need to do it, I'll probably just assume it is and level of the structure like you suggest. Wayne
 
My main problem is that the TC calls out the PK screw and center punch which I won't have on an experimental build and I don't know whether that is plumb to the structure or not. If I can't find out between now and in the long future when I need to do it, I'll probably just assume it is and level of the structure like you suggest. Wayne
You ought to have a set of the Northland PA-18 drawings which are available in the Super Cub Store: https://store.straight-level.com
Or here: The Unofficial PA-18 Drawings Viewer - Terms (supercubproject.com)
Drawing 12183 indicates the front firewall station is plumb. Placing a digital level between the upper and lower engine mount locations on the firewall should be plumb. Additionally if you have a stock landing gear or one which is not swept forward, a plumb line dropped from the wing L/E measured to the axle centerline should be 2" per the TC. 1A2 page 10 item 202.
 
I am truly embarrassed that I didn't think to look at the drawings. :oops: Good grief. Thanks much. Also, nice catch on the TC and the moment arm of the axles, although I won't have standard gear. .

Wayne
 
You need to get up pretty early to get a head of Skywagon 8A


Sent from my iPhone using SuperCub.Org
 
I am truly embarrassed that I didn't think to look at the drawings. :oops: Good grief. Thanks much. Also, nice catch on the TC and the moment arm of the axles, although I won't have standard gear. .

Wayne
Is your gear going to be swept forward or is it just a different length? If just a different length, the measurement should still be 2".
 
Truth is, I don't know. Like I said, I'm pretty new to super cubs. I ordered 3" extended but I don't know the geometry of that gear.

Wayne
 
Piper drawing shows a horizontal reference line dimensioned from the fuselage references - engine mount holes in front and a distance from lower longeron at tailpost. I'll try to find the dwg tomorrow. I have the one for PA-12, but not doubt it's in the supercubproject.com drawings somewhere also.
 
I thought the tubing along the throttles was level? May have been told that.

Gary
There are several places which should be level or plumb according to the drawings. Due to the construction process, welding etc those locations may not be accurate. On a PA-18, the 2" aft of the leading edge for the main (unmodified) axle is the best starting point. If the fuselage is built in an accurate fixture, the firewall locations will be correct. Not necessarily the vertical tubes, but the firewall between the engine mount fittings. I recently did some work on a Bearhawk Patrol which has the lower longeron listed on the drawing as the leveling location. Due to large welded clusters at each end of the tube, there was no place which was accurate. Another place which was correct was located.
 
Piper drawing shows a horizontal reference line dimensioned from the fuselage references - engine mount holes in front and a distance from lower longeron at tailpost. I'll try to find the dwg tomorrow. I have the one for PA-12, but not doubt it's in the supercubproject.com drawings somewhere also.

A friend of mine went to weigh in a highly modified Pacer that he'd built,
no PK screw or screw hole anymore for leveling as per the TCDS.
He was told that both the firewall & tailpost should be plumb for leveling--
unfortunately they didn't agree with each other.
He agonized over this for quite a while,
until someone mentioned the factory drawing's horizontal reference line (HRL).
So that's what he used.
Ditto a friend of mine's rebuilt PA12.
 
^^^^ The Horizontal Reference Line shown in 12183 passes through the tubing that supports the throttles. But as Sky notes it may not be accurate. I know for a fact that the firewall mounts/tubing on my last PA-18A was not vertical after a previous accident.

Gary

Edit: How did the factory determine the level line and then punch the plumb dimple?
 
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I think, but don't actually know, that the horizontal reference line is established by definition. The -18 HRL is considerably different than the -12 HRL, as referenced to fusleage locations. Maybe due to the different wing incidences?? Maybe it will actually be essentially level in cruise flight? Dunno, but maybe somebody here knows.
 
Leveling the plane also allows for trim range to be checked from some neutral or level point. Again...how did Piper do it when the plane was new? Someone here has to know what the Factory did. Or Cub Crafters later?

Gary
 
Firewall vertical/tail post vertical/left window tube level. Get it as close as you can with all three and call it good. It is a cub stop overthinking it. If you run out of trim move some stuff around, pretty simple. DENNY
 
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