View Full Version : PA-12 Leveling
01-19-2004, 12:05 PM
Hi folks --
I need some advice on leveling a PA-12 --- I know the plumb bob way from the door post to the fuselage dimple --- but I rebuilt my door frame and forgot to put in the screw hole that is in the upper door frame.
Any advice on other good leveling points -- then I can put the screw hole back.
I think the bottom of the butt rib should be a good fore/aft level, but is there a good side to side level point, such as the upper windshield cross tube? Or how about a level across the lifting tabs on the wing root?
01-19-2004, 12:40 PM
A couple of places are supposed to be plumb when the airframe was built. Easiest and most reliable will be where the engine mount is bolted to the front of the frame. The firewall is supposed to be vertical when the airplane is leveled. You can use a spirit level for a longitudinal vertical referance line on the cabin side of the front tubes behind the firewall. The rudder post is defined as a good vertical, too, but you'll find a lot of airframes that aren't straight after 60 some years. Most will lean forward some with old age. If you lost the door frame hole, your better off believing the verticals of the firewall tubing than the tailpost.
Side to side, most reliable definition of level needs to be the forward wing attach fittings. Once you determine the centerlines of these are level, doublecheck any carrythrough you can get at. If the airplane is assembled, then easiest is probably underside of the top wing spar carrythough. The headliner shouldn't affect putting a level on there, but make sure there aren't any folds, seams or lumps of cement goofing up your positioning. Once you go to all this trouble, for Goodness Sake redrill the doorframe hole. Its supposed to be 4-1/2 in forward from the rear, but they vary a little sometimes due to production tolerances.
If any of these things are in major disagreement, well, you have a twisted airframe, and it just might be time to go back to the drawing board. Ugly thought.
01-19-2004, 03:30 PM
FAA AC 43-16, dated August 1997 is entitled "Alternative leveling means and rigging of wing washout for Piper PA-12, -12S, -14, -18 series, and PA-19 airplanes". This is an approved document. I will list some excerpts.
For lateral leveling reference on PA-12, PA-12S, and PA-14, the AC43-16 (not to be confused with the AC 43.13, or "43 dot 13") directs one to use an "...18-inch spirit level on top of the member that supports the front edge of the rear seat and adjusting the heights of the jacks under the main landing gear axles to bring the bubble to center. Level the airplane longitudinally by placing and 18-inch spirit level on the cabin floor between the front and rear main landing gear attachment points. Position the level outboard of the front seat(s) on one side of the cabin, so that it is facing directly fore and aft, and place a 33/64-inch block under the level's rear end. Raise or lower the tail to bring the bubble to center. Repeat the procedure with the level positioned outboard of the front seat(s) on the other side of the cabin. If any difference in the tail height required to bring the bubble to center exists between the two sides, adjust the tail height so as to divide the difference evenly."
01-19-2004, 04:33 PM
Just pulled out my PA-12 Fuselage Print. It shows the horizontal reference line drawn through the top fuselage engine mount bushing running to a point on the aft side of the tailpost, 3 1/2" up from the center of the lower longeron.
So mark the tailpost point, use a water line level between there and the center line of the top fuselage engine mount bushing, and you should be right on.
The previous post method should work fine too. But I think the longer reference points from the engine mount to the tailpost would be more accurate than an 18" spirit level on the floor.
01-20-2004, 12:49 AM
Thanks folks --
When I did the rebuild I slightly changed the floor near the front seat, so the slope of that is probably not "factory; also I welded a tube to hold the front of the rear seat (and the fiberglass fish/tool box) so it is not factory either --- although I think I got it fairly level.
So probably the response from Frank T, using the water level will be the best one for me to look at.
When I did the rebuild, I did have the frame in a 12 jig, so I know it is straight -- I was just a dummy and forgot to put in the hole for the leveling screw, and didn't notice it till the whole thing was done. :oops:
It would be interesting to hear other ideas also ---
thanks for the help :D
01-23-2004, 10:36 PM
A good starting point is that the fuselage firewall mounts are supposed to be perpendicular to the horizontal line.
01-24-2004, 12:50 AM
You were looking for ideas. Just a thought here. On the 18 it shows the axle 2.25 inches from the leading edge. Make a mark on the floor from the axle to where the leading edge should be. Then drop a plumb bob off the leading edge. Bring the tail up until the plumb bob hits the mark. You didn't say they had to be good ideas---just interesting ideas!!!! This is assuming they ref the leading edge/axle on the 12.
01-25-2004, 08:03 PM
The axle on the PA-12 was 1.5" aft of the leading edge when it left the factory. PA-18 gear seems to change that a bit, it looks like approximately 1" on mine with "standard" length heavy duty gear.
"note added after TJ's reply"
it might not be the PA-18 gear change that caused the change on my plane. At least one current day STC drawing shows no change in location of the axle relative to the A/C.
01-26-2004, 11:14 AM
I'm looking at exactly that prospect very shortly here on a -12 with a fresh -18 gear alteration. My plan has been from the beginning that when the airplane is leveled again on the scales for weighing, I will drop a plumb bob from the W.L.E. and get the actual arm to the M.L.G. centerline, then compute the Empty Weight CG the "long way" instead of plugging in the numbers on the Piper PA-12 W&B form. This won't be for a month or two, but if nothing else happens between now and then, I'll report back what the actual change is in inches. Maybe someone has done this before, but I will do it that way to get an "actual" difference when the time comes. I'd do it again on the next one, too. Just to be sure.
01-27-2004, 02:08 AM
That's one of those questions that my lawyer may need to answer.
Seriously though, the -18 gear were installed a long time before I bought the plane and the original drawing did not make it to my possession.
I spent quite a bit of time using the original fuselage drawing trying to make the A/C as level as could be, and when I was done the A&P's measurement to the front axle was less than 1.5". How to do a W&B then? Use the actual measurements, and actual weights, figure the CG relative to the leading edge, same as if the gear were 10" aft of LE or any other distance. The "official" axle arm on my latest W&B is 1", based on the tape measurement done by the A&P (while the firewall was vertical and the tailpost ...).
And I probably shouldn't put the "blame" on the PA-18 gear because there are a lot of mods on the plane, just seemed like a likely candidate.
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