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strut fork adjustment-pa18

G

garyh

Registered User
3gv
i need to extend the 5/8" strut fork on my left wing to get the correct 1* dihedral. already extended out 9 threads(about 5/16") but need to go out about 6 more threads(1/4") to obtain correct dihedral. how far out can fork extend and be legal?
thanks
 
I am not sure of the number of threads that must be in the strut. How many are showing on the opposite side? Make sure your fuselage is level.
Mark
 
cubman,
yes, laterally leveled. 3 threads on right and measures 1* dihedral with smart level. 8 threads showing on left and measures 0.6* dihedral. logs show 1986 damage to left side fuselage and repair. guess this is why more thread exposure on left. just wondering if i can go out about 6 more threads and stay legal, that would be a total of about 9/16" thread exposure on left side to have the 1* dihedral.
 
Gary

I always use .7* not 1 for dihedral (may or may not be right)--if that will help. Give a call if you like and I can run down how I would go about the rigging and see if it makes sense and suits you.

Thanks

Mark
 
Sounds like you got it figured out. I got this from my Clyde Smith notes.

15 threads max. showing on lift struts.
 
Steve that is good info. I have not seen one out that far that I can recall.
Mark
 
great, i learned two things from this thread.
1) wing dihedral with smart level is +0.7*
2) maximum 15 thread exposure on strut fork
good information, thanks to all
 
I have read all of the various posting on leveling/dihedral/washout and figured this posting is as good as any to ask my questions. First, my airplane was wrecked back in the '60's and when leveling, the plumb bob to the punch mark option does not agree with leveling across the gear attach fittings. Which would you use? I am thinking gear attach fittings and have done that.
After I use the gear attach fittings to level and I adjust for the 3" measurement across the wing attach fittings, I have 11 threads exposed and +0.7 degrees dihedral in the left wing and about 6 threads exposed and +1.2 degrees dihedral in the right. I am thinking it would be better to have equal dihedral at 0.7 degrees, as I have read in other postings, in each wing irregardless of the measurement across the wing attach fittings? I am hesitant to turn out the left side any further to compensate for turning in the right. Which is more correct, 3" clearance across the fittings or equal dihedral at +0.7 degrees?
Or how would you attack this? Jim
 
cruiser said:
......Or how would you attack this? Jim
Click to expand...

Since you know the airplane was wrecked and you are questioning where to measure, I would not trust the gear fittings to be the correct location to level. Try using the front spar cross tube above the windshield or make a leveling block which goes across the top of the fuselage picking up on the front wing attach bolts. Plumb the firewall for the other direction.
 
skywagon8a said:
Since you know the airplane was wrecked and you are questioning where to measure, I would not trust the gear fittings to be the correct location to level. Try using the front spar cross tube above the windshield or make a leveling block which goes across the top of the fuselage picking up on the front wing attach bolts. Plumb the firewall for the other direction.
Click to expand...

The wing attach bolts are in agreement with the gear attach bolts. Thanks, Jim
 
You want a min. Of 1.5 times the diameter of fork inside the barrel . That equals about 13 threads showing.
 
15 threads is in the rigging instructions section of the owners handbook for PA-18-95, PA-18-135. I am trying real hard to not have that many threads exposed but I cannot seem to get a handle on why it is so different from one side of the airplane to the other. The airplane looks pretty straight to me. Will get back at it tomorrow. Jim
 
Just for the fun of it, measure the strut lengths. Maybe for some reason they are different? An old repair maybe?
 
cruiser said:
The wing attach bolts are in agreement with the gear attach bolts. Thanks, Jim
Click to expand...

If the fuselage is wracked out of square just a little, these measurements could be in agreement and still be wrong. That will throw your strut length adjustments off. You need to cross measure to be sure. I know, that will be difficult with an assembled airplane.

Maybe you can level the fuselage at the wing attach bolts. Then drop a plumb bob and measure in or out to the gear attach bolts. Both sides should be the same.
 
Last edited:
I've got plumb bob's and strings going in all directions and nothing is obvious. The struts are new Univair but measuring them was next. Thanks, Jim
 
Jim, just get both wings the same and as close as possible and go fly. Ask Mead about this we went through this with Daisy.
 
You want a min. Of 1.5 times the diameter of fork inside the barrel . That equals about 13 threads showing.
Click to expand...
Steve Pierce said:
That makes sense. I have always used 15 threads max. showing on lift struts according to Clyde Smith. Not sure where that originated.
Click to expand...


For the shear strength to tensile strength ratio of typical steels, 1.5 diameter thread engagement will develop the full strength of the fastener.
 
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