• If You Are Having Trouble Logging In with Your Old Username and Password, Please use this Forgot Your Password link to get re-established.
  • Hey! Be sure to login or register!

Spar Drilling

PA18project

Registered User
Missoula MT
In laying out my spar blanks for drilling I ran into a discrepancy between the piper drawings and the piper spars I have as patterns.

The Piper Drawings show that the third hole (from the bottom of the spar) for the strut attach fittings for both front and rear spars should be 121.25 inches from the end of the spar (wing attach side). The piper drawings for the front and rear spars are 14388 and 14453 respectively.

The PA-18 front spar I have has the third hold (from the bottom of the spar) at a distance of 123.5 inches. The rear spar has its third hole for the strut attach (again from the bottom of the spar) at a distance of 122.25 inches.

Both of the spar patterns I have came from the same PA-18 (unknown horsepower or year). Before I do any drilling I want to be sure about where things should be. Any help would be appreciated. Thanks.

Brad
 
Correct me if I'm wrong, but the way I read the drwgs. is the measurement of 121.25" is from the centerline of the #1 drag strut hole for the front spar, then for the rear spar, measure from the centerline of the second hole from the top at the wing attach end. Or you could add
2-1/4" to all your hole centerline measurements and measure from the end of the spar to get the same results.
Again, I could be wrong.
wirsig.
 
Wirsig,

You might be right. I have to admit that I have not drilled a set of spars before which is why I am questioning the measurements I am getting. Do you have the numbers for the drawings you are referring to? The ones I am looking at appear to be pretty clear on the strut attach holes but I may be misreading the measurements.

Brad
 
In looking at the drawings I guess its a little confusing to me. The distance from the end of the spar to the #1 drag strut hole is 13 7/16 but otherwise the drawing appears to me to measure (oddly enough) from the end of the spar. Where do you get the 2.25 inch measurement? That seems to make up the difference but I dont quite know how that is derived from the drawing. Again, I admit I am probably missing something in the drawings. I would attach the front spar drawing but I my luck attaching things has been pretty hit or miss (mostly miss).
 
I just finished laying out my spars so I know what you're going through. I spent about half a day scratchin' my head about the measurements not working out, especially the heights for the centerlines of the drag bracing from the bottoms of the spars. I'm using the drwgs from the northland cd 14383 and 14453. I think drwg 383 shows the spar for J3 up to -18. You'll want to use detail fig. 5 to end up w/ the shorter spar. Look real close at the drwgs and you'll notice the measurements to the holes don't match up w/ the overall spar length. Then you'll notice that the starting reference line comes off a centerline for a bolt hole instead of the end of the spar. The diff should be around 2-1/4" if I remember right. If not, let me know because I'm planning on drilling tomorrow.
Wirsig.
 
I've got such a slow connection I just got your last post. That 13-7/16 is for the j-3 you need detail fig 5
 
To get 2-1/4" for the rear spar look at the overall spar length 193-7/8" and subtract the hole layout length total of 191-5/8". For the front spar look at detail fig. 5 and add the radius for the first hole in the spar of .5" to the distance to the first drag strut attach hole 1.76" and you get 2-1/4" plus or minus. I can't drill within .01" so 2-1/4" is close enough for me.
 
Something else to watch for is the rib attach holes for the left and right wings are something like 3/4" different. I think the nose rib holes are a little less.
 
Well I'm finally drilling my spars and I have the inboard ends shaped and have drilled the front spar with 1/4" pilot holes for the hinge and at the 5 compression strut holes. I was getting ready to drill the rear spar for the # 1 compression strut location (It's actually the #2 hole in the H spacer for the hinge mounting). Here's the problem: The holes in the front spar are located "up" from the bottom 1 29/32 while the rear spar hole is 2 1/2 " up. About a 5/8" difference. I went and checked a wrecked wing at the airport today and indeed the rear spar is lower by about 5/8" difference! What this means is that if you mounted both spars perfectly vertical it would be necessary to place a 5/8" spacer under the front spar at both ends to have the spars lay vertical and for a parallelogram. I guess you would use a 1" at the outboard rear corner (for a 3/8 wash front to rear) , a 5/8 spacer at each front spar corner and only the inboard rear corner would sit flat. Am I correct about this? I always thought 3 of the corners lay in the same horizontal plane. I don't remember this discussed before. Dakota calls for a 1" spacer for wash but all the wing assy instructions seem to indicate the inner rear and both front spar sit flat on the saw horses.
As a side, on the example spar ends tcraft sent me, the rear spar has an extra hole 5/8 lower that was used on whatever cub it came off of. This is not on the diagram and would have required cutting part of the H spacer away to get the bolt on. That spar would would have had all 3 corners in the same plane as I thought they should be.?? Any thoughts??
 
Last edited:
I am not following all that, and have never rebuilt an 18 wing. But the J-3 wing has the compression tubes and drag wires all in a plane, while the spar itself has a twist - that is, on the J-3, the compression struts are not all the same distance from the edge of the spar. Did not discover that until we drilled new wood blanks. Makes sense. The engineers from the 1930s have my admiration.
 
I am not following all that, and have never rebuilt an 18 wing. But the J-3 wing has the compression tubes and drag wires all in a plane, while the spar itself has a twist - that is, on the J-3, the compression struts are not all the same distance from the edge of the spar. Did not discover that until we drilled new wood blanks. Makes sense. The engineers from the 1930s have my admiration.

It still has the twist but the #1 compression strut fits to the rear spar 2 1/2" up on it and just under 2" up on the front spar. In my book the two spars cannot lie in the same plane and the compression strut be horizontal. The wing I examined today on a PA-18 and rebuilt by Cubcrafters a few years back is offset as I describe. When I discovered this apparent discrepancy I started searching posts on here to see if it was mentioned anywhere else and I didn't find any mention of it so I thought before drilling any more holes I would bring it up and see if anyone agreed with what i have found before I drill the rear spar.

Perhaps it isn't noticeable when building the wing on sawhorses but when you tighten the #1 compression strut bolts the tops of the two spars should tip ever so slightly forward of vertical if the compression strut bolts and foot are square. I've put off drilling the rear spars for two days trying to convince myself I am correct and I'm about 90% sure I haven't missed anything but why hasn't someone noticed it and mentioned it on here somewhere before?

Bob my "graphic" in the first post didn't work. The text got moved around and I'll try and delete it.
 
Back
Top