Thorp Air Command - T18.net

All,

I have read through the posts, references and newsletters and I am about to final drill to size the attachment lock pin holes in the 215 and 315 fittings. My fittings came pre-drilled to 1/2" hardened and plated with only a very slight offset in one of the holes on the RH wing side (likely from heat treating). My plan is to step drill to 17/32, 35/64, possibly ream to .5615 and then final ream the holes up to the .5625/9/16" pin size.

My set up for drilling and final reaming is going to be similar to ljkrume's shown from his gallery album, shown below during drilling.

I am struggling to see any other way to do this. Everything I have read and the references I see say to "ream to final size on assembly". Well, once inner and outer wings are assembled it will be very difficult to final drill/ream. This would have to be done by hand . . .not my idea of fun. If I go ahead with assembling the 315 to the outer spars which are built minus the 315/rib mounting holes then I will kind of lose any way to do a slight adjustments when drilling/mounting the 315 to the spare and doing dihedral checks as they are already assembled. These 215/315 fittings seem so close to the correct angles when clamped together and temp assembled (they are tight!) and sitting on the catches that I believe they will be good to go.

My set up was going to be with my smaller bench drill press on top of my work table, but I am going to use my larger standing press as it will be more stable and is a better drill press than the smaller one. I will just clamp things together and level the inner spar and shim/clamp the pieces together.

Any recommendations from anyone who has built a T-18C or S-18 wing that has some words of advise or a better approach to this other then as done by ljkrume or how I am suggesting? I think it will be fine as long as everything is level and clamped tight when drilled/reamed. I am just a nervous item to do as it is not an area you want to screw up on. I am finding out on some of this stuff you just have to read everything you can, ask questions in hopes of getting some advice . . . but in the end you just do it and learn as you go . . not a lot of T-18 builders around in my area to consult with.

I am asking as there will bound to be someone who is building a new project from Jim at Thorp Central that will have the same questions!

Rob

That should work. Be advised that there is slop in how the brackets mount to the spar caps. So if those have been drilled, you need to tak that into account.
Cubes

Hey Cubes,

I thought about that realizing that the dimensions/tightness of the two 215 pieces do change slightly as you install/torque down the AN4 bolts. My inner spar 215 cap holes have been drilled. So, I am thinking as long as I put in 2-4 AN4 bolts top and bottom to hold the 215's in place on the caps I could drill/ream the locking pin holes to size and then when doing the final install of the 215's on to the inner spar I could do it with the pins securely installed in the holes and the 315's temp installed. I believe this should keep alignment for the final install of all the AN4 bolts in the 215's. Agree?

Alternative would be to install every mount bolt on the 215's permanently and then drill/ream the pin holes to final dimensions. I was going to delay the final install the 215's to be able to get a bucking bar on the top and bottom spar caps rivets for the skins. Then slide the 215's/spacers in place (supposed to be installed "wet") and complete the install and final torque of the AN4's (I may have a few AN174's in lieu of AN4's). This is where I would install the pins in then 215/315 to keep that tight alignment.

Does this make sense?

Rob

I built a seven degree jig to clamp the two top spars. Then built everything around that.

_________________
Bob Highley
N711SH
SN 835
KLAL

That will work. But you also need to think about the sweep of the wing. I have seen wings built that do not match inner to outer because the spar cap bolts were not taken into account.
In other words, you not only have dihedral to think about, but also do the leading edges of the inner and outer line up and is there any unintended wing sweep. Do that by eliminating as many variables as possible. Measure the distance from the inner wing fittings to the leading edge. Then compare that with the outer.
Are the aft spars drilled? If not, you can account for sweep back there. You can also account for any unintended twist there as well. Lee Walton came up with a beautiful cam setup to adjust twist via the trailing edge attachment fitting.
Cubes

Some photos of my drilling and reamer of the right hand side 315/215 fittings.

Used a .5 bushing to locate/center holes and get everything aligned. Too damn hard to align by feel/sight when you are clamping things down etc.

All in all it went very well. The right side was a tighter fit as the original .5" holes were slightly more misaligned than the LH side. I enlarged the .5" holes with a 35/64 drill. Then reamed undersigned with a .5615" reamer, then a very quick final ream with a 9/16/.5625 reamer. Very good, tight fit with the pins when done. Tapped them in cautiously with a small brass hammer. first pin went in to lock in the reamed hole, then the other hole was done the same way. I know the pins are in backwards . . . but was kind of hard to drive them in from the bottom . . :-)

Happy with the results . . .went easier than I thought. Words of wisdom, check level in every direction . .check again . . ensure hole is still aligned with bushing, then recheck . . .then clamp/tighten everything down . . . and repeat. Will post a few more pics when I do the LH side.

Rob

Well done. I know drilling very expensive parts is something you do not want to do again.
Cubes

Yes, well said Jim.

Left side completed tonight. Had one pin that was a little bit harder to go in. Looks it may have had a little extra nitride/zinc conversion coating on it. Went in....

I assume it is okay to use my small brass hammer to get these in and out until I make or buy a slide puller. Seems harmless.

You are not going to hurt a pin with a brass hammer. All good.
Cubes

I use a small Craftsman soft faced mallet , hard rubber on one end and hard plastic on the other end to install my pins . Not a problem in 13 years and ZERO play in the wing . I use the plans made extractor to remove the pins . Works "Most Excellent !" One last note , I marked my pins with a "T" for top and a "B" for bottom . Probably not necessary , but it makes me feel Muey Bueno !

RB
NX115RX
2019