wandersen
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I've been working on my wings for the last couple of weeks. One of the details is to get the flying and landing wire pins prepared with a flat surface to accept the nuts on threaded portion of the wires. The internal wire Skybolt design has the threaded end of the wires go through the pin and nutted on the back side.
An issue that has come to light for me is that the flying and landing wires don't come off these pins perpendicular, but may have up to a 3.5 degree angle. This has been discussed on other threads along with ways to accommodate this. My thought was to ream the far hole to the standard 5/16" size for the threaded portion of the wire, and ream the near hole to 7/16" which will allow for up to a 4 degree offset without bending the wire or chaffing the threads. Other thoughts were to file the near side hole with an offset to accommodate the angle.
The pins per design are 7/8" diameter by 0.188" wall 4130 tube with a 1" 0.065" collar rosette welded (see picture 15 below). The overall length is 3-1/2" with the wire holes 7/16" from the end.
The question is whether cutting the flats on the pin, and reaming the near side hole out to 7/16" would weaken the pin enough to be of concern. After doing a couple back of the envelop calculations, it became clear that a test would be the fastest and easiest way to answer this.
The pin I used for testing was a production reject, but materially identical to the production pins. I cut flats not only on the side for the flying/landing wire nuts, but also cut flats on the other (near) side. The flats cut into the test pin were wider than what would be cut into the production pins. The width of the test pin flats were at least 9/16". The production pins have flats less than 1/2". With flats on both sides instead of just one side, the test pin should be weaker than the production pins. I did not add the 1" collar around the test pin to make it easy to slide in and out of the test fixture. I don't think the collar would make any difference in strength. The near side hole on the pin was reamed to 7/16". The far side hole (nut side) is reamed to the standard 5/16".
The Skybolt spar is 1" thick spruce with 1/4" ply doublers on each side making a total thickness of 1-1/2". The aluminum spar plates are 1/8" 6061T6 that hold the pin in place.
To simulate the spar, I cut two 3/4" x 5" s 9" red oak boards I had laying around and glued them together. I had a couple of spare aluminum spar plate production rejects floating around the scrap bin. I drilled the 7/8" pin hole, along with the 1/4" holes for the spar plate bolts, and bolted the assembly together using 1/4" grade 8 bolts. Torqued the 1/4" bolts to 70 in-lbs.
I ran three tests.
Test 1 is using grade 2 - 5/16" threaded rod. Tensile strength 3900 lbs.
Test 2 is using grade 5 - 5/16" bolts. Tensile strength 6300 lbs.
Test 3 is using grade 8 - 5/16" bolts. Tensile strength 8750 lbs.
For reference, the Burntons AN675AC 5/16" streamline tie-rod wires show a minimal tensile strength of 6900 lbs. However, on the skybolt these are mated at the fuselage side with AN665-46 clevis terminals that are rated at 4600 lbs. The clevis terminals are the weak link so the system should thus have a strength of 4600 lbs each side.
The test using two grade 5 and grade 8 bolts should have a higher ultimate strength than two of the the installed Burton tie-rods and clevises.
For a test fixture, I used some 3/16" square tubing and a 12 ton press. The ultimate load using the grade 8 bolts should be around 9 tons (assuming the bolts are the weak link).
I loaded this up parallel with the simulated spar wood grain. The actual loading would be 30 degrees off this angle. As the spar and plates didn't fail or significantly distort, this worked out fine.
The final results are that the pin is plenty strong. After the three successive tests, including breaking loads of the grade 8 bolts, the pin came out perfectly straight. With a little buffing, it probably could be reused.
The failures were progressively more spectacular. The grade 8 bolt failure was particularly impressive. There was nothing left on the test stand and the shroud got a workout. I wish I had a video of this to post.
My conclusion is that adding a flat to one side of the pin along with reaming the near hole to 7/16" to accommodate the wire angles up to 3.5 degrees does not make the pin the weak link in the system.
I hope this information is informative. (No warranty expressed or implied....)
I'd be interested in comments and feedback.
Ward
Picture 1 - Components for test rig
Picture 2 - Setup for grade 2 threaded rod
Picture 3 - Testing rig
Picture 4 - 12 ton press
Picture 5 - 12 ton press with safety shroud to keep all the bits contained upon failure
Picture 6 - Test 1 - Grade 2 threaded rod test results
Picture 7 - Test 2 - Grade 5 bolt test setup
Picture 8 - Test 2 - Grade 5 bolt test results
Picture 9 - Test 3 - Grade 8 bolt test setup
Picture 10 - Test 4 - Grade 8 test results (note the aluminum compression distortion and slightly elongated hole. Also - yes - the heads of the bolts are ground round - needed to fit through a 1/2" hole on the test fixture because I could only get 5" bolts, not 6" bolts.)
Picture 11 - Pin after all testing (no distortion or damage)
Picture 12 - Other side of the pin after testing
Picture 13 - Pin showing the two flats cut on both sides.
Picture 14 - Flat cutter that worked surprisingly well
Picture 15 - Production pins showing the sleeve and the flats on the single side.
Pucture
An issue that has come to light for me is that the flying and landing wires don't come off these pins perpendicular, but may have up to a 3.5 degree angle. This has been discussed on other threads along with ways to accommodate this. My thought was to ream the far hole to the standard 5/16" size for the threaded portion of the wire, and ream the near hole to 7/16" which will allow for up to a 4 degree offset without bending the wire or chaffing the threads. Other thoughts were to file the near side hole with an offset to accommodate the angle.
The pins per design are 7/8" diameter by 0.188" wall 4130 tube with a 1" 0.065" collar rosette welded (see picture 15 below). The overall length is 3-1/2" with the wire holes 7/16" from the end.
The question is whether cutting the flats on the pin, and reaming the near side hole out to 7/16" would weaken the pin enough to be of concern. After doing a couple back of the envelop calculations, it became clear that a test would be the fastest and easiest way to answer this.
The pin I used for testing was a production reject, but materially identical to the production pins. I cut flats not only on the side for the flying/landing wire nuts, but also cut flats on the other (near) side. The flats cut into the test pin were wider than what would be cut into the production pins. The width of the test pin flats were at least 9/16". The production pins have flats less than 1/2". With flats on both sides instead of just one side, the test pin should be weaker than the production pins. I did not add the 1" collar around the test pin to make it easy to slide in and out of the test fixture. I don't think the collar would make any difference in strength. The near side hole on the pin was reamed to 7/16". The far side hole (nut side) is reamed to the standard 5/16".
The Skybolt spar is 1" thick spruce with 1/4" ply doublers on each side making a total thickness of 1-1/2". The aluminum spar plates are 1/8" 6061T6 that hold the pin in place.
To simulate the spar, I cut two 3/4" x 5" s 9" red oak boards I had laying around and glued them together. I had a couple of spare aluminum spar plate production rejects floating around the scrap bin. I drilled the 7/8" pin hole, along with the 1/4" holes for the spar plate bolts, and bolted the assembly together using 1/4" grade 8 bolts. Torqued the 1/4" bolts to 70 in-lbs.
I ran three tests.
Test 1 is using grade 2 - 5/16" threaded rod. Tensile strength 3900 lbs.
Test 2 is using grade 5 - 5/16" bolts. Tensile strength 6300 lbs.
Test 3 is using grade 8 - 5/16" bolts. Tensile strength 8750 lbs.
For reference, the Burntons AN675AC 5/16" streamline tie-rod wires show a minimal tensile strength of 6900 lbs. However, on the skybolt these are mated at the fuselage side with AN665-46 clevis terminals that are rated at 4600 lbs. The clevis terminals are the weak link so the system should thus have a strength of 4600 lbs each side.
The test using two grade 5 and grade 8 bolts should have a higher ultimate strength than two of the the installed Burton tie-rods and clevises.
For a test fixture, I used some 3/16" square tubing and a 12 ton press. The ultimate load using the grade 8 bolts should be around 9 tons (assuming the bolts are the weak link).
I loaded this up parallel with the simulated spar wood grain. The actual loading would be 30 degrees off this angle. As the spar and plates didn't fail or significantly distort, this worked out fine.
The final results are that the pin is plenty strong. After the three successive tests, including breaking loads of the grade 8 bolts, the pin came out perfectly straight. With a little buffing, it probably could be reused.
The failures were progressively more spectacular. The grade 8 bolt failure was particularly impressive. There was nothing left on the test stand and the shroud got a workout.
I wish I had a video of this to post.My conclusion is that adding a flat to one side of the pin along with reaming the near hole to 7/16" to accommodate the wire angles up to 3.5 degrees does not make the pin the weak link in the system.
I hope this information is informative. (No warranty expressed or implied....
)I'd be interested in comments and feedback.
Ward
Picture 1 - Components for test rig
Picture 2 - Setup for grade 2 threaded rod
Picture 3 - Testing rig
Picture 4 - 12 ton press
Picture 5 - 12 ton press with safety shroud to keep all the bits contained upon failure
Picture 6 - Test 1 - Grade 2 threaded rod test results
Picture 7 - Test 2 - Grade 5 bolt test setup
Picture 8 - Test 2 - Grade 5 bolt test results
Picture 9 - Test 3 - Grade 8 bolt test setup
Picture 10 - Test 4 - Grade 8 test results (note the aluminum compression distortion and slightly elongated hole. Also - yes - the heads of the bolts are ground round - needed to fit through a 1/2" hole on the test fixture because I could only get 5" bolts, not 6" bolts.)
Picture 11 - Pin after all testing (no distortion or damage)
Picture 12 - Other side of the pin after testing
Picture 13 - Pin showing the two flats cut on both sides.
Picture 14 - Flat cutter that worked surprisingly well
Picture 15 - Production pins showing the sleeve and the flats on the single side.
Pucture
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