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I have 34 hours on the plane so and I decided it was time to check to make sure my wires were still up to the tensions I set them to. I have been noticing at 4.5 to 5 G that my landing wires were slacking a bit and I also noticed it in snaps. As most of you know who have rigged a pitts it is impossible due to differences in geometry of the flying wires to get landing wires to the same tension as the flying wires. So therefore I had to set my landing wires toward the top end of the 750 lb that pitts manual calls for and therefore my flying wires were some where toward the bottom at 650 lb or so. Anyway I thought it would be a good time to check a fish scale formula I that got off the Akro Exploder years ago that you use with a fish scale. http://acro.aerobaticsweb.org/threads/1020283/msg000002.html
Edit: (I had it backwards it was late last night the equation was corrected Thanks Tom!)
Tension = (P*S)/(4*D)
Where:
D = Pulling Distance (in)
P = Force (lb)
S = Pin to Pin Wire length (in)
T = Tension in (lb)
As it turns out it seems to match my tensiometer from Holloway engineering that uses a torque wrench. As you may wondering why I seem surprised by this I have a few issues with the way that this formula has been derived. The most important is that this formula makes the bold assumption that bending strength of the wire is negligible when compared to the overall tension of the wire. This may be fairly true for small flying wires that most of use use with round part being 3/16, 1/4 and 5/16" I would not use this formula for the drag wires as I feel the 3/16 round wire size will skew the actual tension value quite a lot. Also it seems you have to pull 1" for this formula to work for the 1/4" wires.
Also as listed in my S1-11 drawing there is a note that says pull the center of the wire to 50 lb it should deflect to 0.875 +/- 0.063 Using the above formula and the landing and flying wire length on the S1-11 I get 780 for the flying and 880 for the landing which is right in there for the 750-1000 lb range for the wires. So that checks out in the ol' sniff test if you will.
At any rate the reason why I'm posting is that for my Pitts S-1 with 1/4" flying wires I compared the value I get using my supposedly calibrated Holloway Engineering Flying Wire Tensiometer (This actually has calibration tables for each sized wire it does change quite a lot) I get very similar and comparable results. So for those that don't want to spend the money you can measure your pin center wire length and get a $20 scale a ruler and be done with it. That said I bought my wire tensiometer years ago and haven't regretted it yet. It is expensive I know but It does make the job very easy as compared to the fish scale.
EDIT: By the way I do trust the guy who did the statics to get the equation. However, after I saw the derivation I just had to see it in action so I thought it a good re-post on the forum with some new data to go along.
Cheers!
Edit: (I had it backwards it was late last night the equation was corrected Thanks Tom!)
Tension = (P*S)/(4*D)
Where:
D = Pulling Distance (in)
P = Force (lb)
S = Pin to Pin Wire length (in)
T = Tension in (lb)
As it turns out it seems to match my tensiometer from Holloway engineering that uses a torque wrench. As you may wondering why I seem surprised by this I have a few issues with the way that this formula has been derived. The most important is that this formula makes the bold assumption that bending strength of the wire is negligible when compared to the overall tension of the wire. This may be fairly true for small flying wires that most of use use with round part being 3/16, 1/4 and 5/16" I would not use this formula for the drag wires as I feel the 3/16 round wire size will skew the actual tension value quite a lot. Also it seems you have to pull 1" for this formula to work for the 1/4" wires.
Also as listed in my S1-11 drawing there is a note that says pull the center of the wire to 50 lb it should deflect to 0.875 +/- 0.063 Using the above formula and the landing and flying wire length on the S1-11 I get 780 for the flying and 880 for the landing which is right in there for the 750-1000 lb range for the wires. So that checks out in the ol' sniff test if you will.
At any rate the reason why I'm posting is that for my Pitts S-1 with 1/4" flying wires I compared the value I get using my supposedly calibrated Holloway Engineering Flying Wire Tensiometer (This actually has calibration tables for each sized wire it does change quite a lot) I get very similar and comparable results. So for those that don't want to spend the money you can measure your pin center wire length and get a $20 scale a ruler and be done with it. That said I bought my wire tensiometer years ago and haven't regretted it yet. It is expensive I know but It does make the job very easy as compared to the fish scale.
EDIT: By the way I do trust the guy who did the statics to get the equation. However, after I saw the derivation I just had to see it in action so I thought it a good re-post on the forum with some new data to go along.
Cheers!
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