ORDERING FLYING WIRES
by
JOSEPH R. MARIDON, P.E. (Reprinted without permission)
The recent experience of a friend of mine ordering flying wires for his antique airplane restoration project has prompted this article. If you look up the catalog information on flying and brace wires, two lengths are given for each nominal wire diameter. The first is the wire length without the terminal end fittings. The second dimension is the pin center length which is the dimension from the center to center of the terminal pin holes. This dimension, pin center to pin center, is the one that must match the aircraft fitting center to center of the mounting holes for the flying wire. That all seems simple enough. However, what is represented by the catalog pin center to center length of the flying wire? Is it the maximum length the assembly of the wire and terminals can be adjusted to or is it the minimum length the assembly can be adjusted to or something in between?
One current catalog says the pin center length measurement is taken with the terminal screwed half in and half out so there is equal takeup and let out. This statement conflicts with the data provided and measurements taken on a 5/16" wire. The catalog "K" dimension is 2-1/8" for this size wire with "K" being the difference between the rod length and the pin center length. The distance from the inspection hole in the terminal and the pin hole is 1-1/4". If the terminals are installed on the 5/16" wire just to the inspection hole the pin centers will be 2-1/2" longer that the wire. This means the let out amount is only 3/8". This suggests that the catalog pin center dimension is close to the maximum length for the assembly.
New wires have considerable adjustment. For example a 5/16" wire will have 2 inches of adjustment. Plus the right hand threaded end will have 1/2" more of thread length than the other end. This allows for cutting this end to further shorten the wire if necessary. What you want to end up with is an assembly that when installed it is near the mid point of its adjustment range. There are several reasons for this. If the wires are installed on exposed fitting such as on a Stearman, the installation will be more aesthetically pleasing by reducing the exposed threaded portion of the wires. Some aircraft such as a Waco UPF-7 have the fitting inside the wings so only the streamlined portion of the wire is exposed. In the case of a major repair or rebuild in the future there will be some adjustment for manufacturing tolerances of new or rebuilt parts.
If catalog data detailed the adjustment capability of the wires, this would be a simple process but such is not the case. My recommendation is to obtain all the detail dimensions of the terminal end fittings, have the supplier give you the length of the threads on each end of the wire and make your own detail layout of maximum and minimum lengths, determine what wire length will give you the required pin center length to match the aircraft fittings at the mid point of the adjustment range. Remember that the terminal must be screwed on the wire at least past the inspection hole to insure minimum thread engagement. Also, there must be sufficient thread when the terminal is screwed on to the minimum length to accommodate the lock nut. Then order the wire by wire length not pin center length. Be sure the supplier has no doubt about what you are specifying. A sketch of each wire showing this dimension would be useful. A picture is worth a thousand words.
These wires are expensive, long delivery, custom made and not returnable. Be sure you have all the correct information before committing to an order.
by
JOSEPH R. MARIDON, P.E. (Reprinted without permission)
The recent experience of a friend of mine ordering flying wires for his antique airplane restoration project has prompted this article. If you look up the catalog information on flying and brace wires, two lengths are given for each nominal wire diameter. The first is the wire length without the terminal end fittings. The second dimension is the pin center length which is the dimension from the center to center of the terminal pin holes. This dimension, pin center to pin center, is the one that must match the aircraft fitting center to center of the mounting holes for the flying wire. That all seems simple enough. However, what is represented by the catalog pin center to center length of the flying wire? Is it the maximum length the assembly of the wire and terminals can be adjusted to or is it the minimum length the assembly can be adjusted to or something in between?
One current catalog says the pin center length measurement is taken with the terminal screwed half in and half out so there is equal takeup and let out. This statement conflicts with the data provided and measurements taken on a 5/16" wire. The catalog "K" dimension is 2-1/8" for this size wire with "K" being the difference between the rod length and the pin center length. The distance from the inspection hole in the terminal and the pin hole is 1-1/4". If the terminals are installed on the 5/16" wire just to the inspection hole the pin centers will be 2-1/2" longer that the wire. This means the let out amount is only 3/8". This suggests that the catalog pin center dimension is close to the maximum length for the assembly.
New wires have considerable adjustment. For example a 5/16" wire will have 2 inches of adjustment. Plus the right hand threaded end will have 1/2" more of thread length than the other end. This allows for cutting this end to further shorten the wire if necessary. What you want to end up with is an assembly that when installed it is near the mid point of its adjustment range. There are several reasons for this. If the wires are installed on exposed fitting such as on a Stearman, the installation will be more aesthetically pleasing by reducing the exposed threaded portion of the wires. Some aircraft such as a Waco UPF-7 have the fitting inside the wings so only the streamlined portion of the wire is exposed. In the case of a major repair or rebuild in the future there will be some adjustment for manufacturing tolerances of new or rebuilt parts.
If catalog data detailed the adjustment capability of the wires, this would be a simple process but such is not the case. My recommendation is to obtain all the detail dimensions of the terminal end fittings, have the supplier give you the length of the threads on each end of the wire and make your own detail layout of maximum and minimum lengths, determine what wire length will give you the required pin center length to match the aircraft fittings at the mid point of the adjustment range. Remember that the terminal must be screwed on the wire at least past the inspection hole to insure minimum thread engagement. Also, there must be sufficient thread when the terminal is screwed on to the minimum length to accommodate the lock nut. Then order the wire by wire length not pin center length. Be sure the supplier has no doubt about what you are specifying. A sketch of each wire showing this dimension would be useful. A picture is worth a thousand words.
These wires are expensive, long delivery, custom made and not returnable. Be sure you have all the correct information before committing to an order.
