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A little about the engine control fit issue. The reason a lot of the engine controls on the Skymaster get stiff to work is that the original control conduit are made with long lay conduit (many wires twisted in a slow spiral) or a square flattened wire conduit with a armored inner wire. They work well when new, but when they age from many heating and cooling cycles the outer conduit wire stress slowly collapses the inner plastic liner onto the inner wire. This only happens in the control bends. The results of many bends is a lot of friction in overall control operation. The McFarlane controls have a conduit that is made of a two wire conduit with a relatively short lay (tighter spiral). The inner conduit size will not change in the bends as it ages and as a result it will not get hard to work as it ages, but the bend clearance presents its own set of issues.
Ok lets talk about travel problems. As a push pull control is makes a bend, there is a difference between the radius of the outer conduit and the radius of the inner push-pull wire. This difference effects the travel available in the control assembly. When the side clearance of the inner wire in the conduit is greater the difference of the two radii also increases. This difference also increases the effect on overall control travel available. In summary the more free fit you have in the conduit bends the greater the loss of travel. This travel loss is accumulated in proportion to the number of bends and the degree of the bends. When building a control without the airplane, it is very hard to predict the control bends used in the routing. The advantage of the original long lay conduit is that the inside hole of the conduit gets smaller in the bends. This restriction of the conduit inner hole size in the bends limits the loss of travel caused by bending the control. The number of bends in the control routing have only a small effect on the amount of travel available making this type conduit easier designer friendly. With the McFarlane control, some travel is lost in each bend do to inner wire free running side clearance. (This is what gives us our ageless smooth operation). This loss of travel must be compensated for by adding inner wire length. The amount to add depends on the number of bends and the degree of bends in the routing. We have gotten better at estimating the amount of travel compensation to give the 337 controls, but there are different routings for the different models and the routing has been modified on some airplanes. If we give to much compensation for bend loss, we run the danger of improper overlap of the telescoping control ends and the push rods being to long to fit in the space allowed. The problem is exacerbated by the sheer length of the rear engine controls and many bends required. We collect data for each one we build to try and perfect the fits but it is a continuing challenge due to model and airplane variations. It is an engineering challenge with some trial and error learning. McFarlane can not simply match the original and make it work, but we have perfected it for many models. We recommend that you measure the travel used by your working control on the airplane before you remove it. Before you install a McFarlane control, simulate the routing bends required (it might take several people) and then check to see that there is enough travel available. If the travel does not match the old control, return the control and we will re-make it with a different travel allowance. It is important that all bends are simulated. This process will prevent the hours of labor required to uninstall the control assembly.
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