Tail Wheel Tensioning Spring Trap

//Tail Wheel Tensioning Spring Trap

Tail Wheel Tensioning Spring Trap

In the November 1999 issue of the Flying Wire, there was an article concerning the possible cracking and breakage of the rudder/tail wheel idler arm. When this arm breaks, there is a loss of control of the tail wheel, while full rudder control is maintained. However, the tail wheel tries to turn the Stearman in the direction opposite the broken idler arm. That causes considerable excitement during a landing and consternation in taxiing. There is another situation that occurs all too frequently that has the same result.

Just behind the idler arm there is a tensioning spring in the cable leading to the tail wheel. There is a loose safety cable around the spring so that there will be some semblance of control if the spring breaks (not much though).

Even though the rudder cable and the tail wheel cables are tensioned properly, under certain conditions, the tail wheel cable on one side will go slack and drop down to the point where it contacts the guard plate on the birdcage arch. When that happens, and you do not have the correct tensioning spring installed, the loop on the end of the spring can fall off the clevis, and the spring is unhooked and “out of the loop”.

After the landing excitement is over, you look through the baggage compartment and see the unhooked spring and cable being held only by the safety cable, lying on the bottom of the birdcage.

All too often the judgment is made that the tail wheel cable must not have been tensioned properly. The spring is reattached, the cable tightened up some more, and unfortunately the trap is set again.

Fig.1 - Tail wheel tensioning spring with poorly formed attachment loops and large gap. These are trouble!

Fig.1 – Tail wheel tensioning spring with poorly formed attachment loops and large gap. These are trouble!


Fig.1 - Tail wheel springs per the Boeing drawing Extended attachment loops and minimum gap to the first coil

Fig.1 – Tail wheel springs per the Boeing drawing Extended attachment loops and minimum gap to the first coil

The real problem is the configuration of the spring! There are some aftermarket springs and hardware store substitution springs that are made like the one shown in Figure 1. These springs have the end attachment loop made by bending up and cutting-off, the last coil of the spring. This forms a loop which has a length of about ½ the diameter of the spring. In addition, there is a significant gap between the end of the loop and the first coil of the spring. When the spring and cable go slack, and they always will, the clevis can easily slip out of the attachment loop on the spring.

The correct spring (part # 73-2705) as shown in Figure 2, has specially formed attachment loops with ½ inch straight sides before the circular loop starts. The cut end of the loop has a gap from the first coil of 1/16 inch. Since the loop is so long, the clevis cannot accidentally move down to the bottom of the loop, and even if it did there is no gap for it to slip through. These are the springs that you need to have, and they are available at some Stearman parts suppliers. This one was borrowed from Vintage Aeroplane for the picture.

But, you say, you have 25 pounds of tension in your tail wheel cable, and yours are not going to go slack, and two new springs are going to cost $xxx and all the trouble to put them in. Why do you suppose Boeing put those guard plates on the birdcage arches right under the spring? If you don’t have the guard plates, are the arches kind of chewed up?

You can see for yourself how the cable goes slack by running this test:

  1. With your plane on the ramp, swing the tail sideways to the left until the tail wheel is angled to the right the maximum without breaking the tail wheel loose.
  2. The rudder will be cocked to the right and the right rudder pedal will have moved forward.
  3. Have someone hold the tail wheel at that angle.
  4. Open the baggage compartment and undo the zipper so that you can see the tensioning springs.
  5. Now with the tail wheel being held, have a third person get in the cockpit and start pushing slowly but firmly on the left rudder pedal.
  6. Since the tail wheel can’t move, the left tensioning spring starts to expand, and the right cable starts to go slack. If your friend can hold the tail wheel long enough, the right spring will go slack enough to hit the birdcage and guard plate, and you will be able to see how the clevis could slip out of the attachment loop on the spring.
  7. The holding of the tail wheel simulates what happens in landing or taxiing when the tire is on the ground and you go from one extreme rudder pedal position to the opposite one. The tire and wheel, due to friction on the ground and inertia, can’t follow the rudder pedal reversals, and first one side and then the other of the spring and cable goes slack. This is the most severe with fast rudder reversals, or with little or no forward motion while maneuvering on the ground.

Well enough theory, check your springs, and if they aren’t like the drawing, just change them! Some important information on Pedal Hangers is that the front cockpit pedal hangers are straight, while the rear pedal hangers are curved outward about 3/4 inch. (See picture on upper right) The front, straight ones are becoming hard to find since they were removed by the Dusters and thrown away. (The one shown was hacksawed off sometime in its past.) If for expediency, and availability, rear hangers are put on the front, the pedals will interfere with the brake cylinder when they are in the forward adjustment holes. “Straight in the front” and “curved outboard in the rear.”

While on the subject of front pedal hangers, the rudder cable that goes from the left rudder rack, forward through the large pulley, across the firewall, to a pulley, turnbuckle, and shorter cable and on to the right hand rudder rack has to have a precise length adjustment. It is adjusted so that the forward end of each rudder pedal adjustment rack is 3-3/4 inches from the stop which is mounted to the forward fuselage box beam. This may be accomplished by fabricating two wood blocks 3-3/4 inches long. The front cable is then adjusted until the blocks on each side are held between the rack and the stop. This sets the proper deflection of the rudder and prevents putting excessive force on the rudder control horn. Of course the rear rudder cables have to be loose when this is done, and repositioned and tensioned after the “wood block” measurement. In various references, you will find this dimension to be 3-3/4, 3-11/16, and 3-23/32 inches. I think the differences are negligible. Pick one and make them both the same.

By |2016-11-13T09:32:51+00:00July 26th, 2016|Flying-Wire|Comments Off on Tail Wheel Tensioning Spring Trap

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