Maintenance Matters

Tailwheel maintenance and setup.

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This is an Alaskan Bushwheels version of the Scott 3200. Note the raised ears on the steering arm. This part is a drop-in replacement for the standard steering arm and may be helpful when there is a steep angle on the steering cable.

The Scott 3200 tailwheel and the copy now made by Airframes Alaska under their Alaskan Bushwheels brand provide the rolling part at the rear end of many light plane taildraggers. So, with apologies to Lang, Matco, and others that ably compete for this business, we will focus on the 3200-type tailwheel here.

A Scott 3200 tailwheel assembly taken apart, cleaned, and ready to put back together. Alaskan Bushwheels now makes the same tailwheel under its own name, and the parts are interchangeable.

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Common Problems

The most frequent complaint I hear about tailwheels is shimmy. This isn’t so much a maintenance problem as a setup problem, but a sagging tailwheel spring can lead to shimmy problems as a maintenance issue. Other than that, the bolt that holds the tailwheel assembly to the spring can fail due to excessive wear. In fact, most active bush pilots will replace that bolt yearly as a precautionary measure. Flyers who place fewer demands on their plane should include the inspection of the bolt in their yearly condition inspection routine.

Another problem you may encounter is bent arms where the steering cables attach to the tailwheel. This is caused from improper setup and is most likely when using compression springs such as you commonly find on Maules. If the steering cable is too short, it will either break the links that tie it to the steering arms or bend the arms.

Of course, tire wear is a recurring problem, as the small tailwheel tires do not seem to enjoy a very long life. Making sure you do not allow any shimmy problems to develop will go a long way toward getting the maximum life out of your tailwheel tire.

Let’s look at each of these in turn and then consider routine maintenance.

An exploded view of all the parts in a Scott 3200/Alaskan Bushwheels tailwheel. Alaskan Bushwheels sells a kit that has replacements for all the typical wear parts, except for the wheel bearings and races. Those are sold separately.

Shimmy

Worn parts can lead to shimmy, but the main cause is an incorrect angle of the kingpin such that a line downward through it will point toward the rear of the plane. This will force the tail of the plane to move up and down when the tailwheel turns from one side to the other.

The angle between a line through the kingpin and the vertical is called caster or caster angle. In the case of a tailwheel, if the line through the kingpin when projected to the ground comes out ahead of the vertical line through the steering head then the caster is said to be positive. This is what you need to have to avoid tailwheel shimmy. If the line through the kingpin hits the ground aft of vertical, caster is negative, which is not good.

This angle should be checked in the normal resting position and in the loaded position where the maximum safe weight is on the tailwheel. Load your plane up to the maximum safe rearward CG loading to achieve this. You can then push down on the tail to move it a bit farther, if you can do this without damaging the plane. Be careful with this. Watch the angle of the bolt when it is on the maximum load condition. A downward line through the bolt should remain pointing forward, or at worst vertical. If it doesn’t, you can install a wedge between the spring and the tailwheel assembly to restore the proper angle. This may require a longer attach bolt.

If this angle is correct, you should not have shimmy problems. If it persists, look for badly worn parts in the tailwheel assembly and replace as necessary. A sagging spring can cause the angle of the kingpin bolt to change over time, leading to shimmy. Sometimes a spring can be restored to the correct angle by a spring shop, but don’t count on it. Heating it up with a torch and bending it will only weaken it further. It is usually better to replace the spring.

Use a brass punch to push out the bearing and seal on the opposite side of the tailwheel. If you let the bearing push the seal out, you are less likely to damage the seal.

This bearing and race from the tailwheel need to be replaced due to rust on both parts. Regular servicing, including periodic greasing, will help prevent this.

Steering Cable Setup

There is more to installing the tailwheel steering cables than merely taking out the slack and swaging on some thimbles. Here is the proper procedure: Lift the tailwheel clear of the ground and turn it to one side until it reaches the release point. Turn the rudder in the same direction until it stops. Adjust the length of the chain until the slack is just barely taken out of the connection. If you are in between links as you take up the slack, err on the side of looseness. There should be very little to no tension in the spring at this point. Do the same in the other direction for the other side. The springs may be slack when everything is returned to center, but so be it. Especially if you are using Maule-type compression springs, it is important not to get things too tight. The arc of the tailwheel as it goes through its travel will likely tighten the springs at some point. If they are too tight they can bend the steering arms, something I have verified through my own personal experience.

If you have a steep angle coming into the tailwheel, you might want to use a steering arm with the tabs bent up on the ends. This is common when you have a short distance between the steering arm and the rudder horns, such as you see on most Cubs. This part is available from Alaskan Bushwheels.

A one-inch socket works well to press the wheel bearing race into the tailwheel. This is preferable to using a punch and hammer because it is less likely to damage the aluminum wheel.

The old-style wheel bearing seal is shown center top. To its left is a spacer ring that goes in after the bearing race but before the seal. If you use the new-style seal, shown on the lower right, you do not use the spacer ring.

Servicing the Tailwheel Tire and Wheel

To change the tire on your tailwheel airplane, first remove the bolt that goes through the tailwheel. Remove the cotter pin and use a -inch wrench to remove the nut. The bolt should come out with minimal effort. You will need a new cotter pin later, but be sure to save the locking tab near the head of the bolt. With the wheel removed let the air out of the tire by removing the valve stem. You will need a valve cap with a remover on it or a valve stem tool. These are available at any auto parts store. Once the tire is deflated you can unbolt the wheel halves. Discard the worn tire and tube. The tire cannot be recapped, and you should not reuse an old tube.

With the wheel apart use a brass drift punch to push out the bearing and seal on each side. Insert the punch from the opposite side and gently tap on the bearing to push out the seal and bearing. The seal can be reused if it is not damaged, but it should be replaced if there is any sign of damage or wear. Clean the bearings with a low-flammability solvent such as Stoddard solvent and inspect the bearings and races. Replace any bearing and race that shows signs of pitting or rust.

Dropped arms on the rudder of this Champ work well with the standard steering arm of this tailwheel. Note the grease fittings on the top of the kingpin and the axle bolt. They should get a few squirts of grease every 50 to 100 hours, depending on service conditions.

If you need to replace a bearing race, the best way to do it is to heat the wheel up with a propane torch and then strike it on a hard surface, bearing side down, to get the race to move away from its seat in the wheel. Once there is some clearance, you can use a punch to remove the bearing the rest of the way. In some cases, you may be able to just catch the edge of the bearing race with a punch and remove it without heating the wheel. Be careful not to damage the wheel or get the race cocked as it comes out. The new race can be installed with a press or large vise. Use a one-inch socket or other properly sized object to push the new race into place. With the race seated in the wheel, pack the bearing with good-quality grease and install it with the seal. Alaskan Bushwheels likes Mobil SHC 100 grease, but AeroShell 22 is another good choice. Once in service, the tailwheel assembly should be greased through the zerk fitting every 50-100 hours, depending on operating conditions. Do not over-grease to the point that grease squirts out the sides of the assembly.

Once the bearings are squared away and the wheel is reassembled, it is time to mount the new tire. Make sure there is nothing inside the tire, then insert the tube with a little tire talc. Be sure there are no wrinkles in the tube. Assemble the wheel halves around the new tire and bolt them together. Inflate the tire without the valve stem to seat the tube, then insert the valve stem and inflate the tire to 45 psi.

If you are ready to remount the tire, place the spacer into the seal on either side and slide the wheel into place in the tailwheel assembly. Insert the through bolt with the locking tab, then properly tighten it all up with the washer and castle nut on the other end. The nut should be tightened just to take the side play out of the wheel in the fork and enough to seat the bearings and seals. The wheel should turn freely but not have any looseness in the fork. With that done insert a new cotter pin and you are finished.

his is the pawl that slips into the slot on the steering arm. Note that the half-round pin is longer on one side than the other. If you install it upside down it won’t work. If you wish to have the tailwheel swing farther before it releases, a longer pawl is available from Alaskan Bushwheels.

When reassembling the steering arm, insert the pawl so that the top of the half-round pin ends up flush with the top face of the steering arm as shown here.

Servicing the Tailwheel Assembly

There are a number of parts sandwiched between the tailwheel head and the fork. They need to be inspected and greased annually, or possibly more often if operating under harsh conditions. The parts need to be disassembled, cleaned, greased, and reassembled at this time, with worn parts replaced as needed. Alaskan Bushwheels makes a rebuild kit that includes all wear items except the wheel bearings and races.

When you have completely assembled the tailwheel mechanism, the last step is to tighten the kingpin nut. Tighten it until it firmly bottoms out, then back off two slots before installing the cotter pin.

It is easier to service the tailwheel assembly if you first remove it from the plane. You should do this anyway to inspect the bolt that secures it to the spring. You should also remove the tire and wheel from the fork. Disassemble the fork and head by removing the nut from the end of the kingpin. The entire assembly will come apart with this nut removed. Be careful to not lose the little springs that go on the thrust plate. In most cases there will be three of them, but sometimes five. Clean and inspect all of the parts, replacing any worn or broken parts. The rebuild kit is a good way to go if there are signs of significant wear or damage. Be sure to grease the parts as you assemble them.

This Scott 3200 tailwheel has been serviced and is ready to go back on the airplane. Be sure to check the condition of the attaching bolt before you reinstall the tailwheel.

Once you have reassembled everything, tighten the nut down until everything seats into place. Then back off one or two slots in the nut and install the cotter pin. The fork and head should move without binding, but should not have any extra play that would allow the fork to rock from side to side.

A properly set up and maintained tailwheel assembly should give you years of trouble-free service. Keep an eye on your tailwheel tire pressure and condition, perform your required annual maintenance, and monitor the kingpin angle, and you should not have trouble with your tailwheel.

Check tailwheel cable tension by deflecting the rudder to its stop and by moving the tailwheel to its release point in the same direction. There should be only the very slightest tension in the cable on the opposite side (right, as shown here). This lack of tension is especially critical if you are using Maule-type compression springs. When in doubt, make it looser.


Dave Prizio has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed three—a GlaStar, a Glasair Sportsman, and a Texas Sport Cub—and is helping a friend build an RV-8. When he isn’t building something, he shares his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also an A&P mechanic, Designated Airworthiness Representative (DAR), and a member of the EAA Homebuilt Aircraft Council.

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Dave Prizio
Dave Prizio has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed three—a GlaStar, a Glasair Sportsman, and a Texas Sport Cub—and is helping a friend build an RV-8. When he isn’t building something, he shares his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also an A&P mechanic, Designated Airworthiness Representative (DAR), and was a member of the EAA Homebuilt Aircraft Council for six years.

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