The original stock tail wheel strut for the Stearman is a reliable and long lived device when assembled and maintained properly.
It absorbs the energy on the tail wheel when landing, doesn’t feed the energy back in a rebound, and quickly repositions itself in the extended mode if another touchdown is imminent. It uses a hydraulic piston that employs both hydraulic fluid and compressed air.
The seals to maintain the air and oil are five chevrons with a large packing nut to give them the right pressure against the cylinder and piston surfaces.
There is about 1/2 pint of oil in the unit and the remaining air is pressurized to – now get this – approximately 400 PSI.
This is very important when you decide that it is time to disassemble the strut and check on the seals and the condition of the internal surfaces.
- It looks simple
- Remove the snap ring in the packing nut in the top of the cylinder
- Use the spanner wrench that fits in the top of the packing nut and start to unscrew it
You may hear a bit of hissing, and when you reach the last thread, all hell can break loose.
The piston, packing nut, chevron retainers, chevrons, piston head and a some hydraulic fluid will exit the cylinder at a very high velocity, being propelled by 400 PSI compressed air, and maybe more depending on how it was pressurized the last time. If any part of you is in the way when it blows, your next stop will be at the emergency room.
If any part of your Stearman is in the way, your next stop will be at the Airport Lounge to drown your sorrows.
And who cares about the hangar roof? It has happened this way at Chino Airport and several others that I have been told about by people it happened to, or watched it happen.
Before you go anywhere near the strut with a spanner wrench, you first remove the valve cap and depress the valve stem tip until there is no more air coming out.
Hold the strut with the piston up and a rag around the valve stem while you depress the tip. Oil will come spraying out with the air. If air comes out and then dies down, that is a good sign. If you didn’t hear any air, then be cautious and suspicious. Then remove the valve stem.
Then remove the body with the hex flats that holds the valve stem. All of these steps need to be done with the valve pointed in a safe direction (like a loaded gun.)
If you hear air coming out in any of these steps, it is a good sign. Now you should be able to push the piston into the cylinder, which will insure that either there is no more pressure inside, or that the parts are so corroded that they won’t move. One accident of this kind happened to a very knowledgeable Stearman owner, who is absolutely sure that he removed the valve stem body before unscrewing the packing nut.
He made the emergency room trip. So there is the possibility that some sludge or debris on the inside of the cylinder could result in a blockage that would prevent the compressed air from exiting when the valve stem is depressed or when the valve body is removed. Scary, huh!!
When it is finally deemed time to remove the packing nut, continue to treat it like a loaded gun, and point it in a direction that will not do anyone or anything harm when it turns out that there was still a few hundred PSI inside.
In the old days, the strut was pressurized with a special hand pump tire pump, but it is far from that. It consists of three nested cylinders that operate in three pressure stages to produce the 400 PSI and more. It takes a lot of pumping, but given enough endurance, it can be done. Now days, one more often used a compressed nitrogen tank with the proper regulator. This method is a lot easier, but it is also easier to get a lot of over pressure into the cylinder and maybe not be aware of it.
The amount of pressure in the strut that is correct is measured by the distance that the piston is extending out of the cylinder with normal load in the plane and at a three point attitude.
The Navy is clear on this and says the distance from the center of the Schrader valve to the face of the packing nut should be 3 and 5/8 inches.
The fact that you have a boot on the strut to keep the shaft clean (which you should) makes measuring to the face of the packing nut, to the nearest 1/4 of an inch, a real challenge. This is while having a friend sit in the pilot’s seat and with 60 pounds in the luggage compartment. So you start adding pressure with the hand pump or the nitrogen bottle-slowly (Not a problem with the hand pump.)
After a while, the tail isn’t raising and generally nothing is happening. So you go around and give some tugs and jiggles on the rear fuselage handle. Suddenly the piston goes to full extension with a bang and the tail jumps up. Not to worry, just let a little air out and come down to the mark. After letting some air out in several small spurts, the piston suddenly goes all the way in and the tail drops. Start over.
The way I do it, which isn’t found in any of the manuals, is to put a small ladder by the left rear fuselage handle. Another friend stands with one foot on the ladder and the other in the black fuselage handle. As I am pressuring up, when I ask, he shifts his weight to the foot in the handle and bounces up and down a few times.
We continue this act until the piston is out about the 3 ½ inches and stays there after the bouncing. It also shows that when he puts his weight on the handle, the piston compresses a bit, and returns when he removes his weight.
This is where I finally invoke the “that’s good enough” principle. You have to use a new copper washer under the valve body and a high pressure valve stem. That 400 PSI is really good at finding leaks.
Check the valve and body and at the junction of the shaft and the packing nut with a soap solution to check for leaks.
If there are leaks around the shaft and packing nut, and the chevrons and surfaces are good, then you can remove the snap ring and tighten the packing nut a 1/4 of a turn with the spanner wrench. Put the snap ring back in.
The POGO Stick Mod
How about just putting a large spring inside the housing and forget about hydraulic oil and pressurizing. This was a Duster expediency and gave them one less thing to worry about. Well it destroys the whole design principle of the strut to absorb energy and limit rebound, chews up the inside of the housing, and is a terrible thing to do to an original tail strut. Unless the inside of the housing or the shaft is so damaged that it won’t hold oil or air pressure. Then you could add the spring while you look for a good or repairable original style strut.
Since the spring does not absorb energy on a landing, it just stores it momentarily and then feeds it back to produce the Pogo Action.
With the proper smooth finish on the shaft and the inside of the housing, new chevron seals can result in the strut holding oil and air pressure for many years.
The top two inches of the housing are the most important for sealing on the outside diameter of the chevrons. The seals do not move on this surface. The movement is between the inside of the chevrons and the shaft.
The protective boot on the top of the strut is important and will keep the shaft surface clean.
There is a good manual available from Dusters and Sprayers giving step-by -step directions for overhauling the strut.