General Functioning of the gear strut This Oleo-spring Aerol is designed to perform the following necessary functions during the landing of the airplane:
- Absorption of Impact Energy. The impact energy of the landing airplane is absorbed by the Aerol Fluid and by the compression of the spring during the compression stroke.
- Absorption of Taxiing Loads. The loads developed In the Aerol while the airplane is taxiing are carried mainly by the spring.
- Snubbing. Snubbing effect, which is necessary to prevent too-quick rebounds, is produced by forcing the Aerol Fluid to pass through restrictions on the extension stroke of the Aero.
Details of Operation This Aerol is of the cantilever upright form falling Into Aerol Type X. The cantilever form of Aerol sustains all ground loads to which the landing gear is subjected. Type X indicates that this Aerol is an example of the Spring/Oil Type, the functioning of which is much like Oleo-Pneumatic Aerols (the air-oil types) except that the compressed air in the Oleo-Pneumatic types used to support the weight of the airplane is replaced by a helically coiled steel compression spring. The major internal combination of parts that makes this Aerol different from others of its type consists of the Orifice Plate, the Packing Gland, the Metering Pin, the Oil Relief Valve, and the Helical Spring.
Compression Stroke Compression stroke of the Aerol is the initial movement and occurs when the impact load of the landing airplane is applied to the landing gear. The lower or inner cylinder moves further into outer cylinder, thus reducing the volume of the lower chamber.
The Oil Relief Valve is closed instantly by the upward pressure of the moving fluid.
The fluid leaves the lower chamber by passing through the Orifice. The flow of fluid through the Orifice is automatically controlled at all points of compression and expansion by the variable section of the Metering Pin which projects through the Orifice.
This Metering Pin allows the load developed in the Aerol to be controlled throughout the compression stroke in such a way that the greatest possible amount of energy may be dissipated.
As the compression stroke continues, the fluid level continues to rise above the Orifice Plate, in the top chamber . Since the natural extension of the Helical Spring is existent when the Aerol is in the static position, compression beyond this position creates mounting resistance to the telescoping motion.
Compression of the Aerol stops when the entire Impact load of the landing airplane has been absorbed by the fluid and by the compression of the Helical Spring.
Extension Stroke Extension of the Aerol takes place after a sufficient compression stroke has been made to absorb the impact energy of the landing airplane. As the impact load decreases, the Helical Spring which has been compressed beyond its static position expands causing the Aerol to extend itself. The extension of the Aerol Increases the volume of that part of the chamber below the Orifice Plate in the Piston. This forces the fluid above the Orifice Plate to return through the Orifice Plate into the chamber below the Orifice Plate.
The Oil Relief Valve is opened instantly, allowing the fluid in upper chamber to more quickly flow into the lower chamber.
Quick and easy return of the fluid is necessary to take full advantage of the spring action in extending the Aerol.
This is explained by the fact that the Spring extends itself to only the static position of the Aerol, and in order to fully extend the Aerol for the next shock load, the initial momentum of the spring must be fully utilized in addition to the force of gravity, which tends to pull the Aerol to the extended position.
This fluid movement continues until the energy of the expanding Spring which is tending to thrust the Aerol downward equalizes the weight of the plane and the ground forces thrusting upward. The Piston then comes to rest in what is known as the “Static Position.” Of course, it is understood that when an airplane lands, oscillations consisting of compression and extension strokes reoccur with diminishing Intensity until the airplane comes to rest.
OK, what does all of this mean?
When the plane is sitting on the ground, the strut is compressed to the point where it is in contact with the heavy helical spring and has compressed the spring about 1 3/4 inches to match the weight of the plane.
The fluid has filled the upper chamber above the orifice plate, just up to the fill hole.
When taxiing, the plane is riding on the heavy helical springs. When taking off, on lift-off, the helical spring pushes down on the strut for about 1 3/4 inches, and then gravity takes over to move the strut all the way down to its full extension (about 4 inches of movement). At the same time the oil relief valve on the orifice plate opens and allows the oil to drop into the lower chamber.This condition remains until it is time to land.
On landing, the load on the wheels forces the strut up. The fluid tries to move up above the orifice plate and in doing so closes the oil relief valve. Now the oil has to move up through the orifice where the profiled metering pin is blocking part of the opening.
This restriction of the oil movement acts as a shock absorber and cushions the landing force. After about 2 1/4 inches of this movement, the strut comes into contact with the heavy helical spring, which adds a lot more cushioning force.
In a hard landing, the spring will be compressed beyond its static at rest length, and it will then push the gear back down with some decreasing oscillations.
If the plane bounces back into the air and unloads the wheels, the spring pushes the strut down, the oil relief valve opens allowing the oil to move freely into the bottom chamber, and gravity finishes the job of pulling the strut all the way down to get ready for the next contact with the runway. When the plane comes to equilibrium on the roll out, the spring is now once again compressed about 1/3/4 inches, the oil is in the top chamber, and the oil relief valve has dropped down in the open position.
OK, so far so good.
Now if you are one of those Stearman owners who got tired of the seals leaking and just stopped refilling the gear legs, you are running the strut essentially dry. This means that the only cushioning you have during landing is the helical spring which starts working only during the last 1 3/4 inches of strut compression. The first 2 1/4 inches, you don’t get any help, and the spring only gives you “springy” cushioning and no energy dissipation. So if your gear legs have fluid all the way up to the fill hole with the plane at rest on the gear, everything should be all right? Maybe not!!
In order for the gear to be ready to provide the proper help in landing, the strut has to extend to its full length at sometime after takeoff. Preferably just at liftoff.
If the seals have too much friction, or everything is gunked up inside, the gear may not extend more that the 1 3/4 inches that the spring pushes it down – if that.
Tightening up on the brass gland nut to try to stop the leaking, can put so much expanding pressure on the chevrons that they can keep the strut from moving or at least hinder it. How does one know or find out if the gear is extending? Tough to do, and you can’t see it in the air.
An observer in another plane can only give some rough idea as to the extended length.
One way it can be done is to jack up one side of the gear with the jack under the top outside scissors nut, using a piece of wood to protect the nut. Be sure and chock the other wheel, and make sure nothing is under the other wing. First rock the wings to be sure that both gears are settled at rest. Then measure the length of the strut that is showing below the brass gland nut. For the Bendix gear, this will be about 3 1/4 inches to the first weld on the bottom of the strut leg.
Now jack up the top of the scissors about 4 to 5 inches. When the strut extends its full length, you will hear a slight metal to metal contact as it bottoms out.
If the strut stops extending after about an inch and a half (that is when the coil spring stops pushing it out), then there may be a problem. Push on top of the tire and see if that brings it out further. It will measure about 7 1/4 inches from the nut when it bottoms out with the tire off the ground. Try sitting on the tire. If it takes that much or that doesn’t do it, there is definitely a problem. You would have to do some aggressive aerobatics with “g” loading to ever get your gear down. Sometimes a “clunk” is heard in the pull up portion of a loop. That is the gear extending all the way. If it isn’t coming down, you are probably getting help from only the spring on landing.
Requesting permission from the tower to do a loop on downwind to get the gear down on a Stearman is often met with some puzzlement and resistance, and as such has suffered a decline in popularity.
Something inside the gear leg is keeping the strut from dropping down under the gravitational force, and it is probably the seals. (See Changing the Seals article).
If the gear comes down with a hand push, it is up to you to decide if that will suffice during a flight. On actual lift-off, the coil spring gives the strut a shove which may keep it going to the full extension. You don’t get that with the jacking test. However, if the seals are sealing, the strut tries to pull a low pressure on top, and that also acts as a resistance to the gear coming down.
We are trying to come up with a method of monitoring the gear extension with a test device, which would give better information. Any ideas?
Another measurement that will tell the story is the distance from the inside of the center scissor pivot to the strut. When at rest on the ground, this distance is 4 7/8 inches. When the strut is fully extended, the distance is 3 1/4 inches.
It is comforting to know that those teeth jarring landings may not be the fault of the pilot at all. At least that’s my story and I’m sticking to it.