During the summe of 2010, my homebuilt, Melmoth 2, which had been flying for almost eight years, began to display a mystifying symptom. As I turned off the runway after landing, the main landing gear oleo on the outside of the turn would seem to collapse. The wing on that side would slump toward the ground, and because Melmoth’s wings have relatively little dihedral and are completely wet from root to tip, fuel would sometimes even dribble out of the tank vent at the wingtip. Once or twice the local controller at my home field told me that I had a flat tire, but after seeing me taxi off listing drunkenly to starboard a few times, he must have decided that nobody could have that many flat tires.
But here was the bizarre thing: As I continued to taxi, the strut would slowly return to its normal height.
When I returned to my hangar I would bounce the wings up and down. The struts felt and looked normal, and would settle at their proper extension. There was no sign of leakage, either of hydraulic fluid or of air.
Now, an oleo strut — at least the kind of strut likely to be found on a light airplane — is a very simple thing. It should not harbor mysteries. It consists, basically, of a cylindrical chamber — the visible external shell of the strut, inside which a piston (the chrome-plated shaft to which the fork and tire are attached) slides up and down — sealed by an O-ring.
Within the cylinder is a quantity of hydraulic oil and some empty space filled with air. As the piston moves up into the cylinder, it displaces the hydraulic oil and reduces the volume of the air. The oil is incompressible, but the air is not, and it acts as a spring. The air pressure in the cylinder at rest is whatever is required to support the airplane. In the case of my struts, the piston is 1.75 inches in diameter and each main gear leg supports a static load of about 600 pounds. The cross-sectional area of the piston being 2.4 square inches, the static pressure in the strut has to be 600/2.4, or 250 pounds per square inch.
If you make a very hard landing, the piston is driven far up into the cylinder and the air is squeezed to a much higher pressure. The piston then rebounds, and might toss the airplane back into the air were it not that the displaced hydraulic fluid has to pass, in both directions, through a small hole. This orifice, as it is called, produces frictional resistance, so that the piston’s movement is damped: It does not bounce back with as much energy as drove it into the cylinder in the first place.
Now, taxiing does not subject an oleo strut to large loads, and a gentle turn off a runway doesn’t either. So why was my strut first compressing and then gradually re-extending itself? In fact, how could it overcompress at all, since in order to compress it would be fighting against ever-increasing air pressure? It seemed physically impossible, and yet there it was — it happened again and again. And it happened only just after landing; it would never happen while taxiing before takeoff, and would never happen a second time once the strut had regained its proper height.
I discussed this conundrum with a number of people who had extensive aviation experience, including pilots, mechanics and engineers. One of them had even designed oleo struts professionally and had written quite a complex spreadsheet for oleo sizing. Nobody could explain what might be going on. Eventually, I took the right main strut out of the airplane and dismantled it, expecting that I would find something highly peculiar inside. Instead I found nothing unusual, but I did realize something about my struts that I had forgotten.