As a follow on to the article on landing gear struts in the February issue, we devised a method of determining with pretty good accuracy, how far the gear strut is extending after liftoff.
As the article points out, if the strut is not extending all or most of the way, the cushioning provided by the gear on the next landing will be somewhere between limited and “none.”
Many methods of measuring the extension were considered, but the one chosen was to determine the angle between the two scissor elements at rest on the ground and during flight.
Two main plates were fabricated from 1/8 inch steel that just fit in the bottom cavity of each scissor element. Two keeper plates were fitted on the other side of the scissor and bolted to the main plate to hold it in place. A 5/16 in diameter stainless rod was bent into a circle 5 inches in diameter and about 7 1/2 inches long. One end was threaded about 1/2 inch and mounted to one of the main plates 2 1/2 inches in from the pivot point between the scissors. A 3/8 inch hole was drilled in the other main plate the same distance out. The curved rod then passes freely through the hole in the other plate as the angle between the scissors changes.
It ended up being best to bend the rod, measure the diameter and then position the mounting and hole accordingly rather than trying to get exactly 5 inches for the circle.
Two grommets with 1/4 inch holes work for the tell-tale slider that remembers the maximum excursion of the scissors in both directions.
Mount the plate with the hole in the bottom cavity of the top scissor using the keeper plates and the two bolts.
Slide one grommet on the curved rod, and then feed the rod up through the hole until the rod plate settles into the bottom cavity of the lower scissor. Bolt the bottom plate in place with the two keepers and the bolts.
Before final tightening, adjust the two plates so that the rod is pretty well centered in the hole in the top plate. Slide the second grommet onto the end of the rod. Now jack up the gear leg using the top scissor nut with a piece of wood to protect the nut. Lift it up high enough so that the gear, with some help, is fully extended and the tire is a little off the floor.
Push the top grommet against the upper plate and either make a mark on the rod or measure the distance to the end of the rod. This will show you where the grommet should be after a take-off and landing, with full gear extension. You could take measurements for 1/4, 1/2, and 3/4 extension if you are feeling pessimistic about the flight test.
Now for the test. With the plane at rest on the ground, push the top grommet down against the plate and push the bottom grommet up against the plate. Now take the Stearman around the pattern for a couple of landings, taxi back to the hangar and look at the position of the grommets on the rod. If the top grommet is at the same mark or distance that you had in the calibration step, then you had full strut extension. If it is significantly down from that position, then you are not getting the strut extension that you need for full absorption of landing forces.
When the plane contacts the ground on landing and the strut compresses, it will compress the strut spring beyond that position that it has at rest, giving you a little rebound.
The lower grommet will be moved some distance below the plate, and that will tell you how much you over-compressed the spring in your landing. This will probably be 1/2 to 1 inch. If the lower grommet is smashed, split, or missing, refer back to Tom Lowe’s article on landing techniques. I built this tool because I wanted to know if my landing gear was extending all of the way, and if not by how much.
The last picture shows the actual positions of the grommets after the test landings. The top grommet was within 1/8 inch of the calibration mark, and the bottom grommet was 1/2 inch down from the plate. The three landings were “mild” and that represents about 1 inch of over compression on the spring. This tool is calibrated for the Bendix gear. At full extension, the grommet is 5/16 inches from the end of the rod.
I then switched the tool to the other gear and got the exact same results. The original plan was to make two tools and test both gear at the same time. After making one, I took the easy way out and decided to just switch the tool over.
I’m sure the curved rod could be aluminum as well as the plates. There isn’t any stress on the parts except what it takes to move the grommets. For a member that wants to make this test and doesn’t care to fabricate a tool, I will loan this one for the shipping cost and the “assurance” of getting it back quickly.