|bonded gear legs|
Pultrusions are similar to extrusions, but as the saying go's "You can't push a rope" They make pultrusions in 5000' lengths at a time. This is the amount of material on a creel bobbin. The fibers and gathered up in redirection combs until they come together at the opening of a die. Prior to entering this die, they are impregnated with resin. Once in the die the resin is cured by either progressive heating or microwaves. The product that exits the die is one tough piece of structure. Pultrusions have the highest ratio of resin to fiber than any other composite. Typically around 85%. There is a great deal of tension applied to these fibers as they are pulled through the die and this tension is trapped into the fibers. Typically this is around 15,000 psi. This means that for all applications where the pultrusion is in a beam in bending, the pultrusions placed in compression, actually see "less tension" for the first portion of the loading. This makes them very stable in compression. The Robin uses these pultrusions for the spar cap, and for the first 1 G of positive loading, there is no compression in the upper spar cap.
This is a bit of a digression, but the reasoning is exactly the same. One of the greatest military secrets of the British Navy in the days of sail, was the design of their masts. The British ships were well known for their ability to take tremendous punishment when the rigging of their masts were shot out. French and Dutch ships masts tended to break in similar circumstances. The reason was that the British insisted on using whole logs for their masts. The Dutch and the French would laminate up their masts from smaller cut sections. This is actually the reason Britain colonized Australia, to gain access to their virgin timber. When wood grows the outer rings are the growth rings and the core is the old wood, or compression wood. When a mast is fully rigged, the main loading in the mast is in compression. Once the rigging has been shot out, the masts would bend. The British always stepped the base of their masts such that they would resist bending. When a whole log bends, the outer fibers which contain trapped tension, can deflect at much greater angles because they see "less tension" before they see compression. Cut wood on the other hand exposes wood already in compression and the result is a compression failure. And that is the reason Britannia used to rule the waves.
Back to landing gear. The reason I am using flat springs in a molded beam is because of a phenomenon called creep. All plastics creep. The resin matrix of a composite structure is plastic, and it will creep. Creep is the slippage of the resin molecules relative to each other when a constant small load is applied. If a curved beam is molded, the outer fibers of that beam under a 1 g normal loading have a distributed tension component all along the outside of the curve. This tension load in the fibers is normal to the fibers direction and the vector is pointing away from the surface. This small tension load will cause the landing gear to splay apart. When you use a flat beam, the normal 1 g loading causes the gear to deflect in a concave direction. The distributed compression load along the outer surface is now pointing inward, or into the laminate. Plastics do not creep generally in compression.
|Pultrusions being bonded under vacuum|
|center joint and clamp blocks|
This center joint is bonded to the gears and has a large overlap so as to allow the bending loads to transfer by a wide couple. The ends of the tubes are also chamfered at a 45 degree angle to reduce the stress concentration. The gear itself is clamped up in two fittings and pin jointed to the frame. There is a compression/tension load that is induced into the frame due to the fact that both sides are clamped and not allowed to slide or translate. This loading was accounted for in the design of the frame. The ends of the
|Landing gear being rigged in position|
gear are designed similar to the ferrule of a chisel. They are bonded to the ends of the gear and have the axle and brake supports added. This area has undergone a little trial and error. As has the actual gear itself.
I have revised the end fitting design and the gear height. I just finished building a brand new gear because I was lacking enough ground clearance for the new GSC prop I ordered. I had estimated that I would use a 48" prop, optimised for
|original gear drawing|
soaring, but I decided to use the 56" prop because I am in love with rate of climb (see Wren page!!!) I have verified that the calculated deflections exactly match the actual deflections. The earlier gear was test loaded by sand bagging the pilots seat to 2.5's. a 1/2 g above the design ultimate load.
|original gear at 3 point|
|center tube support and fitting|