Landing gear construction.

Landing gears are an especially tough component to design. They add useless weight to the aircraft, but are absolutely essential. Imagine designing a landing gear that was designed to strike the ground at the VNE (Velocity never exceed) of the airframe and then stroke to a point where all of the energy is absorbed and the airframe is neither over stressed or the landing gear breaks. Congratulations!! you have just designed the Peregrine Falcon talons. This  remarkable piece of engineering  like the design of  Spruce wood, was the work of a genius! and I adore him as a matter of fact!!! I have a design manual set by Dr Jan Roskom of the university of Kansas. The sections on preliminary design are the bible for the industry. He has accumulated an immense amount of information, or rather his Grad students have LOL!!!. In the section on landing gear he has a carpet plot that equates the weight of the landing gear to the gross weight of the aircraft. There is a line that holds true from the lightest (J-3 Cub) to the heaviest (C5 Galaxy) aircraft sampled that has the gear weighing around 3.5% to 5% of the gross weight. My landing gear complete with wheels, tires, axles, springs and all fittings weighs 7.2 lbs for the main and 1 lb for the tail or 1.3%. Because of this, this is actually an area of risk on the Robin.

The landing gear spring is the heart of this design, its constructed as a sandwich composite beam.



bonded gear legs

 Spruce wood is used as a core and strips of Graphlite rectangle flats are used as a spar cap. The whole bond assembly is then encased in a woven 45 degree knitted 8 oz Graphite sock. This resists the torsion loads induced by braking. The Graphlite sections were purchased from Goodwinds in Mt Vernon Washington. They are a refinement of the original Nepco pultruded carbon rod product developed many years ago for the Copperhead guided artillery shell program. The original .050 diameter rod was coiled up on a bobbin at the base of the shell. The outer case would shed after firing and the .050 rod would spring out and become a trailing wire antenna. Later versions of these rods were made in various diameters, the most popular being .125" diameter. This is because the bend radius is 19 inches and  means that they can be coiled for inexpensive shipping. The .125" rods were used by Bell Helicopter in the V-22 wing skins as pre-cured unidirectional reinforcement for the wing skin hat stiffeners. They later abandoned that process, because it was cheaper for Bell to lay graphite tape by robotic tape laying machines. Nepco went out of business or sold the Pultrusion business. It has changed hands a few times, but  is being manufactured by Goodwinds, a high performance kite company. The kite fliers and model builders were early adapters of the .050 product when it hit the surplus market.
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

 An added benefit of using pultrusions is the relatively high modulus of elasticity, it is however not as high as Aluminum or Steel, so careful attention must be paid to the amount of deflection the gear has, especially sitting on the ground at 1 G or during taxi. It is desirable that the axles be parallel with the ground in this condition for easy ground handling. This leads to a gear that has a great deal of camber when unloaded.

The amount of deflection of a landing gear determines its ability to absorb energy. These types of gears do not actually absorb energy, like a rubber bungee gear would. Rather the energy is stored and added back to the airframe in the form of a rebound load. Actual energy will be absorbed by the lateral scuffing of the tires when contact with the ground is made on landing. On the Robin I decided to isolate the airframe from the bending loads of the gear. This would have required a much heavier support frame to react the bending loads. I have connected the two gear halves together with a welded rectangular tube section. The center joint is capable of transmitting moment from one gear to another. By using this method, I also get higher gear deflection and soften the landing loads.





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

The tail spring has undergone a design evolution also. originally I designed and sized a tail spring using 4130 Steel. I did not want to pay for heat treating so I sized it for the normalized condition. Eventually this proved to be inadequate. All of my hanger flying started to creep the gear such that I noticed the tail wheel caster angle starting to increase. My first try at a composite gear was to use the same Graphlite flats that I used in the main gear combined with a sandwich core of NEMA grade C fiber Glass plate. This not only proved to be heavy, but I completely neglected the torsional loading that would occur with the tail wheel kicked over.

This design would actually make a damn good torsion spring. So I went back to the Goodwinds catalog and found a pultruded graphite tube of 3/4" diameter x .125 wall. I bought this tube and the purchased some 7 oz knitted carbon sock and bonded it over the outside . The tube would probably have been able to resist the torsion loading, but would have done so by differential bending. This is an inexact load path and could have had eventual delamination issues. I sized some 4130 steel tubing to slip over the outside and act as a ferrule. The tube passes through a solid plywood bulkhead where 1/2 the bending is reacted. The center of the gear has a sleeve tube of 4130 welded to  a support fitting. The tube is bonded into the 4130 tube, after appropriate surface prep, and this mount reacts not only torsion, but the also couples with the bulkhead and reacts the bending



center tube support and fitting