New Landing gear cont.

I have finished the gear fitting installation to the fuselage. The first step was to locate the fittings using the gear as a guide. I loosely bolted the gear to the fittings and then located the fittings to the fuselage. Once located I back drilled the through fitting through the fuselage bearing blocks. I made a makeshift drill jig by drilling a 1/4" hole through a 2.5" thick block of aluminum. It was very critical to drill these holes perpendicular to the outer flange so the holes would line up with the inside flange. Normally the inside flange would be left un drilled or undersize so the final hole would open up both flanges. I was betting that I could drill a hole accurately enough so the drill would align with the inside flange.

homemade drill guide

I used to work in Wichita at Boeing Commercial aircraft, they had the most wonderful surplus yard I have ever seen. I bought a ton of forged aluminum block and plate stock. This is just a small little piece. There are commercial drill jigs, most notably the "Egg Cup " design. all of these will work, but I didnt have a 1/4" drill bushing for my Egg Cup. 


After the 1/4" holes were drilled, I chased then through with a 1/2" twist drill. Opening the holes up to just slightly under 1/4" (-.003) I then fabricated eight 1/2" diameter aluminum bushings. The 1/2" aluminum stock is slightly larger than 1/2" (+.002) this makes for a nice tight transition fit between the wood and the bushings. I sanded the outside of the bushings and then cleaned them with MEK.

cleaned bushing and epoxy adhesive

once cleaned, I coated the bushings with epoxy adhesive and tapped them into place. The bushings were made with the center hole undersize (.223") so that on final assembly they will be opened up again to .250" The adhesive is used to stop the bushing from rotating when it is drilled out.

installed bushings

the bolt loads are such that they needed to bear into the wood with greater area, this is the reason there are 1/2" diameter bushings.




The landing gear fitting is square and parallel to the landing gear. The fuselage has a slight amount of contour . The load path from the gear to the fuselage is through a mechanism called "Heel and Toe" The design condition is a 1G breaking load applied to the tangent edge of the tires. This creates a large moment that needs to be reacted by the two outer bolts on the gear fitting. Since the fitting is wider than the bolt pattern, the reaction will actually  be between the edge of the fitting (heel) and the opposite bolt (Toe) . Therefore the gap between the fitting and the fuselage must be filled with a material capable of withstanding compressive forces. The fitting is "Bedded" with a mixture of epoxy and cotton fiber (Flox) . Flox is mixed to a consistency of dough. I spread it under the footprint of the fitting and then installed the fitting. The inside of the gear fitting was coated with mold release wax so the fitting could eventually be released if needed.

flox bedding

once the fitting is installed, the excess bedding is scraped away.

When I installed the gear for a trial fit I saw that the clamp block was deflecting and bridging over the gear. To fix this I designed 4 more bushing that were exactly .003" taller than the thickness of the gear. They would crush to equal the gear thickness after installation

clamp bushings

after the clamp up bushings were installed, the bridging stopped and a more even clamp pressure was developed.

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New Landing Gear installation

My new gear finally arrived from Grove Aircraft.I have finished all of the fuselage structural modifications. I have built all of the attach fittings and I have prefitted all of them. The gear is designed to clamp up between two radius filler blocks. The purpose of this design detail is to limit induced bending into the fuselage. If the gear were to be clamped with just flat plates, every time the gear deflected, the gear moment would be transmitted into the truss.

new gear

this is one heavy chunk of metal!! Makes me want to think about a mono wheel again.

attach fitting and radius plate

This image clearly shows the radius plate. The gear is clamped with two 3/8" dia bolts. I plan on silver soldering the bolt head to the inside of the steel fitting. This is so the bolts wont rotate when they are torqued. there will be a 3/4" counter bore in the fuselage that the heads will sit in. There is a slight amount of contour mismatch between the fitting and the fuselage truss. I will assemble these fitting with a bedding of Cotton Flox and epoxy. The primary load direction is in compression, with the exception of the fore/aft braking load .

3/4" counter bores

The area under the counter bore has been padded up such that the width of the gear bearing plate is supported.

The attach bolts that pass through these pads are bushed with a 1/2" diameter aluminum bushing.

After the gear is installed, I will need to fabricate a fabric rib that will sit on the front and rear of the gear. An aluminum cover will connect the ribs and fair in the gear

New Landing Gear

I have been patiently waiting for my new Aluminum Landing gear from Grove Aircraft in San Diego. My friends Mike and Neal Lafrance actually own the hanger next to Grove..  Even with that kind of pull, I still have to wait 8 weeks for the gear. As Robbie explained it to me, its out of his hands because the gear has to go to a heat treat shop 3 times during its construction. I have 3 more weeks to wait!!! I decided that the simplest way to add a landing gear to the Prototype was to attach it to the outside of the fuselage in the same location of the old one. I needed to add some additional bearing blocks and increase the area of the two diagonal stiffeners . Truth be told, I should have done that for the original gear.My goal is to have all of the modifications done and the Fuselage recovered by the time I get the gear. I want to be able to just bolt it in and then flip the plane over.

reinforced gear frame

in this picture the extra doubler on the two diagonal members are shown. all of the vertical loading of the gear is reacted by these members. They also need to react the aft braking load by a heel and toe reaction between the gear fitting bolt and the back wall of the fitting. underneath the plywood is 1.5" of birch plywood bearing blocks. .

outer landing gear fitting

This picture shows the outboard fitting for the new landing gear attach fitting. There will be a narrower fitting that is bolted to the inside of the gear frame. Both plates will be welded to a .125 plate that will act as a bearing plate. I will have two 3/8" bolts permanently brazed to the 1/8" plate. I will counter bore into the birch doubler to clear the bolt head. Sitting on top of the plates will be a radius block. The gear will be sandwiched between two of these radius plates. The isead here is to limit the gear bending moment to the gear itsself. The radius blocks will act like two fulcrums.

Unfortunatly this new gear will add 8 lbs to the Robin and may put the prototype out of the part 103 category. I'm sure with the redesigned wing and the single cylinder 2 stroke engine, Part 103 will still be possible. After my talk on Labor Day, the universal opinion of everyone I talked to was to scrap the 2 stroke and Part 103. My Friend Murry Rozansky who is the ESA Treasurer, suggested that I look at the 23hp Briggs and Stratton Vanguard conversions. After some research, I have decided that this is the perfect engine to adopt. The Prototype is actually perfect for it from a weight and balance point of view. My CG will move forward to 22% at full fuel. 26% empty. The static margin increases by 10%. This engine will be far more reliable than the 2 stroke. I will however fly this plane on the MZ 34.