- Apr 5, 2019
The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?
If the one piece leg is free to slide cross-wise at one of the the attach points, the increased bending load remains in the leg. The fuselage will just see the increased vertical load. The larger tires will act as springs in series with the leg. (I'm assuming the leg is free to pivot where it attaches to the fuselage either via a real pivot or by rolling around on a radius block.)The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?
Thank you for this, it explains something I've been wondering about. The Hummelbird calls for 3 pieces of 1.5" x 15" x .125" of aluminum for a tailspring, some people have no problem with it, others replace it rather quickly. The plans call for these pieces to be either 2024-T3 or 7075-T6, the 2024-T3, using your math, has half the spring energy at 118 in-lb and 1179 in-lb of energy per pound. I'm guessing those that replace the tailspring used the 2024. I used 7075-T6 on a recommendation without knowing why, this explains it very well.Please do not put words in my mouth. I said what I said in standard English. For those of you who still did not follow it, I will elaborate.
By the time you make your gear leg durable in 6061-T6, it will be heavier than a similarly durable gear leg for the same airplane in 7075-T6. Why does this happen? First let's understand that WEIGHT IS THE ENEMY. When we design landing gear, we are trying to suck up the kinetic energy of the airplane sinking towards the pavement. Since we do not want it to weigh any more than it has to do the job, we will usually design our parts to be absorb enough energy and be strong enough and stop there.
Our figure of merit for spring leg materials is how much energy we can store in a unit weight of spring. If we can put more energy in a unit weight of one material than another, then we expect that we can make a lighter spring with the higher energy storage material. You can store a lot more elastic deformation energy in a unit of 7075-T6 than in that same unit of 6061-T6. 7075-T6 nearly twice as strong with about the same Young's modulus and about the same density as 6061-T6.
Want math? Energy stored elastically in a spring (any spring) is 1/2*k*dx^2, where k is the spring rate, dx is the spring deflection. Do this with a unit mass of material and you have your figure of merit for comparing materials.
Let's take a 1" cube of each, and apply a force just shy of the yield strength of the material squeezing two opposite faces towards each other. The stress is just shy of the yield strength, and the change in cube length is the strain just shy of yield strain. Divide the whole thing by density and you are figures.
6061-T6 is 0.0975 lb/in^3, yield is 40 kpsi, and modulus is 10.0Mpsi. Strain at yield is 40e3/10e6 = 0.0040 = dx. K is 10e6. 1/2*k*dx^2 = 80 in-lb. Divide by density to get 820 in-lb of energy per pound of aluminum.
7075-T6 is 0.102 lb/in^3, yield is 73 kpsi, and modulus is 10.4Mpsi. Strain at yield is 73e3/10.4e6 = 0.0070 = dx. K is 10.4e6. 1/2*k*dx^2 = 256 in-lb. Divide by density to get 2512 in-lb of energy per pound of aluminum.
Yeah, you can store a little more than three times as much energy in a pound of 7075-T6 than in a pound of 6061-T6. Can you make 6061 hold up under 10 fps sink rates? Sure. But you can make a leg out of 7075 that can do the same thing with similar durability at significant weight savings. And since WEIGHT IS THE ENEMY in flying machines, you should do this where you can.
Sometimes it's easier to grasp or to remember this by seeing a picture( or a pitcher, if you do beer math like me!)Want math? Energy stored elastically in a spring (any spring) is 1/2*k*dx^2... Billski
Landing gear legs as the OP is using are solid rectangles. Given a fixed cross section area: the rectangle has more J than a square; The wider the rectangle, the more J it has. Torsional stiffness goes up - not down - as we widen this type of beam.Compare the torsional stiffness of a box section and and an I section of equal bending properties.
I am still trying to figure out why we would want to do this. It will make for a strong gear leg and at less weight, but let's remember that gear leg has TWO major tasks that must be met together:
Bummer on the effort needed. But then everyone needs a hobby - ;-). My work on the truck for the wife's airplane fuel just got deeper, so I have one too.The 701 project that I was talking about in creating this thread arrived at my hangar today. It turns out it is a much bigger repair project than I had thought, even after having seen it in perosn.
(I was not able to spend a lot of time looking at it in person, and it was already packed up on a trailer.)
Today we took it off the trailer and I had the opportunity to see just how much work is involved in repairing this. The landing gear fabrication discussion in this thread can go on for several weeks... while I make a new firewall, perhaps a new tail section, patch a bunch of un-necessary holes in the skin, perhaps replace a longeron or two....
One of the options that I had been willing to discuss, the Pilatus Porter taildragger configuration, is pretty much off the table. It looks like there is simply not enough structure in the areas where there needs to be structure. A more formal engineering study and some added structure would have to be in place before I pursued this idea.
First off, there is no such thing as "completely rigid". Everything has an inherent springiness. Young's Modulus (E)and its closely related Torsional Modulus (G) are present in all solids and determine their elastic behaviour under load.The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?
What's wrong with stronger gear leg and at less weight?I am still trying to figure out why we would want to do this. It will make for a strong gear leg and at less weight, but let's remember that gear leg has TWO major tasks that must be met together:
It does sound like it could be an advantage for homebuilding, but the usual problem in getting a design that works IS getting deflections high enough - too much stiffness is in the way when doing this work.What's wrong with stronger gear leg and at less weight?
If too stiff, then just reduce the leg thickness. I was only suggesting a method of home fabrication without heat treating. Not an exact drawing.