Aluminum Plate Engineering/Forming Question

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Victor Bravo, Sep 16, 2019.

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  1. Sep 19, 2019 #61

    rotax618

    rotax618

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    There are hundreds if not thousands of 701s, Savannahs, Land Africas etc,etc that successfully use 6061T6. Be careful that you are not transferring extra rebound forces through the gear attachments.
    If you want the increased expense and complication, go right ahead.
     
  2. Sep 19, 2019 #62

    Rik-

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    I think your putting to much faith in 7075. It’s not magic. Your wanting to use mass to get strength. Steel tubing will be lighter and stronger.
     
  3. Sep 19, 2019 #63

    pictsidhe

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    Significant weight savings. The weight of an Al gear is inversely proportional to the square of the maximum strain. Landing gear gets a bigger weight advantage than say, a spar.
     
  4. Sep 19, 2019 #64

    flyboy2160

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    For the same size, you get a larger FS strength margin and better fatigue. If you want to save weight, you can make the leg smaller and keep the 6061 FS.
     
  5. Sep 19, 2019 #65

    BBerson

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    2024 is almost as much yield as 7075.
    Deflection is the same for all the aluminum alloys. So a yield might be better than snap off in the jungle. I saw a snapped gear on a Cessna. Looked sad.
     
  6. Sep 19, 2019 #66

    Winginitt

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  7. Sep 19, 2019 #67

    wsimpso1

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    VB told us before that the existing 6061-T6 gear do not hold up. While it might be possible to make a 6061-T6 leg that works, it will be durable at much lower weight in 7075-T6...

    Billski
     
  8. Sep 19, 2019 #68

    Rik-

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    Are you saying 7075 is lighter than 6061?
     
  9. Sep 19, 2019 #69

    Victor Bravo

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    No, even dimwit me can answer that. The durability will be higher at the same weight.
     
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  10. Sep 19, 2019 #70

    rotax618

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    I have flown my Savannah since 2005 and have around 700+ hours, mostly short flights of less than an hour, the Savannah is a little heavier than a 701 but has a longer span, the wheels are still at the same camber as originally built, the UC is the same as a 701, there are pilots that bend 701 and Savannah UCs they can be re formed once or twice provided the deflection isn’t too great. I have heard of 701 owners increasing the thickness of the leg by 1/8” because they overload the aircraft or the normal landing is like on an aircraft carrier. I’m sure there are “pilots” who would bend the UC if it was made of 2” titanium.
    At any rate, you can form a 6061T6 UC using an hydraulic pipe bender, the material is easily obtainable, the corners can be rounded using a Wood router, don’t think this will be possible with 7075.
     
  11. Sep 19, 2019 #71

    pictsidhe

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    With the same yield strength, you'd need the same size press. Dimensions would be different. 7075 would be thicker but narrower.
     
  12. Sep 19, 2019 #72

    proppastie

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    so lets see....plans call for 3/4 6061...makes sense get 7/8 7075
     
  13. Sep 20, 2019 #73

    BoKu

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    I don't know how it is now, but 7075-T6 bar and sheet has in the past been pretty common on the salvage market.

    7075 cuts just fine with all the standard tools. In fact, it is much nicer to drill or saw than 6061. Because of its greater strength, it releases light fluffy chips that clear easily. By contrast, chips from 6061 tend to be jagged gummy hateful things that give turning or drilling it much greater laceration potential.

    Going by published specs, the density of 7075 is about 4% greater than that of 6061. I was surprised by this. But in the grand scheme of things, it's a tiny difference in light of 7075's much greater yield and ultimate strength.
     
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  14. Sep 20, 2019 #74

    Victor Bravo

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    I am not 1000% prohibiting myself from ANY option here. 7075-T6 is known to be the first choice of the majority of the 701 community, hands down. So in this thread I am looking into just what I can do to afford a 7075-T6 gear, which means I may have to shape it differently in order to escape the need for annealing it.

    I am also seeking knowledge and potential resources for doing the gear in Graphlite, in wet carbon tow, in 6061-T6, and in welded steel tube/bungee. But those inquiries are not appropriate for this thread, because I asked for discussion/assistance with 7075 here.
     
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  15. Sep 20, 2019 #75

    wsimpso1

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    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.

    FromASM's Matweb:

    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.

    Billski
     
    Last edited: Sep 20, 2019
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  16. Sep 20, 2019 #76

    wsimpso1

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    The Grove product known to be durable in this application is 7/8" thick... That is our benchmark.
     
  17. Sep 20, 2019 #77

    proppastie

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    Well if he wants to save money (and weight) there are other options .....for example cub shock chord design, or thinner with tube joining each leg....crossing tube into opposite bracket etc. Trying to bend 7/8 accurately if you do not know what you are doing or do not have the proper tooling is going to be difficult and maybe very expensive.
     
  18. Sep 20, 2019 #78

    Victor Bravo

    Victor Bravo

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    (sound of infantile crying, sniveling, and a puddle forming on the ground)

    No-No, no... no, that's OK, I'll be nice and let you off the hook today on your math homework.

    Did I ever tell you about how to make the best glue joints between small sticks of balsa wood?
     
  19. Sep 20, 2019 #79

    Victor Bravo

    Victor Bravo

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    Oh yes, he definitely wants to save weight, money, time, fabrication complexity, installation effort, and he wants Sofia Vergara to deliver the landing gear wearing a negligee and with a cold beer in the other hand.

    Almost any type of landing gear for this application would be something I cannot make in my own shop or my friends' shops.

    The 6061 I might be able to bend on a buddy's shop press, but maybe not even that for 3/4 or 7/8 thick metal. The 7075-T6 full-temper gear, if it turns out to be the best choice, will most likely have to be made on a large industrial plate roller (or bar roller or ring roller, whatever the correct name is for it). It's a machine about the size of a Sherman tank I believe. Unfortunately I don't have one in my T-hangar, and don't have the brian cells to use it without expert help.

    The carbon fiber gears are also something I understand the basics of, but I don't have the fabrication capacity. BoKu was kind enough to offer to ask a friend of his if he knew anyone who could do this on a cash-sixpack-garage basis. I even have a huge roll of carbon tow that can be used, but everyone is aware that bolt holes and clamping hardpoints complicate what would otherwise be a simple part. Sp if BoKu, or his friend, or anyone here on HBA is capable of making a "one-shot" garge level gear spring out of carbon, please PM me and I want to talk abbout the ups and downs of that the same way we're talking about 7075 here.

    The bungee or McPherson strut articulated gear is a third place fall-back, "Plan C" for me. It's easy enough to build, quite probably the lightest, and I have a bunch of friends and associates nearby who can do the welding and fabrication. But the heavy duty bungee/McPherson tricycle gear (in the style of the Fieseler Storch or Pilatus Porter) on a 701 would look ridicuous, and the upper struts could possibly interfere with getting in and out of the door. It would put a lot of "arrestor hook" structure underneath the aircraft (to grab bushes, branches, etc.). The easy and elegant solution is to take that opportunity to make it a taildragger and make a nice-looking Pilatus style tailwheel gear, saving even more weight. However, taking an already high-drag airplane, tilting it 20 degrees to make more drag, will likely hinder the acceleration to takeoff speed measuably. I have one report from a known 701 guru type of guy that he's spoken to two or three people who were very unhappy they converted the 701 to tailwheel.

    Anyway, I really do appreciate this discussion and people contributing thought and ideas. Keep it going :)
     
  20. Sep 20, 2019 #80

    rotax618

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    3CEAFBBF-343C-428C-82AE-D46AC676A532.jpeg You can form 7/8” 6061 T6 using an hydraulic pipe bender, I have done it on two previous occasions (not for a 701, for the aircraft I built below) 7/8” 6061T6 is more than adequate, the max TOW of the 701 is 500Kg, the Savannah ‘S’ uses the equivalent of the standard 701 UC, has a 622Kg max TOW.
    In my experience the weakest part of the 701 UC is the nose leg, the problem seems to be the inadequately sized inverted horizontal tail which can and does stall at low speed high alpha, dumping the nose on the ground - seen it happen twice, the last guy replaced the 701 tailplane and elevator with a Savannah tail, that is after he repaired the nose.
     
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