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 22, 2019 #121

    Winginitt

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    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 ?
     
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  2. Sep 22, 2019 #122

    rotax618

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    This is interesting conjecture, but really the 701 is a very simple easy to build STOL aircraft. There are thousands of them and their like flying, sure some pilots bend the UC because of accidents, overload or bad technique.
    The solution is simple, if you are going to be flying into unmade airstrips and/or flying heavy, then fit tundra tyres and increase the the leg thickness by 1/8” - large tundra tyres/tires allow the UC leg to be shortened by an inch and the track to be narrowed by 1-1/2 to 2 inches. Tundra tyres absorb a large amount of the landing shock.
    If you want to fly fast, remove the slats fit VGs, fit smaller wheels and increase the leg thickness by 1/8”.
    This is not the thread to discuss the shortcomings and improvements to the 701 (apart from the UC) but the 701 is much improved by increasing the span of the wing and tail, removing the slats and fitting VGs to the top of the wing and underside of the tailplane.https://stolspeed.com/
     
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  3. Sep 22, 2019 #123

    flyboy2160

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    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.)
     
    Last edited: Sep 22, 2019
  4. Sep 23, 2019 #124

    12notes

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    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.
     
  5. Sep 23, 2019 #125

    Victor Bravo

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    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.
     
  6. Sep 23, 2019 #126

    flyboy2160

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    Sometimes it's easier to grasp or to remember this by seeing a picture( :) or a pitcher, if you do beer math like me!)

    The math part above with the 1/2 is like taking the area under the stress strain curve - which represents the energy used to deform the material. In the stress strain curve below, imagine that the lower curve is 6061 and the upper curve is 7075. The area under the curve to yield is triangular, so the area is 1/2 x the base x the height. The 7075 curve goes higher because it's stronger. The base of its triangle is wider. Thus, its area is greater.

    [​IMG]
     
    Last edited: Sep 23, 2019
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  7. Sep 23, 2019 #127

    wsimpso1

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    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.

    A discussion of flanged and box section beams is thread drift in multiple ways. Best if we not...

    Billski
     
  8. Sep 23, 2019 #128

    wsimpso1

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    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 must suck up both the airplane's vertical kinetic energy and the potential energy of the airplane setting onto the gear while all the energy is absorbed;
    • It - and the rest of the airframe - must be strong enough to not get damaged while doing all this.
    The old FAR Part 23 (which does not apply to homebuilts and LSA's, but is still a pretty good idea) specified:
    • Minimum sink rate based upon wing loading which gives the kinetic energy;
    • Minimum of 2.67 g's at peak, and generally maximum of 3.67 g's;
    • Allowed the wing to carry 2/3 of the airplane's weight during the landing stroke of the gear, so the gear has to stand whatever the peak is minus 2/3 times the airplane weight at max stroke;
    • The gear system and the rest of the airframe have to take whatever vertical load that results.
    In homebuilts and in LSA, we have no such restriction on the g's or the sink rate. But we do have to suck up energy (force over travel) and not damage the leg or the airframe. The tires suck up some of that energy and the gear leg has to be springy to suck up the rest.

    It appears that the plans leg is prone to failing in use, while the Grove part seems to be fine in the field. The Grove part is thus a decent target for strength and energy absorption in landings. I am skeptical that the wide flange beam, while no doubt strong, will be springy enough...

    Billski
     
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  9. Sep 23, 2019 #129

    wsimpso1

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    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.

    Billski
     
  10. Sep 23, 2019 #130

    TFF

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    I hope you document your repairs here. I have seen Rockiedog fly his and it’s impressive. Into a good wind takeoff was about 75 ft.
     
  11. Sep 23, 2019 #131

    wsimpso1

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    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.

    Now if you do make the gear really stiff (triangulated steel tubes and the like), you still have the tires. For some airplanes, that is enough, but for most of us, well, we better have some sort of suspension.

    The stiffer you make the springs, the higher the reaction loads we get in sucking up a given amount of energy. See post 128. Little airplanes do happen to be in a region where leaf spring gear legs can work.

    Billski
     
  12. Sep 23, 2019 #132

    BBerson

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    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.
     
  13. Sep 23, 2019 #133

    wsimpso1

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    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.
     
  14. Sep 23, 2019 #134

    Victor Bravo

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    Looked at the forward fuselage, not enough metal there for a bungee Cub or Pilatus style gear.
     
  15. Sep 23, 2019 #135

    BBerson

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    I was thinking the caps could even be 1/8" thick. Another advantage is splicing thin sheet isn't that difficult. So ordering and band sawing shorter lengths is convenient. I have a piece of 1/8" X 3" 7075...
     
  16. Sep 23, 2019 #136

    BBerson

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    An I beam with the same moment of inertia should deflect the same as a flat bar of the same material.
     
  17. Sep 23, 2019 #137

    blane.c

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  18. Sep 23, 2019 #138

    Winginitt

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    ↑Winginitt said:
    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 ?

    Bilski reply:
    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.

    Winginitt: Agree, there is no such thing as completely ridgid. My concern is that the new gear might be "so ridgid" that it transfers sufficient additional forces to the fuselage which might then suffer damage. As I mentioned, Zenith refers to it as a "landing gear spring" which implies they designed/calculated it to have a certain amount of give. Its common sense that virtually all landing gears have a certain amount of movement, so I'm not questioning or asserting their gear is somehow different from other gears, only pointing out that they took the trouble to call it a spring. Changing the "spring rate" may save the gear but could harm the fuselage.

    I posted several alternative ideas (videos) that show ways to absorb the energy and dissipate it in a way that is less harmful and in my mind a more desirable solution. I realize that VB is trying to improve his plane and keep costs to a minimum. Since he's a bright guy, I suggested he look at those designs and see if he might originate his own ideas and possibly come up with a better solution. Maybe the thicker/stronger gear works well and doesn't cause any fuselage problems....I don't know. I did however find the alternative solutions interesting. Anyone who ever raced dirt bikes can attest to the fact that as the bikes evolved, the solution to good "landings" and travel over rough terrain became increased travel with adjustable/controlled dampening. Since VB is virtually starting from scratch so to speak, I hoped he would consider other things. Apparently it is not something he wishes to consider, but he should at least be aware that if his landing gear proves to be too ridgid, he may redamage the fuselage while the gear remains unharmed.


    Billski:
    Now if you do make the gear really stiff (triangulated steel tubes and the like), you still have the tires. For some airplanes, that is enough, but for most of us, well, we better have some sort of suspension.

    Winginitt: The trend today seems to be toward gear that has lots of dampened travel. As the pictures showed, there are even modifications to adapt longer gear to Cubs. Personally, I'll take all the advantages I can get.


    Added note: Watch the gear compress on the Just Superstol. After it plays, another video pops up that can be watched. Its a Zenith and it lands at about the 0:50 mark. Watch how ridgid the landing is and how little travel occurs.



    Bilski:
    The stiffer you make the springs, the higher the reaction loads we get in sucking up a given amount of energy. See post 128. Little airplanes do happen to be in a region where leaf spring gear legs can work.
    Billski

    Winginitt: Yes, obviously leaf springs have worked for many years on many airplanes. Apparently the Zenith design is having more than its share of problems causing builders to search for more durable gear. Inadequate design or overstressed by poor landings....maybe both ? Again, I don't know how well the thicker gear actually works, but apparently there are some builders using it successfully.
     
    Last edited: Sep 23, 2019
  19. Sep 23, 2019 #139

    BoKu

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    I don't know, but I get the sense that braking loads are going to result in much more toe-out than if the gear were secured across the bottom of the fuselage as usual.
     
  20. Sep 23, 2019 #140

    BBerson

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    Gear on bush planes is normally ripped off from hitting stumps and snow berms. I repaired a Citabria that hit a snow berm on skis. It's much more difficult to repair the fuselage (especially in the bush) than replace a bent gear.
    So making the gear stronger than the fuselage might not be optimal. (as winginit said)
     
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