Composite landing gear

Discussion in 'Composites' started by hiryu, Jul 12, 2006.

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  1. Jul 25, 2019 #21

    Lendo

    Lendo

    Lendo

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    I recently did an exercise to evaluate a one piece Carbon main Landing gear leg, with the help of a Engineer and Math wiz - no names mentioned to protect the unfortunate person who helped me.
    Bottom Line 13 lbs for the leg, 4" wide, to withstand a hard landing of 6G for a 1320 lb Light Sport aircraft. All Uni with a sock (or two) of Uni Carbon, to contain the Uni.
    The biggest issues (after flex and strength) seemed to be the mounting of the Carbon leg, clamping was problematic and the mounting position was critical for least stress to the radius turn of the point of the mount. I forget the correct engineering terminology.
    To clarify, when the leg twist upward and rearward the is a lot of stress on the mount point. This upward bending causes a downward bending of the leg between the the mount points, to minimize these stresses the mount point must have a swivel (turning on a bolt) position and degree, that minimizes there stresses.

    Mounting of wheels directly on to Carbon, would be most suited to wrapping Carbon tow around stainless steel bobbins to form a bolt hole. this tow interwoven with the UNI tapes. OR the whole leg made up of Tow. Drilling through the Carbon at this point would seem counter productive IMHO, hence the bobbins.

    Unlike steel the Carbon or Glass legs can't be clamped in their position as a clamp damages the materials, research what others have done to see different approaches.
    I can't comment on timber or timber composite legs, never looked at them, but would be interested in weight differences.


    I figured 13 lbs was a good result for that exercise - compared to steel and Aluminium.

    Cheapracer, Seems about 63% of your glass weight.
    George
     
  2. Jul 25, 2019 #22

    BJC

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    Glasairs use uni glass gear legs, wrapped with cloth.


    BJC
     
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  3. Jul 25, 2019 #23

    opcod

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    Many just use an3 or just m6 drill through as a fixation.
     
  4. Jul 25, 2019 #24

    Voidhawk9

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    A one-piece carbon gear would be very stiff, would it not? A bit of flex, as in glass, might be worthwhile?
     
  5. Jul 25, 2019 #25

    pictsidhe

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    You adjust the thickness to get your max allowed strain at your desired deflection. Then you adjust the width to get the desired stiffness. Glass gear ends up much thicker than steel, but narrower. Al and carbon sit between the two.
     
  6. Jul 26, 2019 #26

    Lendo

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    pictsidhe - I couldn't have said it better.
    Voidhawk9 - correct about stiffness, but it will flex. I've seen videos of other non-aircraft type tests on legs on others things and the Carbon flexes very well, surprisingly - which gave me the idea that it would work. If you considering lightest weight, it may be an option. I'm considering winding it in Tow with Carbon Tube socks for final wraps. Including the Bobbins naturally, as Carbon doesn't take to holes drilled into it at such a stressed area.
     
  7. Jul 30, 2019 #27

    wsimpso1

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    Everything has a load-deflection curve. Steel has higher E than unidirectional high modulus graphite-epoxy, and it works nicely for landing gear on many thousands of Wittman type gear. The trick in any material is in designing enough gear travel for your landing conditions while staying within strength. Then maybe you explore all of the materials and pick either the lightest one or the lowest drag one or a combination of low drag/low weight.

    I went 4340 mostly because of packaging. 7075 would not go in the space I had, nor would fiberglass. There is no graphite yet in my bird, why start now?

    If I were designing a graphite leg, it would be Graphlite rectangular rods and graphite biax cloth. For everything, including the lift tabs.

    Billski
     
  8. Jul 31, 2019 #28

    wsimpso1

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    Not quite so straightforward... You adjust thickness to get close to Max Strain and Angular Deflection. Then you integrate Angular Deflection over the length to get Linear Deflection.

    Not quite... Then you adjust width to get adequate strength. Vertical load at the wheel divided by vertical deflection is then spring rate (stiffness). Then you get to check the landing deflection of the system, the stresses/strains/failure criteria under the deflected shape, the system weight, and then iterate the design until you find it acceptable...

    When you deflect the gear leg in landing, the wheel slides outward, increasing the horizontal space between mount and tread contact, increasing the moment along the leg. Yep, small deflection assumptions are violated with regard to the moments carried in the leg. So you loop the calculation using the revised shape at max landing energy until the deflection curve settles down. Then you check deflection and spring rate against targets and stress/strain against whatever failure criteria you are using.

    In more rigorous terms:
    1. Moment is the integral of Shear over the Length;
    2. Angular Deflection is the integral of Moment divided by EI over the Length;
    3. Linear Deflection is the integral of Angular Deflection over the Length.

    There are more stressors in the beam than just vertical load at the wheel, and they are significant. The wheel/brake/tire is very quickly spun up from zero to landing speed as the tread patch is thrust onto the runway. This acceleration produces a vigorous aft force at the axle, both bending the gear leg towards the rear and twisting the gear leg. Is this a big load? It can approach the max braking aft load. These stresses are additive to the bending from vertical force, driving you to track stress and strain at not just top and bottom surfaces, but at all four corners of a rectangular section. Streamline shapes get more complicated to track, but are still doable.

    On this topic, you might visit Harmon Lang's website for the round rod gear. He has five pages on his site and one is dedicated to how to ship your bent round rod gear leg to him for anneal, straighten, heat treat, and return ship. It seems the round rod gear legs are prone to being bent... From what I hear, Van's does not have this issue with the flat plate 4340 gear legs they supply. Both gear legs are Vans' designs. Cessna has historically used both flat beam and rod gear (from thick wall tube) in their singles with no particular issues with either. You may draw your own conclusions.

    Some may point out that the torsion should be zero by designing the center of the gear leg to intersect the tread patch. Well, it is a nice idea, but the leg will only be straight at one load, and curved over the entire rest of the range seen in the landing cycle, so you will not achieve the ideal everywhere. You can reduce torsion in the leg by designing with this in mind, but designing the lower end of the leg means you must chose among compactness, frontal area, axle and brake attachment, reducing torsion at max energy, and reducing twisting under braking... Being as you have to include bending due to aft forces from spin-up and stress/strain at critical spots on the section anyway, it is straightforward and easy to include torsion too. Then no matter where you put the tread patch you can know the stresses.

    Or you can include ALL of your external loads in a large deflection FEA and keep iterating until your load/deflection is adequate and failure criteria is satisfied.

    The metric for spring material's effects on compactness and weight are Stored Energy per Unit Volume and Stored Energy per Unit Mass. When I ran the numbers for Energy per Unit Volume, the order is 4340, Graphlite Rod, Uni E-Glass/Epoxy, and 7075. Running Energy per unit mass, the order changes quite a bit . The order is Graphlite Rod, Uni E-Glass/Epoxy, 4340, and 7075. If this were a coil spring, you would be done, but leg length plays such a large role in all of the stresses that the differences in achieved design weight and bulk are not as large as the metrics. Nonetheless, if you need compact (either to package the thing or keep frontal area down or both), 4340 heat treated to 260 ksi is best of the group. If weight is the critical thing to you, Graphite/epoxy is your friend.

    One material not discussed is the range of Titanium Alloys. Worthy of evaluation should low weight be much more important than cost. Enough for one day...

    Billski
     
    Last edited: Jul 31, 2019
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  9. Aug 6, 2019 #29

    flyboy2160

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    If your design will allow for a flat tapered Ti design and if you're willing to design custom axle mounts, you can beat the price of a custom Grove bent aluminum gear set by a lot. You can beat the price of a custom bent Ti set by an enormous amount.

    In my case, the cost of a set of flat waterjet cut Ti main gear legs was ~ 1/3 the price of a custom bent set from Grove and about the price of an OTS bent Grove gear (for another plane). Don't get me wrong; I think Grove makes good stuff. But with some design accommodations, you can use Ti for a 'reasonable' price.
     
    Last edited: Aug 6, 2019
  10. Aug 6, 2019 #30

    Victor Bravo

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    Flyboy I'm afraid you have no idea how much work, time, and the number of fabrication/machining/heat treating operations there are in a Grove gear. His price is incredibly cheap if you know what really goes into it. Full Disclosure: Robbie Grove is a personal friend of mine, and I really want him to be a billionaire. But I also have personally seen what goes into his landing gears, it's something like three or four weeks of time and several trips back and forth to and from the heat treat shop, and that's AFTER the initial (primary) gear fabrication was done.
     
  11. Aug 7, 2019 #31

    flyboy2160

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    VB, I do know what his bent legs go through. I already said I wan't knocking Grove OTS legs; they are a great value. But custom bent legs from him are very expensive - justifiably so given all the custom manufacturing steps. I easily beat the price (~1/3 to ~1/4) of custom bent Grove legs with flat waterjet cut Ti legs.
     
  12. Aug 7, 2019 #32

    Victor Bravo

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    OK, fair enough, but I am not understanding how your flat Ti gear legs work mechanically.... if there are no bends in them how do they attach to the axles and how do they attach to the airframe? Can you posta photo or drawing of the type of installation yopu're talking about using the flat gears?
     
  13. Aug 7, 2019 #33

    FritzW

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    ...this is kinda related

    I think I'm getting a little infatuated with the Whing Ding II landing gear.

    WDII gear.jpg

    With a single bend it's different than most laminated gear. I could see something like this that was just a wooden version of *rigid VP Grove gear. I'm a huge fan of Grove gear but the quote I got for custom gear for the ranger was almost twice what the rest of the airframe is going to cost.

    This could be very light, strong, inexpensive and simple to make.

    *rigid gear is perfectly fine for the kind of airplanes I'm interested in ie. Volksplanes, MiniMax/HiMax, Fly Baby etc...
     
  14. Aug 8, 2019 #34

    Doran Jaffas

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    I actually used to own one. Did a lot of high-speed taxis and got it up in ground effect. It would be a viable airplane now with the newer lighter power plants. We have one at a local Museum that I may try to purchase from them. Call me if you like I will be happy to talk to you about it.
    Doran Jaffas
    616-350-1463. eastern time
     
  15. Aug 9, 2019 #35

    Speedboat100

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    I love it..is it just wood ?
     
  16. Aug 9, 2019 #36

    FritzW

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    Doran Jaffas,
    Did yours have the wood gear? The plans show two different gear options: wood (drawing above), and conventional tube.

    I don't know if the wood gear gets wrapped with fiberglass? The plans I have are the low res copies from cfair and I can't read the details ;(
     
  17. Aug 9, 2019 #37

    Doran Jaffas

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    My gear was 4130 truss type. Built in 1976. The Hovey Wing Ding 2 had the honor..though short...of being the smallest airplane. Then ultra lights came into being and it fit the bill.
    It would not hurt to wrap the gear in fibreglass if wood is used. The fuselage tube was thin wall filled with expanding foam to stiffen it. Flattened at the end for vertical and horizontal stab placement. Sign board for the surfaces. Piano hinges for elevator. Cloth hinges for rudder. Door skins for the front fuselage epoxied onto the tube. The front fuselage also filled with expanding foam. Also door skin material used as gusset material attachment to the rear tube. Wing warping for roll control which is very effective.
    Bob Hovey jokingly said his altimeter was a 75 foot tape measure. At least I think he was joking.
    Doran Jaffas
    Note...the ribs are aluminum tube formed over a wood form and run through holes in the mid spar and plugged into the front spar. Held in place by fabric tension and dope. Wouldn't hurt to wrap the rib tubes with rib stitching.
     
  18. Aug 10, 2019 #38

    Rockiedog2

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    Yep. Exactly what I got.
     
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  19. Aug 14, 2019 #39

    flyboy2160

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    VB, here's a belated reply. (Ha Ha I'm in La La Land, so this is typical La La Land 'on time.')

    Here's the flat tapered leg with a custom steel fitting to allow the use of a standard bolt on axle. The green one is a Grove.
    https://www.flickr.com/photos/140193367@N07/48543708957/in/dateposted-public/
    https://www.flickr.com/photos/140193367@N07/48543713977/in/dateposted-public/
    https://photos.google.com/photo/AF1QipMGKU2-VN0rTUw0uYWTKdWX5TwZqI_o1azS5Zhi
    https://photos.google.com/photo/AF1QipPLKPT0fxoEV-F9L2eEfRi16zLwdM_xCNOc5Wuv
    [​IMG]
    [​IMG]

    [​IMG]
    Here are some pics of one of the teeter-totter mounting schemes on my the steel tube fuselage. The top of the leg rests against a steel 'radius block' that is welded onto the bottom of the longeron. The bottom of inboard end of the leg rests on a shelf in the crotch between to V tubes that are perpendicular to the leg and take the downward load of the inboard end (as well as other loads.)
    https://www.flickr.com/photos/140193367@N07/48543563351/in/dateposted-public/
    https://www.flickr.com/photos/140193367@N07/48543562941/in/dateposted-public/
    https://www.flickr.com/photos/140193367@N07/48543712937/in/dateposted-public/
    https://photos.google.com/photo/AF1QipOru-tyzGXpypg0q6zZVR3q3UkieodY1SPuI-UO
    https://photos.google.com/photo/AF1QipOY6pm5JbFYnQQ_c_9F-4T2nrG_Vupy0iPUJ4Sa

    rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr I'm having trouble inserting the photos, so the links are there for now......

    [​IMG]
    [​IMG]
     
    Last edited: Aug 15, 2019
  20. Aug 15, 2019 #40

    Victor Bravo

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    I can't see the photos either, without giving a stool sample to Google and signing up for their service. Whenever you can post them on the forum, I would love to see them.

    What part of LA... and which airport?
     

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