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Thread: Spring Steel versus Spring Aluminum Landing Gear

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    Spring Steel versus Spring Aluminum Landing Gear

    Does anyone have engineering data on the performance of spring aluminum versus spring steel in a landing gear application for a 1000 lb. gross weight aircraft? I have not been able to find a source for spring steel which is specified in the plans. I can get a supplier to make the gear legs out of spring aluminum, but I would like to understand the risks. (I understand the weight savings.) Any comments?

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    Super Moderator orion's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    The most common source for spring steel main gear legs for light aircraft is an automotive wrecking yard - leaf springs as used on trucks. Just match the dimensions as close as you can to those specified in the drawings.
    "To live is to learn; to learn is to live" (author unknown)

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    Re: Spring Steel versus Spring Aluminum Landing Gear

    The thing to remember about aircraft landing gear is that it has to carry certain loads, and it has to absorb certain amounts of energy. Think about it - It is supposed to take a certain vertical velocity on landing, and absorb the energy without going over a certain load...

    So, it has to be strong enough and have the right load vs deflection curve. Change from steel to aluminum, and the section has to get a lot thicker to carry the load, and then the load deflection curve will be too stiff to suck up the energy. T

    Anyway, heat treated steel can do what you want at one weight, aluminum will weigh more by the time you change the design enough to both carry the load and absorb the energy. Now, this is not to say that you can not design an aluminum gear leg that is a strong as the steel one specified in the plans, but if it saves weight, you will either bend the gear or bend the airframe in a landing where the steel one would have just sucked it up.

    We can talk more. The EAA has a design spreadsheet that can take you through spring leg gear calculations, but then you are becoming a landing gear designer when you do that.

    Billski

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    Re: Spring Steel versus Spring Aluminum Landing Gear

    Thanks for the comments. Aluminum does require a thicker section. The weight of the aluminum gear legs is 50% of the spring steel gear gear legs. In my case, that is about 17 lbs. in weight savings. What is not clear is the deflection forces comparison. I would very much like to capture that weight savings as I am looking into changing the design from conventional gear to tri-cycle gear. I'll check the EAA for the information that you mentioned. Thanks very much.

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    Registered User wsimpso1's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    Like I said, if the Al alloy leg saved you weight, it will have a lot less energy absorption. If you NEVER do a hard landing, you will be OK. Me, I know that I will land a little hard sometime...

    Bill

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    Re: Spring Steel versus Spring Aluminum Landing Gear

    Lots of aerobatic planes today, and many other homebuilts, use aluminum gear legs. This includes the Stephens Akro I have. I have flown large numbers of aircraft with both steel and aluminum legs, there is little inherent handling difference between the two. Both are "bouncy" compared to oleo strut gear systems. Neither aluminum nor steel has much internal damping, they both make fairly efficient springs (that's why your car has shock absorbers). The key with either material is to design it properly to have adequate stiffness for the gross weight of the aircraft. A gear system with inadequate stiffness will wallow badly and make ground handling very uncomfortable. With these systems, stiffness and strength are closely related; so, a gear with inadequate stiffness is also likely to have inadequate strength to survive a hard landing. Steel is easier to fabricate yourself. As has been said, you can modify leaf springs from a car or truck. Aluminum must be heat treated after forming to get the needed stiffness. That is not a home operation.

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    Registered User wally's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    There is a company named Grove that makes and sells aluminum gear for homebuilts. You might see what they have.

    I have "drop tested" my Steel tube, rubber bunge cord sprung landing gear on my Pitts a couple of times. A very light design and stout. My G meter showed 3Gs is not a problem for landing.

    Not a pretty touchdown but it didn't break! A very firm arrival.
    Wally

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    Super Moderator Midniteoyl's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    The Express went from steel to Al legs with an increase in usefull load. Believe Grove makes them.

    They are stong/stiff enough to wreck the wings on a real hard landing.
    Jim

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    Registered User wsimpso1's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    Wouldn't you rather the gear legs bend and need replacing instead of having to build new wings?

    This has been the entire conversation I have had with Grove. They tell me that their gear will replace the steel gear that Van's sells, and save weight. When I asked about energy absorption, they told me that they pass the 3-g requirement. When I try to get them to address the issue of energy absorption, they brushed me off. I don't know why, but they clearly did not want to talk about it openly. Maybe it is ignorance, maybe they just don't care if someone damages their wings when the base legs would have held up...

    Designing a gadget to a certain load requirement is easy, designing it to also suck up a certain amount of energy is another entirely. Both are included in FAR Part 23 for a reason. Experimentals do not have to meet Part 23, but it is a good idea. Van's gear is designed and tested to meet the rules, and will suck up a pretty hard landing. Change to an aluminum leg that only meets the strength requirement and saves weight (Grove sells gear legs for RV's claiming a weight save), well, I would bet that you wreck things in a landing that the factory gear legs would have shrugged off.

    Billski

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    Super Moderator orion's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    The trade-off between just the right amount of spring and failure mode is a pretty critical one and usually it tends to go toward the stiffer side, which does often lead to structural damage in case of serious mishap. For instance, most certified aircraft have gear that are quite durable but when the load exceeds reasonable limits, a bent gear leg is almost always accompanied by significant structural damage. The Cherokee gear for instance, is attached to the web and a rib (I don't recall the exact mounting but it struck me the first time I saw it that there sure didn't seem to be enough material there) - should that wheel ever hit a significant pot hole or other obstruction, substantial wing damage is almost a guarantee.

    Cessnas are also subject to this in that by the time you bend the gear, you're almost guaranteed to destroy the structural box in the fuselage. And I've been told that that's almost more expensive to repair than wing damage.

    The one gear mounting I really liked was the configuration used on the Midget Mustang. If you can find a set of plans for that one you might want to use a similar idea.
    "To live is to learn; to learn is to live" (author unknown)

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    Registered User wsimpso1's Avatar
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    Re: Spring Steel versus Spring Aluminum Landing Gear

    The Cherokee gear strut is bolted through the main spar web. The system more than meets Part 23, but if you exceed Part 23 loads and energies (drop heights and velocities), you do wreck the wing.

    My point was not what happens when you exceed Part 23 loads and energies, but when you take a gear like what Van's sells, that does meet Part 23 for drop heights and loads, and replace them with aluminum ones that will exceed Part 23 loads at considerably less energy than Part 23. Now you have a system that can wreck your airplane under considerably less severe circumstances than was originally intended. Does Mkinser3 really want that in his airplane?

    Billski

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    Re: Spring Steel versus Spring Aluminum Landing Gear

    Thanks for the responses and comments. My gut check was that Aluminum would not perform as well as Spring Steel in a hard landing, but would suffice for "normal" landing loads. I think the comments have confirmed that sense. At any rate, it does not look like anyone has the hard engineering data from Grove. Thanks again for the comments.

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