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Thread: HELP - Wittman landing gear on UL?

  1. #1
    Registered User olgol's Avatar
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    HELP - Wittman landing gear on UL?

    I am trying to find any information about Wittman type of landing gear and I can't find any specifics. Are there any special design considerations, gotchas, things to look out for, difficulties in implementation, installation, etc. of such gear design? There are a lot of praises and credits to Wittman on this (and the leaf spring) gear design, but why are there not a lot of tailwheel airplanes using this gear design?
    I understand that it will be more difficult to implement than a straight rod or leaf spring gear - the angle of the leg and the angle of the axle relative to the leg are related and will set the camber and toe in of the wheel. Is this a major pain in practice?
    The reason I want to use this gear design on my future UL (if it ever gets beyond the drawing board) is to avoid having any major structure in the fuselage between the firewall and the pilot seat. Installing a straight rod/leaf gear would require some attachment points in that location.
    I would not use a steel rod on my UL probably, but likely a fiberglass rod with a steel axle attached to it at the end. Attaching it at the firewall with a significant angle will move the wheels back to the proper (almost) location. Using a straight rod/leaf gear installed at the firewall will put the wheels too far forward. I assume this is not good for already unstable tailwheel gear design.
    Thanks for any advice!
    Oleg.
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    Registered User djschwartz's Avatar
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    Re: HELP - Wittman landing gear on UL?

    The RV-3,4,6,7,and 9 all use rod style gear legs. You could probably buy a set from Van's; though, even the RV-3 legs are probably too stiff and too heavy for an ultralight

    To fabricate these the steel rod must first be machined for the taper and correct diameters of the axle and mounting socket, then the lower end of the leg is bent to get the correct angle for the axle, then the leg is heat treated for strength. This is a bit beyond the typical home operation.

    Burt Rutan's designs typically used fiberglass gear legs. They are well proven and would be a valuable source of reference material if you want to try to design your own gear. Watch out for the stress riser that will occur where the metal axle assembly attaches to the fiberglass gear leg. Without proper load spreading the leg will fracture at that point at far below the ultimate strength of the gear leg itself

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    Registered User wsimpso1's Avatar
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    Re: HELP - Wittman landing gear on UL?

    Gotchas? Yeah, it has to be engineered to both carry the weight and to absorb landing energy without overloading the attaching structure. They are really simple. Cessna has used them on everything, including retracts, and they have used both tapered tube and tapered leaf spring versions. RV's use 'em. Once you have it figured out and installed, it is the ultimate low maintenance gear system.

    Pazmany's book on landing gear is worth getting, although he does not tell you how to size the things... EAA has a spreadsheet for designing them. If it is no longer available, get back with me, and I will see what I can do.

    Billski

  4. #4
    Registered User olgol's Avatar
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    Re: HELP - Wittman landing gear on UL?

    Thank you kindly, guys.
    Yes, I take back my statement about not many airplanes using the spring rod gear.
    Why is the tapered and bent rod critical? Other than weight considerations, why couldn't a straight non-tapered rod be used? I suppose the weight gain will be dramatic. Why couldn't one weld the axle to the rod instead of bending it to the proper angle?
    Billski, if you can send me the gear spreadsheet, that would be great. I canceled my EAA membership a couple years ago and have not renewed since...
    Oleg.

  5. #5
    Registered User olgol's Avatar
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    Re: HELP - Wittman landing gear on UL?

    One thing I forgot to ask - is there any special requirement regarding the geometry/stiffness of the back angled spring rod gear versus the straight spring gear? When the back angled rod bends, the wheels get some additional toe-in, correct? But then the rod will also twist, giving the wheels some toe-out, correct? Seems that the ratio of the torsional to bending stiffness may be important.

    Oleg.

  6. #6
    Registered User PTAirco's Avatar
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    Re: HELP - Wittman landing gear on UL?

    Quote Originally Posted by olgol View Post
    Thank you kindly, guys.
    Yes, I take back my statement about not many airplanes using the spring rod gear.
    Why is the tapered and bent rod critical? Other than weight considerations, why couldn't a straight non-tapered rod be used? I suppose the weight gain will be dramatic. Why couldn't one weld the axle to the rod instead of bending it to the proper angle?
    Billski, if you can send me the gear spreadsheet, that would be great. I canceled my EAA membership a couple years ago and have not renewed since...
    Oleg.

    The reason for tapering is not simply weight: using a spring landing gear to absorb the landing shocks, you want ALL of the material to work in absorbing this energy. In a straight , non-tapered leg the stress would be concentrated that the attachment point and then fall of rapidly to next to nothing at the axle end. In a perfectly designed leg the stresses would be equal at all points, at full defelction, or as near as we can practically make it.

    Welding an axle to rod is also a no-no; The leg is heat treated to something like 220,000 psi ultimate strength - you would never get a weld to that strength, even with subsequent heat treatment.

    For my own little single seater, I intend to machine the leg to the correct taper, but use a seperate welded axle socket fitted to the straight leg, to avoid the problem of bending it to the correct angle. If I goof and get the toe-in wong or something, I can always make a new socket cheaply.

    The EAA spreadsheet is really useful, but it would help if you get the accompanying article to help you along. It is not particularly user friendly if you don't know all the conventions and symbols the author used. But it does save a hell of a lot of napkins.
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

  7. #7
    Registered User wsimpso1's Avatar
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    Re: HELP - Wittman landing gear on UL?

    Olgol,

    Send a PM with your request and your email address.

    Billski

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