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Thread: Wheel alignment

  1. #1
    Registered User Bob Kelly's Avatar
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    Wheel alignment

    I read something in the completed projects that has got me to thinkin'
    its about wheel alignment ...
    Not ever having experience with aircraft before
    ( this is my first ) in my haste to get her up and rolling I may have neglected an important part .... the wheel alignment !
    .... i know on cars and motorcycles how important wheel alignment is ...
    on trikes however i have never had a problem
    of any sort ( vw trike , side cars ... etc )
    However, in a tail Drager aircraft the more i think about it the more important it becomes...
    as you only have 3 wheels on the ground ...and they leave the ground too.... the transision when they leave the ground could be all important if one wheel is pulling harder than the others ..... you may be compensateing with the stearing to keep it going stright but
    when that traction goes away there could be a supprise and a change of drection in your travel..... ofcorse all the wheels should leave the ground at the same time... but we all know that doesn't happen .... miliseconds may count big here.
    ..... so my question is.... on a tail drager aircraft, are the front wheels pointing exactly stright ahead ? is there caster and camber on these wheels figured in ?
    are the wheels toed in ? ( which would seam logical ... ) if so how much any idea ?
    the Artical i read said he had one "Squirly airplane.... " then he fixed it .
    .... um...well my aircraft is squirly too ...
    not near as bad as it used to be ...but its not like you point it and it goes that way and never changes..... it wanders quite a bit.....

    My Dads Robenson B1RD is worse than my gyro is at this .... hard to keep the thing going stright ! Being another ultralight perhaps wheel alignment would help this eh ?

    any coments would be greatly appreaciated thank you !

    Bob......
    --------------------------------------------------
    My Momm'a alwayse told me "the impossable is only a little bit harder Son ! "... and ya know ... i do believe She is Right !

  2. #2
    Super Moderator orion's Avatar
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    Wheel alignment is a function of several variables, most of which have to do with the physical design of your main gear. In other words, how does the gear structure react to the various loads it encounters during a typical landing. For instance, if you install your wheels just right under static conditions, you will need to predict how the gear will behave when you touch down hard or a bit askew.

    Ideally, I think most of us would like to install the wheels perfectly straight. However, if you have a flexible gear (such as I've seen on most RVs and many ultralights), a landing or operating on uneven ground might cause an assymetrical toe-out condition, which might make the airplane uncontrollable. The primary forces created by the wheel, and for which you need to set the geometry, are "spin-up loads" and "braking loads". In both cases the force vector is aft, which could twist the wheel's axle backward, inducing toe-out. As such, a bit of toe in is necessary. How much? I've heard from as little as 1 deg. to as much as 4 deg. For bolt-on axles I've seen tapered shims available that will preset the axle to different angles, depending how the spacer is installed.

    The same discussion applies to the angle of the axle in relation to the ground. Ideally you'd like the axle to be horizontal (or parallel to the ground plane) and in airplanes that utilize Oleo type landing gear, this is not an issue.

    For spring gear however, the angle will be some form of a compromise since it will vary with respect to the aircraft's weight and the severity of the landing. If I recall correctly, most folks seem to want the axle to be parallel to the ground with the vehicle at or a bit above gross weight.

  3. #3
    Registered User Bob Kelly's Avatar
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    thanks Orion !

    i think i'll shoot for 2deg toe in ... axel horizonal ... with max load.

    so i got work to do.... standing looking at the beastie from the front the left wheel 's top is in a bit more than the bottom .... other than that they look ok.

    sense this is solid suspension... no give but in the tires , there shouldn't be much flex.

    i think i'll re work the spindles again ... just to be sure

    thanks again Orion !

    Bob....
    --------------------------------------------------
    My Momm'a alwayse told me "the impossable is only a little bit harder Son ! "... and ya know ... i do believe She is Right !

  4. #4
    Registered User Craig's Avatar
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    Wheel alignment

    Bob -
    After doing a lot of work with sports racer set up, most involving suspension, toe-in, camber, etc., I insisted on having my Duce's wheels aligned also.
    I made a chalk line along the centerline, then bungeed two 3-pipes to the wheels.
    After making sure that both wheel centers were equidistant from the center line, I used the pipes to set toe-in at 1/16" over the 3' length of the pipes.

    Realizing that probably the most critical part of the landing was the touchdown, I weighed the plane down to about 2,000 lb (max gross is 1650), and adjusted the loaded camber to be about 1.5 degrees negative (in at the bottom). Unloaded, there is probably 3 degrees of camber.

    The airplane tracks very straight, especially with the new tailwheel, which does NOT have a bent axle - and it took me a while to figure that one out!!! Race cars don't have tailwheels.
    Craig
    Bakeng Duce NX96CW
    Jupiter, FL

  5. #5
    Registered User Bob Kelly's Avatar
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    Smart move useing the extra length of the pipes to help make it acurate...
    i used a similar method when setting the rotor pitch .... instead of the small 8" protractror they recomend i used a 3' level on a jig i made up to be perfectly level on top of the rotor blade.... then when you stand back you can Eyeball the angle againxt another level laid on the rotor bar.... its so much more acurate doing it that way ....

    when dealing with angles i alwayse try to get a stright edge to help me out ...the longer the better.... ....after all seeing 2degrees is tough
    anymore.... need my specs fer dat ! ...
    but translate that into 3 feet and its like an inch and a half easy to adjust to !

    thanks for the sujestions......
    i'll remember to do that on my spindles !
    great idea !

    Bob.....
    --------------------------------------------------
    My Momm'a alwayse told me "the impossable is only a little bit harder Son ! "... and ya know ... i do believe She is Right !

  6. #6
    Registered User flyoz's Avatar
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    Re: Wheel alignment

    My 2 cents worth . My Jab had zero toe in . Direction stability was like a lame duck ! so i tried 1.0 deg - too much - kept trying to track straight . Then i tried 0.5 deg and it worked great . Landing gear geometry is complex and i dont have all the answers just some practical results from my own building and testing . First i found the centerline of the axles , set the fuselage at level then took off the wheels and placed the gear on 2 skatebords to allow it to naturally move sideways as the load is applied . Used weight bags in the pilot and passenger seats to simulate full load . On the jab the axle is bolted to the composite gear with 4 bolts . To set them i used alloy bolts and nuts then filled the gap with cotton flock and Araldite 420 resin . I drew a line from the centerline of the axles ( checking the wings corresponded to that line ) to an equidistant point further than the tips of the wings and placed a vertical boards at both ends . I used a cheap key ring laser which i machined a piece to fit on the end of the axle and thats how i could accurately get the Down angle and set the Toe in . You need a ball joint on the laser and rotate it until you can zero it at any rotation to get accuracy . When the resin was cured i drilled out the alloy bolts and replaced them with the correct aircraft grade . The resin "spreads" the loads on the axle . I have used it like this for ten years now on mostly grass and gravel strips and most of my tyres wear evenly indicating i got it just about right i think .
    Flyoz
    Attached Thumbnails Attached Thumbnails Wheel alignment-landing-gear.jpg  

  7. #7
    Registered User BBerson's Avatar
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    Re: Wheel alignment

    A taildragger airplane needs a bit of TOE-OUT to help make the unstable taildragger design slightly more easier to handle.

    I know this controversial.... if you don't want to trust my 30 years maintaining and flying taildraggers.... then perhaps check with EAA.

    I think EAA will say " set the wheels on a taildragger at 0 degrees or maybe a bit of toe-out, toe-in should not be used."


    Here is short explanation:
    Imagine you are swerving off the runway with a sharp turn to the right. (This is called a groundloop) Centrifugal force will make the inside wheel will come off the pavement about half way into the right hand swerve and the outboard left wingtip might hit the runway. With only the left wheel on the runway, it is desirable to have a bit of toe-out to help stop the swerve to the right before in gets out of control. Toe-in will make it worse.
    Toe-out helps add stability for the taildragger, which is always unstable and requires constant pilot attention.

    The tri-gear airplane is stable and could use a bit of toe-in like an automobile.
    BB

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    Re: Wheel alignment

    Quote Originally Posted by BBerson View Post
    A taildragger airplane needs a bit of TOE-OUT to help make the unstable taildragger design slightly more easier to handle.

    I know this controversial.... if you don't want to trust my 30 years maintaining and flying taildraggers.... then perhaps check with EAA.

    I think EAA will say " set the wheels on a taildragger at 0 degrees or maybe a bit of toe-out, toe-in should not be used."

    BB
    Well, I certainly haven't seen a consensus on toe-in/out, but I certainly place high value on the advice of someone with practical experience like yourself. Do you have any insight on other factors such as gear width and wheel camber? The gear I've built is wider than the original design--I'm wondering how this will affect handling. Also, I have way too much camber (long explaination of how that happened). I'm wondering if the excess camber will have any adverse effects other than really ugly tire wear.

  9. #9
    Registered User BBerson's Avatar
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    Re: Wheel alignment

    The Cessna 180 service manual says: Ideal setting is zero toe-in and zero camber at normal operating weight."

    It is just my opinion that a bit of toe-out for a taildragger helps. I have adjusted my airplane that had toe-in when I bought the airplane. I installed some tapered shims to give a bit of toe-out and improved the stability.
    BB

  10. #10
    Registered User PTAirco's Avatar
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    Re: Wheel alignment

    Maule manual states that wheels should have between 0-1/4" toe-IN, measured across the gear, between front of tire and rear (on a 7.00 tire, or 0-3/8" on a 8.50 tire).

    Each to their own, I guess.
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

  11. #11
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    Re: Wheel alignment

    The world may never know. I've Googled the issue and come up with a score of three to one--tow in. And each team can explain definitively why their side has the right answer. Surely someone out there must know why there's two schools of thought--some not-so-obvious reason like wet grass vs. dry pavement or wide gear vs. narrow gear or spring vs. oleo, etc, etc??

  12. #12
    Registered User BBerson's Avatar
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    Re: Wheel alignment

    It depends on what you want.
    For minimal tire wear, a bit of toe-in is best because the drag will pull the tire to zero, which has the least tire wear.

    But for stability at the expense of some tire wear, toe-out can save the airplane by adding stability.
    BB

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