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Thread: Wheel Alignment

  1. #11
    Senior Member jtpitkin06's Avatar
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    Default Re: Wheel Alignment

    Jim,

    Back in September you asked for specific degrees. I can’t tell you what will work best for your airplane as only by doing several taxi test will you find the best. However, we can use information from certified airplanes as to what works for them.

    Cessna’s Service letter SLN 46 concerning wheel alignment for the 120, 140, 170, 180, 190, 195 tail draggers recommends a range of 1/16 inch toe in to 1/8 inch toe out. On a 6 inch wheel 1 degree is 1/10 inch. The reason you can have 1/16 inch static tow in on a tail dragger is the drag on the wheels when in motion will force them to tow out. Within that range I have a personal preference for tow out on tail draggers. As I stated back in September, tow out improves tail dragger stability.

    http://www.teamkitfox.com/Forums/sho...23&postcount=7

    Start with 1/2 degree tow out and see how it tracks in taxi. If the aircraft wanders, try a little more tow. If you have tire scuffing but it tracks OK then back off the tow a bit. This is assuming you have correctly set the tail wheel.

    Far more critical than tow in or tow out on a taildragger is the tailwheel plane of rotation. (The vertical swivel axis) If it is not correct you will have horrible handling, excessive shimmy, or even an uncontrollable airplane. The tailwheel swivel must be vertical or tilted slightly forward. This ensures the tailwheel will naturally seek a position in trail with weight on the wheels.

    If the tailwheel swivel is tilted aft the aircraft tail must be raised to allow the wheel to center and it is inherently unstable. As the pilot tries to steer the airplane the tailwheel will flop back and forth over center or even lock to one side.

    There are frequent posts on this forum by owners installing big balloon tires. Doing so raises the front of the airplane and changes the tailwheel plane of rotation. If you change tire sizes at some point in the future, it is imperative that you reset the tail wheel plane of rotation.

    Hope this helps,

    John Pitkin
    Greenville, TX

  2. #12
    Senior Member HighWing's Avatar
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    Default Re: Wheel Alignment

    Quote Originally Posted by jtpitkin06 View Post
    Far more critical than tow in or tow out on a taildragger is the tailwheel plane of rotation...
    Good info, but a couple of things I might disagree with based on my personal experience.

    Early Kitfoxes were notoriously unstable on the ground. Factory recommendations and fleet experience focused on wheel alignment, regardless of tailwheel geometry. Whenever tailwheel geometry came into the discussion it was always regarding wheel shimmy. I personally dealt with both - about 200 hours with the original bungee gear and the rest with a Grove analogue. In my experiece, a degree or so toe out will help greatly in landing stability.

    A tailwheel tilted slightly forward will introduce a shimmy tendency. Tilted too far aft will indeed result in the tail being raised when taxiing straight ahead, but I never found it to be an issue except possibly when doing tight turns during slow taxi and when hand moving the airplane with the side handle - and neither were sufficient to motivate me to make adjustments. I have had the tilt to the point that if sitting straight and the tail is nudged a bit it would slip to the side, but just like John mentioned with very slight toe in, dynamic forces when near flying speeds tend to move things where they should go. Then keep in mind that when landing in three point, conventional wisdom is to keep the stick full back during taxi which will essentially add a downward force to the tail which will collapse the spring a bit moving the pivot forward. For those with shimmy problems, the quick fix was to move the stick forward to reduce tailspring flex - uprighting the pivot. For me, I would rather have stability at the point of first touch, than stability when slow taxiing or moving the airplane into or out of the hangar.
    Lowell

  3. #13
    Super Moderator Av8r3400's Avatar
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    Default Re: Wheel Alignment

    I'd like to add a little more to this discussion. Toe in/out is always a matter of debate and discussion, so here we go again...

    What I want to bring up is the idea that these toe adjustments of the gear helps with taxiing. These planes should only taxi to 10 or at the absolute maximum 15 mph. Anything more than that and you are entering into the transition to flying phase of operations. (Also the reason why "Highspeed Taxiing" is a serious game of Russian Roulette) At the slow speed of taxiing, toe adjustment is almost irrelevant.

    On takeoff, acceleration is so brisk that by the time speed instability issues arise, you will be flying away from the runway.

    Where it becomes critically important is at the moment of touchdown and the following rollout. I tend to agree with Geowitz's above statement of zero to slight toe out (on a tail dragger) and his reasoning. A tri-gear IMO should have zero to slight toe in.

    Toe out is inherently unstable. In most cases it exaggerates turning tendencies - this is why road racing cars use zero to toe out more than in, like your street car would. Taildragger airplanes are that famous exception to the rule.
    Av8r3400
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  4. #14
    Senior Member jrevens's Avatar
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    Default Re: Wheel Alignment

    Quote Originally Posted by HighWing View Post

    A tailwheel tilted slightly forward will introduce a shimmy tendency....
    Lowell
    I agree with Lowell 100%. It is my experience, having flown many different types of taildraggers, that the swivel axis should definitely not be angled forward. This is counter-intuitive to many people, but is a fact. I believe that the best is a perfectly vertical axis with the airplane loaded to max weight, with the corresponding spring deflection. This will necessitate it being angled back when lightly loaded or empty. I learned this the hard way with tailwheel shimmy so bad that it caused damage.
    John Evens
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  5. #15
    Senior Member jiott's Avatar
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    Default Re: Wheel Alignment

    You guys convinced me I should check wheel alignment on my new SS7 with the Grove gear (taildragger). I did that today and found that the right side was perfect at neutral toe-in/out. The left side was also nearly perfectly neutral within 1/16" (on a 16" tire). This is with the airplane leveled in a flight attitude. This is as close to perfect as I can reliably measure so I am going to leave it as-is.

    I am not surprised at the accuracy of the Grove gear. After all, the gear legs axel mounting pads are machined as are the axels themselves. It would be pretty sloppy manufacturing if they were not dead on. On welded gear, like the Kitfox bush gear, I can see much more chance for alignment error.

    Oh well, my mind can rest easy now even though I think that it was kind of a waste of time to check brand new Grove gear. The process was educational as I studied up on recommended alignment for taildraggers. You all were also very helpful.

    Jim

  6. #16

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    Smile Re: Wheel Alignment

    Hi Guys

    Must say John is very accurate for without weight there will be a positive camber and with weight a negative camber, therefore because you mostly fly with weight it is always better to adjust a total toe of 2mm toe in depending on the true camber of wheels on a full load. That way you will get the most out of a set of tyres and increase your landing life.

    also to watch out for is one side must not have a positive camber where the other side have a negative camber (very important) because it will make your aircraft known as (CRAB) and will decrease your handling.

    Front wheel and known as steering it came to my attention that allot of aircraft's have a high negative castor witch is also bad. if you look at a shopping trolley to make it easy. with the high negative castor when you push it fast the two front wheels shake from side to side and the same with big aircraft's when empty load or half load sometimes it feels shaky when landing.

    i have been studying and doing research on my own for a few years now to do and increase not just Tyre life of aircraft's but also the handling and controlling of aircraft's to make it better and more stable on taking of and landing. but as I read over the internet there is allot of people with this problem but no one to give a change to help them increase it.

    i am originaly from South Africa and currently in New Zealand and have 14 years of commercial vehicle wheel alignment and axle straightening experience and are still looking to get in the market to help airlines/aircraft's to achieve the best out of there suspensions.

    have a good one guys

  7. #17
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    Default Re: Wheel Alignment

    Ok, I have been reading all these posts for wheel alignment on the Grove Gear, but I don't see any numbers. I thought that Paul Lederbrand at S&R told me the SS should be slightly toed in. If so, to what degree? The Grove Gear website has shims in 1/4 degree, 1/2 degree & 3/4 degree increments. Neutral to slightly toed in seems appropriate, but what is slightly? Thanks for all your input. Also, today I was discussing this issue with a couple of airport bums and they said that I should use grease plates to make my measurements. Any thoughts on that idea?
    Last edited by inzersv; 02-08-2017 at 10:01 PM.
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  8. #18
    Senior Member jiott's Avatar
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    Default Re: Wheel Alignment

    I think grease plates are good for ultimate accuracy, but I dare say very few of us used them. Just jockey the plane around before you take measurements to relieve any locked-in deflections on the landing gear. I doubt very much that grease plates would help toe in/out measurements on the Grove gear; camber yes maybe.

    INMHO "slightly" would be 1/4 degree or less.

    I case you haven't seen it somewhere in the wheel alignment threads, I like and used John McBean's suggested method for measuring toe in/out, using a 4' straight edge (level) bolted at its center to the flat surface on the tip end of the axle. This machined surface and therefore the straight edge is now accurately parallel to the wheel plane of rotation. Then you can measure from the fore and aft tip ends of the straight edge in to the aircraft centerline chalked on the floor. This 4' spread gives you 0.20" difference to equal 1/4 degree (tan 0.25 degree x 48" = 0.20").
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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