Hands-off does not equal Perfect Rigging

Discussion in 'Pitts Wings' started by cwilliamrose, Dec 19, 2017.

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  1. Dec 19, 2017 #1

    cwilliamrose

    cwilliamrose

    cwilliamrose

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    My new to me S-2A came with two thumbs up from excellent sources in terms of the state of rig. Both the owner and the ferry pilot report hands-off or near hands-off behavior. Of course one of the first things I'm likely to do is to grab the rigging boards for a quick look. The fact that there were trim tabs on both ailerons just made me that much more curious.

    Turns out the rigging isn't too bad compared to some I've seen. But perfect it is not and I think I see at least one reason why there are tabs on the ailerons. The airplane may have also had a slight tendency to drop the right wing during an upright stall.

    DSCN3905a.JPG
    Upper left wing tip. I'll guess this is about 3 washers worth of error in the direction of rolling left. Note that you can see the far rigging board in good alignment with the other two height-wise. This tells me the top wing is pretty flat (but with some 'twist').

    DSCN3906a.JPG
    Upper right wing tip. I'll give this 2 washers worth of right roll. Again, the third rigging board is just visible in the background indicating a flat top wing.

    DSCN3908a.jpg
    Lower right wing tip. This one earns 3 washers of right roll.

    DSCN3910a.JPG
    Lower left wing tip. This one is almost perfect. Call it one thin washer rolling right.

    When all the errors are added together you get a 2.5 washer error in the direction of right roll. This is a guesstimate at this point but it does show a reason why the aileron tabs were added. Once the wings are true the tabs may not be necessary or maybe a small wedge under one aileron will replace the tabs.

    DSCN3911a.JPG
    Left aileron trim tab. A fairly significant amount of left roll from this tab.

    DSCN3912a.JPG
    Right aileron trim tab. I'm not sure how effective this tab is given the direction of the deflection, the small angle and the fact that it is behind the TE and not in clean air. Still, it is deflected in the direction of left roll.

    I prefer to use the rigging boards visually. I don't use levels (digital or otherwise) if I can see all the boards. It is fairly easy to see a thin washer's worth of error by eye and since that's the smallest practical amount of adjustment available there's no need to try to measure anything smaller.

    Notice I didn't level the airplane. For purposes of a touch-up to the rigging you don't need to have the airplane level. If you want to confirm that the wings are level to the fuselage you will need to go back to square one and level the fuselage, etc.

    I did not check the tail surfaces yet. It is possible there's additional roll coming from there.

    I have not removed the I-strut fairings yet so I don't know what's there washer-wise. I suspect I'll be adding washers at the front spars which means loosening the wires to allow access. I will also use an Aviat wire tensiometer to measure the existing tensions more out of curiosity than anything else. I normally use the G-meter to adjust wire tensions.

    More to come -- after I tweak things..........
     
    Last edited: Dec 19, 2017
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  2. Dec 19, 2017 #2

    EAABipe40FF

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    I just love to see a man in the middle of his element. Add those washers, tension those wires, get that checkout and get your new beauty in the air......:D
     
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  3. Dec 19, 2017 #3

    IanJ

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    Bill, that's a great set of pictures, it really drives home how the rigging boards are used. Hopefully I still remember any of this when I get to that stage. :D
     
  4. Dec 19, 2017 #4

    TFF1

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    I would fly it some first. Fly some aerobatics and see how the plane tracks as it speeds up and slows down. Then start twiddling. One of those things where it probably is out, but you might end up where you are now, too.
     
  5. Dec 19, 2017 #5

    raymoeller

    raymoeller

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    I normally use the G-meter to adjust wire tensions.


    Bill, just wondering about this statement and how/what you mean.
     
  6. Dec 19, 2017 #6

    cwilliamrose

    cwilliamrose

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    I will probably true the wings before I do much flying. The wings are rolling the airplane right and the aileron tabs are rolling the airplane left. Those two things are in opposition when they shouldn't be. An aileron tab serves only one purpose in my mind -- it causes the ailerons to fly in trail where they would not otherwise do so by themselves. I'm not sure if that was the original reason for adding the tabs on this airplane or not, the only way to know will be to get the wings as true as possible and see how the ailerons fly.

    Ray, The wires need enough tension so that when you are at limit load the unloaded wires have only minimal tension. If they have less tension than is required the (landing) wires will loose their tension before you get to limit load.

    So you need to load the airplane to gross weight and go pull some G. The initial wire tension can be fairly low or you can maybe get it in the ballpark with a tensiometer. I do it by feel or by ear based on my previous experience and I used to be able to get it pretty close. The flip side also works -- go pull 6G (or whatever the limit load is) and if the landing wires are still snug you can try releasing some tension.
     
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  7. Dec 19, 2017 #7

    raymoeller

    raymoeller

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    Ahh... so in essence you are trying to flex the wing and see if they start to vibrate/shake?
     
  8. Dec 19, 2017 #8

    cwilliamrose

    cwilliamrose

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    You're actually stretching the loaded wires and that allows the wing to flex. If you stretch the loaded wires enough the opposing wires will loose their tension.
     
  9. Dec 20, 2017 #9

    acropilotbret

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    Just to add to Bill's above comment above on the trim tabs which are usually required to correct aileron float due slight variations in aileron build or wing profiles. So its possible to have a perfect straight flying and stalling airplane that rolls off when you take you hands off the stick but if you fair the ailerons flys straight. You can't rig aileron float out with washers. You can rig it out with spades trim tabs, trailing edge angles, welding wire on the trailing or many other devices. I had even heard that Curtis used to squeeze the aileron trailing edges to shape them differently. That said I have no idea why there are two trim tabs but other than they needed to correct that much float and didn't want a monster tab on one side or the other.
     
    Last edited: Dec 20, 2017
  10. Dec 20, 2017 #10

    Chris McMillin

    Chris McMillin

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    I'd agree they were loading each surface equally or trying. I too wonder if the rigging makes it fly hands off.
    Chris...
     
  11. Dec 20, 2017 #11

    cwilliamrose

    cwilliamrose

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    And you shouldn't try to do so, that's how flying pretzels are created. Thankfully, this airplane falls well short of the definition of a "flying pretzel".
     
  12. Dec 20, 2017 #12

    taff

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    Bill.
    I second those that said that this series of photo's and description is a great rigging class for some. Very interesting. I wish this was available before I got myself involved with rigging mine.

    I love the color, red and gray.

    Now I need to know, what is a G meter tool that is used during wire tensioning?
    If anyone has a photo, that would be good.
     
  13. Dec 20, 2017 #13

    cwilliamrose

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    It's the one in the instrument panel. See post #6.
     
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  14. Dec 20, 2017 #14

    Larry Lyons

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    The G meter tool is hydrocarbon! ;)
     
  15. Dec 20, 2017 #15

    EAABipe40FF

    EAABipe40FF

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    G meter tool is called the stick.:rolleyes:

    I'd like a stick with a break-out device like a torque wrench so I could avoid putting too much G on my aching tail bone. When I complained to my instructor that loops hurt by back he logically suggested that I simply roll inverted and push instead of pull.......:D
     
  16. Dec 21, 2017 #16

    will moffitt

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    Bill, I am missing something here. I am familiar with the approach on the top wing. In wood working they are called winding boards. Used to check twist in a board or level a carcass. But on the lower wing what are you referencing it too?

    will
     
  17. Dec 21, 2017 #17

    cwilliamrose

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    With the lower wings all you are doing is making sure there is no twist. Since the root end is fixed by the fuselage and is not adjustable (except with a welding torch) you simply make the root and tip agree. The upper wing is really the same, the center section incidence is fixed by the cabanes so you need to remove any twist. The difference is only that upper wing is meant to be straight (no dihedral) while the lower wings have dihedral which is set by the length of the I-struts.
     
  18. Dec 21, 2017 #18

    will moffitt

    will moffitt

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    I get it now, I did not look close enough. The little white line is the white rigging board. I was wondering what you were referencing the red board to as I could not see the other board because of the I-strut.

    Thanks
    will
     
  19. Dec 21, 2017 #19

    Larry Lyons

    Larry Lyons

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    YES! I went through this rigging Miss Smith. Rigging boards are the greatest thing since sliced bread. So much more accurate than any level.
     
  20. Dec 21, 2017 #20

    Gmovies

    Gmovies

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    I rigged mine awhile back and got it to where it stalls wings level (ball centered - very important) and also flies hands off cruise speed. I've read that the top wing on a Pitts has more influence on stall than the bottom wing so that is where I started adding/removing washers when trying to get a level stall. I believe I did end up doing a slight adjustment on one lower wing for stall behavior but it did seem to have less influence than the upper wing. Once I got the stall straightened out it was a simple matter of tweaking the one trim tab on my lower left aileron to get wings level cruise hands off.
     

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