Landing gear placement

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by fulcona, Sep 11, 2017.

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  1. Sep 11, 2017 #1

    fulcona

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    Trying to determine the proper placement of the main gear on my original design (taildragger). An EAA publication, and most others that I have read, state to draw a line through the thrust axis, place the plane in level flight attitude and determine the CG. From this CG, draw a line 15 degrees forward and where this line intersects the ground, that's where the wheels should touch. This is fine except no article I have read mentions which CG they are using. ( empty weight CG, gross wt CG, ect ). And if the plane were to have a ,say, 5 inch CG flight range, where along that 5 inches would you begin to draw the 15 degree line. To make matters worse, I ran into another article that says not to use the thrust axis, but at the planes vertical CG, which complicates matters . Suggestions? Thanks
    Neal
     
  2. Sep 11, 2017 #2

    Vision_2012

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    Gross weight CG would matter most, don't you think? A plane can't land or take off without the pilot and fuel.
    If I were applying the rule across 5 inches, I would use most aft maximum CG and check that the applied elevator moment can lift the nose.

    And if I was designing a plane with commitment to build, I would compare to similar designs. Sometimes this may include buying plans in order to reverse engineer what is done or buying more than one book on design. Education may have a cost to it, your life doesn't.
     
    Last edited: Sep 12, 2017
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  3. Sep 11, 2017 #3

    Winginit

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    Since its a tail dragger, I would try to locate the main attachment points as far forward as you can, and possibly cant the gear forward rather than straight out from the plane. Then I would do a preliminary weight and balance to see how long the airplane needs to be to counteract the engine and main gear. The farther forward the tire patch touches the ground, the less tendency to groundloop. If you are using in wing tanks, I'd try to put them on the CG so there is little change in CG as the fuel is depleted.
     
  4. Sep 11, 2017 #4

    Aesquire

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    With airplane level, axles directly under the leading edge. With swept wings like a Nieuport 17, midspan leading edge.

    If that lines up with the 15 degree rule you are darn close. See the Kansas Dawn Patrol reports on the subject about their Nieuport replicas. They flew for years with the wheels too far forward and paid for that with squirrel tendency and multiple repair jobs.
     
  5. Sep 11, 2017 #5

    harrisonaero

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    Place the main gear axles directly below the leading edge of the wing when the aircraft is level. Empirical evidence over a century says it's the best compromise between handling and braking.
     
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  6. Sep 11, 2017 #6

    Victor Bravo

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    Uhhh... no.

    The farther "forward" the tire patch contact area is, then the farther forward this contact patch is from the aircraft center of gravity. The farther forward the contact patch is from the CG, the GREATER length of mechanical arm (leverage) that the mass of the aircraft has to swivel the aircraft around backwards (groundloop).

    If on the other hand the tire contact patch was directly below the aircraft CG, the mass of the aircraft would never have any leverage or impetus to swivel or groundloop the aircraft. But in this case the slightest application of brakes would make the aircraft nose over and whack the propeller on the ground.

    So taildragger aircraft design has always been about the balance between directional instability (which comes from having the tire patch forward of the CG) and the ability to brake without nosing the aircraft over.
     
    Last edited: Sep 12, 2017
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  7. Sep 11, 2017 #7

    fulcona

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    Thanks for the hints. It was not lost on me that the RV aircraft have their main gear about under the wing leading edge.
     
  8. Sep 11, 2017 #8

    Rockiedog2

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    I did what Harrisonaero said and it worked OK on my original design(once I got the tail big enough). But I think it's something of a guess and see how it does then adjust if necessary. I didn't do any of the cg/thrust line/15 degree stuff just went with what has been successfully done so many x. If I'm not mistaken Leonard Milholland designed his planes with the axles under the LE and that worked on the 2 UL's but he later adjusted the gear forward (I think it was 6") on the DE cause of a nose over tendency.
    In my experience, there aren't many planes that have a tendency to ground loop...it's usually the PIC that has the ground loop problem.
     
    Last edited: Sep 11, 2017
  9. Sep 11, 2017 #9

    Rockiedog2

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    from VB
    >>>So taildragger aircraft design has always been about the balance between directional instability (which comes from having the tire patch forward of the CG) and the ability to brake without nosing the aircraft over

    Yeah, VB.
    I think that if you ask Leonard if his DE developed an increased tendency to ground loop after moving the gear forward he'll tell you no, the tail is so big it didn't matter. That wouldn't be the case on this SS1 of mine, it's already named Squirrel. LOL.
     
  10. Sep 11, 2017 #10

    Pops

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    Rule of thumb is axle at the leading edge is a good place to start. Its what I did on the SSSC and it was perfect. If you had the stick back, you could brake hard with no danger of nosing over . Yet I could slow taxie with the tail up and fuselage level with a little power and dragging the brakes and full down elevator. That is needed for an off field airplane so you can walk the tires around the bigger rocks and such.
     
  11. Sep 11, 2017 #11

    TFF

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    15 deg is a general rule. Real long or real short gear will change that. Remember the flight controls more the airplane around the CG/NP range. Too far forward and the rudder does not have enough authority to bring it back if it swings. Too close and it has too much authority. Too far forward and the tailwheel has lots of weight on it; great for three pointers; bad for wheel landings. Too close and you are on your nose. The original gear on Starduster Toos were too far forward; the gear placement has been moved back and lengthened twice to make it modern. Skybolts have a similar revision update. Some is the change from most GA fields being grass to pavement. WW1 plane is fine on grass; probably wrecked on pavement. If you have a wide CG range, probably smack in the middle is where you would design it.
     
  12. Sep 11, 2017 #12

    harrisonaero

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    Also depends on the "flavor" of taildragger you want. Easy to handle and mostly longer or smoother strips- put the mains further back. Want to stand on the brakes, land super short on rough ground, and/or keep the tail up during ground maneuvers- put it further forward. Use the "leading edge" guideline as a nice intermediate.

    Now if you want a real religious taildragger argument- toe in or toe out to reduce groundloop tendency? (before you ask- I suggest using zero toe-in, just keep it straight)
     
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  13. Sep 11, 2017 #13

    Rockiedog2

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    Yessir I agree on zero toein/out. I use a long run of tube(all the way across), jig it in place plumb and solid and after getting it finish welded then cut the middle out and use it somewhere else.
     
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  14. Sep 12, 2017 #14

    Pops

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    This is the jig I made to line up the axles on the three Bearhawks. The 2" steel angle was one piece and welded at both ends and then the middle cut out

    BTY-- A Baby ACE built to plans will go over on its nose very easy. Even some C-120-140's have a fix to extend the axle forward about 3".
     

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  15. Sep 12, 2017 #15

    Aerowerx

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    No one mentioned Pazmany's landing gear book???

    And I would use the most forward CG position.

    Pazmany's book will explain all this.
     
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  16. Sep 12, 2017 #16

    Rockiedog2

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    Pops I think that jig oughta do ok. LOL.
    we had a 140 with the regular gear...another undeserved rep...we thought it was fine, I was taught that all TDers can be stood on the nose just some easier than others. As so often, the PIC was usually the problem; I flew that plane all over the country and never a problem and I had about 100 hours total most of it in a Luscombe. Same with the Luscombe...said to be a squirrel...ours did just what it was told and nothing else and did it when it was told; great plane airborne and on the ground. More PIC problems. The Tcraft was said to be a "floater"...ours loved to sail in ground effect unless the PIC figured out what speed it wanted and flew that then no more float. PIC again. I was taught to get to know the plane and adjust to it.
    who was it that came up with the old sayin "there aren't any bad planes, just bad pilots"? Well, I've always looked at the Gee Bee and wondered if whoever said that had ever seen a GB. That thing looks like it would be a little mean. I wonder if Delmar landed it on 50 ft wide paved runways.
    Sorry didn't mean to get off on all that. But it was LG related, wasn't it.
     
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  17. Sep 12, 2017 #17

    BBerson

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    The Helio puts the mains way forward. It has a huge rudder and good tail wheel.
    I asked the pilot at Oshkosh and he said it works fine. Start with 15° from CG for a lighter plane.
    The CG is the 3D single point where horizontal CG and vertical CG and lateral CG meet.
     
  18. Sep 12, 2017 #18

    Rockiedog2

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  19. Sep 12, 2017 #19

    Pops

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    My daughter had C-140 with the wheel extenders. I took them off. My grandson got a share in a Luscombe when he was 16 years old. The only thing I hated about it was in the winter it would freeze you to death with all the air blowing up through the holes around the main gear. +20 felt like -20. I could just take so much.
     
  20. Sep 12, 2017 #20

    Pops

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    I always wondered about the Helio, the main gear is way forward .
     
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