Over Sensitive Rudder

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Kyle Boatright, Jun 12, 2018.

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  1. Jun 12, 2018 #1

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

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    I was reading the flight review of the BD-4 in the latest kitplanes and the article indicated (in more words than I'm using here) that the rudder was overly sensitive.

    How do you correct something like that?
     
  2. Jun 12, 2018 #2

    Chris In Marshfield

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    Reduce max throw, maybe? Only other thing I can of is changing the “gear ratio” between making rudder horns longer and/or cables closer to the pivot point on the rudder pedals.
     
    Last edited: Jun 12, 2018
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  3. Jun 12, 2018 #3

    Pops

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    The rudder on my Falconar F-12 was very light with no breakout force. If you took your shoes off and touch the rudder pedal with your toe where you could feel it, you had some rudder input. I always flew it with my shoes off. After shutting down the engine, I would open the gullwing door on my side and put my shoes out on the wing. Got a few funny looks. Really hard to have to much rudder. The only time you have to much power is when you run out of rudder.
     
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  4. Jun 12, 2018 #4

    Kyle Boatright

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    My sense from the article was that there was very little breakout force and with just a little persuasion, the rudder would give a substantial amount of yaw. Looking at the pictures, it doesn't look like it has a huge aerodynamic balance, so I wonder if there is something that could be done with aspect ratio or trailing edge thickness?
     
  5. Jun 12, 2018 #5

    Turd Ferguson

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    Depends on who and how they decided it was overly sensitive. They may be fighting freeplay and trim band with rudder centering outside the trim band. That would give the impression the slightest rudder movement is yawing the plane. Friction, builder variation, etc. can all impact rudder response.

    The original BD-4 had a Wittman style rudder, the rudder was not the full length of the vertical fin and it was changed because pilots complained there was inadequate rudder response.
     
  6. Jun 12, 2018 #6

    wsimpso1

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    If the problem is too light control through the whole range, yeah, changing the ratio is one way. Another thing you can do is a trim tab geared to be anti-servo (like a Cherokee stabilator trim tab) - When the rudder moves to one side the tab moves further that way.

    If the problem is very low on-center feel (very low breakout forces), you can thicken the trailing edge (trim wedges on both sides), sharpen the corner of the trailing edge, or apply Gurney flap both ways - a little T or two angles with the vertical leg of the T attached to the rudder with the top bars sticking out into the airstream (these likely stick out 1/4" or less on each side and may not need to be more a few inches long. All of these can be tried by taping them on with high speed tape for test hops, and make them bigger or smaller as you see fit, then do a permanent change.

    Oh, I can think of one other way. Centering springs. They give a kind of artificial feel, but they do increase the control gradients and making one anchor adjustable gives rudder trim. They are used on the Piper Cherokees and all of the derivative models.

    Have fun...

    Billski
     
    Last edited: Jun 12, 2018
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  7. Jun 12, 2018 #7

    BBerson

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    Install a hydraulic damper. My Grob has a damper installed to prevent flutter, required by Airworthiness Directive.
     
  8. Jun 12, 2018 #8

    wsimpso1

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    Hmm, I can see it reducing susceptibility to flutter of a control surface if it is mounted between the surface and the fin or fuselage... That would add force resisting movement in proportion to movement speed. Which means it would resist a lot if you are trying to move the rudder fast, but would resist only a little if you are squeezing on the rudder slowly - Yes? That would tend to damp off flutter, which is fairly fast movements, but have little effect on most small movements, like normal control inputs.

    So how would you apply a hydraulic damper to make the rudder less sensitive to control inputs?

    Billski
     
  9. Jun 12, 2018 #9

    BBerson

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    The absorber is under the seat. Pushrod to pedals and pushrod to rudder. It's a taildragger. I was concerned about the slowed rudder response after installing it. About 1 second to full deflection from stop to stop. But it didn't seem to matter any after a few flights. The aircraft isn't very fast to respond like a short span Pitts.
     
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  10. Jun 12, 2018 #10

    Pops

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    Maybe springs in the rudder cables like on a Stearman. The F-12 had so much rudder I removed the tailwheel springs for a free swivel tailwheel, no problem down to a slow taxie. Even in a slow taxie a little blast of power and put the tail where you wanted.
     
  11. Jun 12, 2018 #11

    Sockmonkey

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    Does it use two control horns sticking out to the side or a single one sticking forward?
     
  12. Jun 12, 2018 #12

    Dana

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    A larger vertical stabilizer would increase yaw damping and make the rudder less sensitive.
     
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  13. Jun 12, 2018 #13

    wsimpso1

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    Hmmm. In an ideal world, the damping would be at the rudder, picking off all of the rudder vibration and slowing any inputs to the rudder. Two bad parts: Packaging it without adding drag, and CG shift. Putting it under the seat minimizes CG shift, but leaves the springiness of the rudder cable system in the loop with the inertia of the rudder. The spring rate of the rudder cables is pretty high so remote damping probably works pretty well anyway. Preventing sharp rudder inputs comes along for the ride and will prevent flutter due to sharp inputs, which is a flutter trigger. So it works nice for flutter suppressing.

    A sensitive rudder is one that gives more airplane yaw response than wanted for a given deflection or force on the pedal. With the pedal moved slowly or just held deflected, a hydraulic damper mounted conventionally will not change the yaw response to pedal input ratio... Is there some other way to add the hydraulic damping that reduces the ratio?

    Billski
     
  14. Jun 12, 2018 #14

    TFF

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    The question is what is sensitive? Control harmony is always the sign of the last details being covered. Very few planes have perfect harmony. Sensitive rudder; is it sensitive for a C150 pilot or an Extra 330? I learned to fly RC planes before computer radios. The first adjustments on a new model were changing the pushrods on the servo or flight control arms to respond like you wanted. Very much lost on many of the computer radio people; I have had to fix many new fliers planes. A homebuilt could require some major surgery. Re-welding pedals or control horns. At minimum drilling new holes. Please forbid having to slice open a composite control or drill out a bunch of rivets to get in there. A control rod or cable that may have had clearance is now sawing into something. Most just learn to live with it unless it is flat dangerous.
     
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  15. Jun 12, 2018 #15

    BBerson

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    Cessnas have nosewheel shimmy dampener which slows the rudder somewhat. Tail wheel planes usually have springs to the tail wheel which tend to damp.
    The BD-4 lacks both, I think.
     
  16. Jun 12, 2018 #16

    N804RV

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  17. Jun 12, 2018 #17

    Pops

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    I do not like a swept rudder. There is a yaw and pitch coupling when using rudder on a swept rudder. I can demonstrate the coupling with a straight tail C-172 ( 1956--1959) and the swept tail C-172 ( 1960-up) I demonstrated the coupling to a IA friend of mine that had a 1965 C-172. We flew my 1959 and then his 1965 and he seen the difference. He was used to the pitch coupling of his 1965 and never really noticed it. Where you can see it the best is trying slow flight without stalling and using rudder to pick up a wing.
     
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  18. Jun 12, 2018 #18

    BBerson

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    So that's why the scale C-182 RC model I flew today didn't roll well. :)
     
  19. Jun 13, 2018 #19

    Aesquire

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    Shows that some people will not like the control feel others love.

    On an already built plane, with no other issues, changing the shape of the trailing edge of the rudder is the easiest, and probably safest way to deal with it being too sensitive.

    Going from a "slow" handling plane, with low roll/pitch rates and high stick forces ( or yaw/pedal forces as here ) to one that is faster and lighter can be a challenge, and not just the shame of a PIO on takeoff in front of others. Imagine going from a Murphy Moose to a BD-5. I've scared myself. The trick is to not clench up, and use fingertip control... easier said than done.
     
  20. Jun 13, 2018 #20

    Pops

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    Love the controls of the RV-4. So people says the RV-3 is better, I don't know, never flew one.
     

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