Aircraft designed for rudder only (aileron-less) turning

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by danmoser, Mar 16, 2010.

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  1. Mar 16, 2010 #1

    danmoser

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    Some aircraft are designed to accommodate rudder-only turns.
    My first RC model airplane had this, and my first hang glider did too (Quicksilver B).
    Generous amounts of dihedral seems to be the key ingredient in making reasonably successful rudder-only turns..
    But I can't find much in the way of design guidelines for this type of aircraft.
    A friend of mine is getting close to flying a prototype hang glider that is strikingly similar to the Quicksilver, and I'd like to have some design assurance that he is using the right amount of dihedral, rudder area, rudder lever arm, etc.

    Happy landings!
     

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  2. Mar 16, 2010 #2

    bmcj

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    Many factors will induce roll-yaw coupling. Dihedral is very effective means. Wing sweep is another.
     
  3. Mar 17, 2010 #3

    piperpilot1363

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    I once made an rc aircraft with EXTREME dihedral dual axis control (rudder and stabilizer). Needless to say, it turned on a dime, and crashed after wing failure.

    Andrew
     
  4. Mar 17, 2010 #4

    BBerson

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    My very low aspect ratio models turn very well with rudder only and no dihedral. Rudder causes instant bank and yaw at the same time. My high aspect ratio full size glider on the other hand has very little reaction to full rudder.
     
  5. Mar 17, 2010 #5

    lr27

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    Well, you could look at the proportions of the Skypup for starters. I hear it flies well.
     
  6. Mar 17, 2010 #6

    danmoser

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    Actually, I was looking for some design guidelines based on sound engineering fundamentals.
    Obviously, we could just copy the Quicksilver dimensions, but who knows what they were based on?... guesswork?.. trial & error?.. TLAR (that looks about right)?..
    Not that those aren't valid ways to get the job done, but surely there are some sound ways to engineer a new design from scratch without too many iterations and prototype crashes.. human flesh & blood are at stake here.

    This kind of aircraft design just hasn't been done very often ..
    Early Quicksilvers, Weedhoppers, SkyPups, and a few others were rudder-only turn controlled aircraft.
    There are admittedly some limitations, especially strong crosswinds.
    And I'm sure there are those who want to post "Why not just use ailerons?"
    .. believe or not that actually has occurred to me.

    However, the topic of this thread restated is:
    How do you properly engineer an aircraft that turns with rudder alone.. no ailerons or spoilers?
     
    Last edited: Mar 17, 2010
  7. Mar 17, 2010 #7

    lr27

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    Keep in mind that although I'm an engineer and an aircraft fanatic, I'm not an aero engineer.
    One thing you want to be very aware of is the moment of inertia of the wings in yaw. The higher that is, the longer it takes to start or stop a turn. Also, I bet that contributes to Dutch roll. I guess you'd need to know that in roll too. So you'd look at the force generated by the rudder vs. that moment of inertia. Plus you'd look at the dihedral effect. I'm sure there's some formal way in the books, but if the wings were long I bet the main effect is the change in angle of attack due to yaw.

    If you figured out the moments, etc. for the Sky Pup it would give you an idea of some acceptable relationships, but of course that doesn't tell you what the acceptable range is.

    Wings with a lot of dihedral probably need more damping in yaw to avoid dutch roll. Or at least that's been my experience with models.

    Note the two extremely well designed models at the following URL's:
    http://charlesriverrc.org/articles/bubbledancer/PDFs/bd_V3.pdf
    http://charlesriverrc.org/articles/allegrolite2m/Allegro_Lite.pdf
    These are probably a bit better damped than necessary, but note the values for Vv. One is .024, the other .025. I just did a guesstimate of the same value for the Sky Pup and got 0.020, plus or minus a lot. Damping goes with the square of the tailboom length, actually, so handling may be better if the tail is small but further back. However, the Sky Pup's tail is close in and it works.

    I have not seen Vv formally, but it appears to be the area of the fin times the tail moment between 25%MAC for wing and fin, divided by the wing area times the wing span, i.e. (a*m)/(A*span)

    Note that the equivalent dihedral angle in both cases is just over 12 degrees, which is a traditional value for models. On the Sky Pup, however, it's only 6 degrees. That seems like a big difference. Probably it goes along with the smaller tail volume. (Vv)

    For the first model, moments of inertia in each axis are given. You could convert those to radii of gyration and compare with a number for tail damping, which is probably proportional to the area of the fin times the square of the moment (i.e. distance 25%MAC wing and tail), divided by moment of inertia of wing, I guess. I think you'd also want to multiply the whole mess by the square of the speed because damping will go up with aerodynamic forces.

    Again, I'm sure some book has a far more formal development of this.
     
  8. Mar 17, 2010 #8

    Michealvalentinsmith

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    Dana has some interesting insights into the limitations of rudder only control in the Quicksliver. There's a thread somewhere where he talks about it.

    I know from personal experience when ridge soaring at least, it's more important to get the directional heading you want right now, than the bank angle. If you get gust turned back into the hill you're headed for a fast downwind crash. If you can get the nose back into the wind quickly you can usually do OK.

    I think the problems start with ground handling such wings as there's no way to level the wings.

    Mike Sandlin quickly found ailerons essential for his rolling start machines. Without an ability to level the wings he found his rudder only machines ground looped.
     
  9. Mar 17, 2010 #9

    Dana

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    Rudder-only planes do, of course, have a long history in R/C models and ultralights. It can work, and it's simple, but you lose maneuverability, and the ability to cross control for a crosswind landing, which depending on where you fly may or may not be important.

    The early Quicks had only rudder (coupled to the swing seat for some weightshift roll effect), and weightshift for pitch control. In the one I restored and flew, the rudder seemed to be adequate, but pitch control was woefully lacking for a high engine powered aircraft, leading to my crash.

    Later Quicks added an elevator for two axis control and were quite successful, along with Weedhoppers and similar aircraft. Later, Quicksilver added "spoilerons" for roll control, though they were so ineffective that they put them on the pedals and left the rudder on the stick. Even when they added real ailerons, the Quicks had (and still have) so much dihedral that a small rudder application will override full aileron.

    If you're building an aircraft that is "striking similar to the Quick", I'd suggest using their proportions. Yes, it was doubtless a TLAR thing with no formal analysis, but it worked... and that's what engineering really is, anyway.

    -Dana

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  10. Mar 17, 2010 #10

    bmcj

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    We flew our Quicksilvers in some very strong winds and some crosswinds too. Being a relatively slow plane, we could usually adjust our landing direction to better align with the wind, even if it meant landing diagonally across the runway. We even flew in steady 30+ knot winds that blew 90 degrees across the runway, but that called for driving it down under power (to keep from 'backing up') and having some big guys out there to grab the flying wires on the ground and walk us in.

    The point is, the Quicksilver was light and slow enough to land across the runway during crosswinds, but a faster aircraft would not be able to, so would need some sort of roll control to keep the wing down during a crosswind approach.
     
  11. Mar 17, 2010 #11

    danmoser

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    My friend is building the Quicksilver-esque design as a hang glider, so high engine thrust line and added weight are non-issues. In my own un-powered Quicksilver B flying experience, it seemed that the rudder effectiveness should have been greater.. and I think the C model had a bigger rudder and wing area, both welcome improvements. The fixed horizontal stab. and weight shift pitch control were limiting, and I never got the chance to try to thermal with the Quicksilver and explore small radius turning... but it was great for boating around in ridge lift.. very stable and forgiving.

    In fact, my friend has more or less copied the dimensions of the Quicksilver, which is the easiest and least risky way to go.. (some people call this reverse engineering, but copying ain't the same as engineering). But it would be nice to know how to go about design modifications on this type of aircraft. For example, what if we wanted to increase the span and aspect ratio? Then what dihedral, rudder specs., etc. should you use without undo risk of flying an unnecessarily dangerous prototype?.. models could be helpful, but not definitive.
     
    Last edited: Mar 17, 2010
  12. Mar 17, 2010 #12

    BBerson

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    For increased span, you might consider polyhedral. (bent up outer wing panels)
    Works well for RC gliders.
     
  13. Mar 17, 2010 #13

    Dan Thomas

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    A rudder-only airplane will induce a skid at first, before the roll starts. A skid at low airspeed is asking for a spin. I'd sure be tempted to have all three axes under my control.

    Dan
     
  14. Mar 17, 2010 #14

    danmoser

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    As far as I know, skids don't cause spins by themselves, and neither do stalls.
    BUT skidding while the wing is stalled will probably do the trick..
    Note that this is the case whether it's a conventional 3-axis controlled or rudder turn only aircraft.
     
  15. Mar 18, 2010 #15

    Dan Thomas

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    Skidding while close to stall speed, especially while in a descent, will cause the inside wing to stall first. A spin results. We demonstrate this (at altitude) to hammer home to the student the importance of maintaining coordination in the circuit. He pays close attention to the ball after that.

    Dan
     
  16. Mar 18, 2010 #16

    Michealvalentinsmith

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    Any reason why flying wings with tip rudders would be any different in this regard? I flew the Fledge copy - the EF 5 for awhile and don't recall any stall or spin issues at all.

    It also had to yaw before it would roll, but I think yawing about a tip doesn't induce skid in the same manner as a central rudder. Though they needed about the same amount of dihedral so this may just be my imagination.

    It's interesting that the last thing the Wright brother developed was the movable, coupled rudder. You'd think, given the precedents, that a dihedral, rudder only wing would have been the first step.
     
  17. Mar 18, 2010 #17

    lr27

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    There must be a way to set up aircraft so a skid does NOT cause a spin, as I've never noticed the effect in many years of flying rudder elevator models. The principles are the same. In fact, at low speeds the rudder elevator gliders seem a bit more forgiving, and they continue to respond to the controls in about the same way. As airspeed decays, ailerons lose their power a lot sooner than the rudder does.
     
  18. Mar 18, 2010 #18

    Dana

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    The Wrights saw that others had tried rudders and dihedral in the attempt to make a stable airplane that could be steered like a ship. Their chief accomplishment was realizing that an airplane should be less stable (theirs was downright unstable), and the pilot should actively keep it level, like a bicycle rider.

    -Dana

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  19. Mar 18, 2010 #19

    Dan Thomas

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    Models often have far more washout (and dihedral) than real airplanes, especially rudder-only affairs. Washout inhibits the stall progression.

    Dan
     
  20. Mar 18, 2010 #20

    bmcj

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    For whatever reason, I never experienced roll-off of a wingtip in the Quicksilver, and I put them through their paces with just about every combination of airspeed, roll rate, attitude, etc that you can imagine. I don't remember if there was significant (or any) washout built into the wings. I suspect that a strong yaw-roll coupling may somehow help the wing stay below it's critical angle of attack (no scientific basis for this assumption).

    Bruce :)
     

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