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Thread: Aircraft designed for rudder only (aileron-less) turning

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    Registered User danmoser's Avatar
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    Aircraft designed for rudder only (aileron-less) turning

    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!
    Attached Thumbnails Attached Thumbnails Aircraft designed for rudder only (aileron-less) turning-02241002.jpg  

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    Registered User bmcj's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    Many factors will induce roll-yaw coupling. Dihedral is very effective means. Wing sweep is another.

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    Registered User piperpilot1363's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    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

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    Registered User BBerson's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    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.

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    Re: Aircraft designed for rudder only (aileron-less) turning

    Well, you could look at the proportions of the Skypup for starters. I hear it flies well.

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    Registered User danmoser's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    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 by danmoser; March 16th, 2010 at 09:28 PM.

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    Re: Aircraft designed for rudder only (aileron-less) turning

    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/b...PDFs/bd_V3.pdf
    http://charlesriverrc.org/articles/a...legro_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.

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    Re: Aircraft designed for rudder only (aileron-less) turning

    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. #9
    Moderator Dana's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    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

    1. Programmers are expensive.
    2. Press releases are cheap.
    3. Therefore, it's cheaper to explain the bug than to fix it.

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    Registered User bmcj's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    Quote Originally Posted by Dana View Post
    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.
    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.

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    Registered User danmoser's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    Quote Originally Posted by Dana View Post

    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.
    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 by danmoser; March 17th, 2010 at 01:01 PM.

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    Registered User BBerson's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

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

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    Registered User Dan Thomas's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    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

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    Registered User danmoser's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    Quote Originally Posted by Dan Thomas View Post
    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
    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.

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    Registered User Dan Thomas's Avatar
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    Re: Aircraft designed for rudder only (aileron-less) turning

    Quote Originally Posted by danmoser View Post
    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.
    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

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