Duncan's got a Flea

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Aug 3, 2017.

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  1. Sep 27, 2017 #461

    Topaz

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    Differential ailerons may have an adverse effect, since they result in a slight net lift reduction when applied. "Conventional" ailerons are lift-neutral, and shouldn't affect the stall behavior at all.
     
  2. Sep 27, 2017 #462

    cluttonfred

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    Hmmm, I was trying to stay out of this thread as I've already said my piece, but now you've confused me, Topaz. Logically, don't all ailerons effectively increase or decrease the local angle of attack of the wing section in front of them? Therefore wouldn't a wing with the ailerons in any position but neutral tend to stall the section in front of the "down" aileron before the section in front of the "up" aileron? I have always understood that the utter lack of ailerons contributed to stall/spin resistance in Mignet types.

    power-off-deflected-ailerons.jpg

    EDIT--Added image.
     
    Last edited: Sep 27, 2017
  3. Sep 27, 2017 #463

    Topaz

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    Hi Matthew,

    Sorry, didn't intend to confuse the issue! You're absolutely right, a downward-deflected aileron increases the effective camber and angle of attack of that portion of the wing where the aileron is down. That portion of the wing will stall before the rest of the wing. The "up" aileron de-cambers its section, and decreases its angle of attack.

    Recall that what's critical for stall is the angle of attack of a the wing surface involved. For any tandem-wing airplane (including canards), it's critical that there be no situation where the airplane can be trimmed such that the rear wing is near enough to its stall angle for it to be in danger of stalling, and that includes any situation with a deflected control surface on that rear surface. Basically, it's a matter of maintaining sufficient stall margin on the rear wing that it can't be stalled in any circumstance where the airplane is in trimmed flight.

    For a regular canard airplane (or a Quickie), that means that the rear wing still has stall margin (angle of attack to stall angle) in any situation where the front wing is near stall. Same for a Pou. With the latter, the explicit angle-of-incidence change makes this at least easier to visualize, and perhaps easier to analyze, too (I've never tinkered with a Pou design).

    Adding ailerons should make no real difference in the design process - the amount of "additional" stall angle margin they require should be absolutely minimal, and well inside the "normal" stall-angle margin of the rear wing. Upwash and downwash interactions make determining the margin difficult in the first place, so designers tend to be fairly conservative here, since the consequences of a rear-wing stalling while the front wing is still flying are so utterly drastic.
     
  4. Sep 27, 2017 #464

    cluttonfred

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    Thanks, Topaz, that all tracks with my understanding. By all accounts, the strong downwash effects of the front wing on the rear one are a somewhat tricky aspect of designing any high-low tandem (Mignet, Delanne, etc.) and not really an issue with the low-high tandems (Mauboussin, Rutan Quickie, etc.). That does not change, of course, the fundamental point that you emphasized, that the forward surface should always stall well before the rear one, otherwise things get ugly fast.

    Duncan, for some reason I had it in my head that you were thinking of ailerons on the rear wing, not the front one, so basically Cosandey flaps tied to the stick, is that correct? One other point to keep in mind is that, while they might be fine for trimming out some crosswind to reduce the crab angle when landing, ailerons on the rear wing are unlikely to be very responsive. With the rule of thumb that the front surface should always have a wing loading at least 50% greater than the rear one, and the generally larger span and area of the front wing, then ailerons the rear wing have less total lift to play with in order to provide roll control. Ailerons on the variable-incidence front wing, while certainly possible, could introduce their own complexities.
     
  5. Sep 27, 2017 #465

    rtfm

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    Hi Matthew,
    Why is it that some errors seem to outlast repeated corrections? They seem to enjoy a perverse imortality. Like the 75 year old fallacy that Pou du Ciels are dangerous. Or that there is a definite slot effect caused by the front/rear wing positions in a Flea. Or this one - that there is a strong downwash over the rear wing caused by the front wing. Yes, there is some downwash - but it is nowhere near as great as is so frequently asserted. Check out the worst case scenario below.
    smoke deflection of airfoil 16 deg AoA.jpg

    Actually, I had envisaged rear wing ailerons on the outer panels.
    Mmmm I'm not so sure about this - well - I'm conflicted about how effective they would be. I don't think there is any danger of them stalling the rear wing (they only decrease the lift on one half of the wing anyway) and there is a healthy extra margin for error in the 6 degree fixed incidence of the rear wing anyway. But I think, overall, it will be simpler to have rear wing ailerons rather than putting them on the front wing. I can certainly see that front wing ailerons would be far more effective, but what we are looking for here is some added roll authority - the rudder is still there, doing its job. The ailerons simply kick the roll off immediately.

    Yes, this is my concern also... Although the Watts Bridge Pou (the one built by Byron) has the Cosandy flaps on the front wing, and while Patrick says he seldom or never uses them, he didn't indicate that they introduced any weird effects when he had used them.

    Mmmm
    Still thinking about this one...

    Duncan
     
  6. Sep 27, 2017 #466

    rotax618

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    Ailerons on the outer panels of the rear wings will not give you an aerobatic roll rate but they will give 3 axis control. If the ailerons are rigged for UP only, they will not make the rear wing more prone to stall.
     
  7. Sep 27, 2017 #467

    cluttonfred

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    I hardly think that I am someone who keeps alive a "75 year old fallacy that Pou du Ciels are dangerous," quite the opposite, in fact. What airfoil is that, under what conditions? The point is that, despite the need to keep the rear wing flying at all time, the rear wing incidence is substantially greater than one might expect from lift calculations alone. As someone who has been around the Mignet community for 20+ years, including corresponding with and speaking by phone and in person with some of the true experts, I'd personally suggest trying the Mignet approach unchanged as a starting point.
     
  8. Sep 27, 2017 #468

    Topaz

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    No, but you would get a pitch-trim change with every aileron deflection. Remember, the rear wing is aft of the CG. If the ailerons only go up, you get a net reduction in lift with every roll input. Reduce lift aft of the CG, and the nose goes up.

    Best to have ailerons that have equal deflection up and down, in this case. The span is short enough that I doubt there will be much adverse yaw to worry about. Certainly the typical Pou rudder would be more than enough to deal with it.
     
  9. Sep 28, 2017 #469

    BBerson

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    I think two small ailerons interconnected to a small separate additional rudder might be useful. Limited crosswind cross controling could be managed by cross controlling the ailerons and rudder.
    But with these small ailerons interconnected only with the small rudder, stick movement would always provide coordinated roll so no tip stall.
     
  10. Sep 28, 2017 #470

    Topaz

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    Why go to all the trouble? Ailerons and rudder are a solved problem - we do it every time we fly a conventional airplane. Providing the aft wing of a tandem-wing airplane with sufficient stall margin despite it having ailerons fitted is a solved problem - every LongEZ, Quickie, Q2, Q-200, Dragonfly, etc. works this way.

    There's no need to overcomplicate this.
     
  11. Sep 28, 2017 #471

    BBerson

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    I am not sure aileron and rudder is a solved problem on conventional aircraft.
    And I am not sure tandem and canard are the same. But with no experience with the latter I guess I will quit speculating.
     
  12. Sep 28, 2017 #472

    rtfm

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    Hi Matthew. I seem to have hit a nerve. My apologies. I know very well that you aren't a member of the "Dangerous Pou" clan. I was merely drawing attention to how folk lore tends to cling to the Poux like **** to a blanket.

    However, having said that, I must go on to say (and again, this is not directed at you, but at all the good old boys in a thousand hangars on both sides of the Atlantic) that I find it easier to believe Barnaby Wainfan's wind tunnel tests which concluded that there was no observable slot effect operating with the Pou configuration - than the received wisdom regarding the slot effect being the culprit in the unrecoverable nose-dives of the HM-14. As he goes on to explain, those dives can be explained in terms of airfoil choice, amount of front wing pivot angle and CG location. Barnaby does not give reasons for his conclusion, but I suspect it is because even at 16 deg AoA on the front wing, the airflow deflection experienced by the rear wing is only 3 deg.

    Friends?

    Duncan
     
  13. Sep 28, 2017 #473

    rtfm

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    Hi,
    I'm not sure I follow. What do you mean by a "small separate additional rudder"? Do you mean splitting the rudder into two parts, the smaller of which is linked to the ailerons, and controlled by the stick, and the larger part controlled by foot pedals?

    Duncan
     
  14. Sep 28, 2017 #474

    BBerson

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    Yes. That would work.
    Or two separated rudders. I haven't thought the idea through yet, but think it might have merit for conventional aircraft as well.
    It seems to me the ailerons should go on the larger wing nearest to CG. Rutan tried ailerons (elevons actually) on the small forward surface on the VariEze prototype and found marginal and later moved the ailerons to the large wing. Makes sense to me that ailerons should be on large wing as a rule of thumb. But that invites aileron tip stall if the ailerons are not inter-connected like a Ercoupe. So that's why I suggested interconnection for the ailerons.
    But since the Flea is set up for rudder actuated roll control with big rudder and large dihedral, it makes sense to keep the roll control mostly with rudder.
    For an Ercoupe, an auxilliary small third rudder could be fitted and with rudder pedals installed for this small rudder. So some crossed control slip could be done but not enough to cause loss of lateral control from crossed controls.

    Edit. Not sure if the stick should control rudder or ailerons. Depends on pilot, I guess. An old Flea pilot might want set up like an old Flea. If you fit ailerons then that's a choice you must make also.
     
    Last edited: Sep 28, 2017
  15. Sep 28, 2017 #475

    rtfm

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    Oops... Double post [deleted]
     
  16. Sep 28, 2017 #476

    Aesquire

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    I'll strongly recommend you stick with conventional controls on an airplane with conventional control surfaces.

    Nothing to unlearn. Other pilots need only type training checkout, vs. Conditioned reflex reprogramming. ( insert backward bicycle video here ) And there aren't enough Pou pilots that can't deal with conventional controls easily to make that a factor.

    The original Quicksilver hang glider controlled no differently than the company's Rogallo gliders. Despite the rudder.
    The modern ATOS, ditto, despite the spoilerons.
    The Pterodactyl ultralight deleted the rudder pedals as does a Pou, both 2 axis birds.

    A aileron equipped Pou with the pedals controlling the ailerons just seems a bad idea ergonomically.

    But if you prefer a Wright early set up with one fore & aft lever controlling elevator/front wing and another fore & aft lever controlling rudder, and shoulder yoke for ailerons. .... your plane. No one else can successfully steal it. Crash it on takeoff? Every time. ;)
     
  17. Sep 28, 2017 #477

    Victor Bravo

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    Goodfellas Over-Complicate.jpg
     
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  18. Sep 28, 2017 #478

    rtfm

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    Ha ha. :ban:
     
  19. Sep 28, 2017 #479

    rotax618

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    Just remember Henry Mignet was no Kelly Johnson, he was a well meaning enthusiastic amateur who wasn't able to pilot an aircraft with conventional 3 axis controls. I'll bet he was as surprised as anyone when his first Flea flew and was controllable.
    Henry never lost his dream that anyone could build an aircraft and fly it, even after there were some accidents, and continued refining his original concept - but because of his own lack of coordination all of his design have been 2 axis.
    The basic idea of Henry's Fleas is sound, and if you feel comfortable with the lack of roll control, build it as Henry designed it.
    If someone can build a safe aircraft using the Mignet formula of simple construction and stall resistance, compact and foldable, but with conventional controls it will be attractive to many more amateurs, I know I would reconsider the flea.
     
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  20. Sep 28, 2017 #480

    rtfm

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    "The proof's in the pudding" - as they say in NZ and Aussie. I have no idea why they say this, because it doesn't even make sense! The expression is: "The proof of the pudding is in the eating", but when I mention this to the locals, they look at me as though I'm from another planet.

    Anyway. A three axis Pou is on my workbench as we speak. Let's see how it turns out. Sacre Bleau!
     
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