biplane stalls

Discussion in 'Rules and Regulations / Flight Safety / Better Pil' started by jgnunn, Sep 18, 2006.

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  1. Sep 18, 2006 #1

    jgnunn

    jgnunn

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    In theory, during a stall, the top wing would stall first being further forward. For those flying/flown pitts/skybolts, how true is this in practice? How would one know? and are they so close that it doesnt really matter?

    I am asking because I was thinking of fitting an Angle of Attack Indicator, but prefer to mount it on the lower wing [less hassle] but literature Ive found suggest they be mounted on the upper due to the above reason...
     
  2. Sep 18, 2006 #2

    Craig

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    Angle of incidence

    Beej -

    Generally, the top wing stalls first because it is usually installed at ZERO deg. incidence, while the bottom wing is mounted at about 2 deg. nose up.

    A swept upper wing might decrease the stall break just a bit.
     
  3. Sep 18, 2006 #3

    orion

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    Actually if that alone was the cause then the aft wing would stall first since the higher angle of incidence would allow it to reach the stall angle before the top wing.

    There exist a flow interaction between the top and bottom wing on any biplane. This interaction changes the behavior of the wings due to changes in the pressure gradients and lift distribution. The incidence angles (and airfoil choices) are set in such a way that allows both wings to come to stall at nearly the same time. If the stall is slightly staggered then it is of paramount importance that the forward wing stalls first since if the aft wing stalled first the lift vector would move forward, possibly pitching the airplane into even a deeper stall, resulting then in a spin.

    Regarding the angle of attack indicator, the wing it needs to be on really depends on what type of indicator you're looking to use. Either way though, it still needs to be calibrated to the airplane. A simple dial indicator can be anywhere.

    The pressure port indicator should be on the wing that stalls first and yes, that should probably be the top wing since its upper surface flow is not affected to any great degree by the lower wing (it actually is a little bit due to the induced angle of flow created by the bottom wing).
     
  4. Sep 18, 2006 #4

    Craig

    Craig

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    Simple

    Ah, Orion - Here I make something simple, and you gotta go and explain all the forces at once - good job, by the way!

    Yes, ultimately, both wings stall at about the same time. And the only bipe I'm familiar with that has the top wing to the rear is the old Beech. As I recall, the LOWER wing on that was set to zero. Course, I am just relying on memory, and never measured the incidence on either wing.

    Thanks for the "engineer's" explanation!
     
  5. Sep 18, 2006 #5

    jgnunn

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    thanks for the replies..
    the AOA in question is this:

    Liftreserve.com



    i would like to install this on the lower wing...
     
  6. Sep 18, 2006 #6

    orion

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    Since the indicator is on an externally mounted blade and not dependent on the airfoil, it should be able to be mounted on either wing, as long as it's correctly calibrated and adjusted.
     
  7. Sep 18, 2006 #7

    CAB

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    Just thought I'd toss a fly in the ointment- the Pitts and Steen designs use different airfoil sections set at the same alpha to achieve the same effect!:p:

    CAB
    Bearhawk#862
     
  8. Sep 19, 2006 #8

    orion

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    Out of curiosity, has this changed in the Pitts' evolution? The reason I ask is that many years ago we did a modification to an earlier Pitts S1 for a customer whose organization (Legal Eagle Air Racing Assn.) was looking to enter the bipe racing circuit. The work included quite a few modifications to the engine installation, the cowl, cockpit and tail. It also included a set of new all graphite wings.

    The wings were however installed by their own mechanic. Even though I recommended that both new wings were to be set at zero deg. incidence, the mechanic set them at zero for the top wing and plus three for the bottom - he indicated that this was the recommended Pitts installation.

    During flight testing the pilot fortunately did all the maneuvers above 5,000 feet. One of the first maneuvers was a stall - due to the incidence setting the bottom wing stalled first. This resulted in three things happening almost simultaneously: First it resulted in the loss of roll control since the ailerons were only on the bottom wing. Second, almost immediately after the lower wing stalled the airplane pitched up abruptly resulting in a deep stall on the top wing, which in turn resulted in the airplane almost immediately entering a spin.

    According to the pilot the spin came upon him so fast and was so rapid that the airplane did almost full three turns before he could even react. Fortunately once the airplane picked up speed the recovery was relatively straight forward although the pilot estimated that the airplane completed at least seven turns (and lost over 2,500 feet) during the few seconds throughout which this occured.

    The incidence setting also caused power off descent rates of over 1,500 fpm, which was quite exciting on final.

    Needless to say the incidence was corrected to zero-zero and the airplane flew fine. (The new wings used the same section top and bottom.)
     
  9. Sep 20, 2006 #9

    CAB

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    The info I got was from Bud Davisson. According to him, Pitts has a patent on the design I mentioned where the wings are different airfoils set at the same angle of incedence. Steen (according to BD) ignored the patent and used the same trick. Big issue for the Pitts purists.

    Don't the Pitts have 4 ailerons? The S-2 I saw @ RMR did.


    CAB
    Bearhawk#862
     
  10. Sep 20, 2006 #10

    orion

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    If I recall right, the choice of ailerons is up to the builder. The racing team elected not to use four since the interconnecting control strut was just added drag and the graphite wings were so much more efficient and stiffer than the fabric covered wood wings that the four surfaces were really not necessary.

    The two airfoil sections is an interesting bit of data - when I modified the wings I had the old set there and I did compare the upper and lower ribs - they sure looked the same. But this was an older Pitts S1 - we think it was originally built between the late sixties and the mid seventies so it could be that back then it was built using the same rib section.
     
  11. Apr 20, 2008 #11

    bmcj

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    Hey Beej,

    Check out the diagram near the bottom of thei page to see flow patterns induced by biplane wing interaction:

    http://www.melmoth2.com/texts/CFD.htm

    This is just a guess on my part, but I think that as long as you place the AOA gauge in stable airflow (air that is out of the propwash and doesn't vacillate, you can rely on the readings you get. After all, you just want to establish reference points for things like stall AOA. The gauge might have different readings for different mounting locations, but since you will not be moving it around, all of your readings should remain consistent.

    Bruce :)
     
    Last edited: Apr 21, 2008
  12. Apr 20, 2008 #12

    Dana

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    Waitaminute... you'd want the forward (usually the top) wing to stall first, wouldn't you? To move the center of lift aft so the nose drops? Or is it a matter of drag, stall the bottom wing and the lower drag creates a nose down pitching moment?

    I can see that on an aircraft like a Pitts, which you want to be just as happy inverted as upright, you'd have to design it very carefully.

    -Dana

    If we wish to "restore" respect for the law, a good start would be to pass only laws that people will respect.
     
  13. Apr 20, 2008 #13

    JMillar

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    Kind of the same issue of a canard, except less exaggerated, isn't it. Canard must stall before the main wing, so the nose drops. So I guess you want your ailerons on the forward wing, whichever that is. Flaps, have to go on both I guess?? Imagine stalling one wing as you landed.... this layman doesn't think that sounds fun.

    Dana, that would agree with Orion's first comment above. If the front wing's the top wing, yeah.
     
  14. Apr 20, 2008 #14

    Topaz

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    The forward-most wing should stall first, as you say. This will tend to drop the nose and soften the stall. Having the rear-most wing stall first would have the opposite effect: the airplane will want to nose-up in a stall. Nasty behavior. This is very similar to a canard, as JMillar says. In fact, one can visualize a biplane with significant stagger as a three-surface airplane, or a tailed tandem-wing (the Quikies were tailless tandem-wing airplanes), depending on how you want to name it.

    Personally, I think that if the top wing is forward-most, I'd want the ailerons on the lower (rear-most) wing. That way they keep working even when the top wing is partially stalled. This layman likes good roll control right through the stall! ;)
     
  15. Apr 20, 2008 #15

    JMillar

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    That's right topaz, I got it backwards there. I had it nicely pictured mentally, and I went and wrote the wrong thing.
     
  16. Apr 20, 2008 #16

    Topaz

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    I hate it when I do that! :gig:
     
  17. Apr 21, 2008 #17

    RacerCFIIDave

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