Stall progression

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by pictsidhe, Mar 25, 2019.

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  1. Mar 29, 2019 #41

    BBerson

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    The DH53 used patented DeHavilland differential ailerons that were effective through and beyond stall. (source, 1923 FLIGHT archive).
    The strut attach at 4.5 feet from the fuselage should stall the root first so not a bad feature.
    The sharp leading edge wasn't good.
     
  2. Mar 29, 2019 #42

    pictsidhe

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    Rectangular wings inherently stall much more pleasantly than tapered wings. So, I've put some effort into finding a pair of airfoils that work together at my Re and are less likely to give a bad stall. I also went for a pair that will smoothly linearly interpolate and not give some funky mid span shape. I now have a pair of airfoils that XFLR5 thinks are a big improvement over the originals. The originals looked like they'd stall first about 75% out. Thanks, but I really did not like that idea. Maybe my extensive flying experience is making me overly cautious? The new ones don't even look very different, which tells me that tolerances and finish are important. The only thing that might jump out to someone without fancy measuring gear is that the root is now 21%. Try as I might, I couldn't get my desired combination of CLmax and stall margin over my speed range with a 19% airfoil.
     
  3. Mar 29, 2019 #43

    Aesquire

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    Mr. Riblett started his research in the wind tunnel, making small changes and measuring the results. Later he moved to a software approach, informed by that experimentation. As often happens, he was not a lone genius, but stood on the shoulders of others with help from his contemporaries. Kitplanes magazine, Sept. 2013 had an article on this subject.

    The desire for safe stall characteristics is a smart choice. Corkey Meyers's book on test flying for Grumman & his experience with the competing machines of the day, is biased by a pilot who was responsible for Naval Carrier planes. How a plane stalled was very high on his list of plusses and minuses. Hindsight, in the raw numbers of students killed in WW2 in training on planes like the P-40 & F6F, show he was very right to do so.

    My personal guess/opinion is that differences in airfoil may shift how much a wing platform affects the stall, but a plain rectangle vs. tapered wing all else being equal, will still show the same trends.

    Taper, twist, changes in leading edge shape, & changing airfoils all move lift distribution and therefore bending stress & stall characteristics.

    It's only rocket surgery.
     
  4. Mar 29, 2019 #44

    Vigilant1

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    If a designer is willing to use a rectangular wing planform with no change in thickness, the resultant wing is almost certain to stall at the roots first, right (assuming there's no flex/twist in the wing that increases the angle of incidence toward the tip)? This is just due to the lift distribution of a rectangular wing. This alone gets us a >long< way toward good stall behavior. The only important characteristic remaining is the abruptness of the stall/presence of stall warning, and that can be addressed with the selection of the airfoil, with a burble strip as on the Glassair or even (less ideally) with an effective stall warning indicator (AoA or lift reserve indicator).
    The Thorpe T-18 has a rectangular wing and I don't recall reading that it tended to roll/spin when entering a stall, but the airfoil did stall abruptly. Lu Sunderland's revised airfoil fixed that, and the planform has remained rectangular.
    For a plane like the proposed Micromaster with a rather short chord (approx 30"), I think a rectangular wing may be useful in helping to avoid some possible Re issues at the tip. Oh, plus easier fabrication (avoidance of need to incorporate twist, etc).
     
    Last edited: Mar 30, 2019
  5. Mar 29, 2019 #45

    Heliano

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    Pictsidhe,
    As reference about slotted ailerons, you can look into Abbott's "theory of wing sections" chapter about high lift devices (when it comes to 2d airfoil data it does not matter if it is aileron or flap) and you will see CLmax for all kinds of slots. There is also an old report by NACA (NACA Report 422 - "Wind Tunnel Research Comparing Lateral Control Devices, Particularly at High Angles of Attack - Part II: Slotted Ailerons and Frise Ailerons"). Keep in mind that a slotted aileron will likely require hinges protruding downward. If your intention is to design a sleek, speed record breaking aircraft, slotted ailerons may not be the way to go. But if you want a docile, benign and very safe aircraft they are worth considering. Of course - and most designers do - you can combine the slot with frise-type aileron. Frise ailerons, invented by a british engineer by the name of Leslie Frise, reduce reverse yaw and improve flight qualities. One interesting aircraft which had slotted ailerons and was very advanced in its time was the Messerchmitt BF108, predecessor of the 109.
     
  6. Apr 4, 2019 #46

    Scheny

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    XFLR5 has the limitation, that as soon as one section cannot meet the required Ca, it stops computing, although the remaining wing could theoretically support the weight at higher angle.

    Because of this, you cannot calculate partially stalled wings...

    It makes sense it works that way, but still you won't know the post-stall behaviour which could prove quite interesting.

    BR, Andreas
     
  7. Apr 4, 2019 #47

    12notes

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    § 91.15 Dropping objects.
    No pilot in command of a civil aircraft may allow any object to be dropped from that aircraft in flight that creates a hazard to persons or property. However, this section does not prohibit the dropping of any object if reasonable precautions are taken to avoid injury or damage to persons or property.

    As long as you drop it in an area away from people and buildings, and pick it up to avoid a littering charge, you're good.
     
  8. Apr 5, 2019 #48

    pictsidhe

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    I wouldn't drop a gallon jug, I'd empty it. that can be done in seconds and will result in a little rain. Not usually known for damaging much. Though I suppose it could cause a Californian to die of shock, but I won't be flying there...
     
  9. Apr 5, 2019 #49

    lr27

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    I would guess that's not true, and I know it's not true for model airplanes.
     
  10. Apr 5, 2019 #50

    Scheny

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    At least true for this gravity defying Depron planes ;). The cutest I ever saw were "cloud-sheeps" with dangling feet. But also the flying Snoopy house or flying lawnmower show that at some point aerodynamics is overrated compared to power density.
     
  11. Apr 5, 2019 #51

    PiperCruisin

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    Sounds like how the Avid Flyer airfoil was made. 2 aluminum tube spars with one fixing the leading edge...a spline for the top and one for the bottom and ...Voila! An airfoil that served its purpose.
    I think Wainfan helped them select/develop an better/more efficient airfoil, but was never implemented. Best not to over think things lest analysis leads to paralysis and one stays firmly grounded.
     
  12. Apr 5, 2019 #52

    TFF

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    With a model you can get away with a lot. I think in the 70s someone did take a flip flop and used the curves but narrowed it to a reasonable thickness on an Ugly Stik. I had a fun fly airplane that had the camber on top. The idea is trimming upright and rolled over required no retrimming. Meh. For the Ugly Stik Geek like I am, Phil Kraft described the airfoil as flat bottomed. The bottom is flat right behind the spar for ease of building on a table; the top has an ever slight curve. Greatest RC airplane ever designed.
     
  13. Apr 5, 2019 #53

    lr27

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    Well, yeah. If you put 500 hp on your ultralight and give it a Hershey bar wing, then you can probably get away with anything. As an RC glider flyer, I care about aerodynamic performance more than that.
     
  14. Apr 5, 2019 #54

    MadRocketScientist

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    If you have aver had the chance to see Jurgis Kairys' custom built aerobatic plane, the wing airfoil is out there, almost a large rounded leading edge and then straight back to a point for the trailing edge... on both sides! A wedge and half circle.
     
  15. Apr 5, 2019 #55

    BJC

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    Such airfoils have been the standard for competition aerobatic airplanes for 30 years or more.


    BJC
     
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  16. Apr 6, 2019 #56

    pictsidhe

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    What makes the ice cream cone airfoil so good for aerobatics?
     
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  17. Apr 6, 2019 #57

    MadRocketScientist

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    Pure speculation on my part would be that it gives a better stall over a completely flat section but flies like a fun fly foamy RC plane when needed:p
     
  18. Apr 6, 2019 #58

    BJC

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    There is a decent paper here http://www.eaa1000.av.org/technicl/onedesaf/1desaf.htm addressing the subject. Except for a nit-pick or two, I agree with the paper. IIRC, they don’t comment on the benefit of using a symmetric section. A symmetric section makes entry speeds, handling, and appearance of stick-forward maneuvers the same as stick back maneuvers.


    BJC
     
    Last edited: Apr 6, 2019
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  19. Apr 6, 2019 #59

    Sockmonkey

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    Don't differential ailerons that deflect up farther than down give much the same beneficial yaw effect?

    I've seen reverse tapered ailerons on swept flying wings where the neutral position has both slightly "up" as a simpler substitute for washout. Seems like that would work well on a straight wing to reduce tip drag, but would it also delay tip stall?
     
  20. Apr 6, 2019 #60

    BJC

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