Decalage angle

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Eugene, May 29, 2017.

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  1. May 30, 2017 #41

    BBerson

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  2. May 30, 2017 #42

    vtul

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    The confusion between angle of attack and incidence probably contributes to more misunderstandings than any other aeronautical terms.

    Decalage is the difference between incidences, not angles of attack. The geometrical difference can be between two surfaces, eg. wing and tail, or upper and lower wing. It is not slang, but it can apply to various plane surfaces.
     
  3. May 30, 2017 #43

    BBerson

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    Sure there is confusion. But I guess you are saying there is only geometric decalage and no such thing as aerodynamic decalage.
    That's like saying there is only geometric pitch on a prop. There is geometric pitch, effective pitch, nominal pitch, standard geometric pitch of the whole propeller, uniform geometrical pitch, experimental pitch, zero thrust pitch....

    The designer had to account for all the aero effects of wing downwash, prop thrust axis etc.
    Peter Garrison's article link in post#41.
     
  4. May 30, 2017 #44

    vtul

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    No, it is saying that decalage has a specific meaning.

    Accounting for aerodynamic effects starts with clear understood conventions for terms for the geometric shapes and dispositions of the parts of an aircraft. Like those described by the terms incidence, wing area, aspect ratio, decalage, dihedral, thrust line, airfoil, etc, and then seeks to predict the behavior of those elements together in flight conditions. Using a term properly does not negate anything.

    If you want to talk about the effective angle of attack of a horizontal stabilizer as a result of a wing's downwash -- that is perfectly adequate terminology for the subject.

    Here's the reason why mixing up geometry with effects matters: when a designer talks about setting the decalage for a biplane's wings, for instance, a specific angle is given. It may appear on plans, it may even appear in design discussion. But if decalage is re-defined as the effective difference in angle of attack, it becomes meaningless. At what speed, who measures it, what would it mean to set decalage at all? How could anyone set rigging for any specified decalage if it is indeterminate?

    Now it's easy to engage in theoretical discussions about a tail's angle of attack as a result of wing effects. Just talk about that directly in so many words. Your comment after I first mentioned decalage shows exactly the problem created by misusing a conventional term. We aren't in argument on flight dynamics. But it appears so in your statement. Now we've gone back and forth on it and you seem to be saying I can't comprehend flight dynamics because I use a geometrical term in its conventional sense? That's silly. I make a distinction between the two, that's all.
     
  5. May 30, 2017 #45

    pictsidhe

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    Does your airfoil have a different pitching moment to the original? That would affect the decalage.
    A wing will produce X amount of lift at speed Y and incidence Z. If you change one of XYZ, at least one other will change. The tail has a similar task as the wing, except that it needs to push down.
    Possibilities for a strange wing AOA.
    Incorrect washout/washin
    The wing is twisting more than it should under flight loads.
    It's am optical illusion.


    If the tail AOA is out, it is dealing with an incorrect moment. The main wing could have greater pitching moment due to mis shape, changing the thrust line will also change the tail moment. When yoy changed that, you increased the moment, but the effect was masked by the increased airflow over the tail making it more effective.

    I'd follow the designers advice before trying others. I suspect that it just looks weird myself.
     
  6. May 30, 2017 #46

    pictsidhe

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    Another possibility that the fuselage or something forward is producing a substantial amount of negative lift. Other than weight, that's the only way to get a high AOA in level flight.
     
  7. May 30, 2017 #47

    Eugene

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    I end up contacting Peter Garrison directly. We did exchanged few emails. I even allowed myself to disagree with him (stubborn russian).
    He agreed that airplane flying with excessive angle of attack after doing his homework online. He was telling me that 4412 wing at 90 mph and 1000lb load should be flying at approximately 1.5-2 degrees angle of attack. So bottom of my wing should be just about leveled to the horizon in cruise.

    But after that we disagreed. He believes, that only way to change angle of attack is to increase speed with more power, reduce drag or lose some weight. He didn't thing that horizontal stabilizer plays any role for angle of attack. The fact alone that if I managed to make my wing level to the horizon - horizontal stabilizer, which has humongous leverage, Will be up by 10° didn't concerned him at all.

    I was completely lost after that. How is it, that so many people, so disagree about something fundamental, that we are doing for over hundred years???
     
  8. May 30, 2017 #48

    Eugene

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    negative-dynamic.jpg

    I believe, this is what happened to me on my test flight
     
  9. May 30, 2017 #49

    BBerson

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    Vtul, the word decalage isn't mentioned in Perkins and Hage - Stability and Control text. So I don't know a source of a conventional definition. Hence my reason for calling the word slang.

    To the OP, it doesn't matter what the angle of the tail is if it flies hand off (in trim). The optimal "decalage" is only good for one flight condition.
     
  10. May 30, 2017 #50

    BJC

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    Someone else already mentioned it, but have you checked the wings for proper washout?


    BJC
     
  11. May 30, 2017 #51

    vtul

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    Also, check your ailerons for reflex at neutral stick.
    Check gap seals in place and intact.

    re. thrust line changes, that mainly affects the difference in trim required between power off and full power.

    re. phugating, check carefully the location of the CG by doing an accurate weight and balance with pilot aboard. Have a friend check the scales.
     
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  12. May 30, 2017 #52

    Dana

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    Peter is correct. Forgetting declage for a moment, the only way you can be at a high AOA in level flight is if you're flying an a speed low enough to require such a high AOA (i.e. high Cl). If you increase airspeed, AOA must decrease or you'll climb.

    Dana
     
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  13. May 30, 2017 #53

    Eugene

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    So, tale doesn't have anything to say in this situation????

    And it's very possible to fly around with tail up by 10.2°???

    I have pretty good high-definition video in my head about airplane with power changes and therefore changes of speed lifting its tail every time you added more power. But that video about normal airplanes with about 4° difference between tail and the wing. And I am okay with fact that most general aviation airplanes flying with its tail up buy 2 or 3° at high power settings.

    10° was probably perfectly okay with 50hp, but is it still okay for 100hp?

    Or, in other words if I wanted to fly more efficiently I need to look at propeller, Engine cowling, wheel pans and so on. And forget about this tale and its angle, because in your opinion it doesn't present much drag at all?


    If we take two identical airplanes, but one with decalage of 4° and one with 10°. with short rope we will tie one of them to the pick up truck and get on the highway. Sort of like flying the kite! Which airplane Will allow us to drive faster? Is 4° triangle Will be easier to pool through the air then 10° triangle?
     
    Last edited: May 30, 2017
  14. May 30, 2017 #54

    BBerson

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    Yes, you have neutral or negative stability with aft CG flown solo.
    Moving the CG forward will add stability but increase drag.

    The pusher plane has many things causing nose down moment:
    1) Wing negative moment
    2) high thrust line with high power.
    3) possibly "propeller normal force" lift adds a negative moment?
    4) short tail distance from wing and prop?
    5) too small tail area that needs excess angle to work?
     
  15. May 30, 2017 #55

    vtul

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    I would stop worrying about theory, Eugene, and do the physical checks already suggested, accurately. They will probably lead to an answer.

    1.) Washout-- how much, if any.:
    If the airfoil is known to be consistent, root to tip, tape two sticks along the bottom of the foil, one near root and one near tip, stand back and sight along them. Are they canted in relation to each other? Do this for both sides of the wing. Then compare to the designer's spec for washout, (if required by design).

    2.) Reflex/deflex of aileron -- Set the ailerons even and lay a short stick along the top and bottom of the foil crossing the aileron. Does it appear that these are reflexed (or deflexed)? Compare with designer's spec.

    3.) Check the designer's requirements for gap seals on all moveable surfaces, and make sure they are in place and un-damaged.


    Finally some additional questions:

    a.) one thing that hasn't been very clear from your descriptions so far: how is the fuselage riding at cruise? You have mentioned repeatedly the wing's apparent attitude, but not the fuselage's attitude. Is it level, or is it flying at some other attitude in cruise? If it's flying at an angle, it will add a lot fo drag. Which will slow you down, thereby producing less lift. This is the main reason people adjust the incidence of the H stab. The fuselage should generally speaking fly level at cruise speed.

    b.) Also, I caught an early mention of your airfoil having been different than the stock high lift airfoil originally designed for the plane. I also see some conflicting descriptions of the wing area for this type of plane. Is it possible that you have a shortened "speed wing" compared to an original stock wing? Can you repeat for us the exact specs (airfoil, span, and area) for both your present wing and the original wing?

    The reason we are concentrating on the wing rather than your own focus on decalage is that the bottom line is, your wing is taking it's present flight attitude because it must do that to provide enough lift for all of the forces opposing that lift. I think all of us, though describing the problem in possibly different ways, are in agreement on that.

    Your stab's incidence would normally be set to allow the fuselage to fly in a level attitude at cruise. If that is how it is now, then the only possible problem is that the wing is not providing enough lift to fly at a lower angle of attack.

    Reasons for that could be peripheral problems, like washout, aileron setting, leading edge dents, faulty gap seals, etc. Or they could be systemic, like simply not enough wing area, or a change to a low(er) lift airfoil, or not enough speed ("cruise" speed is too low), or too much weight. A dragging fuselage could be a possible cause of too low a speed, as well.

    Ideally you would eliminate the peripheral causes by inspection before concluding anything further.
     
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  16. May 30, 2017 #56

    Eugene

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    Thank you for your help! And I will go and check everything. But I am connected to Skyboy group and as far as I can tell my description of how this airplane flies is exactly in line with everybody else's experience.

    You can pretty much see everything on any video or pictures on Internet. They all flying like that. Original designer told me repeatedly to stay away from any modifications. It is a Quicksilver competitor! This airplane had completely different purpose, with a different design goals, different airfoil and different power.

    I will do little bit more homework and they will post some different data that I can find. But I am starting to think that for 100hp Engine this airplane needs to get longer to provide more distants between propeller and horizontal stabilizer.
     
  17. May 30, 2017 #57

    vtul

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    If they all fly that way, that's different. My understanding was that you were experiencing an unusual problem specific to your airplane.
     
  18. May 30, 2017 #58

    BBerson

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    Check your tail volume coefficient which includes the tail arm distance and tail area. Compare with similar aircraft.
    I think if your tail area was larger the angle would be more normal.
     
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  19. May 30, 2017 #59

    Eugene

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    Almost all of them (about 60) were shipped to this country with 80 or 100hp and 4412 wing. There is guys out there with two stroke engines and P3 airfoil. They are very happy with what they have, because they were looking for quicksilver with inclosed cockpit.

    Nobody ever question angle of attack, wing swept or trust angle. Until this crazy Russian was looking for ELSA and purchased airplane that flies 30 miles an hour slower then advertising numbers.

    Designer of this aircraft was trying to tell me that pieces in this puzzle - are not correct pieces for 100 engine or higher speed. At some point in Europe they decided to install 912 Engine to satisfy demand from flight schools, because they didn't want to rebuild two stroke engines every 300hr

    IMG_7668.jpg
     
    Last edited: May 30, 2017
  20. May 30, 2017 #60

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