Decalage angle

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

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  1. Jun 21, 2019 #1081

    BJC

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    Please explain how an airplane can fly as fast at a higher angle of attack / at two to four times the lift, with the same HP, as it can in unaccelerated flight.

    Thanks,


    BJC
     
  2. Jun 21, 2019 #1082

    poormansairforce

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    I was simply passing along something that was learned by someone actually doing something constructive. The point was that it didn't lose nearly as much airspeed as they thought it would. Apologies for not being more precise.
     
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  3. Jun 22, 2019 #1083

    Eugene

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    Thank you !!!

    Screen Shot 2019-06-21 at 21.45.05.png
     
  4. Jun 22, 2019 #1084

    proppastie

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    So if you go into a flat spin with a too far aft CG will you have a chute?
     
  5. Jun 22, 2019 #1085

    Eugene

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    Screen Shot 2019-06-21 at 22.27.59.png
    I have a little problem with this picture. Sure we can use this logic to size tail for Kitfox. But how about airplanes with large vertical component between center of drag and engine position above the wing.

    fullsizeoutput_1457.jpeg

    Will aircraft with high mounted engine need larger tail?
    .......Yes it will........
    Will position of that tail (high vs low) have some kind of effect on sizing?
    ........Yes it will........
    Will airfoil tail vs flat deck be more effective?
    ........Yes it will, up to 30% at some AOA.....
     
  6. Jun 22, 2019 #1086

    plncraze

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  7. Jun 22, 2019 #1087

    mcrae0104

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    The graph is still generally applicable; it is saying that the larger the tail volume, the wider the allowable CG range (and the further aft the neutral point).

    Why, in your drawing, do you assume the drag is centered on point B? I do understand the point you're getting at, but don't forget that there are also moments (and lift) acting on the aircraft, so you have something of an incomplete picture.
     
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  8. Jun 22, 2019 #1088

    Eugene

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    Yes picture is incomplete. I used it only to show that engine placement will generate additional moment that ignored by everybody.

    Point B does represent center of drag. If you find the way to retract landing gear = this point B will go up = smaller forward pitching moment = smaller horizontal tail load needed.

    If we take Cessna 172 and mount engine on top of the roof like LA-200. We will need to sweep wings back to get CG in to correct place. Will this new aircraft need larger tail? Yes it will!!! But nobody is talking about it.

    Every book I have automatically assumes that you building Kitfox, Tailwind or RV.

    Skyboy designer, who is 1000 times smarter then stupid russian immigrant, he is taking tail from Skyboy, installing it on mid-wing tractor configuration with all points A, B and C on one horizontal line. And he is telling me - " look, nothing wrong, it's big enough, works perfect on my airplane, you don't know what you are talking about!!!!"

    But I see it as apples vs oranges.

    Just like Peter Garrison was telling me 100 times that its perfectly OK to fly around with 10-12°of decalage. I am not smart enough to understand why that would be OK.
     
  9. Jun 22, 2019 #1089

    plncraze

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    Who is a stupid Russian immigrant? When I read this thread I am seeing someone doing a heck of a lot of conceptualization and modification in a real short amount of time. My understanding is you are bothered by the trim drag created in order to provide enough moment to handle the high thrust line.
    The gyrocopter guys get pretty serious about stuff like that. That's why some of the gyros you see have those long gear legs and what looks like a low thrust line.
     
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  10. Jun 22, 2019 #1090

    Eugene

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    Thank you for cheering me up!!!

    My biggest problem, is fact that for every theoretical question I am sending to 5 very smart different engineers = I am getting 7 different answers. Who is right and who is wrong here??? I can't argue with them, they spend a lifetime to learn about this.
     
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  11. Jun 22, 2019 #1091

    akwrencher

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    No, but
    You can learn a little from each answer and decide/figure out which ones apply best for you. Keep up the good work, you are living the expirimentors dream! :)
     
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  12. Jun 22, 2019 #1092

    plncraze

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    I have the same feeling when I read airplane design books which are aimed at the amateur.
    One has to pick through a lot of books to grasp the whole of the subject matter and sometimes you find that many of the "real" designers don't care about certain things. The Formula One guys are going crazy over stuff that the guys at Piper don't even think about.
     
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  13. Jun 23, 2019 #1093

    Eugene

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    I understand that aviation has no limits. But we not talking here about best way to land on dark side of the moon.

    We are talking about very basic fundamentals in airplane design. So basic, that some "real" designers not talking to me no more. Because I am not smart enough to understand them.

    Everybody standing in front of Tailwind and wondering why is this boxy thing flying so fast? Well, this guy understood this basics so well, that his aircraft is flying with only 1° decalage.
     
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  14. Jun 23, 2019 #1094

    plncraze

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    The thrust line of the Tailwind probably runs parallel to the longitudinal cg and maybe slightly above it. Wittman wanted to go fast on little power and he worried about some of the same things you do. Wittman also has thin wings but he could apparently pull the thing into tight turns according to Jack Cox who rode with him. He was sharp and understood how to get what he wanted from a design.
     
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  15. Jun 23, 2019 #1095

    Hephaestus

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    And the pod and boom style pushers are less well understood compared to a tailwind style layout to most engineers.

    100 years of development versus what 40ish on the pod and boom style?
     
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  16. Jun 23, 2019 #1096

    Eugene

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    So, can we say that if you move thrust line up = you will end up with balancing drag = you will need larger tail to do this balancing = as result will loose efficiency = loose speed.

    So, why I couldn't find this logic in my books?
     
  17. Jun 23, 2019 #1097

    Eugene

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    Yes, so many books will show you how to size horizontal tail, but they don't ask were this tail will be installed in reference to your propeller, will you have additional moment from high thrust line, what kind of airflow you will provide to this tail behind wide side-by-side cabin?

    Like it doesn't matter. Like its OK. Nothing to worry about.
     
  18. Jun 23, 2019 #1098

    Hephaestus

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    No argument, but the knowledge base just isn't there. The more I play with the Carmichael racer design the more I realize how much thought he had put into it. (Like the U shaped tail - out of the prop wash) I don't think I've ever seen another one except inverted on twin booms (like the old dehaviland jets)

    I keep following this thread as I think you're on the right track, just not sure what that track is myself. ;)
     
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  19. Jun 23, 2019 #1099

    plncraze

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    Oh yes! Just look at his bibliography in those articles!! Some of the folks who read Bruce had probably never even seen that kind of stuff in Sport Aviation before. Of course Bruce invented or discovered some of this so he wanted to share it.
    For the same reason Bruce wanted to share his info. Light airplane design is a different pathway in airplane design. Bill Welch wrote a lot for Kitplanes and he did a story on why small airplanes are different than big ones and why the generalizations in textbooks don't always match up. In a small airplane the pilot is a significant part of the load so design becomes an issue of handling larger masses in proportion to the empty weight. Drag becomes the same issue. Riblett had a formula for calculating trim drag in earlier versions of his Airfoil books but he eventually dropped it in what became his final edition.
    Try some of Darrol Stinton book "Design of the Aeroplane" for something that may pay more attention to various types of drag. The trouble with becoming an expert is that you have to find what others have done to try to solve the problem in the past. In Orion's "Airfoil" sticky he talks about pitching moment in airfoils and how some airfoil sections pitching moment are excessive and how that can lead to having to "carry" a lot of trim in cruise to deal with it.
     
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  20. Jun 23, 2019 #1100

    BJC

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    In addition to what plncraze wrote, above, it is difficult to write a book on how to design something - anything. It is easy to list the skills needed to design an airplane, it is easy to write a procedure that is generalized (they are available), and it is easy to document a specific design process, but there are too many variables to have a useful, specific guideline that eliminates the need for experience and judgement in many diverse areas.

    Therein is part of the reason that HBA is interesting: the variety of skills and experience in the many diverse areas needed to design a homebuilt are represented here. The challenge is separating the wheat from the chafe.


    BJC
     
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