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Thread: Decalage angle

  1. #796
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    Re: Decalage angle

    I find myself in the uncomfortable position of being an advocate for large enough tail surfaces, and wing area, and cautioning against trying to squeeze more speed by making tails as small as possible.

    So even though I fear you aren't correct in your logic here, and really don't want to hear you had a failure because of a modification you made chasing a few more knots of speed, it is in a way against my own statements to tell you not to make the tail bigger. Sigh.

    I still think the thrust line is wrong on your plane, but you said the designer said it was right and not to change it. So I must be wrong, and I'd quit listening to me if I was Me.

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    Re: Decalage angle

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    Re: Decalage angle

    It seems like on my airplane for every small change in power setting horizontal tail assuming immediately different negative position. It makes sense to me, because tail is undersized and with 10 decalage there is so many different options to choose from. And I am constantly compensating by adjusting trim lever.

    I think if my airplane would have longer and larger tail with needed negative angle only 1 or so. Maybe aircraft would be more stable and not so trim sensitive. Tail will be more or less level at any power setting. But I don't know what I'm talking about here.

    And it's not about speed at all. Its about getting it right. I am learning that horizontal tail needs to be large enough to do its job at reasonable angle 2-3. If you need 6-8 = tail is undersized.
    Last edited by Eugene; February 27th, 2019 at 11:03 PM.

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    Re: Decalage angle

    See post 52 here: http://www.homebuiltairplanes.com/fo...l=1#post386320

    Some aircraft such as yours may need double the tail area of a Cub because of the high thrust line and the massive drag wake from the uncowled engine. A tail in a wake doesn't work well.
    But your tailboom is not designed for double the tail area. A smoother flow to the tail might help.

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    Re: Decalage angle

    Quote Originally Posted by BBerson View Post
    See post 52 here: http://www.homebuiltairplanes.com/fo...l=1#post386320

    Some aircraft such as yours may need double the tail area of a Cub because of the high thrust line and the massive drag wake from the uncowled engine. A tail in a wake doesn't work well.
    But your tailboom is not designed for double the tail area. A smoother flow to the tail might help.


    Someone tell me that symmetrical airfoil shaped horizontal tail can be up to 30% more effective in comparison to flat deck with same area. So, plan is to install 2 new little wings and increase area by 20%. Old stab/elevator will be converted someday to 0009 airfoil as well.

    Hopefully combination of this 2 improvements will have noticeable positive effect.

    P.S. Found many dangerous reasons against to small tails. Nothing about tails that is too big.Click image for larger version. 

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  7. #801
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    Re: Decalage angle

    Peter,

    Forgive me if I asked this question before. I looked in old messages, but didn't see it.

    In the past 2 years I talked to different Skyboy owners and they all confirmed to me that this airplane flies completely different with passenger vs solo. Best way to describe is to say, that with passenger airplane feels overloaded. You need much more nose up trim. Trim is not sensitive anymore. You can move trim lever back and forth by 1/2 inch and feel very small gradual changes in climb or descent. Otherwise flying solo aircraft would respond immediately with up or down by 500 ft/min.

    In addition ailerons with passenger becoming very lazy and wiggling your wings will turn in to slow motions. I call it flying cow. One time I was flying C-172 with 200 lb over-gross and 160 HP with very similar feel in controls.

    I was told by some engineers that this kind of behavior is direct result of under sized horizontal tail. Wondering what do you think.

    Thank you!



    Peter Garrison
    12:46 AM (1 hour ago)

    to me

    Eugene,

    The lack of trim effectiveness would be expected if the horizontal stabilizer were already operating near its maximum ability with two aboard. Do you have any photos of the empennage in flight while carrying two people? Is the 2-person loading the most forward CG case? It would then be the most statically stable, meaning that stick forces would become heavier. There would be some diminution in roll response because the moment of inertia about the longitudinal axis would be greater, but I'm surprised that the effect would be as large as you describe.


    Peter,

    This picture was taken with 200 lb lead ballast on passenger seat, at 85 MPH, CG at about 23% MAC, stabilizer at about negative 8 AOA and elevator at 10-11 AOA.

    This aircraft was certified in Germany with CG limits 21% - 36.6% MAC. This is same CG range as Aeronca Champ with same NACA 4412 wing and you feel very little difference between solo flight and with passenger. Aeronca empty about 100 lb heavier and same cruise speed with 85 HP engine. My Skyboy has 100 HP engine.

    I remember during our first flight with LSA instructor right after takeoff he decided to abort our lesson. On downwind he was telling me that something is wrong with this airplane and he not comfortable flying with me and checking me out in it. To prove his point he was moving stick left and right with 6" spread, but aircraft was flying straight and didn't respond.

    Only later after talking to different Skyboy owners I found that this behavior is normal for all Skyboys and we start flying again.

    Thank you!





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    Re: Decalage angle

    Peter,

    Just wanted to add something, that I think it's important. Skyboy was going through German certification in 1992 as ultralight with 50 HP engine, larger wing 168 sq. ft. with different P3 airfoil.

    For American market 100 HP engine was installed with smaller wing 139 sq. ft. and 4412 airfoil.

    Same 5 ft chord was used on both wings and same empennage without changes.

    Thank you.




    Peter Garrison
    2:21 PM (6 hours ago)

    to me

    Eugene,

    Reducing the size of the wing while keeping the horizontal stabilizer the same would normally be expected to increase longitudinal stability. On the other hand, I am inclined to think that power is destabilizing in this airplane, and so reducing power would increase stability and reduce the balancing load required because of the high thrust line. But I cannot guess the magnitude of either change.

    Lack of roll response, with a large dead band in the middle of the stick travel, is not uncommon, and can be due to adverse yaw or to poor aileron design. Many airplanes, especially older ones, required rudder to initiate a turn, with ailerons later adjusting for coordination and maintaining back angle.




    Peter,

    Ok, I will not worry about slow roll response anymore. On the other hand, it's only slow with passenger.


    So, with engine above the wing, more power should be compensated with larger horizontal tail. This is what I understand from your message. With reduced power down to 50-60 HP or so, airplane very stable, slow 65-70 MPH, with 6-8 AOA and very relaxed. Because at that point aircraft in its design configuration.

    I was trying to find what needs to be done to match this aircraft to 2 times larger 100 HP engine. I was told by many that correctly designed 100 HP - 100 MPH, LSA aircraft should have decalage of 4-6, and not 10-12 like on my Skyboy. Without knowing any better I was after decalage.

    Someday I will try to convert my flat tail to 0009 airfoil with increased area by 20%. Hopefully it will make some difference.

    Thank you very much for your help!

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    Re: Decalage angle

    Look at this decalage angle: https://www.homebuiltairplanes.com/f...l=1#post465394 (post 656)

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    Re: Decalage angle

    Its hard to tell exactly on this picture were to measure angle for horizontal tail. Thank you. Very interesting airplane!

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    Re: Decalage angle

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  12. #806
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    Re: Decalage angle

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    Seems to me that during level flight horizontal tail will be above and outside of turbulence that generated by wide cabin and above wing down the wash. From that standpoint looks like a very good design

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    Re: Decalage angle

    Yeah. The tailboom is angled upward.

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    Re: Decalage angle

    Quote Originally Posted by Eugene View Post
    Seems to me that during level flight horizontal tail will be above and outside of turbulence that generated by wide cabin and above wing down the wash. From that standpoint looks like a very good design
    Down was comes from above the wing too.


    BJC

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