Decalage angle

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

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  1. Feb 4, 2019 #781

    rotax618

    rotax618

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    I personally have gained a lot from Eugene’s thread, it reinforced a lot of my previous beliefs. It is very difficult to design a low speed side by side pusher sport plane, the designers of the Skyboy and the Rans have designed aircraft that are safe and stable given the difficulty of the task.
    It is relatively easier to design a tandem pusher, as 1/2 of the variable payload is directly under the CG and the other half (pilot) balances the aircraft, the variation of pilot weights is only say 25kg, this is easily compensated by making the seat slide forward for smaller pilots, increasing the wing chord (increase CG tolerance) and increasing the tail volume (usually involves increasing horizontal tail size rather than long tail boom which moves CG back).
    The side by side can have a payload variation of say 120kg (passenger or no passenger, large/small pilot), this is very dificult to design for in a 550kg TO weight sportplane, the above solution don’t apply so it is necessary to set the longitudinal trim for worst case (pilot + passenger).
    The trim drag is always going to be a problem for side by side pushers, it can be reduced by selecting a low pitch airfoil ( may require more span and area to compensate for loss of CL) increasing the area of the tailplane/elevator requiring less deflection to control pitching moment (trim drag) and ensuring that the thrust line is as close to the centre of drag as possible (power on/off pitching moment).
    Having said all that this configuration is best avoided in low powered sportplanes.
     
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  2. Feb 4, 2019 #782

    Eugene

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    fullsizeoutput_164d.jpg

    I don't see anything wrong with cables looking at this numbers!
     
  3. Feb 5, 2019 #783

    Aesquire

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    I never said there was anything wrong with cables. In fact, while a solid rod may be stronger in tension than a cable of the same diameter, a nick or kink in the rod will cause total failure. One piece, one break. With a cable, damage is more survivable.

    OTOH, streamline rod can reduce drag on several feet of bracing. Note that the rod offered uses rolled, not cut thread. That's important. Any competent machinist can make you cut threads on relatively inexpensive bar stock, but each section is a notch failure waiting to happen.

    And, I object to turnbuckle ends on structural bracing even though it's common in architecture. So I'm probably wrong & paranoid. :)

    I still would use thimble ends from tang to tang on at least the top wires on a horizontal stabilizer with no turnbuckles. In any use of turnbuckles use best practices and be sure of proper thread engagement. Hanging your life on less than three turns of engagement? Bad idea.
     
    Last edited: Feb 5, 2019
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  4. Feb 5, 2019 #784

    Eugene

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  5. Feb 6, 2019 #785

    Eugene

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    fullsizeoutput_164f.jpg

    Wondering if this could be possibly good option. If I take tubing and flatten out the ends and drill some holes. Bolts and nuts can be used vs only pins. This tubing will be almost completely covered with symmetrical airfoil made aut of fiberglass and the whole thing will become one solid piece. No flutter possible. No idea what diameter tubing should be used and what kind material.
     
  6. Feb 7, 2019 #786

    Eugene

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    fullsizeoutput_1655.jpg

    So, I finally got confirmation from someone with a lot more experience and need education.

    This aircraft if would be manufactured with 5 different engines, will need to have different wing incidents on the wing to match different projected cruising speed.

    Horizontal tail however, if calculated correctly, should be able to stay same size and at approximately same 1-2° angle and be able to provide needed down pressure.

    Final adjustments can be made during test flights, but if tail sized correctly changes should be within 1° or so. With more engine power and more speed, more drag will generated larger nose down pitching moment. But increased air velocity over horizontal tail should provide pressure needed to balance aircraft at new higher speed.

    As you can see that very slow model with smallest engine and somewhat miscalculated tail can easily have 8 or 10° of decalage. It will be very draggy and slow aircraft, but you have no another options if you using small 50 hp engine to carry 2 people. And if you decide to install 2 times bigger engine some day then this aircraft would fight back and wouldn't want to fly any faster than it was design for. On top of that in pusher configuration at higher speed air separation behind fuselage will also slow you down dramatically.
     
  7. Feb 16, 2019 #787

    Eugene

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    Peter Garrison


    to me

    Eugene,

    The drawing you sent has no physical basis. Speed is roughly proportional to the cube root of power; it is not directly proportional to power, as you seem to think. The speed with a 100-hp engine would be greater than that with a 60-hp engine by a factor of the cube root of 100/60, or 1.18. You obviously know your actual speed and your actual power, but I have forgotten both.

    I'll skip the math, but it's clear to me that the difference in angle of attack at cruise between the 60-hp engine and the 100-hp engine would not be more than maybe 2.5 degrees, not the 4 degrees shown in your drawing. You should not refer to this as an "incidence," which means the angle of the wing to the fuselage centerline. The least-drag attitude of the fuselage is unknown, and so it doesn't make much sense to talk about changing incidence. All that matters is the angle of attack of the wing in cruising flight.

    The nose-down pitching moment of your airplane is composed of one fixed element (CG location) and two that vary with speed (wing pitching moment, high thrust line combined with varying thrust). The balancing force on the horizontal stabilizer is composed of a two variable elements, namely propeller slipstream (which diminishes as speed increases) and airspeed. It is next to impossible to pry these apart. But in any case the trim drag is mainly due to the high thrust line, and there's nothing you can do about it.

    You insist on blaming the high drag of the Skyboy on its decalage. As I have told you over and over, that is not the cause, the decalage is obviously fine since the elevator is more or less in trail in cruise; but apparently no matter how often I say this you keep hunting around for somebody else to tell you otherwise. You yourself know little enough to be unable to tell who is reliable and who is not, and so you imagine that someone with a title like "EAA technical advisor" must know a lot. Maybe, maybe not. At any rate, your latest drawing makes no sense.

    Just build the cowling, and put baffles in it.




    Peter,

    Thank you for answering to me.

    Numbers I used to represent engine power, angle of attack or speed only for illustration purposes. I am simply trying to understand what will happen to aircraft that was designed for low power and low speed if you decided to install 2 times bigger engine.

    Nobody is offering correct explanation, but everyone is telling me that airplane that was born as ultralight never can be a good flyer in LSA world. They don't know why. But I am looking for answers.
     
  8. Feb 16, 2019 #788

    BJC

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    Eugene:

    The answer is in Peter’s letter above, as well as in a dozen or more posts from HBA’ers in this thread.

    Time to just go and enjoy flying your airplane. If it isn’t what you want, sell it and buy or build something else.

    Good flying.


    BJC
     
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  9. Feb 16, 2019 #789

    TFF

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    I think you did some good work but it's time to accept the plane and fly it. I get the fun part of modifying, but you are at the point of looking for the last five percent. Every racer knows that it's where the big money is spent. If you started with a faster plane, getting the last 5 percent is still going to take way more work than building another airplane. If you are having more fun working on it than flying, start building another plane while you fly that one.
     
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  10. Feb 17, 2019 #790

    Eugene

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    I decided some time ago for my research only to talk to real aviation designers/ingeneers who actually designed and build airplanes. They trying to make me to understand some fundamental things about designing typical LSA with 1300 lb MTOW:

    #1 - properly designed aircraft in level flight at target cruise speed of about 100 MPH and 75% power should have wing AOA between 1-3°. Most airfoils will have relatively small drag at that angle. If you see 6-8° AOA and only 70 MPH, then you miscalculated something = to much drag, engine not developing rated power, overloaded, wing area to small, propeller efficiency and so on.

    #2 - same logic is used for designing horizontal tail. In level flight at 100 MPH good design should have only 1-2° negative angle. If you see aircraft at design cruise speed with tail at negative 6 - 8°, = designer made mistake and tail is to small and producing to much drag.

    #3 - my designer was telling me that original angle for stabilizer on my Skyboy was much smaller. But during German certification it was changed to 5.7°. This is proof that tail was miscalculated and undersized . Instead of increasing area they simply changed angle.

    #4 - original Skyboy was certified with MTOW 992 LB and 5.7° tail. To this country this aircraft was shipped with MTOW 1320LB, 100 HP engine and exactly same tail without any changes at all!!! Absolutely wrong approach!

    #5 - very best engine cowling will give you at best 5 MPH. Open engine is no way represent 50% of total aircraft drag.


    So, who is right and who is wrong? Only one way to find out. Time will tell
     
  11. Feb 19, 2019 #791

    Eugene

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  12. Feb 22, 2019 #792

    Eugene

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    Peter,

    I decided some time ago for my research only to talk to real aviation designers/ingeneers who actually designed and build airplanes. They trying to make me to understand some fundamental things about designing typical LSA with 1300 lb MTOW:

    #1 - properly designed aircraft in level flight at target cruise speed of about 100 MPH and 75% power should have wing AOA between 1-3°. Most airfoils will have relatively small drag at that angle. If you see 6-8° AOA and only 70 MPH, then you miscalculate something = to much drag, engine not developing rated power, overloaded, wing area to small, propeller efficiency and so on.

    #2 - same logic is used for designing horizontal tail. In level flight at 100 MPH good design should have only 1-2° negative angle.

    #3 - my designer was telling me that original angle for stabilizer on my Skyboy was much smaller. But during German certification it was changed to 5.7°. This is proof that tail was miscalculated and undersized . Instead of increasing area they simply changed angle.

    #4 - original Skyboy with MTOW 992 LB and this 5.7° tail was shipped to this country with MTOW 1320LB, 100 HP engine and exactly same tail without any changes at all!!! Absolutely wrong approach!


    Wondering if you agree with all this statements



    Peter Garrison
    12:09 PM (2 hours ago)

    to me

    Yes.
     
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  13. Feb 23, 2019 #793

    jedi

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    I have been lurking here but only half following since about the beginning. I did get a brief flight in a Sky Boy when they were being sold and understand your issues. I have also been following an early version of a similarly configured amphibian, the Mermaid, with similar problems and no real solution.

    It has been recently reinforced that the down wash from the main wing increases the effectiveness of the horizontal stabilizer. Please confirm the Sky Boy has both flaps and ailerons. Have you checked the rigging on flaps and ailerons and do you have specifications on the allowable limits? It would be an interesting experiment to rig the flaps down and ailerons up to increase the down wash over the tail at cruise. (Perhaps flaps down 6 degrees and ailerons up 4 degrees when measured with simulated flight loads.)

    I am suspecting the manufacturer may have drooped the ailerons in an attempt to lower the stall speed at the higher max weight. I would trade a higher stall for the improved cruise if this were the case. You might play with a little cruise flap before changing the aileron rigging. This will also improve the lift distribution and may reduce drag in slow cruise which is where you appear to be operating in general.

    This is likely a far out proposal but does not involve major changes to the structure. If I remember correctly the Sky Boy does not have protruding surfaces on the flaps or ailerons. (flow off the wing does not impinge on a slightly thicker surface and therefore tends to separate.) It may be necessary to install vortex generators on the upper surface ahead of the flaps and lower surface ahead of the ailerons to assist the flow in responding to the changes.

    Before trying any of the above I would install vortex generators on the lower side of the horizontal stabilizer if this has not already been done.

    You all be careful out there. This flying as test pilot on experimental aircraft is serious business.
     
    Last edited: Feb 23, 2019
  14. Feb 23, 2019 #794

    Eugene

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    No flaps, only ailerons. Flaps on this aircraft not giving you anything at all. It was installed on some Skyboy's in response for demand from flight schools. It feels like you have flaps and they fully extended. Guys who have flaps report that they changing stol speed by only 2 MPH or so.

    Vortex generators on lower side of stabilizer installed already. Trim lever moved slightly forward in level flight after I installed them.

    Hope increasing tail volume will make some difference.

    FUM%GYJTSYeTe30cs+cIcA.jpg hqiyFosuQPOxAWyLcaQ7TA.jpg fullsizeoutput_1669.jpg
     
  15. Feb 24, 2019 #795

    BJC

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    Reported
     
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  16. Feb 27, 2019 #796

    Aesquire

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    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. ;)
     
  17. Feb 27, 2019 #797

    Eugene

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  18. Feb 28, 2019 #798

    Eugene

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    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: Feb 28, 2019
  19. Feb 28, 2019 #799

    BBerson

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  20. Feb 28, 2019 #800

    Eugene

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    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. XtYzePfpQHuzNwiyLfk5mw.jpg
     

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