Decalage angle

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

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  1. Jul 4, 2019 #1141

    Hephaestus

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    Your wife has threatened each of us and said that's not to happen. We're all afraid of her too.
     
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  2. Jul 4, 2019 #1142

    Eugene

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    508B8A17-6F9F-48A5-AA78-7F48D4A42206.jpeg

    So, on retractable Skyboy point B will move up a little, speed will increase a little, angle of attack will get smaller and tail will move up.

    Moment between point A and point B will stay the same because power setting did not changed. But horizontal tail physically will be positioned little bit higher, will get a little bit better airflow and at faster speed will need smaller angle of attack to control this moment generated by A and B.

    I do understand that there is more to consider like wing pitching moment Mz, but I am ignoring it for now on purpose.

    And altogether this aircraft in flight will look much more normal. Without crazy angles and total decalage of 10°
     
  3. Jul 7, 2019 #1143

    Eugene

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    Last edited: Jul 7, 2019
  4. Jul 7, 2019 #1144

    BBerson

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    Might need to try that while gliding with the prop stopped. Each time a blade passes it swirls the flow some upstream.
     
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  5. Jul 7, 2019 #1145

    Doggzilla

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    Probably not a very safe idea.

    It's pretty clear the flow is not laminar, which is exactly what would be expected on such a thick object.
     
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  6. Jul 8, 2019 #1146

    Eugene

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    Well, I will not be doing this experiment over lake Michigan or something like that.

    Our longest RWY = 5100 feet. I can do extra long and high final. Turn engine off, trim for 90 MPH and do power off landing.

    This should give us same velocity and same picture as level flight
     
  7. Jul 8, 2019 #1147

    Eugene

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    31F4D5AE-3461-41E6-A51F-F619986B3D75.jpeg DC7589C7-0A91-4F7C-B7E4-6C47AA99F0BA.jpeg

    During my test flight yesterday I was doing some thinking. Like I always do. And it hit me that all this time I was thinking about my airplane as A-B-C triangle. But in reality there is no mechanical connection between point A and point C.

    There is connecting brace going from point A to point D. This is a problem area on all Skyboys and needs to be inspected periodically for loose connection. It is apparently high stress point. If you don’t pay attention and your connection at point D is getting loose, you will have unpleasant surprise in flight.

    All my connections are solid, but I still get very small sequence jerking that reminds me of driving small car with big trailer. I was complaining about it all along, but unfortunately nobody ever listens to me. I do believe that tail boom is flexing inflight up and down under constant load changes in bumpy rough air conditions. That is why it was always so much more enjoyable to fly at night for me when air is stable. Airplane feels 100% solid and nobody’s constantly jerking on your tail.

    I would like to try installing two cables brace bailable two strut connections on the wings. I believe very similar cables you can see on Boorabee.

    https://drive.google.com/file/d/0B9ZeEB_79LEPUFZNbmdoOWNVVDA/view?usp=drivesdk

    Screen Shot 2019-07-08 at 07.23.04.png Screen Shot 2019-07-08 at 07.21.59.png Screen Shot 2019-07-08 at 07.20.02.png Screen Shot 2019-07-08 at 07.19.03.png DC7589C7-0A91-4F7C-B7E4-6C47AA99F0BA.jpeg 31F4D5AE-3461-41E6-A51F-F619986B3D75.jpeg
     
    Last edited: Jul 8, 2019
  8. Jul 8, 2019 #1148

    BBerson

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    That flexible tailboom is an interesting clue.
    Maybe your flexible tailboom is bending down 5° in flight and that's why the decalage appears excessive on the ground. The decalage in flight might be less. Your tailboom was designed for a slow ultralight.
     
    Last edited: Jul 8, 2019
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  9. Jul 8, 2019 #1149

    Eugene

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    Wow !!!

    Why didn’t I think about it myself ???

    Maybe not so smart after all !

    Thank you !
     
  10. Jul 8, 2019 #1150

    poormansairforce

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    Put 100 lbs on the stab and see if it deflects.
     
  11. Jul 8, 2019 #1151

    BBerson

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    I bought a 5" used irrigation tube to test years ago. It deflected so much with my 160 pounds I decided to reject it.
     
  12. Jul 8, 2019 #1152

    Doggzilla

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    If you have to trim so much it means the angle is not enough anyways.

    The angle should be high enough that no trim is required while solo and only trim has to be added when carrying someone else too. Since solo is the lowest takeoff weight possible and should not require trim at all at minimum weight, only as weight increases.
     
  13. Jul 8, 2019 #1153

    BBerson

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    I estimated the negative pitching force is about 300ft/lbs for my 400 pound design. So if his is 1200 pounds the force would be about 900 ft lbs or say 90 pounds. But then also need to add the high line thrust force.
     
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  14. Jul 8, 2019 #1154

    Eugene

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  15. Jul 8, 2019 #1155

    Eugene

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    Or, horizontal tail should be large enough and effective enough (NACA 0009) to do its job at somewhat reasonable 2-4°AOA
     
  16. Jul 8, 2019 #1156

    Eugene

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    Even 1° or 2° is better then nothing. But most importantly cables will stop vertical motions of horizontal tail inflight.
     
  17. Jul 9, 2019 #1157

    Doggzilla

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    With the thrust mounted so high that greatly increases the angle required.

    You can find this out accurately by marking the trim tab with engine at full power and then mark it again with throttle pulled back and diving slightly to test it at the same airspeed without power.

    Then on the ground you move the trim to the settings you marked, then measure the difference in angles of the elevator.

    This will tell you exactly how much of your trim angle is caused by the engine thrust and allow you to adjust the tail accordingly.
     
    Last edited: Jul 9, 2019
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  18. Jul 9, 2019 #1158

    Eugene

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    I am hearing something different.

    With the thrust mounted so high = that greatly increases -horizontal tail volume -required. Angle of incidence should be somewhat normal = 2°- 4°, for lowest drag possible.

    I am learning that increasing angle only to get what you need = not very smart way to do things. And should not be used on 100 MPH aircraft. But for 50 HP = 60 MPH was probably OK thing to do.
     
    Last edited: Jul 9, 2019
  19. Jul 9, 2019 #1159

    Eugene

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    So, I can use something like 120-150lb to do this test?
     
  20. Jul 9, 2019 #1160

    BBerson

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    I don't know what the tail load from thrust is. Just guessing, it could be 100 pounds from thrust and 90 pounds from the pitching moment.
     

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