Decalage angle

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

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  1. May 19, 2019 #841

    Doggzilla

    Doggzilla

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    Ya, as it says, if you can hit 5800 it's under pitched and you'll lose top speed.

    Back it down until you hit 5600 and you'll get your missing speed back.

    As for your stability issues it's just an effect of having such a thin elevator. Thin airfoils are very angle sensitive and will slowly move upwards and downwards as vortexes shed off the surfaces in an alternating fashion. They do not have laminar airflow and are prone to vortexes forming along the leading edge during any change in AoA.
     
  2. May 19, 2019 #842

    Doggzilla

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    The change in pitch is normal during change in throttle. This is due to the vertical offset of the engine and it being a pusher as well. Totally normal.

    Reduction in throttle should cause nose up. A high pusher engine pushes nose down when throttle increases.

    Is that what you are experiencing?
     
  3. May 19, 2019 #843

    Eugene

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    For prop: - In 3 years I changed 3 different props and changed pitch setting must be 100 times by now. Right now I have 70" - 3 blade Powerfin with pitch of 12.7°. I am pretty sure that I got most cruise performance out this one. Spend a lot of time playing with it, with 0.3° increments and test flying in between.

    For stability: - many people here and at my local airport don't understand my desire to fix or find problem and recommending to sell this airplane and get better one. If we buy house with badly performing heating system, we don't think about selling it right away and get another one. Same goes for used cars. Most people will fix problem and keep the car.

    After talking to many smart people I have pretty good understanding about Skyboy situation. Or I think that I do.

    Correctly designed LSA will cruise 80 - 120 mph with wing AOA about 1 - 3° and CG at 25% MAC. Most airfoils do have least amount of drag in this position. If you see 4 - 6°, you made mistake in your calculations.

    Similar logic applies for empennage. Horizontal tail should do its job at reasonable 1-3°negative AOA. If you need 6-8°, your tail is to small, in wrong place or wrong airfoil.

    So, normal decalage for this type of airplanes should be about 4° to 6°or so. On Skyboy we have 10.2°. Only way to get CG to 25% is to take 250 lb passenger. Aircraft will fly with wing level to the horizon, tail will be up by 10°with very overloaded feel, like Cessna 172 with 200 lb over-gross. Tail is simply to small to handle pithing moment from 100 HP engine!!!

    Skyboy was designed in 1992 as 50 HP ultralight with 2°negative wing swept, 4.5° wing incidence and about 3°negative stabilizer incidence.

    During German certification stabilizer angle was changed to 5.7°and aircraft was certified with CG range 21 - 36.6% MAC.

    All that was probably OK in 60 mph 50 hp ultralight world, but last Skyboy was shipped to this country in 2007 as LSA with MTOW of 1320 lb, heavy 4-blade 62" prop, 100 HP engine, same 5.7° horizontal tail with Vh = 0.34 and negative wing swept.

    Here in North Carolina on brand new aircraft from Europe right after installing wings they needed to install 20 lb ballast in front of rudder pedals to get your CG for solo flight forward to 36% for standard pilot.

    So, when I am all done with horizontal tail with symmetrical airfoil and 2° angle with VH=0.45-0.5, I will move wings to 2-3° positive swept to get CG to were I need it.
     
    Last edited: May 19, 2019
  4. May 19, 2019 #844

    Eugene

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    Screen Shot 2019-05-14 at 23.06.36.png Screen Shot 2019-05-14 at 23.02.11.png
     
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  5. May 19, 2019 #845

    BBerson

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    None of those are pushers with a huge draggy engine wake or CG problems with people in front of the CG instead of near the CG. Large CG changes need more.
     
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  6. May 20, 2019 #846

    Eugene

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    I need clarification about some things:

    - I understand that aircraft with high mounted pusher engine will make additional moment and will need larger horizontal tail to balance in level flight.

    - I agree that engine without cowling creating drag and reducing propeller efficiency. Many designers however telling me that it is relatively small factor. Searay designer was telling me that I will not see any difference at all.

    - I agree with Peter Garrison that wide cabin reducing tail effectiveness. But disagree that 10° decalage is no problems at all.

    - "CG problems with people in front of the CG instead of near the CG. Large CG changes need more." This one is confusing to me. If there is aircraft in level flight with CG at 25%, why would it make any difference how did we get CG to this point and who is sitting were??? In front of the wing or behind? We can do it in so many different ways (moving pilot and passenger, using ballasts, changing wing swept). My CG range 36.6% - 21% = 15.6%. In real life situation solo or with passenger CG moves only 10% or 6". Not sure if this 10% or 6" is consider to be big changes or not. One time in my life I was looking at V-tail Bonanza with CG range of only 3.5".
     
    Last edited: May 20, 2019
  7. May 20, 2019 #847

    BBerson

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    Sure, ballast works. But inconvenient. If you drop off a heavy guy and don't have any ballast to fly home what will you do?
    Most pilots don't want to bother with ballast and choose conventional airplanes with big enough tails to handle the CG range without ballast.
     
  8. May 20, 2019 #848

    Eugene

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    Yes, 100% agree, ballast is only temporary fix during experimental test flights. When I am all done experimenting with my tail = I will know how heavy it will become, I will do wing swept to get my CG solo to about 32% and with passenger about 22% or so.

    Screen Shot 2019-05-13 at 22.11.06.png
     
  9. May 20, 2019 #849

    Doggzilla

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    Everything is correct there except the tail area helping trim. That is exclusively caused by the moment of the high mounted engine.
     
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  10. May 20, 2019 #850

    Doggzilla

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    Was just looking back through this thread and saw that the rear of your aircraft does not look like any others of the same type I can find online.

    Looks like the rear cowling is only half length instead of full length for some reason.

    Its almost certainly operating as a huge diffuser when set like that, and will both increase drag massively, and interfere with the airflow on the bottom half of the prop arc. Which will exacerbate the issues with the moment of the thrust being so high as well.

    There is probably a very good reason that rear cowling design was discontinued. I would recommend looking into getting the standard rear cowling if I was you.
     
  11. May 20, 2019 #851

    Eugene

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    I was thinking about it for some time 0JV0l6pnR8mDszCbJoz4DQ.jpg 5YzZ40vFQzuRv0LBkwLqyw.jpg CKygMujESwi81PuSM9346Q.jpg Qkvue9CATHaDN1DpJbJjmA.jpg %ohUfdt9SuaVJuVNP37Q1g.jpg
     
  12. May 21, 2019 #852

    proppastie

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    you are adding more weight aft again.....better put some tape going forward...or that forward wing sweep will be really big.
     
  13. May 21, 2019 #853

    poormansairforce

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    I thought about posting this a long time ago but just never did.
    I think this is exactly what is happening. Look at the bottom of your fuse. Its the top half of a wing thats inverted and traveling backwards. This has a terrible pressure recovery zone at the rear. Lots of drag there! And it operates as as reflexed wing but upside down so a lot if nose down force being applied along with inverted lift being applied.

    Again, horrible pressure recovery. We would never fly a wing backwards. Look at the plane from the front in the pics in the following quoted post:

    Look at the 3rd pic in the following quoted post. A side view pic shows it as well. I just never had a better pic of the bottom until now. Lots of nose down forces being applied. Some fairings to flatten the curve would go along way to help keep it from hunting as well as reducing the drag and killing the inverted lift.
     
  14. May 21, 2019 #854

    Eugene

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    Did little test flight and really didn't like it at all!!!

    1st. - steel needed a lot of "nose up trim". Now I really think that my diagonal wings did much better job.

    2nd. - whole airplane feels like small car pooling big heavy trailer and trailer is constantly jerking airplane back and forth. Not as bad like on this video,https://drive.google.com/file/d/0B9ZeEB_79LEPUFZNbmdoOWNVVDA/view?usp=sharing but its coming from same source = unbalanced elevator. Maybe on this aircraft we need 100% elevator balancing at all times.

    So, I will remove additional plates from elevator since they didn't help with up-trim anyway.
    https://drive.google.com/file/d/11pJ-YuKziNcxifV7tKASLBXa_RrJoICP/view?usp=sharing

    I did look at the bottom of my fuselage at one time, but only from stand point of air separation. I will look again. Thank you!
     
  15. May 21, 2019 #855

    Eugene

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  16. May 21, 2019 #856

    poormansairforce

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    A row of VGs across the bottom may help pinpoint whats going on. That is a severe curve!
     
  17. May 21, 2019 #857

    poormansairforce

    poormansairforce

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    That tapering quote is referring to fuse/wing intersection on a low wing. Even so, a curve that severe won't help any either.
     
  18. May 21, 2019 #858

    Doggzilla

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    Maybe it's an optical illusion, but do the ailerons both droop?

    The angle of the trailing edge does not seem to match other aircraft of the type, but maybe this is just an illusion from that angle.

    If the leading edge had trailing edge have opposing angles of incidence that will likely cause some drag and possible buffeting.
     
  19. May 21, 2019 #859

    Eugene

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    No, aircraft fully symmetrical I checked measurements many times.
     
  20. May 21, 2019 #860

    Doggzilla

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    Im not saying there is a difference between the sides, but that they appear to be set downward when they should be flush with the rest of the trailing edge.

    [​IMG]
     

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