Decalage angle

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

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  1. May 30, 2017 #21

    Eugene

    Eugene

    Eugene

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    I have only about 20 hours in this aircraft. So, I can't say anything about pressure on the stick, during gliding. But I do know from original designer, that stick pressure was some kind of requirements in order to pass German certification. I wanted to say that almost no matter what you do and how you fly, stick pressure is a very minimal, if you trim properly.
    Trim - on another hand different issue altogether. It is unbelievably sensitive!!! I wanted to say the Very minimal movement of approximately 1/16 or so, can make you climb or dissent 300 ft./m.
    I do like "decalage thinking" - because it feels with high power settings, that airplane under such tension, like wing under 90hp trust is trying to get into efficient cruising mode, but tail is pushing Down so hard.
    There is no harmony here with 10 degrees apart!!!
     
  2. May 30, 2017 #22

    lr27

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    500 lbs on the tail? Just eyeballing the tail in the drawing in the owner's manual, I'm guessing that the tail area must be about 12 percent of the wing area. That would requre a Cl of something like 1.75. You can't get anywhere near that with a flat plate type "airfoil". Maybe 0.6? 0.8? So, just before stall, you couldn't have more than maybe 200 lbs. And it's not safe to fly with the tail almost stalling. It IS quite draggy, I suppose. I guess if the elevator was up a few degrees, you could get a bit more downforce, but earlier in the thread it seemed that the elevator angle was quite modest.
     
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  3. May 30, 2017 #23

    BBerson

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    The prop blast (download)on the tail might be more like 150 mph at 75 mph cruise.
     
  4. May 30, 2017 #24

    lr27

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    If true, that would mean the prop was VERY inefficient.
     
  5. May 30, 2017 #25

    Eugene

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    AFH_Figure_4-1.JPG this picture doesn't show correct Extreme Elevator deflection, but this is a essentially how I fly my Skyboy.

    I learned to fly in Cessna 172. I was told to land with full flaps every time. I remember when I forget to bring flaps up after take off - how air plane was flying and feels. This is about how I feel now, every time I fly my Skyboy.
     
  6. May 30, 2017 #26

    BBerson

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    Well yes, because a prop same size as the original 50 hp prop of limited diameter but now with 100hp and still limited diameter but higher pitch and slipping will be less efficient.
     
  7. May 30, 2017 #27

    vtul

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    There's a difference between angle of attack and incidence. Incidence is the relationship of the wing to a design datum.

    Angle of attack is how the wing rides relative to the airflow. And in level flight, is physically determined by the amount of lift needed to support the wing's proportion of the load at a particular speed.

    Ordinarily, for level cruise, changing the wing incidence a small amount doesn't change the level flight angle of attack by an equivalent amount. The wing must always take a necessary angle of attack to support its share of the load at a particular speed. The main effect on a well trimmed plane would be to change the angle the fuselage is flying at, relative to the flight line. Normally the incidence is designed to keep the fuselage level at designed cruise speed.

    Changing the wing incidence, and therefore the fuselage angle at cruise does affect the elevator's angle of attack and the engine's effective thrust line. So the trim needed to hold level flight at a particular speed will have changed, as will fuel economy, depending on whether drag has been increased or decreased as a result of the trimming change.
     
    Last edited: May 30, 2017
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  8. May 30, 2017 #28

    Eugene

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    Propeller has its own little story....

    Originally on takeoff Engine was spinning up to 6000 RPM. Properly load a engine, I was told, needs to have WOT about 5600 RPM. This is a when I discovered that my three bladed warp drive (62") is already at 16°. I did turn it up to 18 to get to WOT = 5560 rpm.

    Since most normal people flying in with 13°, I and up calling Warp Drive. They didn't tell me, that they don't have anything to help me , because combination of three blades, 62 inches and 100hp does not to work! They do however Believe that at this point I am not losing any efficiency in cruise, but on takeoff if you exceed 18° propeller possibly will cavitate.

    This is why on my Christmas list with very fat permanent marker with big letters - five bladed carbon fiber 62 inch propeller from POWERFIN follow price of $1500.

    But even with new propeller Best I can hope for efficiency increase buy 5% or so.
     
  9. May 30, 2017 #29

    TFF

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    The stick force the government made them check was to see if it was too light as you know its sensitive. It could be even more, if allowed! With the glide we are trying to see if the plane trims at the same spot. If there is much difference in trim between flying 80 at cruise and 80 in a shallow dive, the incidences are a big compromise. I would think if you could ballast the CG more forward it will tame it some of the trim changing. In CG range and perfect CG spot are different things.
     
  10. May 30, 2017 #30

    Eugene

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    FullSizeRender 20.jpg

    I was told by Peter Garrison that this is a essentially two identical airplanes, with different pilot position. I like to think that all flying is done between two flying surfaces and fuselage is just a connection between them two. If there is no harmony with 10° decalage, there is no efficiency!
     
  11. May 30, 2017 #31

    vtul

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    That drawing shows two hypothetical cartoon fuselages with the same wing and tail angles of attack, and different wing and tail incidences.
     
  12. May 30, 2017 #32

    vtul

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    Both also show the same decalage.
     
  13. May 30, 2017 #33

    lr27

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    Air props don't cavitate, but they can stall.

    With a water prop, you can get the pressure so low that the water boils, or maybe it's dissolved gases coming out, creating a cavity or bubble. If the bubble collapses, it damages things. A nick on a water prop can trail bubbles that collapse on the blade downstream, smashing the paint off! This doesn't happen with an air prop.
     
  14. May 30, 2017 #34

    TFF

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    Back in the days when most model airplanes were freeflight or single channel RC. The decalage was used to trim the plane and hold it at one speed. Freeflight especially fixed tails, not with timers, had to be able to climb and the glide. Single channel RC was set to glide power off and climb power on. Rolling the plane slightly into a turn would hold altitude, but rolling it into a dive added speed, then you leveled off, and the excess speed would loop the plane. To look good on exit at the bottom of the loop, you turn the excess energy into a roll. Speed settles and the model resumes a slight climb. Look at WW2 planes and they have a bunch of wing incidence. They are pretty heavy considering a fighter is around the same size as a Cessna 172 , but weighs 4-5X as much. When scaling down to model size, the scale incidence is excessive as models dont fly at scale weights they are much lighter, so the wing incidence has to be taken out.
     
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  15. May 30, 2017 #35

    Eugene

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    I am no expert by any means, but point I was trying to make is - decalage is important, but position of fuselage irrelevant
     
  16. May 30, 2017 #36

    BBerson

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    The wing down wash adds decalage with a negative flow to the tail. The lower tail likely gets more down wash.
     
  17. May 30, 2017 #37

    vtul

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    Decalage is the difference in the physical incidences set on a plane, which is a straightforward geometrical relationship.

    Downwash can not add decalage. Though it may make a particular decalage desireable, or not.
     
  18. May 30, 2017 #38

    BBerson

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    The geometrical relationship is straightforward. The aerodynamic relationship includes down wash.
    For example, an aerobatic airplane that has the engine axis, and a symmetrical wing and the horizontal all on the water line at zero relative incidence will have decalage in flight from downwash. In fact, because the airfoil is symmetric, it has decalage while inverted as well.
     
  19. May 30, 2017 #39

    vtul

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    Decalage is the relative geometrical angle of two surfaces. You are using the term to mean the difference in effective angle of attack under flight conditions. Decalage is a physical design term, not an experiential term. You set decalage, just like you set incidence.
     
  20. May 30, 2017 #40

    Eugene

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    This is how I see it : airplane is a puzzle . If you have correct pieces such as wing surface area, Power loading , wing loading and so on. You need to know how to put those pieces together correctly . Good flying airplane is successful puzzle . I am trying to find out what's wrong with my puzzle . Geometry of my puzzle worked very good for German certification with 50 hp and different airfoil . I feel, if you replace some pieces, you should change geometry somewhat . You just have to know how, but I don't, that's my problem .
    That is why my initial attempt was get this airplane out of trimming for slow flight . But I found myself riding pretty aggressive waves and I felt it with my seat .
    Maybe my only answer is to become aeronautical engineer next life ?
     
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