Decalage angle

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

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  1. May 29, 2017 #1

    Eugene

    Eugene

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    Hi everyone!

    New member here with very long story. I will try to make it short and hope that somebody knowledgeable will answer some of my questions.

    Last summer I purchased myself ELSA Skyboy with 100hp Rotax and NACA 4412 wing. It is high wing pusher designed and built in Europe originally as ultralight trainer, side-by-side, with 50hp engine and different very thick high lift airfoil. To American markets they were installing 100hp engines and 138 sq ft wing. Only about hundred of them where are made and sold worldwide.

    To satisfy the insurance company with 5 hour dual training I made appointment with local CFI and one day we were ready for first flight. Previous owner did give me 15 minutes ride and I found this airplane very stable with a very solid field on the controls. My instructor however didn't like this airplane at all. Biggest concern in flight was that at 80 miles an hour and about 5000 RPM bottom of the wing was showing very high angle of attack to the horizon.

    I managed to contact original designer and we were trying to determine if they didn't make any mistakes at the factory in 2001. In the process of checking everything out I found that engine what's tilted forward 4 degrees, opposite from his recommendation. I did rebuild engine mounts and now my engine tilted back by 3° and it's blowing on the tale. However I didn't find that airplane behaves any different.

    Next I discovered that when my airplane parked on perfectly level ground, which supposed to be more or less straight and level flight attitude. When I am holding Digital level to the bottom of the wing, which is almost flat, I can see 4.5 degrees positive angle (tilted up). When I put a level on horizontal stabilizer - I can see negative 5.7° (tilted down) . So total difference between this two flying surfaces 10.2°. we can assume that true decalage to the cord line is about 11° !!!. I did walk around airport and measured everybody else's airplanes and found that most of them with tractor configuration have 4-6 degrees difference between wing and tail. Original designer did tell me, don't change anything, everything is good.

    Of course I didn't listen. Because flying all the time with such high angle of attack is resulting into very poor performance (low speed, poor glide ratio). I talk to many different people, EAA technical advisors, different designers and builders. My conclusion was that airplane configured into slow flight mode to comply with ultralight rules in Europe 25 years ago. Everybody I talk to suggest to raise horizontal stabilizer, do it in small increments and test flight in between.

    So I did. I and up raising leading edge of horizontal stabilizer by 2° and on my first and second flight didn't see any changes on handling or improvements in speed. Angle of attack to the horizon at 5000 RPM = 92hp with 1000lb total was about 6° going 82mph (GPS speed avg. four different directions). Not very efficient way to fly. I was told by many people that 4412 wing at that speed and load should be just about level to the horizon.

    Aeronca Champ with 85hp engine and exactly same wing and 200 pounds heavier with power of setting of 75% (64hp) moving faster than my airplane.

    On my third test flight I discovered, that if I trim airplane for level flight and without holding the stick, controlling direction only with a rudder pedals. After a few seconds or so stick Will start making gentle movements back and forth and if you let it go it will progress, then in the few seconds and you will find yourself in pretty aggressive rocking chair. Pulling power back and pushing stick forward and 200 feet of altitude, Will get you back to normal flight.

    As you probably can guess, I am done with that experiment, and I will put everything back to original position. However I am still looking for explanation of why my airplane needs to fly this way. It feels exactly like flying Cessna 172 with fully extended flaps. Feels like airplane locked in to slow flight mode, with high angle of attack, high-power Setting, maintaining altitude and moving a relatively slow.

    I would appreciate any kind of input.
     
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  2. May 29, 2017 #2

    Swampyankee

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    Can you post pictures of the airplane and, perhaps, a three-view drawing?

    I'm guessing "low-drag" was not high on the list of design considerations, though.
     
  3. May 29, 2017 #3

    BBerson

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    Cruise at high wing angle indicates excess weight or insufficient wing area.
     
  4. May 29, 2017 #4

    BJC

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  5. May 29, 2017 #5

    BBerson

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    Also, your tips might be at a higher incidence than root making it look that way in flight.
    Tips should set a few degrees less than root normally, for tip stall prevention.

    Sometimes owners increase tip incidence for slower flight. Bad idea.

    The listed cruise speed is only 68 mph. Probably the normal wing angle of attack for that cruise speed. At top speed with full throttle the wing angle will be less.
    Adjusting the tail angle won't help. All you can do is set the tail angle for proper trim.
     
    Last edited: May 29, 2017
  6. May 29, 2017 #6
  7. May 29, 2017 #7

    TFF

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    How does the plane fly in a long glide with the power back? Same airspeed as powered cruise. Go up 3000 ft and see how it handles in a long glide. How is the CG? Decalage positions are about CG and weight.
     
  8. May 29, 2017 #8

    Dana

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    11° does sound like a lot, but an airplane with a high mounted engine will need more than a conventional plane to counteract the nose down pitching moment from the high thrust line.

    A lot of ultralights fly with what looks like an excessive AOA, but it works. Also a lot look like they're at an excessive AOA, but it's an illusion due to the small leading radius forced by the tube leading edge. But if the Skyboy has a 4412 airfoil that wouldn't be the case.

    A lot of CFIs with only experience in more conventional GA aircraft get very disturbed by the very different flight characteristics of ultralights.

    Dana
     
  9. May 29, 2017 #9

    Eugene

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    Thanks to all,

    Every picture or video online will show high angle of attack on this airplane.

    As soon as I learn how to post pictures, I will do so.

    Geometry for this airplane was developed 25 years ago for European ultralight market. I can simulate that kind of flying by pulling power back to something like 3000 RPM. But my engine, I was told, designed to spend most of its life between 5000 and 5500rpm. And also, if you have 100hp available to you, why not use it. But at 5000 RPM you get this feeling of pushing big piece of plywood against the wind. You are wasting a lot of energy for nothing. I was just looking for reasonable explanation, why that is.

    Trying to move wing through the air at 5° angle of attack requires a lot of energy and creates a lot of drag. As a result of this drag -very poor gliding capabilities. When you pull Power back to idle - you are going down at 45° angle. If your engine quits on this airplane, you will be going down right now with very little time for planing.

    Seems like I need 200 hp to force my wing into a level flight, but at that point tale of my aircraft Will be up in the air by 10.2° acting like a giant speedbrake with a lot of down force from incoming air. Since tail on the airplane has a lot of leverage naturally, I was hoping to change that and start flying more efficiently. I wasn't looking for speed, because I understand the design speed of this airplane not to exceed hundred miles an hour.

    Icon A5 doesn't fly with high angle of attack. But of course there is additional zero to that price tag.

    So John and Paul were wrong! All you need is money! Not love! And your angle of attack will be absolutely perfect.
     
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  10. May 29, 2017 #10

    BBerson

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    The angle of incidence of the tail is the combination stabilizer and elevator. The elevator might be fighting the fixed stabilizer as CG shifts with solo or dual. With side by side seats and heavy or light pilots, the CG shift must be large.

    Sometimes the factory demo pilots are 5 feet tall and 120 pounds. (Terrafugia, for example)

    The wing angle is proportional to speed. Not the wings fault.
    Drag of the pusher installation is likely the problem. Need some yarn testing for separation.
    VG's might help.
     
  11. May 29, 2017 #11

    Eugene

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    IMG_5502.jpg IMG_5503.jpg fullsizeoutput_c58.jpg fullsizeoutput_c97.jpg With original designer we did go back and forth many times with CG issue. He wants me to be in between 21 and 36% of MAC. We made the conclusion that with two pilots we are right in the middle. If I am flying by myself then my CG is closer to aft limit, but nothing to worry about. My wings swept forward by 1°. someday I will straighten them out to make nose Little heavier. That was designer and factory recommendation for 100 horse engines. But somehow they didn't do it, just like they tilted engine wrong way, so they can installed bigger propeller.

    In straight and level flight I made marks and get some measurements, so I can reconstruct position of elevator and trim tab. It is relatively hard to get hundred percent correct Picture, but conclusion was that all three surfaces pretty much in line within 2 or 3° (horizontal stabilizer, elevator, trim tab)

    So with another words - there is no fighting, there is no conflict! That is why it's very puzzling!
     
    Last edited: May 29, 2017
  12. May 29, 2017 #12

    BBerson

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    There is also wing downwash, so not so simple.

    With your CG at 36%, I would expect near zero tail incidence and zero tail download.
    And perhaps neutral pitch stability?
     
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  13. May 29, 2017 #13

    lr27

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    Did you try the digital level at several different places along the span to measure twist? If that's ok, you might try lining up a piece of tape inside the canopy parallel to the wing tip on the ground. Or, if not the wing tip, then a line drawn on the bottom of the wing. Then see what the difference is in the air. Also, what angle is the elevator at in trimmed flight? Small changes in decalage will just result in small changes in elevator angle to maintain trim. Shouldn't make a huge difference. OTOH, you are talking about some pretty large angles in the first place. However, if the elevator is pretty close to the same angle as the stab, in trimmed flight, then decalage probably isn't a significant problem.
    -----------

    Using figures from the owner's manual link above, wing area is 145 square feet, gross is 990 lbs, and aspect ratio is something like 6.6. Referring to a chart in Hoerner, the dalpha/dCl is about 13 degrees for that aspect ratio, as opposed to about 10 for an infinitely long wing. The latter can't be measured because you'd need an infinite flat planet to do the measurements with. ;-) The Cl at 80 mph works out to something like 0.42, so we'd expect an angle of attack, relative to the zero lift line, of about 5.5 degrees. According to http://airfoiltools.com/airfoil/details?airfoil=naca4412-il the zero lift angle is about -4.5 degrees. However, from a quick look, I'd guess the bottom "flat" area is inclined a further 2 degrees or so negative. Add this all up and you get that the bottom should be inclined around 1 degree negative. OTOH, if the span efficiency isn't so great, maybe it should be zero or 1 degree up.

    Considering the above, I wouldn't expect the wing to be at a "very high" angle of attack or, rather angle relative to the horizon at 80 mph. At least, not in level flight. Are you sure it wasn't just the fuselage? If part of the wing is at such an angle of attack, I suspect the whole wing isn't. That would mean a twist problem of some sort. Or maybe the ailerons are rigged with a lot of reflex?
     
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  14. May 29, 2017 #14

    lr27

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    P.S. Are you sure the airfoil is really a 4412? That could make a big difference. Could it have been distorted somehow?

    P.P.S. For the most part, that's a clean looking airplane. I suspect that if you could get a good cowling approved, it would be much faster. Particularly if the cooling inlet was close to the leading edge of the wing. It ought to be a little quieter that way too. But that's a separate issue, I think.
     
  15. May 29, 2017 #15

    Eugene

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    FullSizeRender 18.jpg it is for sure 4412. It's not a perfectly flat on the bottom, but from what what I can tell there is approximately 1 ° difference from root to the tip off the wing.

    Just want to make it clear, that there's nothing wrong with the airplane according to original designer. It was designed 25 years ago as Quicksilver competitor with inclosed cockpit. Later on somebody decided to install the bigger engine and different wing. It is steel a quicksilver at heart. "if you change something - you will get killed!!! If you don't like it - go buy something else"

    That is picture with 5000 RPM at 82 miles an hour
     
  16. May 29, 2017 #16

    BBerson

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    Is the stick well forward in a power off glide?
     
  17. May 29, 2017 #17

    TFF

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    You are also in front of the wing. It will look different than sitting under the wing of a Cessna.
     
  18. May 29, 2017 #18

    Eugene

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    fullsizeoutput_caa.jpg this airplane his same wing, hundred pounds heavier and only 65hp engine
     
  19. May 29, 2017 #19

    lr27

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    Doesn't it have a higher aspect ratio? That makes a difference.
     
  20. May 29, 2017 #20

    BBerson

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    I think the increased power mostly is absorbed as trim drag with that much decalage for 100hp.
    The download on the tail under power must be carried by the wing. So the download of 100hp is double of original 50hp.
    Maybe 500 lbs ? Added to wing lift.
    And in power off glide, the decalage angle is wrong so again trim drag.
     

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