Decalage angle

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

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  1. May 30, 2017 #61

    Dana

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    You have two completely separate unlrelated issues (I won't call them problems).

    One, high apparent AOA in cruise flight. For any given airspeed, the wing needs to be at some specific AOA to maintain level unaccelerated flight. The only way to change that is to go faster or increase the wing area. This has nothing to do with your declage angle.

    Two, large declage angle. This is a function of the sum of the pitching moments (CG forward of neutral point, wing pitching moment, and high thrustline all pushing the nose down, tailplane incidence is the only thing counteracting these). If your CG is within limits and the elevator is roughly in line with the horizontal stabilizer, then the declage is correct, don't worry about it any more.

    Dana
     
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  2. May 30, 2017 #62

    Eugene

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  3. May 30, 2017 #63

    Eugene

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    So, in other words - I have very draggy airplane. And nothing I can do about it. I should stop losing my sleep and start enjoying life. All I can do is to build maybe Engine cowling, install wheel pans and so on.
    It's going to take 90hp to go 80 mph and nothing I can do about it.

    Next time I will be more careful with my choices
     
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  4. May 31, 2017 #64

    Eugene

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    Different people were telling me different things during my research. If I put it altogether, it will sound approximately like this -

    - when you're designing airplane on paper, sitting in your office, are You doing this using your best knowledge about Engine position, CG position, incidents for this and incidents for that. And you doing that for one purpose, so you don't get killed on your very first test flight. And if your creation actually gets airborne and later you managed to land this machine without accident - you should be proud of yourself, because you did something right! On your second test flight, when you're not so nervous anymore, you start looking around. You are looking at many things and correct angle of attack in the cruise is one of those things. And if you are only 170 pounds and tank is only half full, and you visually see have About 5° from bottom of your wing to the horizon - this is when you know, that something is not right. And if aircraft doesn't glide very well, then you know it's not a optical illusion. At 80mph reynolds numbers are very low. Your struts and cables not slowing you down that much. Trying to push 140 sq ft of wing through the air at the wrong angle - requires a lot of energy.

    That is why I was looking for answer.

    I agree that in pusher configuration many things needs to be done differently in order to get to that target. And target is a efficient flight. And efficient flight with 138 sq ft - 1000lb and 80mph is About 2° angle of attack. And if you don't have that - you are too heavy, too draggy, or something wrong with your powerplant or propeller.
     
  5. May 31, 2017 #65

    TFF

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    I think you answered your own question on page one; "Quicksilver competitor." That is not bad but just states the target the design was designed to be like. I'm sure you can make it go faster, but loose the LSA qualities. Rotating the wing so you get a few degrees AOA in cruise like you want; the tail and engine will have to fall in line to make it happen. Then you will have a plane that mimics a Cessna 150 at stalls, takes offs, cruise, and landings.
     
  6. May 31, 2017 #66

    BBerson

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    Try holding an inclinometer up in flight to measure the angle.
    My iPad has an inclinometer app. Most smart phones also.
    Hard to measure exact by eye.
    The 172 has washout at tips, so looks less.
     
  7. May 31, 2017 #67

    vtul

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    One thing that continues to bother me is the phugating flight and observations like:

    .

    I'm just really feeling like the CG may be a problem. Not saying that's the cause of the angle of attack situation, as Dana points out, and which I tend to agree with. But those really don't sound like good responses, taken together.
     
  8. May 31, 2017 #68

    Eugene

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    IPhone was way to sensitive and digits were jumping up-and-down. I purchased small digital level and found spot in the cockpit that represents bottom of the wing. It is very hard to trim this airplane in to steady and level flight, so I did it by holding stick and watching GPS. At those moments when GPS didn't change any numbers it was between 4 and 6°. I did this measurements after I moved leading edge on horizontal stabilizer up by 2°.

    To Peter Garrison's defense I didn't feel anything different with airplane behavior or speed increase after I raise stabilizer by 2°. if somebody would do this to me overnight, without telling me about. I would go flying and land normally what same trim settings.


    IMG_5280.jpg IMG_5281.jpg IMG_5282.jpg

    This is TriPacer
    You can change angle on this airplane from cockpit. Takes 24 turns to get from 1/2 ° to 6 1/2° difference in comparison to the bottom of the wing. I figure it out. This guy I Was flying this airplane by himself for 25 years. he was taken of with 3.7 degrees but cruising was usually at 3.1.
     
  9. May 31, 2017 #69

    BBerson

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    Your airspeed indicator might be off, slower than indicated.
    Your wing surface might be rough giving reduced CL.
    Ailerons floating up?
    Aileron gap not sealed?
    Trailing edge large tube instead of thin?
    Not much else I can think of.
     
  10. May 31, 2017 #70

    Eugene

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    Best I can do is to keep posting some pictures, maybe somebody will recognize potential problem someday.

    There is another little story with this airplane. On the very first flight - first takeoff was aborted. My CFI is very knowledgeable guy, very Active at Oshkosh - Best LSA instructor/fanatic you can find. His name is Steve Krueger.
    He was waiting for controls to get lighter and lighter as we were getting speed. Instead of doing that, airplane was getting heavier and heavier on the nose, Felt like it's practically glued to the runway at 80mph indicated with trim all the way down. On second attempt with a lot of back pressure on the stick we are finally became airborne and with little trimming got in two normal climb mode 800 ft./m or so. On the downwind going straight and level at 80 mph, this is where he became real nervous with a high angle of attack. We managed to land and he tell me that it was enough excitement for today and he needs to think this over, but it definitely feels like he doesn't want to fly this airplane ever again.

    So, I was on my own at that point and one day I decided to do high-speed taxi, just to feel what he was feeling. Didn't have any intention to fly, but at 65 indicated with a little back pressure I became airborne. At that point - might as well go flying! Without knowing any better, because I have total only 120 hr in Cessna 172 and 10 in Champ, I didn't feel that's anything wrong with that airplane at all. I made horrible lending of course, my instructor was waiting for me on the ground with radio. This time I asked him to go and fly solo. And he did for 1 1/2 hour. When he landed, he said "I feel much better now, I certainly don't understand why this airplane needs to fly this way, but we can go together and I will check you out".

    Long story short - I extended front fork by 2 1/4 inches to get 7° angle of attack from level ground to the bottom of the wing. Now if I advanced power on takeoff relatively slow with holding stick firm in neutral position - at 45 mph my nose wheel Will get off the ground and airplane will be rolling only on mains. At that point all you need to do is to add more power and you just fly away!!! You should see my big smile when that happened first time!!!

    This event made me very optimistic. That is why I decided to adjust my horizontal stabilizer. But clearly I didn't know what I was doing.

    fullsizeoutput_c63.jpg IMG_5441.jpg IMG_5442.jpg IMG_5443.jpg IMG_5383.jpg IMG_5384.jpg
     
  11. May 31, 2017 #71

    vtul

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    The wing is well behind the cockpit. Side by side seating.
    There's a big change in CG for 1 vs 2 people.
    The tail boom cocks upwards and restricts the pusher prop size.
    Likewise the downwash crosses it at a steeper than usual angle fo a tube boom, adding drag.
    The boom looks short and the tail looks small.
    The stab has an extreme looking negative incidence.
    The plane has 7+ lbs/ft wing loading, but has a low top speed with 100 hp.
    It is supposedly a Part 103 UL "upgraded" with a big engine and lower lift wing for a 900+ lb AUW.
    It phugates, and has extreme pitch trim sensitivity.
    This is how they all fly, apparently.
     
  12. May 31, 2017 #72

    vtul

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    Also I don't get the "Quicksilver" heritage in any way. Maybe CGS Hawk, due to the cocked boom. But that's a true ultralight with more modest power and yet proportionately longer prop for the engine. Dump a 100 hp engine into one, reduce the wing and double the cockpit width, etc, and maybe it will be as weird a performer.
     
  13. May 31, 2017 #73

    BBerson

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    Changing the tail angle won't help. Making a bigger tail might help some. Some outboard wing cuffs might help.
    The problem seems it has so much drag (uncowled engine) it can't get "on step", as my instructor would say.
    That open engine probably creates a mass separation that stalls the inner wing root and interferes with the tail.
    George Pereira had trouble with the engine pylon on his Osprey 1. He found the streamlined pylon didn't work well and went with open strutted pylon.
    What does it weigh?

    Is it experimental exhibition?
     
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  14. May 31, 2017 #74

    BJC

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    Have you checked the washout as several have suggested? Is the fuselage at a streamlined attitude in normal cruise flight? Have you verified the aircraft weight and balance, and verified that all operations have been within the CG range and gross weight limitations?

    By down trim, do you mean aircraft nose down trim? Perhaps you mean that the leading edge of the horizontal stabilizer was fully down, or that a trim tab was all the way down, both of which would be referred to a full (nose) up trim. Please clairfy for us.

    You should calculate the CG for the two takeoffs to see if there is any correlation between CG and the different takeoff speeds and or control systen pressures.

    Have you verified that the thrust line is per factory specification? As others have commented, a high thrust line airplane will have takeoff chsracteristics very different from a C172, and any power or speed change likely will result in a significant change in trim. The thrust angle verses the horizontal is a tradeoff, so I would verify that it matches the factory spec. Please also review the immediate control action required following a loss of power just after takeoff, then practice it at altitude until it is second nature.

    Verify elevator position relative to the stabilizer at normal cruise speed. It should be close to being streamlined with the stabilizer.


    BJC
     
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  15. May 31, 2017 #75

    BBerson

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    Another issue is cockpit angle. It looks like the cockpit, or forward fuselage is angled up in cruise making you think the wing is up also. Ideally, the cockpit could be set a bit lower angle to fly level in cruise. Modern gliders have the cockpit set at a down angle for good visibility and to align with wing up flow.
    Can't change the cockpit angle easily, of course. But vortex generators might help.
     
  16. May 31, 2017 #76

    clanon

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    The huge diference between P3 (R series Tsagii ) and the 4412 may be a clue of the need for such a high wing incidence angle...

    This "flawed" re-designed could surely benefit from a different airfoil and an stabilatorOr a more eficient TAIL.like the one on the Earthstars and titan Tornados...IMHO
     
    Last edited: May 31, 2017
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  17. May 31, 2017 #77

    ragflyer

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    Hopefully this helps...

    1. The incidence of the wing with respect to the fuselage only affects the fuselage angle at a given speed. Ideally you want to set it so that the fuselage is level at cruise thus minimizing drag. This angle has no effect on stability or control. It just affects drag due to fuselage not being aligned with the flow.

    2. The incidence of the horizontal tail wrt to fuselage does not affect stability (common misconception). It only impacts stick position & pressure at a given speed. If you change the tail incidence (while keeping elevator range constant), you want to make sure you have stick range to still slow down the airplane to stall and/or fly at top speed. In general, the incidence is set so that at cruise the elevator is in line with the tail to minimize drag. Keep in mind, the drag difference is small and really should not be an issue in an otherwise draggy ultralight.

    3.The decalage is the difference between the 1 and 2. You set wing and tail incidence based on 1&2 and the decalage is what it is. Do not worry about it. It does not affect stability.


    Your visual of the triangle is not quite right. For a given weight and speed, there is only one wing AOA the airplane will fly in level (non-turning) flight- i.e lift = weight. It is what it is. For a given wing AOA there is only one lift coefficient for the tail to keep the airplane in balance- i.e Cm=0. This lift coefficient can be achieved by changing the tail incidence and keeping the elevator in line or keeping the incidence constant and elevator angle changed to create equivalent lift or some combination of the two.

    In your example of 4 deg and 10 deg elevator incidence it is impossible to know which has more drag. If both have the same wing incidence then for a given speed, the one that has the elevator less aligned ( can be either decalage) with the tail will have slightly more drag. But in a typical ultralight this minuscule increase in drag will be undetectable. If they have different wing incidence then it would depend on fuselage drag.
     
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  18. May 31, 2017 #78

    Autodidact

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    Is it possible that the fuselage is creating some negative lift? That would explain a high wing incidence at higher speeds.
     
  19. May 31, 2017 #79

    BBerson

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    I would adjust the horizontal stabilizer so that the trim tab is neutral in cruise.
     
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  20. May 31, 2017 #80

    Eugene

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    Wow!
    Thanks to all!

    Now, I will need to take day off, to process that much information! I have to admit, some of that information sound like French to me. I do speak 3 languages, but I don't speak French. So, I will keep reading over and over, until I get it right.

    I will do some checks and measurements just like everybody suggests. But I wanted to make something clear. Original designer Jaroslav Dostal who Was very helpful and spend a lot of time with me, going back and forth, to make sure that everything is correct and up to he's recommendations.
    Reason that he give up on me, because In his mine I am trying to build speedrocket out of Quicksilver. This is when he said, that if I don't like this airplane, then I should sell it and find something else. Because there is nothing wrong with this airplane as far is he is concerned. It is 50hp ultralight and if you have double up horsepower, you cannot double up your expectations. In his mind - I am trying to go faster, and I will kill myself.

    I wasn't trying to go faster. Aeronca with 4412 airfoil flies absolutely beautiful with less power, more weight, and higher speed at similar power settings. I was just trying to find out what's wrong with my airplane, or for that matter with all Skyboys. I talked to many owners and we all agree, that this is how they all fly. And if it is nothing you can do about it, then I will concentrate on building engine cowling and so on.

    fullsizeoutput_c88.jpeg
    Thanks again.
     

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