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  1. #1
    ZoomZoom-RCU's Avatar
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    Stubby Fat Plane hates right turns!!

    I have designed a very short coupled stubby fat biplane. Here is whats happening, and any advice on how to remedy it will be much appreciated. The craft has a rather short wingspan, but being a biplane has sufficient lift to fly well. The only issue is, being a biplane, it is quite draggy, so the engine can swing a prop that is roughly 1/3 the entire wingspan, and needs to for flight. As you might guess as well, due to the big prop- small span issue, a decent amount of right engine thrust has been built into the design to counter this leftward torque impulse. So, the issue I'm having is this: she flies well, but fights the right turn. In fact, its downright difficult to get her to turn right. So much so, I haven't yet succeeded in doing so! On the next few flights I will try to combine rudder and aileron more to try and force her into a right turn, but any suggestions to eliminate this resistance are most appreciated.

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

  2. #2
    Moderator BMatthews's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    You need to find a copy of the article that Ken Willard wrote about his Stitts Sky Baby and the issues he had with trimming it. Turned out that the rudder was a key element to being able to fly and maneuver well. So you're likely on the right path with trying to use rudder and ailerons together. Sorry that I can't be of more help but it's simply too many years ago that I read that article.

    Go and find the plans for it on the RCM plans website and once you know what year and month the RCM article was published I'm pretty sure someone could scan the article and send it to you.
    Witty saying to be plagarized shortly.....

  3. #3
    ZoomZoom-RCU's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    Thanks BM. I will do that, and yes, the ailerons are almost innefective on this design. In fact, I have reversed the controls on it in such a way as to put the ailerons on the rudder channel on my transmitter, and vice versa for the rudder, so when I turn the machine it is always mostly with rudder alone. The wings are quite far apart, so when you hit aileron, it sort of tilts the machine on its side,but thats about it, and with no turning influence really. I'm wondering if a 3-blade prop, without so much span might not help. I would think it would produce less torque than the 2-blader, and if I could get equal thrust from it, perhaps would reduce the right turn resisting left torque effect that is what I think is causing the issue now. Again, any ideas/experiences are appreciated!

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

  4. #4
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    RE: Stubby Fat Plane hates right turns!!

    Here's where you problem might be. The engine is turning the BIG prop clockwise (sitting in pilot seat). Remember, for every action there is an equal and opposite reaction. So the big prop is turning clockwise and it's trying to turn the plane counter clockwise. When you turn to the right, the prop is trying to turn the plane to the left. When you turn left, the prop is pushing the plane to the left, therefore left turns are easy, right turns are hard.

    You can test this theory by reducing throttle to idle and then turn right. I think you will find that the plane will turn right or left with equal ease.

    You might try a smaller pitch prop and see if it will help.

    Frank
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  5. #5
    Moderator BMatthews's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    Well, techincally ailerons do not turn any airplane. They simply bank the wings so you can use elevator to "climb" around the side of the cone shape indicated by the banked wing.

    For example on a pattern model that is set to near or at neutrally stable and you bank the wings with no other control input the model will not turn. It'll simply slide sideways all the way to the ground unless corrected.

    That's only an example. Really though there are so many issues with short stubby model designs that you simply can't expect things to work the same.

    You could TRY the 3 bladed prop and it may well give you SOME help. But don't be surprised if something else shows up too along with that change.

    I do recall on Willard's write up on the Sky Baby that the rudder throw needed to be limited to a very small amount on a rather big rudder to avoid gross amounts of overcontrol.

    I also got the impression that, like the full size airplane, the model was never much fun to fly so it was hung up on display shortly after the troubles were worked out.

    The silly thing was that it was 1/4 scale and only had a 3 foot wingspan.... and was darn near as tall sitting on its gear....
    Witty saying to be plagarized shortly.....

  6. #6
    ZoomZoom-RCU's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    Thanks, and yes you are both on target with what I'm saying, in that all of that left torque from the prop is causing the right turn resistance. Its funny Bmatthews that what you said about aileron turns is true, you bank the bird and then up elevator turns it. You should see what this thing does, really its the nicest (ugly fat) knife edge you ever saw. You hit right aileron, she rolls right and hangs there, you hit up elevator, and she goes nose high, and still just hangs there...lol. But, like you two,I'm leaning towards trying a 3blader to assist in reducing this, especially if I can get the same thrust from it. So, A J-Zinger 11x5, which tends to be a bit of a fat bellied prop is what works with this craft. What would an identical thrust 3 blader be?

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

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    RE: Stubby Fat Plane hates right turns!!

    Sounds like it may also yaw opposite the ailerons. Happens often. It's the reason many airplanes are rigged to give significantly more up aileron than down. It's most often experienced with cambered airfoils.

    It's the result of more drag being generated by the wing going up. To correct that, the aileron differential is rigged to cause the down going wing to get more drag from it's aileron. That counteracts the adverse yaw.
    Good flying wit ya today

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    RE: Stubby Fat Plane hates right turns!!

    The adverse yaw thing is definitely worth looking into, although if it turns left nicely that's probably not the main problem.
    No kid, I said break ground and fly into the wind!

  9. #9
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    RE: Stubby Fat Plane hates right turns!!

    On the low sort of aspect ratio that something like this has any adverse yaw won't have much leverage to work with. But with a LOT of deep side area and short stubby wings it may even be a case of the circular prop wash hitting high on the left side and low on the right and creating a strong right sort of torque. Of course this is balance by the left hand torque of the engine but all in all they may not cancel out completely.

    After all free flight models went to a pylon in order to let the circular prop blast lean the models into a right spiral. That being far more stable a manner to climb more or less vertically than letting engine torque take them to the left.
    Witty saying to be plagarized shortly.....

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    RE: Stubby Fat Plane hates right turns!!

    With low stubby wings, you've got a high fuselage right? Ever noticed it's almost impossible to find the CG with suckers like that if you try to do it with them upright? Know why? The CG has the "leverage" to tip the thing over whereas your fingers under the wingtips don't. The wing being way under the CG has as little leverage as your fingers when the plane is in the air flying.

    Does that matter when some other influences come into the picture? You betcha.

    Unique airplanes often have worse personalities and often will suddenly do things that are hard to figure out. Unless you keep in mind how far from normal some of their characteristics are and what those things are. That plane has a CG thats probably S/2 above a wing that has no real span. Short wings have no leverage. CGs do.

    BTW, when you move ailerons into the prop blast you're putting at least one of them into what could be an improved environment. Man, there are a lot of unique things that plane brings to the mix.

    BTW, remember the short little stall strip they slapped on the Corsair to cure it's treacherous landing characteristics? It was close to the fuselage. No leverage right? Sometimes it's not leverage that matters. They put it on just one side right. It created a stalled area close inboard. On one side. Wonder what would have happened if the sucker had had stubby wings and the ailerons were tucked right up inside the prop blast?

    This stuff ain't simple, is it.
    Good flying wit ya today

  11. #11

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    RE: Stubby Fat Plane hates right turns!!

    I'd kinda like to see a picture of this beast to get an idea of what we're dealing with.
    No kid, I said break ground and fly into the wind!

  12. #12
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    RE: Stubby Fat Plane hates right turns!!

    I ususally use rudder in my turns. What I ofter forget is to use rudder only or or at leastmore so than aileron, on landings.
    Glow Head Brotherhood #15

  13. #13
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    RE: Stubby Fat Plane hates right turns!!

    If torque was the problem then rudder will not fix it. Likely it is P factor and slipstream effects.
    Glow Head Brotherhood #15

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    RE: Stubby Fat Plane hates right turns!!

    Picture please!

  15. #15
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    RE: Stubby Fat Plane hates right turns!!

    Ok, but if I post a picture, You all have to swear to secrecy, because this is a design that I plan on selling plans for, and it is quite unique, unsusual and one of a kind. So I will post some pics shortly. Its a cartoonized version of a WWI pfalz. In the meantime, I might add that I've ordered a shorter span 3 blade prop to reduce the torque left, so once that arrives, if it gives me enough thrust, that might help. So far the only prop beefy enough to move this machine is a Zinger 11x5.

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

  16. #16
    ZoomZoom-RCU's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    Ok, your all sworn in....here are some pics.

    I've altered the landing gear a tad with slightly fatter wheels and a smidge lower stance, but that really isn't pertinent I don't think.

    The football shaped fuselage also acts as a "lifting body" which allows it to fly on a OS LA.46. Pretty underpowered engine for its apparent size. Design is pat pend, as is the name, which I will reveal later. Right now, I wouldn't release the design plans for sale anyway, and not at least until I get the flight bugs ironed out.
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    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

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    RE: Stubby Fat Plane hates right turns!!

    The pilot's head is turned to the left...and you thought you could get him to turn right?

    Fun project. My lips are sealed.

    Jim

  18. #18
    ZoomZoom-RCU's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    Lol. Well if it makes you feel better I removed him altogether for testing purposes....so he has no further leftward inclinations.

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

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    RE: Stubby Fat Plane hates right turns!!

    It's got very little side area aft of the CG compared to in front of the CG. That's apt to be a problem. I would make the fin/rudder a whole lot bigger.

    Jim

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    RE: Stubby Fat Plane hates right turns!!

    Thats an interesting point, and do-able. But after pondering it a bit, I was struck finally with the realization, "well, she turns left just fine", and that alone leads me to believe it's not an overall rudder issue. Sure we could alter the rudder if we have no other choice to deal with an unchangeable combo of elements. But if she turns left fine with the current rudder, that means ergo, she then should turn right just fine, unless there's something preventing this. So my first task is focusing on what this "moment" or force is that is preventing normal behaviour. But your solution might be usefull if we can't overcome/counteract whatever is truly at the root of the influence.

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

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    RE: Stubby Fat Plane hates right turns!!

    Three things jump out at me from the pic, two of which are really both parts of the same issue. First, the aerodynamic center of the fuselage is farther forward than the AC of the wings. That's going to make the plane want to drop its tail in a sideslip, which is the opposite of what we normally expect planes to do. Second, the tail is way too small, IMO. I'd have to wonder if the rudder does much at all given that it's so close to the shadow of the big fuselage. Third, the wing design looks like it would have pretty abrupt stall characteristics, meaning that in a turn that wasn't executed just right it would just stop flying instead of obeying control inputs. I'm no expert in any of this, but that's my initial impression.

    On a positive note, kudos to you for envisioning a project and seeing it through to actually fly it. It's neat to see someone working out a design that hasn't been done before.
    No kid, I said break ground and fly into the wind!

  22. #22
    ZoomZoom-RCU's Avatar
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    RE: Stubby Fat Plane hates right turns!!

    On a positive note, kudos to you for envisioning a project and seeing it through to actually fly it. It's neat to see someone working out a design that hasn't been done before.

    Thanks! You guys are the first to see it and I'm glad you're intrigued. It truly looks ridiculous, yet amazing in the air, which was just what I was hoping for. Interesting you would mention the tail surfaces. In the photos, its a bit misleading cause shes sitting tail down, but when flying, the tail rises up out of the slipstream of the fuselage to a level almost even with the top wing. I designed her this way for the exact reason you mentioned, to get a cleaner airflow. When I first test piloted her, I thought the ailerons would be the main turning element. I was WRONG...oh boy. The rudder is VERY powerful, and effectively turns the machine quite well. The elevator also is most effective, in fact I had to reduce the throws. So their influence is quite potent.
    You did mention however dropping of the tail in sideslip, and from the few times I've tried to force the right turn, I think I remember her doing this. Perhaps this is a clue?

    ZZ.
    PAY NO ATTENTION TO THE MAN BEHIND THE CURTAIN!

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    RE: Stubby Fat Plane hates right turns!!

    What we're talking about is not just rudder area but side area. Normally you would have much more aft of the CG, but here the engine housing looks like it's almost as big as the rudder, and the rest of the fuselage looks to have almost as much area forward as aft. Especially when you throw in the area of the wheels and prop.

    I don't think you'll figure out the odd aerodynamic problems without balancing out the proportions. Interactions of so many different forces, slipstream etc., are too complex. Just get some more area to the rear and I bet things will start to improve.

    Fun project, keep it up! And keep posting.

    Jim

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    RE: Stubby Fat Plane hates right turns!!

    Cute fat stubby plane for sure. Looks like a fun airplane.

    When you're developing a plane like that, a very useful trick is to use card stock taped to the surface in question in order to increase it's area. Masking tape and cut up soft drink box sides work great. Cut some of the card stock to slightly overlap the rudder's trailing edge and tape it on. It's strong enough to last a lot longer than you need to see if the added area makes a difference.

    A couple of us just used some 12-pack cola box to figure out how much additional elevator and rudder area we needed to get an old design to spin reliably.
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    RE: Stubby Fat Plane hates right turns!!

    Great suggestion!


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