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Dihedral effects on CL models

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IdRatherBeBuilding:
I was just reading an old thread where Jim mentioned the dihedral on his beloved Dragon.
I started to wonder about it. I'm pretty new to CL, so please bare with my ignorance.
Dihedral effects roll stability, but CL models dont roll. They always fly with level wings relative to their path. And the lines guarantee that.
So, dihedral doesn't effect flight characteristics between upright and inverted flight? Only incidence does?
Also, does Wash in/out do anything? Well other than create drag?


David Miller

Dan McEntee:
   Hi David;
    Welcome to the group!
     In short, the main purpose of dihedral in a control line model is to better locate the lead out exit in relation to the vertical CG of the airplane. If you look st some semi scale stunt models of the P-51, you will see that a lot of them use dihedral in an effort to have the wing low enough to more look like the real airplane, and the real airplane also did have some dihedral for extra roll stability as you have mentioned. If you didn't do that, the airplane would most likely fly with the outboard wing tip down. There are many old designs that have low wings that if built and flown with a straight wing mist be countered with longer, heavier wheels and landing gear to get the model to track straight. I have never had to deal with a sever case of this, so I don't know if tweaking a flap can have a positive or adverse affect on the trim. A few years ago, I built a profile semi scale model of the crop duster airplane that is painted up to look like Dusty Crophopper from the movie "Planes". It was basically a SIG Twister that I modified pretty heavily. In order for the model to look more like the real airplane, which has significant dihedral in the wings, I needed to do that ion the model also. I dropped the wing center line as low as I could with the space I had, which was about 3/4 of an inch. So to make sure the airplane tracked and handled as good as a Twister normally does, I put in the same amount of dihedral in each wing . Once that is done, the model is flown and trimmed as per normal, and you do not notice anything at the handle. Search out images of Al Rabe's semi scal models, as they all had some dihedral in them, and there are many in flight shots of them in circulation that show how they look out on the lines. As far as wash in or wash out, that is never used on a stunt model in the way that it is used on a free flight or R/C model. The closest that a C/L model get to that is the flap tweak that I have already mentioned, but is not the same.
    Control line models do have to deal with roll stability in the manor that they can resist too much rocking and rolling in rough air. That is mainly addressed by working to keep the tips of the wings light and balanced to reduce what is called 'barbell effect", and by wing tip shape and airfoil thickness. As a model goes through rough air and it wants to roll, the less weight keeps the roll rate down, and a straight, clean wing tip and airfoil will hopefully fly through it with minimum effect, and return to wings level in a hurry. I hope this helps.
    Type at you later,
   Dan McEntee

IdRatherBeBuilding:
Thank you for the informative reply and the warm welcome.
Design on CL models are very out of the box engineering for newbies like me. Very interesting, and very sensible. My experience in CL hasn't past 1/2a profiles like the CG stuntman and such. And that was back in the early 80s!

As far as incidence goes, I've noticed stunt designs just shoot for 0* all around. Are there any models, past or present, that deviated away from that rule? Even better, do you know of any CL planes that have adjustable incidence?

   

Brett Buck:

--- Quote from: IdRatherBeBuilding on February 09, 2019, 10:30:04 PM ---I was just reading an old thread where Jim mentioned the dihedral on his beloved Dragon.
I started to wonder about it. I'm pretty new to CL, so please bare with my ignorance.
Dihedral effects roll stability, but CL models dont roll. They always fly with level wings relative to their path. And the lines guarantee that.
So, dihedral doesn't effect flight characteristics between upright and inverted flight? Only incidence does?
Also, does Wash in/out do anything? Well other than create drag?

--- End quote ---

   Unfortunately, the lines don't guarantee that at all, and controlling the roll is a tricky issue that people rarely master. If nothing else, the lines whip around during maneuvering, you have to start a corner with the lines going horizontally, the corner ends with the airplane going vertically, so at some point you have to have the airplane pull them from one state to the other, and that tends to induce roll and yaw. And it's not like they are rigid rods, one end might be going up and the other end mught be going sideways, and they oscillate in all directions.

     Regarding dihedral, you have fairly strong dihedral effects even when the wings are straight. Roll and yaw are very heavily coupled, both through the lines as above, and through the aerodynamics. The spinning prop acts like a gyroscope, causing yaw effects when you (very rapidly) pitch the airplane, that makes it want to yaw, that causes roll, which changes the line tension, etc. The prop also has what is called P-factor, essentially, in a corner, one prop blade acts like it has more pitch than the other prop blade, causing a different yaw torque, maybe in the same direction as precession, maybe against it and always changing.

   You can do a bunch of different things to deal with these issues, passive yaw control, open-loop yaw compensation, and people have at least considered actively controlling it with a gyro and a feeback system.

   This is a very complex subject and we spend more time talking about trimming than almost any other topic, and correctly so. It's a very good question with no simple answer.

    You can use actual dihedral in airplanes, either for appearance or for putting the leadouts in the correct vertical position (so it doesn't have to roll to align the CG with the line tension, with other supposed effects that no one has ever been able to adequately explain). Given the above issues, small amounts of actual dihedral is perfectly fine and causes no inordinate problems if you can solve the rest of the issues.

     Brett

IdRatherBeBuilding:
Is this the reason the out board wing is a tad bit shorter on some designs?


David Miller

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