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Building Tips and technical articles. => Building techniques => Topic started by: Rod Lamer on June 28, 2009, 08:55:57 AM
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On a control line plane why does the airfoil have to be fully symettrical to work? Many R/C planes and real ones don't have this kind of airfoil and do unbelievable tricks and stunts. They enen fly inverted realy well. I just don't understand. Thanks Rod
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I think Al Rabe has done some stunt airplane design using semisymmetrical airfoils. Talking about stunt or combat airplanes, the benefit of a symmetrical airfoil is that it flies at the same angle of attack, upright or inverted, to generate a given amount of lift. With an unsymmetrical airfoil, the airfoil has to fly at different angles of attack, upright vs. inverted, to generate the same amount of lift. This means you have to fly the airplane differently, upright vs. inverted; an unnecessary complication.
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Control line planes don't have to use symmetrical airfoils, but decent aerobatics are tough with flat bottom or semi-symmetrical airfoils. If you look at a plane flying inverted with a flat bottom airfoil you will see that the nose is pitched quite high to mainain level flight. This doesn't leave much room for error. In control line flight, if you have a flat bottom or semi-symmetrical airfoil and you get close to the ground, you need all the down elevator you can get to recover and get back up and over the top of the outside loop so you can return to normal upright flight. If you're already using all of the "down" elevator travel to stay level, you are not going to have a chance at recovery unless you use "UP" elevator and pull out above the ground. The plane and the ground typically will not cooperate in such cases.
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Is this like when my son was flying his sport plane with a semi-semetrical airfoil. He was going to show how to do an ouside loop by diving. Good thing radio planes have ailerons as he had to roll it upright. Can't do that in control line. DOC Holliday
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I believe Al Rabe used an airfoil with about 2% camber. The theory was that the fourth corner of the hourglass really needed some extra lift, since the plane often approaches this corner at high speed due to the long inverted, downwind dive. It's the only seriously assymetrical point in the pattern.
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There is a lot of negative comment here, especially by people who haven't experienced asymmetric airfoils on a truly competitive stunt ship. The last comment about the hourglass corner is correct but the same condition also applies to triangle corners. When pushing the limits of lift, there is a greater need for lift in triangle corners than in square corners and the only triangular corners in the pattern are all insides. I figured that a little extra camber might help, and sure enough, it did, at least to the extent of winning Open Stunt at the NATs twice and the Walker Trophy.
Do they fly noticable different upright and inverted? Well, I'm still checking. So far, I've flown more than four thousand flights with asymmetry and haven't noticed any yet.
To try to put all of this in context, I only use/used asymmetry when I think that a semi-scale design will push the limits of its airfoil to produce enough lift for the tightest possible corners. This necessity ia usually the result of a design using limited wing area for greater semi-scale realism or because of the realistic semi-scale fuselages usually adding more weight even when built using extreme weight saving techniques like molding shells. It is a challenge to design a stunt ship with realistic semi-scale appearance whthout comprimising its ability to fly competitively at NATs, Team Trials and World level competition.
Al
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I wrote more about asymmetric airfols in my two part autobiography published in Control Line World October 2007 and January 2008 issues. I said:
"Asymmetric airfoils
While testing airfoils, it became apparent that thicker airfoils were providing more lift. It also raised the question of whether we need as much lift for inverted flight and outside maneuvers. There was no clear reason why asymmetric airfoils wouldn’t work well for stunt. My thoughts were that the two most difficult corners in the pattern, demanding of lift were the lower right triangle and hourglass corners. These corners are all inside corners. We need more lift for these inside corners than we do for the less demanding inside and outside corners in the rest of the pattern. Through airfoil testing, I selected two airfoils having the same general curvature but differing slightly in maximum thickness for the Sea Fury wings. I used the thicker one for the top of the rib and the thinner one for the bottom. The Sea Fury used this asymmetric airfoil from the wing root, out to the dihedral breaks, then smoothly transitions to fully symmetrical at the tips. In flight, there is no detectable difference in the handling characteristics of the asymmetric airfoil in inside and outside maneuvers.
It raises the question of what would motivate me to use such a radical airfoil on a competition airplane. Remember, at the time the use of a 25% section was unheard of. I was frankly afraid that an airfoil this fat would result in an ugly airplane. Thinning the airfoil by reducing the camber on the bottom half of the airfoil would give a more pleasing appearance to the wing in flight while preserving the highly cambered top for necessary lift in those demanding triangle corners. The wing design met my goal of creating a winning airplane, whatever the effort required."
Al
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Wow! Thanks guys. The reason I asked is I have a wing from a crashed R/C plane and was going to use it for a semi scale C/L plane and people told me it would not work. OK to that but I have flow inverted on a R/C plane and had very little elevator input to keep it level. Since you are in direct control of a C/L plane and all I want to do is make a cool looking plane to help learn on, what would it hurt? How many of you out there would like to see an Eindecker C/L plane? Simple, basic, and 1/2A powered by a .09 medallion; what could it hurt. The only money I would have in the plane is from paint and maybe some stickers! Everything else I already have. H^^
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THANKS!!! You have revived my interest, I had given up hope on this, now I think I will! What the heck, the worst I can do is crash. Best I can do is make a wonderfull bird! I will settle for somewhere in the middle. CLP**
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This is what makes model airplanes fun and challenging. Try things and see if it works. I added flaps to a Scientific American Boy way back when I was trying to learn to do maneuvers. Not knowing what I was doing it didn't help the plane one bit. Now if I did it, it wouldn't look like an AB because the nose would be stretched as well as the tail with bigger tail surfaces. Keep us posted on your findings. DOC Holliday