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Old Apr 06, 2012, 10:58 PM
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1) Would you be kind enough in telling me the references to back this up? - Is the Pitching Moment Curve, Cm, i'm not familiar with reading Cm curves or how they look... so could you please include another example alongside the Clark-YH Cm representation.

Other than texts on airfoil behaviour I'm not sure what you mean by asking for references. If you look at the curves for the Clark YH given by Sparky in the reply a few posts up you'll see that the line for the Cm is sitting at some negative amount below the zero line. That means that despite the upturn at the lower surface just before the trailing edge that it STILL has some negative pitching moment and is not suitable as a self stabilizing airfoil. Now compare this to the image for the behaviour of the MH110;



The scales for the right side curves are a trifle misleading at first glance as the Cm curve is below the center line. But note the value of -0.05 above the center line on the left side of the axis line and the notation for Cm beside it. The Cm curve scale is reversed with the line being positive despite being drawn below the zero line. This is supported by the Cm for the MH110 being given as +0.041 on the description page at http://www.mh-aerotools.de/airfoils/ .

2) If i choose a neutral or + Cm aerofoil then i MAY be trading-off another aspect like high-lift @ low R.E. which is important for my project given the limitations in design like wingspan.

Simply put yes you will. But you do NOT have an option. You need to be able to fly in a stable manner before you can complete any flight assignment. The first rule of winning is that first you must finish. If you can't fly the darn thing then you're just another cadidate for an amusing You Tube moment.

3) Looking at Cl-Alpha curves please explain how to know what is a strong Cm value & what isn't.

Well, you don't care about the Cl-Alpha curve at all. You need to see the Cm-Alpha curve. If it doesn't come out as positive or at least neutral for your "tailed Concorde" then you're not going to make it far from the launch point. The MH110 is a fairly highly self stabilizing airfoil with its Cm of merely 0.041. It is entirely suitable for a simple plank style wing such as shown by Sparky in his military target drone model. So such an airfoil would be more than suitable for your "Concorde with tail" style design. If your wing used some washout to the tips you could get away with a lot less of a positive value. Again borrowing from Martin Hepperle's web site the MH 64 at only 8.61% thick and with a Cm of -0.005 would still work if used with about a 3 degree negative washout at the tips. "Washout" being the term used when we twist the wing to a negative angle compared to some other reference point. "Washin" is the other way for the twist. In your case some washout would aid the tail in stabilizing the rest of the wing since a Delta wing is actually a planform which combines a radical taper ratio with a high sweep angle. So with the wing tips located so far back behind the CG having them angled for some washout will stabilize the wing IF the airfoil used at the root is not too strongly negative in it's amount of negative pitching moment. Did I emphasize the "IF" enough for you

4) Can pitching moment not be corrected by relocating the CG of the battery etc (within the fuselage cavity of the aircraft) rather than choosing another completely different aerofoil?

Simply and plainly NO! ! ! ! ! Let me add a few more... ! ! ! ! ! !

It would only work if you extend the tail and increase the size of the tailplane enough to make your "Concorde with tail" look more like a conventional planform. At that point the tail would have the ability needed to overcome the pitching moment of the airfoil just like every other standard planform aircraft. If you try to shift the innards forward to move the CG then the tail simply would not be able to control anything and it would simply lawn dart itself into the sod just in front of your feet. Keep in mind that the Clark Y isn't trying to raise the nose. It is trying to pitch the nose DOWN. The tail needs to try to counteract this pitching moment. Shifting the inside stuff forward is the opposite of what you want. Shifting it back sounds good at first glance. But the pitching moment is a torque generated by the airfoil which is SPEED DEPENDENT. This means that as the speed slows the nose will want to rise. And as the speed goes up the nose will want to go down. THis is the exact opposite of what a stable model does. For us mere mortals this means that you or I would NOT be able to fly it other than in something suitable for another You Tube moment.

5) Is a reflex aerofoil easy or hard to manufacture...it's a college project... we dont have state of the art machines just milling/CNC machines etc.
How did you make the wing you have now or plan on making the wing with the Clark Y? If you can make a Clark Y wing you can make a wing with an MH110 or MH 64 with some washout.
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