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Old Apr 02, 2012, 04:59 PM
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CM Analysis in Aerofoil Selection

Hi All,

I've been told before that a Clark-Y on a delta wing configuration RC-aircraft is unstable due to the presence of a strong CM (pitching moment). It was suggested to use another aerofoil (I've forgotten off the top of my head what the substitutes are) or the Clark-YH which is a Clark-Y with a Reflex @ TE

My Q...

1) Looking at Cl-Alpha curves please explain (in layman terms aswell) how to know what is a strong Cm value and what isn't.

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

3) I need calculations to carry out in an 'Aerofoil analysis', I've done calc's on Re # of my design comparing CL's at that design-Re for a number of diff aerofoils but i havent even looked at CD or Drag values nor have i looked at Cm's...which now seems to be a concern because the kind members here have prev pinted out that Clark Y on A delta wing config is unstable... hence why i NEED calc's to back this up...

4) 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.
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Old Apr 02, 2012, 06:53 PM
Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
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Sounds like a good project. The best advice I can give you is to look up a good book on this topic. I recommend Dynamics of Flight by Etkin and Reid.
Here is the link to the new version
http://www.amazon.com/Dynamics-Fligh.../dp/0471034185
There is a cheaper Dover version as well.
The chapter on longitudinal stability will work wonders!

You will benefit yourself much more by finding the answers on your own than by us listing them out for you.
If you get stuck, we'll be happy to discuss your problem.

As far as manufacturing, the difficulty depends on your material and technique. I would say that in general most airfoils are similar in complexity and difficulty. Some people find a flat bottom easier.
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Old Apr 02, 2012, 07:58 PM
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You'll be lucky to find a modern presentation of a Clark Y. Most of them will show the position of the "center of pressure", which is an obsolete and useless quantity.
This chart for an NACA 2412 will show what modern data plots look like, with the data plotted at different Reynolds numbers.
The pitching moment is usually negative for airfoils of this basically generic shape.
The CY-H's reflex makes the Cm positive a bit.
It's not usually something to worry about with a model... just put some reflex in the trailing edge of a flying wing, and adjust to your needs as you fly it.
.
Andy Lennon's "R/c Model Aircraft Design" from MAN has more than sufficient data to design any r/c plane, with a good chapter on tailless. Martin Simon's "Model Aircraft Aerodynamics" has more theory, and is also something to put in your library.
.
This site says it has Clark-YH data..
http://forums.x-plane.org/index.php?...&showfile=5802
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Old Apr 03, 2012, 12:55 AM
B for Bruce
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The 'Wack, BC, Canada
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I recently got a new computer as an upgrade so I no longer have XFLR5 at my fingertips. But as I recall it provides a Cm curve as part of the graphic output.

And by the way, it's not that you can't use the Clark Y because it has a pitching moment. You can't use it for a delta wing because it has a NEGATIVE pitching moment. That means that for an increase in flying speed it wants to pitch DOWN instead of up.

So what you need is an airfoil with a neutral or positive pitching moment so your "Concorde with a tail" design will be able to have a proper positive pitching stability.

If you had chosen to use a regular planform instead of a Concorde like wing you would not have this problem. With more conventional wings with aspect ratios of more like 4:1 or greater a normal tail can deal with the pitching moments of the wing. ..... well.... assuming you give the tail a long enough distance behind the wing to have some leverage to use.
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Old Apr 06, 2012, 03:32 PM
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Thanks all for your feedback!

@Bruce: Hi, my friend, I understand about the negative Cm on a Clark-Y when used on a Delta wing configuration.

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.

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.

Also these Q's are open to all...

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

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?

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.

Thank you all!
Will be looking forward to your responses!
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Old Apr 06, 2012, 06:06 PM
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This is a simple UAV... note the airfoil. Not particularly exotic.
4) can a simple airfoil's Cm be balanced by positioning the c.g.?
Probably not. The tail does that by providing the downforce needed, in lieu of any reflex.
3) How much Cm is too much? Any.
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Old Apr 06, 2012, 07:49 PM
greg
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somerset, nj
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these notes may help:The first explains a bit about pitching moments, but draws incorrect conclusions because I misread Simons. The 2nd tries to correctly put all the math together after reading the Etkin book previously mentioned. The latter part looks at why the Cm of the airfoil is important with a tail-less aircraft.
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Old Apr 06, 2012, 10:58 PM
B for Bruce
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The 'Wack, BC, Canada
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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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Old Apr 13, 2012, 07:23 AM
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Take a look at Martin Hepperle's site http://www.mh-aerotools.de/airfoils/index.htm site for more info on "Flying Wings" (two items down on the left side). Once you have reached the flying wings page, choose the linkes under "see also:" for a lot of basic stuff on stability of tailess aircraft.

Way over simplified, you want to move your CG far forward (compared to a tailed aircraft) and choose a (reflex) wing, such as the MH-6x series, which has a positive CM over the flight envelop you will encounter.

Alan
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Old Apr 13, 2012, 08:00 AM
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United States, UT, Salt Lake City
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Chee- it's all a matter of leverage -
no graphs needed
With no horizontal stab they will simply pitch nose down.
a flying wing is nothing more than a very short moment arm aircraft.
The reflex is just UPTRIM which creates downforce at the trailing edge.
ANY airfoil with no UPtrim at the trailing edge will simply follow the path of least air resistance .
why ?
air follows the path which will equalize pressures -unless there is some force emplyed to counteract.( PS, gravity has a role here )
This stuff isn't difficult - why complicate the explanations.
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Old Apr 13, 2012, 09:21 AM
Grad student in aeronautics
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Quote:
Originally Posted by richard hanson View Post
ANY airfoil with no UPtrim at the trailing edge will simply follow the path of least air resistance .
why ?
air follows the path which will equalize pressures -unless there is some force emplyed to counteract.( PS, gravity has a role here )
This stuff isn't difficult - why complicate the explanations.
Can you clarify what you mean here please?
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Old Apr 13, 2012, 12:16 PM
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Dick, it may not seem complicated to you. But to someone just starting out it's like rocket surgery... And you even complicated your own post by suggesting that gravity plays a part.


A great test for all this was done by someone here in the MS forum a few months back. He took a simple wing with a ClarkY airfoil and tried, unsuccesfully of course, to make it fly as a plank style flying wing. He then turned the negative pitching moment into a positive pitching moment by turning the wing upside down. It trimmed to a stable glide with no issue at all. Of course the upside down shape brought a lot of drag and limited the maximum attainable lift coefficient. But it DID glide in a stable manner... just not very far.
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Old Apr 13, 2012, 12:36 PM
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I'm trying to finger out how this guy works!
Take the tail off an airplane, and by moving the c.g. forward and reflexing the trailing edge, you can make fly.
What options would a bird have to do this?
I first saw this guy August last year, and just a day or so ago. Appears to "raven" with the best of them.
I would have expected an injury to have healed with a partial (at least) recovery of the tail feathers, but there's not even the bump where tail feathers grow from!
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Old Apr 13, 2012, 12:58 PM
Grad student in aeronautics
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There is a lot we can learn from nature, but we have to be careful about our assumptions. I have never measured a bird, so I do not know if it has what we would call "longitudinal static stability" (which is what we are talking about when we talk about CG placement). However, do not assume a bird is statically stable because it could still undergo steady level flight using active control.

Take bipedal walking, for example. Using our definition of static stability, this motion is unstable, just like an inverted pendulum. So if I was trying to build a robot that could move around, would it be a good idea to use two legs? That depends.....
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Old Apr 13, 2012, 12:58 PM
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Sorry --- gravity has no part in aerodynamics -
well maybe just a little
ANY flying wing is draggy - the stabilizing force required to hold workable AOA is the same - if on a long boom like a glider - or on a curled up te of the wing .
the problem tho -is that the short moment setup is DEADLY at low speeds - they will crash and kill easily
adding the long lever arm really improves the force setup - ever seen an endurance glider done up as a flying wing?
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