<p>I don't think that being 'consistently more negative' will make a lot of difference. What's more important is that the overall aircraft should pitch down at low speeds (i.e. high angle of attack) and pitch up when flying fast, so a flatter curve would allow a smaller tail.</p>
<p>This nicely illustrates why planes with these sections need a horizontal stabiliser and why flying wings need special aerofoils.</p>

CC BY-SA 4.0
2022-02-02T09:53:55.397Z
2022-02-02T09:53:55.397Z
Robin Bennett
<p>Short answer: Sorry, airfoil charts do not contain conclusive information about the aircraft's handling qualities. So, picking the airfoil requires some more elaboration.</p>
<p>Regarding the last question: Cl of 0.4 (or any value) is not related to pitching motion, as pitching is related to Cm. The Cm values at and around AoA 0 are negative (-0.9), given in the corresponding graph. Without a stabilizing surface (horizontal stabilizer) or another method of pitch control (e.g. canard, elevons, etc) the wing will just try to nose-down. The only way of knowing what the &quot;airplane&quot; does is to design and calculate.</p>
<p>The longitudinal equations of motion and aerodynamic analysis can be implemented with algebraic equations. Therefore, I recommend you to do the sizing comparisons of each profile and corresponding horizontal stabilizers. Then, you might do some simulations (in Excel, or Python, or any other computer program) and see and understand the behavior of the airplane.</p>

CC BY-SA 4.0
2022-03-08T08:15:45.833Z
2022-03-08T08:15:45.833Z
Gürkan Çetin
<p>I'm designing a small RC plane and I'm currently at the first iteration of airfoil selection. I have converged to Clark-Y, NACA-2412, NACA-2415, and S-8036.</p>
<p>In comparing the four, I noticed that all of them have a negative Cm up to the critical angle of attack. Clark-Y's Cm values, however, are consistently more negative for all angles of attack, compared to the other three. I was wondering if this can be considered as an advantage for Clark-Y as less downward pitch would be required from the horizontal stabilizer. Could this be a disadvantage at level flight since it can cause the plane to nosedive? I noticed that at AoA = 0, Cl = 0.4 which could stop the nose from pitching down?</p>
<p>I have included a picture of Clark-Y aerodynamic graphs for your reference.</p>
<p>Thank you in advance for your help<a href="https://i.sstatic.net/ZJay5.png" rel="noreferrer"><img src="https://i.sstatic.net/ZJay5.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2022-01-29T13:07:57.823Z
You might consider asking here https://www.rcgroups.com/modeling-science-136/ .
quiet flyer
CC BY-SA 4.0
2022-01-29T13:08:14.650Z
And btw, I think the question was fine to be on aviation stack exchange, and probably more likely to (eventually) get a good answer than here.  But I know some folks are arguing that posts like this should be move.  If you search through the "meta" page on ASE, you'll see the issue is unresolved.
quiet flyer
<performance><design><stability>
The Effect of Pitching Moment on Aircraft Stability
user3993
# The Effect of Pitching Moment on Aircraft Stability

(Asked by: username_0 on 2022-01-27)

I'm designing a small RC plane and I'm currently at the first iteration of airfoil selection. I have converged to Clark-Y, NACA-2412, NACA-2415, and S-8036.

In comparing the four, I noticed that all of them have a negative Cm up to the critical angle of attack. Clark-Y's Cm values, however, are consistently more negative for all angles of attack, compared to the other three. I was wondering if this can be considered as an advantage for Clark-Y as less downward pitch would be required from the horizontal stabilizer. Could this be a disadvantage at level flight since it can cause the plane to nosedive? I noticed that at AoA = 0, Cl = 0.4 which could stop the nose from pitching down?

I have included a picture of Clark-Y aerodynamic graphs for your reference.

Thank you in advance for your help[![enter image description here](https://i.sstatic.net/ZJay5.png)](https://i.sstatic.net/ZJay5.png)

**Tags:** performance, design, stability
> **Comments:**
> **username_1**: You might consider asking here https://www.rcgroups.com/modeling-science-136/ .  
> **username_1**: And btw, I think the question was fine to be on aviation stack exchange, and probably more likely to (eventually) get a good answer than here.  But I know some folks are arguing that posts like this should be move.  If you search through the "meta" page on ASE, you'll see the issue is unresolved.  

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## 2 answers

### Answer by username_2 on 2022-02-02. Score: 1

I don't think that being 'consistently more negative' will make a lot of difference. What's more important is that the overall aircraft should pitch down at low speeds (i.e. high angle of attack) and pitch up when flying fast, so a flatter curve would allow a smaller tail.

This nicely illustrates why planes with these sections need a horizontal stabiliser and why flying wings need special aerofoils.


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### Answer by username_3 on 2022-03-08. Score: 0

Short answer: Sorry, airfoil charts do not contain conclusive information about the aircraft's handling qualities. So, picking the airfoil requires some more elaboration.

Regarding the last question: Cl of 0.4 (or any value) is not related to pitching motion, as pitching is related to Cm. The Cm values at and around AoA 0 are negative (-0.9), given in the corresponding graph. Without a stabilizing surface (horizontal stabilizer) or another method of pitch control (e.g. canard, elevons, etc) the wing will just try to nose-down. The only way of knowing what the "airplane" does is to design and calculate.

The longitudinal equations of motion and aerodynamic analysis can be implemented with algebraic equations. Therefore, I recommend you to do the sizing comparisons of each profile and corresponding horizontal stabilizers. Then, you might do some simulations (in Excel, or Python, or any other computer program) and see and understand the behavior of the airplane.


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