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A slightly more complicated question about drag.


hellwraiz
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Now I have seen a lot of posts about the fact that the best place for COL is behind COM, but what about up and down? I tried reading some articles about this, but in the articles that I found they only talk about COL being behind COM! And to clear out any confusion, I am talking about planes, where I change the angle of the wings to get COL up or down.

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Hello @hellwraiz, and welcome to the forums! :)

16 minutes ago, hellwraiz said:

Now I have seen a lot of posts about the fact that the best place for COL is behind COM, but what about up and down? I tried reading some articles about this, but in the articles that I found they only talk about COL being behind COM! And to clear out any confusion, I am talking about planes, where I change the angle of the wings to get COL up or down.

The placement of CoL "above" (i.e. dorsally) or "below" (i.e. ventrally) the CoM is much less critical.

Basically, what that will tend to do is affect the roll stability of the craft.  Above the CoM = more stable for roll; below the CoM = less stable for roll.

The reason it's not all that important is that aircraft tend to be naturally pretty stable on the roll axis, unless you do something really unusual to the design.  This is because,

  • there typically aren't any major aerodynamic forces that tend to induce roll
  • aircraft tend to have a lot of control authority for roll, since wings tend to stick pretty far out to the sides, and an aileron at the tip therefore has a lot of lever arm to work with
  • SAS does a pretty good job of maintaining roll attitude

If you'd like to add a smidgeon of roll stability, you can give your wings a slight dihedral angle (just a tiny bit is plenty), and/or mount them higher up on the fuselage than the midline.

Speaking for myself, though, honestly I don't bother most of the time-- I just mount wings on the default midline and everything works fine.  I'll usually give them a very slight amount of pitch up (i.e. positive angle of attack) to reduce fuselage drag, but that's about it.

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4 hours ago, Snark said:

SAS does a pretty good job of maintaining roll attitude

I would disagree with the usage of "pretty good"--In a plane whose wings have AoA, you're going to end up very far off course if you walk away from it for a few minutes.

  • Wings have AoA and zero dihedral. I don't find that dihedral works very well on mid-low-winged planes with wing AoA.
  • SAS set to Prograde.
  • A portion of the tail has zero AoA, and is fixed (not a control surface, or a locked control surface) for passive pitch stability.
  • This configuration will result in SAS Prograde giving the plane a slight nose down attitude relative to the velocity vector.
  • With the above in mind, I give the zero AoA tail 5 degrees of ANhedral, which will passively correct the roll.

 

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I mostly agree with Snark. Vertical displacement of CoL vs CoM does have some effect on the stability, but it's so negligible that it can easily be relegated in favor of other design considerations with no ill effects. Case in point: Here are the first 2 aircraft from my latest caveman speedrun

VAWST3S.jpg

49fdNkT.jpg

Both fly so similarly that I don't notice the difference.

Best,

-Slashy

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One situation where high/low CoL matters is high angle of attack stability for reentry.

Having CoL above CoM means that as AoA increases more and more and the wings transition from being lifting surfaces to dragging surfaces, you end up with a CoP (center of pressure, both lift and drag together) that is now behind the CoM.  This can result in a "pinning" effect, where the craft is fully stable in a range of 30 degrees or so off of prograde, but outside of that it has a preference for fully stalled AoAs.  Sometimes even as high as 90.  With careful adjustment this can be exploited to create a craft that is very neutrally stable over a huge range of angles of attack.

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