Fuse station for wing CG?

[RCL]

Hi All,

I'm trying to design (spelled 'part together') a small foam airplane. I'm sure I need help from gurus that understand this design stuff.

I haven't done any of this yet, but I'm trying to think it through. As example, let's say:
I've selected a general fuse shape (Edge 540) and an airfoil (Cap 21). Then, I've made the wing span and chord large enough to provide sufficient lift (wing loading low).

How will I know where the wing will be placed on the fuse? In other words, if Profili tells me the CG of the wing is at xx location on that wing, how do I figure out what fuse station to put this wing CG?

Thanks,
Mark

[Sparky Paul]

Put the wing where it is on the Edge.
You'll be close enough as makes no never-mind. :)
Tail surfaces the same.
Balance where Profili says from there.

[RCL]

Hi Sparky Paul,

Ya know, I was kinda thinking that way, but one more question.... The chords will be about the same length, but the chosen wing's CG (Cap 21) is at about 15% of the chord, where the Edge wing's CG is at about 30% of the chord.

When positioning the new Cap wing, do I put the LE in the same place as where the Edge wing LE was? Or do I put the Cap wing's CG in the same place as where the Edge wing CG was?

There's over an inch of difference in the wings' CG locations.

Thanks,
Mark

[Sparky Paul]

The 15% c.g. is odd... where did it come from?
Generally a c.g. that far forward is used on flying wings..

[RCL]

Yea, I know,

Do you have the program called Profili 2 ?

Look in the database for Cap 21 ..or just Cap. Am I reading this wrong?

Mark

[Sparky Paul]

Yeah, it's the wing thickness, not the c.g. that is illustrated in Profili. (everyone needs this program. Shameless plug! :) )

[Ollie]

There is only one chord that represents the whole wing. That is the mean aerodynamic chord(MAC). That is the chord of the wing that you should use when comparing one wing with another.

When comparing the CG of one plane with another, it should be in relation to the neutal point which is the aerodynamic center of the whole plane. Its location depends on the tail moment arm in relation to the MAC and the tail area in relation to the wing area.

Rules of thumb about CG location as a percentage of wing chord are not fundamentally correct for all configurations and only work for similar configurations and because there is a range of CG locations that work for any particular configuration. The more similar the configurations the better the rule of thumb works and the more different the configuration, the worse the rule of thumb works.

If you really want to understand what the relationships are you have to understand what the neutral point is and what affects its location.

Model Aircraft Aerodynamics by Martin Simons is easy to read and explains the fundamentals.

[RCL]

Sparky,
You're right... I guess I was looking at a lot of profiles that ended up with the thickest part at about 30%. I guess I was thinking CG at the same time!?!?

At this point, the location becomes easier so thanks! Wish I could swear never to be stupid again <grin>.


Ollie,
I was also using Airplane Design Calculator.XLS by Lee Van Tassle. Your info on the "aircraft's" CG compared to the wing's range is a big help in understanding this spreadsheet a little better. ...Particularly with the huge mistake I was making (shown above).

Thanks a lot guys!
Mark