CG behind AC

[rosco]

Hi there troops,
I was reading another thread which mentioned a CG and AC calculator called A.C. Calculator by Martin Hepperle. I tried the calculator on a flying wing that I have made previously and it shows that the CG is 86mm behind the A.C. !?!?
I guess that the wing is very unstable?
The funny thing is that the wing is great to fly, all my friends think that it is fantastic too.
I guess that my question is, am I miss-understanding the meaning of unstable?
To me, the word unstable means unable to be flown, like when your C.G. is wat too far back.
Any enlightenment would be appreciated.
I have attached the .BMP file that I used

[Sparky Paul]

Dimensions please.. and where did you specify the c.g. to be?

[mharms]

rosco,

The "CG" that program calculates is just the CG that that shape would have if the shape was made of material of uniform density. The really useful thing is the AC position. For your wing to have decent pitch-wise stability, you will need to position your battery, servos, motor, etc, such that the CG is somewhat ahead of the AC.

To get an idea of how far ahead of the AC the CG should be, you need to figure out the Mean Aerodynamic Chord (MAC). You can use the AC program to help you do this.

Take your full shape and divide it in half, so you just have the right side wing. Run it through the AC calculator. The resulting positions of the AC and the CG will lie on the MAC. If you draw a line from the leading edge of the wing to the trailing edge, going through these points, this is the MAC.

If you do a screen capture of the graphical output of the AC calculator and bring it into a paint program, you can measure the length of this line in pixels. I've seen various recommended values, but if you position your CG ahead of the AC by between 2% and 10% of the MAC length, then you should have reasonable logitudinal stability. Another rule of thumb is to position the CG about 20% of the MAC back from the leading edge of the MAC. Either way, you are pretty much in the same ballpark. Figure out this range of CG positions while in your paint program, then scale the pixel measurements to real-world units so you can apply this to your plane.

Mark

[rosco]

Dimentions are root cord 520mm, tip cord 130, and it is a delta (flat across the back...hope you get what I mean)
The A.C. calculator Gives me a AC of 247mm.
I have a CG calculator that I use for all of my wings (and it has proven to be correct for everything that I have made). It also comes up with 247mm. :confused:
I currently fly my new wing with the C.G. at 247mm.

Will the wing be more 'stable' if I move the battery (for example) forward and in turn move the CG towards the front?
If that is the case, I would probably have to give the elevons a little more 'elevator up', hey?
This would be fine exept that it would give the wing more drag...correct?
cheers
rosco

[mharms]

The AC calculator gives you a approximation of the actual AC, so maybe the true location of the AC in your plane is a little back from the reported 247mm. Did you include your elevon area in the graphic for the AC calculator?

You could move the CG forward and adjust the elevator trim, as you say, to see how you like it, but if you are happy with the stability now... why fix what's not broken?

Mark

[peterangus]

In the context of aircraft, the abbreviation "CG" is applied in two different ways, which leads to some confusion.
--------------------------------------------------------------------------
1
CG position is the actual centre of mass.
--------------------------------------------------------------------------
2
CG position is where the centre of mass is required to be if the aircraft is to fly correctly.
--------------------------------------------------------------------------
Maybe we should use two different abbreviations:
"ACG" actual centre of mass.
"RCG" required centre of mass.

[rosco]

Thanks for those bits and pieces fellas.
I did include the elevons into the wing area...thanks for putting that one in the mix.
I appreciate the input, since everything I know is self taught and sometimes I feel that I may have gotten it wrong.
cheers
rosco

[Flight Engineer]

Chaps, All these calculations of MAC are wrong. What you are calculating is Mean Geometric chord. It has very little to do with the force vectors of your wing. I do wish aero modelers would stop using the term MAC or learn how to calculate it properly!!

[Sparky Paul]

Chaps, All these calculations of MAC are wrong. What you are calculating is Mean Geometric chord. It has very little to do with the force vectors of your wing. I do wish aero modelers would stop using the term MAC or learn how to calculate it properly!!
.
Do show us the errors of our ways.

[Flight Engineer]

http://homepages.stmartin.edu/students/SeniorDesign_ME2004/Neutral_Point.htm

http://en.wikipedia.org/wiki/Chord_(aircraft)

[Sparky Paul]

The excessive math approach example uses equal root and tip chords.
Of little use for a typical tapered wing, where the location of the m.a.c. relative to the leading edge at the root is needed.
The usual sites linked for determination of this account for sweep/taper.
http://www.palosrc.com/instructors/cg.htm
http://perso.orange.fr/scherrer/matthieu/english/mce.html

[Flight Engineer]

http://www.aeromech.usyd.edu.au/aero/perf/perf_ac.html

This is a very good web site for the beginner.
What model makers forget is that the force vectors are constantly changing along the span and therfore simply locating the geometric mean chord is not the same as creating an imaginary straight wing with the same total force vectors as the swept wing.

Why not express CoG with ref to a known and measurable datum rather than a very rough and meaningless chord?

I see claims of CoGs at 40 or more % MAC! This is simply missleading and only hinders beginers trying to find an apropriate CoG position for their models.

Just to give you a little real world data. The latest regional jets with fly by wire flight controls have CoG limits between about 7 and 27% MAC. Of course stability requirements for passenger aircraft are very conservative but they give a good idea of where the CoG should be for any aeroplane.

[peterangus]

they give a good idea of where the CoG should be for any aeroplane.
Not for any aeroplane.

Only for 'planes with typical tail area and moment arm.

Aeroplanes of tailless or canard configuration will require the CG to be much further forward.