CG Question

[Fliegenmong]

Hi All :O

I have a question regarding the C of G. I understand it and in my full size flying have of course kept it all within where it should be...that's why I am here today....

Jnr Fliegs and I spend a lot of time together flying model aircraft.....approx 1/3 of the way back from the leading edge is a wonderful little rule of thumb to start out with for straight wing aircraft, adjusting slightly fore & aft until the perfect spot is found.

Shortly we will maiden a model, that seems as though it has the tail feathers, undercarriage & wing of an A330, but with the Fuselage of an A320.

Swept wing........I have never had to find the C of G on a swept wing model.....does it so happen to be around the 1/3 mark back from the leading edge as measured at the wing root?...(to hell with the wing tip).....

I guess what I am asking is...measured from the wing root......can I expect a correct C of G to be 1/3 of the way back from the leading edge?...essentially the same as a straight wing?? Or is it that the C of G should somehow be determined from the MAC (Mean aerodynamic Chord?) and so radically alter from the accepted 1/3 MAC....and I suppose that begs the question....does a critical Reynolds number come into effect????

Thanks in advance to brains far superior to mine :(:8:)

[Mad (Flt) Scientist]

With a swept wing you need to consider the geometry of the whole wing not just the root. Otherwise you'd be putting the cg in the same place for a straight wing as for, say, a 60 degree swept wing, and without even doing any analysis that seems pretty self evidently wrong.

The 33% or 1/3 rule of thumb you are using depends on the relative size/effectiveness of the tailplane - something engineers call tail volume. (Its the area of the tail times the distance between the tail and the wing, divided by the wing area and by the mac of the wing). assuming you have a 'reasonably' sized tail relative to the wing, 33% mac sounds a decent first guess.

If the wing is constant sweep and taper, then you'll get a rough idea of the MAC at the 40% span location - the local chord there is about the mac. (Someone's going to correct me with the exact %age, I'm sure, but since you're in rule of thumb territory it's not critical i expect)

Whether the flow is laminar or turbulent, you still need to account for the wing planform.

[Yanchik]

Here's part of the answer....

1) CG position ? Between 25% and 33% of the mean chord is what I used to use. As Mad Flight Scientist suggests, there are better ways, and somewhere I even had a generic equation that was intended for model flying, but 25-33% is a fine start for "normal-looking" tailplanes.... Beware using actual airliner dimensions as you'll find them a bit under-tailed by model standards.

2) The other problem, finding where that's actually located on a swept wing aeroplane ? Here's a method for you.

i) Make yourself a scale drawing of the wing planform. One side of the aircraft (port or starboard) will be sufficient. Approximate the planform to four lines: the tip and root chord to be parallel to the aircraft centreline, leading/trailing edges make up the other two lines.
ii) Measure 50% of the root chord and mark that point. Measure 50% of the tip chord and mark that point. Join those two point with a straight line. Call it "Line A."
iii) Project a line in the forward direction, parallel to the aircraft centreline, from the leading edge at the root chord. This line to be the length of the tip chord. Let's name the point that it reaches "Point B."
iv) Project a line aft from the trailing edge at the tip chord, again parallel to the centreline, the length of the root chord. It will reach "Point C."
v) Join Point B and Point C with a line, Line D.

Where Line D intersects with Line A, you've found your mean chord. That is to say, the chord section of that part of your swept wing is equivalent in CG location terms to a nice constant-chord plank wing stuck through the fuselage at that point. So, measure 25-33% along that chord section, project it back onto the fuselage, mark it as forward/aft cg limits, and Robert is your avuncular relation.

As MFS said, if this chord section doesn't come out "somewhere about 40% outboard from the centreline" then you or I got something wrong.

This method can be used for wings with different degrees of sweepback on differing panels. However, that's not a description you want me to type out.

It worked on my pusher canard delta (which flew a circuit and a half before the fin fell off and I went to Polyversity...)

Caveat emptor: it's ~20 years since I designed and flew models, ~17 since I completed the degree that taught me how to do it properly, and I'm now a consultant giving strategic advice to senior managers in the industry. So you might want to ask someone who knows what they're doing... and hasn't damaged their memory with several years drinking in Eastern Europe :8

Y

PS. Unless your model is accomplishing something that will make the evening news, I don't think you'll need to worry about Reynolds numbers.