MAC question

[MikeL72]

Could someone explain the MAC dimension in the attached spec table? I would like to know what the IM means and how does this relate to finding the CG?

Mike

[guapoman2000]

I am an Electrical Engineer and so, I made a WEB search under the heading, "mean aerodynamic cord" and this is one that came up...

http://www.geistware.com/rcmodeling/cg_calc.htm

[guapoman2000]

Here's the on-line calculator.....

http://www.palosrc.com/instructors/mac.htm

[guapoman2000]

Wow!

So much stuff out there!

http://www.mmaa-modelairplanes.org/fun_stuff.htm#tools

[MikeL72]

Thanks Guapoman -- fine references. Perhaps I'm not seeing it -- but I don't find what the IM dimension is in the table. But I should be able to get at the CG with the other parameters given.

Mike

[madrob]

Looking at all the other figures, the "IM" in question is a unit of measure and being that all the other units used are imperial I am pretty sure that it is a typo and should have read "IN" meaning inches. MAC (mean aerodynamic chord) is a linear dimension so there is not much else it could be.

Rob

[MikeL72]

The root chord dimension is is given as 10' 4" and the wing is tapered in the trailing edge. If in inches -- the dimension given for MAC would be 7' 6". For a half-span of 18', this could be the location of the MAC from the centerline out. The specs given are taken from a jpeg figure found on the internet. A 3-view is shown with dimensions but the resolution is so poor, I cannot read them. The plane is an OS2U Kingfisher. I don't remember the source on the net.

A typo may be correct, thanks.

Mike

[sblack]

The Mean Aerodynamic Chord is measured at the spanwise location which corresponds to the centroid of an assumed eliptical lift distribution. So if you take the entire lift of the wing and make it act at a point along the span, the chord at that point would be the MAC. The equation for it, based on taper ratio and root chord is:

2/3*rootchord*(1+taper_ratio+taper_ratio**2)/(1+taper_ratio)

where taper_ratio is tipchord/rootchord i.e. usually less than 1.
and where ** means "squared", also sometimes denoted by ^2

The cg for a conventional configuration with normal proportions of tail area and tail arm should be somewhere between 25% and 35% of the MAC (NOT the root chord), depending on how much stability you are looking for.

Of course the centroid of the elipse is always at the same % of semi-span for a straight tapered wing so you can ignore all the above and just go to 41% semi-span. The portion of the wing covered by the fuselage is usually included.

You don't have to be precise on this, since there is a range of cg location that will work and you will probably tune it to your requirements anyway. It is important to get in the ballpark.

I am sure the IM should be IN. You can work it out to confirm with the formula above.