Thanks David
The root and the tip are virtually identical. The shape of the arf wing tip is slightly different but who knows who sanded them out. The root and tip chord as well as the length and sweep are the same. The arf matches the kit plans. If I calculate the MAC and the cg the 3-1/8" cg position is ~25%. When I work it out on the plan version it's about ~33%. I didn't see any obvious changes with the design and couldn't help but be curious as to why Tower chose such a forward cg. I think the ~28% - 30% cg range sounds like a good starting point and that's what I'm going to use.
I pulled all the covering off the arf, so when I had it in the bones I decided to lay it over the plans to see how accurate they kept the lines. The only real noticeable difference was the fuse forward of the firewall, it was shorter. We guessed it was either to fit it in the box better or to accommodate an electric motor. Either way the wing was the same which is where doubt creeped in. I wanted to make sure I wasn't missing the obvious. If anyone actually balanced it at 3-18" I wonder how it performed.
K
Originally Posted by
doxilia
Bullseye,
the first thing I'd check is what the root and tip chords of the two wings (ARF & kit) are as well as the LE taper and sketch them to scale. The TE taper will be given by the closing of the frame between root and tip. If you draw it at ~20% scale (divide all dimensions by 5), each wing should fit on a sheet of 8.5x14. This will allow you to make a proper first view comparison between the two wings.
Once that is done, proceed to find the mean aerodynamic chord (MAC) of each wing. To do this, you need to place two lines the length of the root chord at the ends of the tip chord and vice versa (tips on root chord). Then draw two diagonal lines between the extremes of the added lines on either end of the root and tip chords. The intersection marks the location of the MAC on the wing. Then, locate the CG on the root of the respective wings (4" on the kit and 3-1/8" on the ARF divided by 5) and project this location with a spar parallel (perpendicular to the fuse centerline) line to the MAC. Measure the LE offset of the projected CG onto the MAC and see at what percentage these CG's are located on the MAC. Typically, the CG should be located anywhere between 25 and 33% of the MAC with the former being very conservative and the latter quite aggressive. For aerobatic models like the Kaos a 28-30% MAC CG is probably a good range.
You can compare the projected CG measurements vs the 28-30% MAC CG and see how much they differ. Whether kit or ARF, I would balance the model near the 28% mark for the first trim flights. You will likely be very close to or on the main spar of the wing.
Note that the model flies on the wing so regardless of the nose and tail moments, the distribution of mass about the wing must still check out. So whether an ARF has a shorter nose moment than the kit, it should not change the CG on the wing, only the distribution of mass about it. A shorter nose moement requires more weight in front of the CG in order to balance properly. For example, if one looks at WWI models with very short nose moments, the majority of the models mass is all kept up front with a very light tail. The converse is true as well for a model with very long nose moment or a heavy engine up front requiring more mass in the tail to balance. An example of this would be an SPA classic with an FS-91 up front. The tail boom of such models is typically stretched in order to counteract the heavier (than 60 2s) engine up front.
David