Originally Posted by
Dr Honda
OK... here it is.
According to the manual... the CG is on the tube. The center of the tube, from the leading edge, where it hits the fuse is 5.75" Total chord is 20". SO... if my math is right... it puts it at 28.75%. (?) But like I said... it could probably go back another 0.5" to make the landings better. (but I don't know yet)
Here's a pic of the wing, with the tube in it. That way, you can see where the straight line is. I also put a long ruler on the front, parallel to the tube, so you can get an idea of the tapper. The rear is actually more tapered than the front. (Similar to a Reaction 54)
Thank you.
just so that we all know, the definition of wing sweepback is the angle of sweep of the 25% chord line. That's the line drawn from 25% root chord to 25% tip chord. That's for a simple tapered wing like the
Firebird (double taper means something else). If the sweep of the 25% chord line is zero then the wing is defined as straight, not swept.
That means that the LE is swept back a bit, and the TE is swept forward (by 3 'bits').
A quick measure of your photo leads me to believe that the
Firebird wing has no sweep. That is handy, because a CG position of x% chord can be measured anywhere (e.g. the root) not necessarily at the mean chord.
25% or even 28% is fairly well forward, to compensate for the long nose.
I will be interested in your assessment of stability when you test fly.
Regarding flutter, I am a worrier. I have experienced it a couple of times. The first time was almost 40 years ago on a Gangster, a popular sport aerobatic model of its time. At the time I dug out my old university notes and textbooks to read up on flutter, and I have mass balanced large fast models since. I devoted 3 or 4 pages of my book '
Basic Aeronautics for Modellers' to flutter, and how to avoid it. Flutter is a speed problem. If flown fast enough almost any aeroplane will have flutter. The trick is to put it off to an unreachable speed.
So my own design jets are all mass balanced. I put lead in the forward projection of each control, best seen on the photo (post #17) on snowy background.