ORIGINAL: Campgems
Last fall, I picked up a large pattern plane from a retired club member. The ony thing he knows about it is that it was built by another person. I haven't been suscessful in determining the model so I am unable to get recomended CG and throw info. I've just about got it air ready, but I'm at a loss as to where to place the CG.
I've been playing around with a couple of the CG caculators and I'm a bit confused as one shows a range between 6 and 7.75 inches from the root LE. This looks like a huge range from planes I've built. The wing span is 74", Root cord is 15.75, and tip cord is 11.5 with a 3.5" sweep on the LE. The horziontal Stab is 32" span, 10" root cord, and 6" tip cord with a 1.5" sweep on the LE. The Root LE of the wing to Root LE of the stab is 37"
Based on this and using a 10% static margin, I get the 7" CG from the Root LE. using this caculator.
http://adamone.rchomepage.com/cg_calc.htm
I'm looking for someting that is going to let me get it up and flying without fighting a nose heavy, or tail heavy condition. Based you your experience, does the 7" sound OK???
Don
Interesting question. It really gets to the heart of designing airplanes.
In my experience, finding the Mean Aeodynamic Chord (MAC) and Aerodynamic Center (AC) for the wing and stab are straightforward for all but the most irregular planforms. Finding the Neutral Point (NP) gets involved with several incremental ratios and factors that are complicated to estimate without experiments so the on-line calculators assume built-in constants rather than ask for specifics. This is perhaps why you've encountered such disparity in answers among them.
One of the NP factors is relative lift coefficient ratio between wing and stab. If the calculator didn't ask for this it may have assumed a flat plate stabilizer which is probably incorrect for the plane you have. (Most pattern type planes have airfoil shaped stabilizers.) Another is stab "efficiency". It's another fudge factor that could be from 30% for an obstructed tail on a scale ship like a Gee Bee up to 90% for a T-Tail.
I calculated the points by hand and got the exact same MAC's and AC's as the one in the link you posted but my number for the NP came out at about 7 7/8" from root leading edge, not the 8.42" they suggest. (I assumed a airfoil stab with 60% efficiency.) Allowing a 10% static margin I would suggest a starting CG at 6 1/2" back from root LE.
This plane would likely still be controllable with CG from 5 to 8" from LE without too much drama. However, I always make sure I have more than adequate throws on all control surfaces prior to
any maiden so the pilot has more than needed to wrestle a badly trimmed plane back under control on lift off. The throws can be returned to something more appropriate later.