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General control line discussion => Open Forum => Topic started by: Bootlegger on August 13, 2011, 11:14:15 AM
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Maybe I should have started with Where did I read the way to be able to get a good C/G reading on a new construction (as in with fuse only) so that I'd have a good ball park setting.
Any suggestions??? :! #^
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I would like to know this to.
Jimmy
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I assumed that Gil meant a read on where the CG was going to end up, not where it should be.
I do it all by calculation -- I figure out how much each piece is going to weigh, and what its moment arm is. Then, when I get the airplane finished, I end up hanging a bunch of weight on the nose or the tail.
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Maybe I should have started with Where did I read the way to be able to get a good C/G reading on a new construction (as in with fuse only) so that I'd have a good ball park setting.
Any suggestions??? :! #^
Start by looking at similar size/performance planes. Adjust for the weight of the engine(sticking an LA 46 in a Nobler will make quite a difference compared to an unmuffled Fox 35). Increasing the size of the stab to 22-25% of the wing will allow a more rearward CG, using Fancher's rule of thumb.
Best bet, especially if it is something a bit different, is to design if from scratch, using Wild Bill Netzeband's articles, some weights from other planes for the stab, rudder, weight of finish/sq.in., landing gear, etc. and do a weight and balance ala' Tim's suggestion above. Plan on the first one being a prototype, at least until you get it to the point where you can tape and pin everything together and see how close it is coming to what you figured. If something is out of whack, you can still add or hack where needed to get really close.
The last one I did like this from scratch flew right off the board. The only mistake was that a 3 in. bellcrank couldn't handle 3 in. wide elevators in anything but dead calm weather. I hadn't done WB's calculations for the forces on the pushrod(my bad). Putting in a 4 in. bellcrank, a longer horn, and building a longer span stab with the elevators narrowed to 2 in. wide pretty much fixed everything.
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Build the wing, measure its CG and weight.
Build the fuse, temporarily or permanently install ALL the rest of components, measure the CG and weight of that assy.
Balance those two weights on a seesaw. The seesaw's pivot represents the combined CG of the fuse and wing together.
Adjust the fuse on seesaw so that the pivot is where ever you want the final CG, and that will give you the relative position of wing to fuse.
You may want to compensate for the weight of finish, I have read that 75% of finish weight is aft of CG.
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75% of the finish weight is a pretty good guestimate I would say. Total area(not counting the fuselage, which is pretty small) is about 120% or so of the wing area. 20% is ahead of the typical balance point(25% of the wing chord), 80% is behind.