Balance Point Determination
 

Hi there!

What is the procedure for determining a "starting" balance point for a model with the wing/stabilizer configuration shown (all dimensions in inches)?

RE: Balance Point Determination
 

Here is a link you will find very useful. This firstis to the index of the site
http://www.geistware.com/rcmodeling/index.htm

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

Weight and balance calculator
http://www.geistware.com/rcmodeling/cg_super_calc.htm

Don

RE: Balance Point Determination
 

Those same links are in the sticky thread titled "Aerodynamic Resources and Online Tools" at the top of the forum thread listings.

RE: Balance Point Determination
 

Man, I have to learn everything the hard way. Ifound them on my own. http://www.rcuniverse.com/forum/js/f...sn/biggrin.gif

Don

RE: Balance Point Determination
 

It's not that hard pick up the plane by the wingtips a little forward of center not much about a 1/4 inch.That will get you close.Plane has to be complete and ready to fly meaning engine battery electronics.The plane should hang in a level position.You deffinetly don't want to be tail heavy.It will be totally uncontrollable.

RE: Balance Point Determination
 

Actually, that depends on how tail heavy, and it also matters what you call "tail heavy".

It's always safer to balance on or ahead of the suggested start CG location, and especially if you use a "rule of thumb" to guess where the CG might be.

However, the plane won't become uncontrollable until the CG is pretty far aft of a recommended location. (Recommended by the designer or a reliable CG locator like geistware.com)

It appears that an awful lot of modelers have decided that "tail heavy" is an instant death warrant. It'd serve beginners better if those dire sound bite warnings had a bit of explanation added. Because CG locating and location aren't sound bit simple, but aren't hard to understand either.

RE: Balance Point Determination
 

Since you have a double taper in your wing you'll need a calculator that will take advantage of that. It's not in the sticky mentioned above but if you go to www.TailwindGliders.com there is a FREE spreadsheet that will work with MS Excel or OpenOffice.org called "Sailplane Calc" that will easily make those calculations for you.

All the best.
Curtis
Montana

RE: Balance Point Determination
 

BTW guys, if you'd looked closely at the OP's (original poster) drawing, you could see that the usual applications don't work for him at all.

He has a wing with two planform tapers. He needs some help if he is to use those links. And it'll take some effort to find the MAC and extrapolate measurements that can be input into the CG locator, as it only recognizes wings with straight taper from root to tip.

Time to get out the pencil and paper. Be back in awhile. Anybody with time right now, feel free to beat me to the answers. ;)

RE: Balance Point Determination
 

Sailplane Calc is so easy to use I just input the data for ya.

Curtis
Montana

RE: Balance Point Determination
 

OK,

You need to have just a Root Chord and Tip Chord for those applications. And you have 3 chords. I won't go into how to figure that out right now.

But if you use your 12" that shows on your drawing for the wing's root chord and 8" for your tip chord for the CG locator application (and would then use 4" for the wingtip sweep), the application will work for you.

You should still use the 50.8" half span and your other measurements too.

RE: Balance Point Determination
 

Thanks but to use the CG locator, the MAC isn't needed for input.

Plus I just noticed either his numbers on the tail have an error or his drawing. OK, let's go with his drawing not being right.

But you're right, Sailplane Calc is a great way to find MAC.

RE: Balance Point Determination
 

GEISTWARE.com says that 5.3" would be a good place to start.

RE: Balance Point Determination
 

And if we look closely, you win the trophy for speed to results. And you didn't have to extrapolate.

I gotta start pimping your Sailplane Calc, at least in the sailplane forums. We came up with the same results but you were far quicker and didn't have to figure out anything.

RE: Balance Point Determination
 

And Sailplane Calc says 5.26" aft of the wings root leading edge!

Curtis Suter
"Sailplane Calc Designer"

RE: Balance Point Determination
 

Thanks! That was the entire idea behind Sailplane Calc. There is also Flying Wing Calc too! Also one for V-tails and they are also available in metric.

There is an article on how to use the spreadsheet written for RCSD. It's all at my website for FREE!

Oh, this works for most all conventional planes. Free flight is a different beast all together.

Curtis
Montana

RE: Balance Point Determination
 

I used to make many flying wings from crashed planes and they were 80% nose heavy but flew fine did loops inverted flight fine till the motor ran out of fuel.The glide ratio was very low and the dead motor made it nose heavy again!!

RE: Balance Point Determination
 

You should check out my article for "Tinamou" at my website.
http://tailwindgliders.com/Files.html#Articles


There is another article as well called "Table Top" that show more about Tinamou.
http://www.rcsoaringdigest.com/pdfs/...SD-2010-09.pdf

Curtis

RE: Balance Point Determination
 

Thank you, Curtis.
That is an interesting flying wing,........with flaps!!

Here is the link to the article (Page 66):

http://www.rcsoaringdigest.com/pdfs/...SD-2008-10.pdf

As noted by da Rock, the tail's dimensions are not correct somehow.

RE: Balance Point Determination
 

I drew your wing with Autocad and calculated the area. The right half you have shown has a wing area of 508.2 square inches. If you put the CG five inches behind the leading edge at the dihedral joint, 30.6% of the wing area will be in front of the the CG. That is equivalent to having at 30.6% of the MAC and is safe enough for a successful first flight. Dan.

RE: Balance Point Determination
 

Hi All!

Many thanks for the very helpful responses!! I learned of some new 'calculators' and will add them to my 'list'. I'll do some review, and assemble the model and see how the various locations seem to work.

The stabilizer drawing is just a little misleading, as there is a double taper in the planform; I just ignored it, thinking it wouldn't make all that much difference; the dimensions are correct.

FYI: the surfaces are for a 1/16th-scale electric model of the Boeing Sea Ranger flying boat; I've flown the model several times, but am now trying to sell it and needed to have a better/more accurate estimate of the balance point.

Thanks again for all the help!!

Dave P.

RE: Balance Point Determination
 

Charlie, your reply is just way too vague to be meaningful. First off the last place you often want the balance point to be is "...a little forward of center not much about a 1/4 inch." On a wing that is only about 1.5 inches wide this would work. On a more typical RC model with a wing chord of anywhere from 5 to 15 inches it would be a recipe for disaster because your description would result in a balance point way too far back for most models. A far more "normal" balance point for a run of the mill planform is around 25% to 30% of the wing chord back from the leading edge. But even this is a rather sweeping generalization that only works for a constant chord wing with an average tail length and stabilizer size. This is why you seldom see such a recomendation but instead we all point the folks to one of the online CG calculators for a much more accurate indicator.

I only raked you over the coals on this one because you appear to be trying to get into designing your own models. To be successful at this you need to start thinking to a tighter and higher standard that takes in more of the design details.

RE: Balance Point Determination
 

Please read the article at RCSD linked from my website called "Sailplane Calc Article/Tutorial" and it will enlighten some things about aircraft balance. Also, sailplane calc works on most planes and it's a little conservative, not much but it makes for a nice safe handling model on it's maiden voyage.

Lastly, you cannot accurately balance a model without knowing the area of the horizontal stabilizer and the distance the horizontal tails quarter chord location is from the main wings quarter chord location. Anything else is back to basic rules of thumbs which in some limited cases works well.

Curtis

RE: Balance Point Determination
 

I have already used a few of the computer based softwares and found them to be near to the same results. They are based upon trainer criteria, and reassign the CG to where often makes the new A/C fly like a 1960's box design from a kit.

If the model A/C is intended to fly aerobatic, racer, or w/o power then the CG ought to be noted as in a different location than the set ratio of the computer software. Near zero had an adjustment for different radio gear (and weights) and different power up front.

Before I got into one of the noted above softwares, it did say to fuel up the aircraft and then weigh it very carefully using their $200 scales on each leg of the tri-cycle landing gear and input data. That would tell you as to where the present CG is at, not where it is SUPPOSED to be at, based upon the noted differences.


Now, how did they know I was going the three wheeler route? An assumption?

Wm.

RE: Balance Point Determination
 

How did they know? ;)

Because tail draggers and tricycle gear both have 3 wheels? maybe..... could be..... ;)