Determining CG

[Top End]

I've scratch built a plane, having completed construction I am facing the problem of how to determine where the center of gravity should be located in order to balance the plane. Any help would be most appreciated.

Thanks,
Chad

[2NG]

http://www.radiocontrolzone.com/forums/showthread.php?s=&threadid=87291

[Dave Robelen]

Hi Top End,
You are correct in assuming that every plane should have a CG. In order to help you locate the right place to put it, you need to hand out just a bit more information. Is the wing constant chord? Tapered? Swept? what type of airfoil? Is the tail generous in size or rather small? All of these are factors in locating the CG of a particular design. Glad to help, just please be a bit more generous with the details.
Regards, dave

[Top End]

Thanks guys, here is some more info...

Wingspan 48", Chord is 7" constant, no sweep with a 1 1/4" aileron, I'm not sure of the airfoil name but it is flat on the bottom, 5 degrees of dihedral. The fuselage is 34" long. The horizontal stab is 15 1/2" wide and 3 3/4" at the center and tapers to 2 1/4 at the tip with an elevator that is 2" le to te at its largest point. The vertical stab is 5 3/4" tall, 2 3/4" at the base and tapers to 1 1/4" at the top. The rudder is 7" tall and 2 1/2" at its largest point. I believe the tail is generous, with large control surfaces.

The le of the wing is approximately 7" behind the front of the motor, the bottom of the wing is 1 1/2" above the top of the fuselage.

The fuselage with all the electronics in it weighs in at 24 ounces and the wing at 8 ounces. The plane is powered by a Graupner Speed 400 motor with 2.33:1 reduction gear. I am using an 8 cell 1100 mah battery. Unfortunately I don't have sub-micro electronics but I am using 3 mini servos to control the rudder, elevator and ailerons.

The plane as a whole strongly resembles the SR Batteries Cutie ( www.srbatteries.com ) as that is the model I attempted to copy, albeit only with wingspan, fuse length, weight and pictures to work from.

The plane taxis very well and I believe it is flight worthy.

Thanks for any help,
Chad

[Dave Robelen]

Hi Chad,
Bsalance that beauty at 2.1" back from the LE to start (30% CG). If that flies nose heavy, move it back just a bit more.
Regards, Dave

[Richard Miller]

Funny, but things have flown without tails and without a fuselage or a landing gear, without power or propulsion, even without wings, but nothing ever flew without a CG.

Less funny is the fact that the plans in olden days, when Dave and I were young, omitted the CG, the one thing you really can't do without.

-Richard

[Top End]

Thanks Dave for the help. I'll give it a try.

Chad

[brucefly]

Ok, I have just bought a Bohemia Models Tipsy Nipper and I want to get the cg correct. Ive had a few bad experiences do to bad cg and I dont want it to happen again. First of all I have a 7 cell 600ae pack that came with my Wingo that I will be using primarily. I also have an 8 cell mi-mh 1100 that Im hoping will work as well. So I really have two questions. The 7 cell weighs 4.9 Oz., the 8 cell weighs 5.9 Oz.. Will this plane fly with the bigger 8 cell battery? Also the recommended center of gravity is 55-70mm back from the leading edge of the wing. My problem is that the plane is tail heavy even with either battery completly forward in the fuse. Im assuming I should have to add some weight to the nose of the plane to get the cg correct. the best balance I can get is on the main spar but seems to be further back than recomended in the manual. I havent discovered a real true method of figuring this out yet so any help would be appreciated.

[goofup]

Brucefly: The CG is the point of the wing (length-wise) that maximum lift is created to hold the plane up when it’s flying. The wing is holding up everything, so everything has to balance weight-wise on that point. In other words, if the plane were hung by a string at the manufacturer’s CG (the wings lift point), would it hang level? The manufacturer has shown you where the CG on this plane must be in order for it to fly correctly. With most planes you can move the battery back and forth to get the CG correct, but in your case it looks like you’ll need to either add some weight at the nose (or reduce it on the tail) to get it to balance on the CG.

Oh, you should be fine with either battery.

Goofup

[Dave Robelen]

Hi Brucefly,
The "true" method for determining the location of the proper CG is the location spelled out by the manufacturer. In the case of the "Nipper", you can make a simple stand from a couple of 1/4" dowels in a base plate that are long enough to support the model from the bottom of the wing. Pu one of those wedge shaped pencil erasers on each dowel for a sensitive measurement. Is it possible to reconfigure the battery to get more of the cells forward? Even if it takes some ballast, as you are aware, the CG must be correct fo safe flight.
Regards, Dave