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Post by Lt Swiss on Oct 31, 2009 19:03:22 GMT -5
Okay... I built this ASW-28 (see attachment below) to spec and, with all said and done to the letter, it looks like there is a major CG issue. I don't have a scale for such things, but even with 2 giant 3200ma batteries (about 20oz total) in the nose, it is still is so tail heavy it looks like it would need that much weight again to be on. (Note: All this is without the jet pod) Should I melt down lead car wheel weights and make a cone shaped weight that can tuck in the nose tight? Maybe a giant magnet, but would that freak the radios and such? It seems like it needs about 2 pounds to even it out. Is that freakin' typical? Some help...  I really want to fly this baby! Here's a video of a ASW-28 speed run and landing... Attachments:
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Post by bearcat on Oct 31, 2009 23:30:50 GMT -5
Well, a rare earth magnet does have a strong magnetic field, and the motors inside the servos use magnets as well. But I think as long as the magnet you use in the nose (to affix washers for balancing) is kept as far from the servos as possible, I think you'll be fine. I don't believe the receiver will be affected, but if you want to test it, put in your radio with the magnet in the nose, and start walking away and see if the radio in the plane works okay. Someone can stay behind with the plane and use your cell phone to communicate.
I was thinking about it after our conversation, when I told you about putting a magnet in the nose so you can "stick" washers as needed for balancing. If you really need a lot of weight, why don't you get a steel case, like an Altoids mint case or a lip balm case, and put lead fishing weights in it. That way, it weighs more than washers and is more compact, and you can still vary the weight. And the Altoids case will still stick to the magnet.
Do you have a scale to measure ounces? If so, hold the wing at the intended balance point and put the tail on the scale and measure how tailheavy it is. But you also have to measure your moment arm length. That way, you can predict what you'll need in the nose to balance.
I'll show you the math in person. It's simple, it's just a ratio.
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Post by Lt Swiss on Nov 1, 2009 22:54:40 GMT -5
Don't have a scale, but went to a few forums tonight and found that 22-30oz is the norm. Most said it needed so much ballast that they just put a motor in it instead. Also, the forums said repeatedly that the wing can handle a lot of weight no problemo. I am putting its launch as high priority for next Sunday... I looked into the math and I think I'll be going with the ol' school method of trial and error. I'd love to do it sometime though just to know how. I love math... This looks cool... Online Center of Gravity Calculator www.nasascale.org/howtos/cg-calculator.htmOnline Mean Aerodynamic Cord Calculator www.nasascale.org/howtos/mac-calculator.htm |
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Post by bearcat on Nov 2, 2009 7:46:32 GMT -5
Okay here's the math. Keep in mind that both sides of the equal sign remain equal, and then you solve the ratio for the variable, x.
For example, let's say you held the plane by the correct CG and put the tip of the tail on the scale and it weighed 10 ounces. Then you measured the distance and it was 20 inches from the CG (big plane). Then you measured the distance in the nose exactly where the ballast would go, and that was 5 inches fore of the CG. Then here's your formula:
10 oz x 20"=5"x
Solve for x:
x=(10)(20)/5=200/5=40
So at 5 inches forward of the CG, you should require 40 oz to balance your plane. The fore moment arm has to equal the aft moment arm.
I do want to buy a scale on e-bay or someplace though, since I do so much building. If I do, I'd be happy to share it.
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Post by Lt Swiss on Nov 2, 2009 20:49:18 GMT -5
Awesome... I thought it would be more complex math. Even I can handle this formula. I am going to try it... I would love to get one of those cool CG Machines too. Here's the standard... www.greatplanes.com/accys/gpmr2400.html  |
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Post by bearcat on Nov 4, 2009 13:28:34 GMT -5
Some biplanes, like the Sopwith Camel for example, end up kind of heavy because they require SO much ballast in the nose. The original planes had short noses because of the very heavy rotary engine. So it's kind of hard to balance a scale bipe, unless it has a longer nose like the SE5a or the Jenny. Even those need some extra nose weight. And if you want to fly a tail-heavy plane, you've got no choice but to ballast the nose.
My Jenny turned out a little heavy once I ballasted the nose. I had to put quite a bit of lead in the front, which I wasn't happy about. I'm sure it would still fly, but like I said in another thread, I'd prefer to save it for a lower altitude flying field, at least for the maiden flight.
Another idea might be to gas-power a plane like that, because the gas motors are a chunk of metal and provide more balast probably than you would get out of an electric motor.
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