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Where Is Correct Balance Point

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Where Is Correct Balance Point

Old 06-10-2010, 06:04 PM
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Homer712
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Default Where Is Correct Balance Point

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Can't believe that after all these years I'm asking this question. Possibly it's because I've always balanced my airplanes the old "put finger here" way and things sort of worked out. Now that I have a fancy tool to balance my airplanes I'm a bit confused.

If the plans/manual gives the balance location as 3 3/4" to 4 3/4", do you just pick the middle of the two locations? Only the GP Big Stik 40 that I have actually gives a "start here" location (3 3/4") and then a range of 3 1/4" to 4".

Would appreciate some advice before I start adding weight to my airplanes.

Old 06-10-2010, 06:17 PM
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Gray Beard
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Default RE: Where Is Correct Balance Point

The 3 3/4 will make the plane a bit nose heavy and the 4 3/4 will take it back into the area of being tail heavy. Where you put it is a pilots choice. I start mine out in the center then fly the plane to see if it should be moved one way or the other. Newer pilots will want there planes more on the nose heavy side. Tail heavy planes tend to be a bit touchy and want to stall quicker.
Old 06-10-2010, 06:21 PM
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LargeScale88
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Default RE: Where Is Correct Balance Point

I also use a tool for my balance instead of the holding it by your finger.

From what the plans on my at-6 said, and other planes, the more forward the CG, the easier it will be to fly. The most rear CG, it will perform quicker manuevers such as rolls ect. On my At-6, I have the CG in the more forward area, and it flew fantastic.

Good Luck.
Old 06-10-2010, 06:39 PM
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Villa
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Default RE: Where Is Correct Balance Point

Be careful. I thing Gray Beard gave you the numbers backward. Be careful, maybe I'm the one that is wrong. If you figure it out yourself, you will better understand the problem.
Old 06-10-2010, 07:03 PM
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Tarasdad
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Default RE: Where Is Correct Balance Point

Gray Beard has it right. I thought it was backwards myself for a minute until I visualized what he was saying. LS88 is also right in that a more forward CG will generally provide more stability where a rearward CG provides more responsive flight. In the past where no starting point was given I would generally start 1/3 back from the forward limit shown on the plans and adjust from there. This is just a personal habit, and not something recommended to be done. If possible I would seek info from someone who owns and flies the particular aircraft you're building to see what their experience is.
Old 06-10-2010, 09:00 PM
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Gray Beard
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Default RE: Where Is Correct Balance Point


ORIGINAL: Tarasdad

Gray Beard has it right. I thought it was backwards myself for a minute until I visualized what he was saying. LS88 is also right in that a more forward CG will generally provide more stability where a rearward CG provides more responsive flight. In the past where no starting point was given I would generally start 1/3 back from the forward limit shown on the plans and adjust from there. This is just a personal habit, and not something recommended to be done. If possible I would seek info from someone who owns and flies the particular aircraft you're building to see what their experience is.
Very true and great advice. I had to give that statement about the CG a second thought myself and run it through my minds eye, good thing I'm not too cockeyed. There is a very old addage, a nose heavy plane will fly poorly but a tail heavy plane will only fly once. My own planes are CGed nutral so when I go inverted they don't drop or need almost no down elevator, when someone else flys my planes they usually don't care for them. I do start out on the nose heavy side during flight testing then start moving things back until I hit the magic spot I'm happy with. Getting a plane into trim is fun but it takes a while most teh time. I use the CG machine on smaller planes but when they get up there in size and weight I hang them and use a plumb bob to locate the CG I want. The CG machine has served me for years and still works very well. There are better ones on the market today but the CG machine works just fine, no reason to spend more money when I have something that works.
Old 06-10-2010, 09:04 PM
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LargeScale88
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Default RE: Where Is Correct Balance Point

On my B-17 with removable outer wing panels, I just drilled a 1/4 hole at the cg, each end of the wing, and put a dowl in there with the dowels hooked up to a little PVC Pipe rig that I built, works perfect.
Old 06-10-2010, 09:09 PM
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thrushloader
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Default RE: Where Is Correct Balance Point

Hey Guys, I just got my first low wing trainer. Explorer 60L from nitro. Nowhere in the instructions does it give me a CG. Where do I start? Thanks for the help and fly safe. Thrushloader.
Old 06-10-2010, 09:41 PM
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TedMo
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Default RE: Where Is Correct Balance Point

If CG is not given a standard starting point is about 25% back of ld edge. Usually that is a bit nose heavy therefore you will be able to move it back after flight tests to suit your flying style.
Old 06-10-2010, 10:20 PM
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Default RE: Where Is Correct Balance Point

And that's only with a perpendicular leading edge to the lateral axis. Tapered or swept wings will give different results as you move away from the center of the aircraft.

The plan or with an ARF the C.O.G given in the manual is just a starting point. Unless you built exactly as the designer did and installed identical components in the same locations your results will vary. Start with the suggested balance point and then fly it to determine if it's balanced well. If you trim it for hands-off level flight and then pull up to 45ΒΊ at crusing speed and half-roll inverted then release the sticks does it hold the line? If the nose falls it's nose heavy. If it climbs steeper on it's own it's tail heavy. Knife-edge will tell you about tail-heavy also. When in doubt, start out nose heavy.

A very nose heavy airplane flies poorly. A very tail heavy airplane flies once.
Old 06-10-2010, 11:37 PM
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Gray Beard
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Default RE: Where Is Correct Balance Point

http://adamone.rchomepage.com/cg_calc.htm should take you to a very good on line web site for figuring your CG just by adding your information. If the site is still up??
Old 06-12-2010, 01:21 PM
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thrushloader
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Default RE: Where Is Correct Balance Point

Thanks everyone ! I think I have got it figured out. Thanks for all the good fast help. Thrushloader

Old 06-13-2010, 01:14 PM
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Default RE: Where Is Correct Balance Point

heck yea, graybeard has it right

daisy air guns... keeping kids off your lawn for a hundred years
Old 06-13-2010, 01:20 PM
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onebadhawk
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Default RE: Where Is Correct Balance Point

Graybeard,
Would you please read - Lipo's for a flight battery.
and tell me what you think.
Teddy
Old 06-17-2010, 07:01 AM
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Crazical
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Default RE: Where Is Correct Balance Point



Hi all.  I was hoping to get some CG advice regarding a new Pulse 60.  The plane is super nose heavy.  It's so nose heavy it can't even be held at or near the CG point using fingers.  I built it as per instructions, stock, but using a 75AX engine.  Moving the battery back doesn't help.  I'd really prefer not to add a lot of weight to the tail if possible.  Any suggestions as to what could be wrong or what I could do to fix it?  Many thanks!

Old 06-17-2010, 01:03 PM
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landeck
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Default RE: Where Is Correct Balance Point

I have an OS 61 FX on my Pulse 60. I moved the battery as far back as I could and then added about an ounce of lead to get it to balance. It did not hurt the flying to add the weight. The plane has been flying like a dream for a year.

Bruce
Old 06-17-2010, 04:22 PM
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Gray Beard
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Default RE: Where Is Correct Balance Point


ORIGINAL: onebadhawk

Graybeard,
Would you please read - Lipo's for a flight battery.
and tell me what you think.
Teddy
Well Ted, I'm not the person to ever ask any of the hard questions about batteries. I just built my first electric plane this year and it's only a two channel fun to fly thing. I do use the 3c 11.1V 2100mah lipo in it but I don't even have a charger for them, I had to borrow a friends Triton. If you go over to Wattflyer you can meet some of the nicest people in the hobby and learn pretty much everything you need to know about electrics and/or Lipo batteries and there care and feeding.
Sorry but it is really out of my realm. I had to go to the WF forum myself just to figure out how little I really know, they set me straight. Also, if you have a question for someone you may want to hit them with a PM, I don't subscribe to any threads, you get a hit on your email every time someone posts something. Really can clog up your mail!!
Old 06-18-2010, 09:17 PM
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mikekosatka
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Default RE: Where Is Correct Balance Point

Hey Greybeard,
It kinda scary when you start being considered a Guru, isn't it?
Old 06-18-2010, 10:56 PM
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Gray Beard
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ORIGINAL: mikekosatka

Hey Greybeard,
It kinda scary when you start being considered a Guru, isn't it?
Yep. Considering I have guys here on RCU I go to with problems and info. I'm at a bigger loss then most everyone when it comes to electric power. I am up enough on the old batteries but I'm just starting to learn enough to burn out things properly with these new fangled gizmos they call batteries today. I finally figured out how to charge one without burning down my house. I don't understand it but I can do it!!!
Old 06-19-2010, 07:27 AM
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da Rock
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Default RE: Where Is Correct Balance Point

When a range of CGs is given, and you balance toward the front location, you need to have your elevator deflection match the full amount suggested. Nose heavy models seem more stable because the elevator has to work harder to change pitch.

If you balance toward the rear location, expect the airplane to be quick to respond to the recommended elevator deflection. Matter of fact, expect to re-rig the elevator connection to get less deflection, as you probably won't need or want the over sensitive pitch response that comes with a more efficient elevator.

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