# Balancing staggered wings

My friend needs to balance his Ultimate biplane. As you may know, the Ultimate has swept back staggered wings.
The problem is, the indicated CG on the plan has it just about 1/3
(33%) from the LE under the top wing.
I had thought that for a staggered wings plane the CG has to be at the average 1/3 of the two wings.
Which is correct, 1/3 under the top wing or the average of 1/3 of both wings?
Wan
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Why does your friend not believe the plans? If nothing else, it's a great starting point -- you know, like when all else fails, read the instructions :-)
Cheers -- \_________Lyman Slack________/ \_______Flying Gators R/C___/ \_____AMA 6430 LM____ / \___Gainesville FL_____/ Visit my Web Site at www.LymanSlack.com

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Lyaman, you're right about reading the instructions, but only the CG was drawn in the plans without commentary. If my friend was to go by the given CG in the plans, he'd have to add 36 oz. or 2 1/4 lbs. of weight in the nose section. He would like to avoid dead weight.
If he could use the already balanced CG at the average of 1/3 of both staggered wings, he need not add weight, just some adjustments by moving some inside components.
He is very nervous about getting this correct on the Ultimate, as he had crashed two planes recently.
Wan
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I put 26+OZ in the nose of my , yet to be flown, GP Super Skybolt! I'm understanding your buddy! mk

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I have a DPM Ultimate and it balances between 50% to 60% of the center cord on the top wing. The 60% setting is needed for torque rolling maneuvers. The 50% settiing is quite docile. I think you can use the same ballpark numbers regardless of your model size as lon as it is fairly close to scale. Sizewize(sp?) the DPM Ultimate is 60.5" span, 64.5" length, 1230sq in area, and weighs about 10.5lbs. Using a Saito 150.(180 would be better)
Phil AMA609
snipped-for-privacy@toast.net wrote:

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Don"t forget guys that the 1/3 back thing is for a clark Y airfoil(flat bottom)...the ultimate, if scale, has a fully symetrical airfoil which is not tapered but constant cord swept. If you are gonna err, do it on the nose heavy side that way it will just fly like a dog but at least be controllable.
Phil AMA609 (Lotsa biplanes here.)
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pcoopy wrote:

Actually it's not directly related to the airfoil -- see http://www.zenithair.com/kit-data/ht-90-4.html by a full-scale aircraft designer, who doesn't mention airfoils.
It _does_ relate to the size of the tail and the tail moment, so if you put on an oversized tail or if you put the tail way back then you'll be able to move the C/G back.
It is _indirectly_ related: Because the Clark Y center of pressure ranges around 33% of chord instead of the 'natural' center of pressure at 25% of the chord*, a successful craft with a Clark Y has to have a bigger stabilizer to overcome the moment; this bigger stabilizer means that you can move the center of gravity back.
I mostly agree with your recommendation, however -- as long as it's not so nose heavy that you have to land at 100mph to keep the nose up, when in doubt you should start with the thing nose heavy. Make that nose weight easily removable for test flying, however, and play around with it a bit -- try to sneak up on the right balance (don't overshoot by too much -- that may re-kit the airplane for you).
* The description I've heard is that every airfoil has three factors that impact the center of pressure and coefficient of lift in an un-stalled, subsonic condition: First, inside of every airfoil there's a symmetrical section trying to get out. The result of this is that every airfoil acts like there's a lift force at 25% of the chord that varies with angle of attack. As long as the wing isn't in stall and isn't too stubby this lift varies in direct proportion to angle of attack and airspeed squared. Second, because of camber there's an angle of attack offset, so a cambered airfoil may need a negative angle of attack to have zero lift. Finally, every airfoil has a torque that varies only with airspeed squared -- so a symmetrical airfoil wants to fly straight, while a Clark Y wants to pitch down. You can use this to advantage in a flying wing: by building an airfoil that wants to pitch _up_ and putting the center of gravity ahead of the 25% point you'll have a craft that's stable in pitch even without an elevator.
--

Tim Wescott
Wescott Design Services
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Tim,
Thank you for posting so often and contributed so much here. Also for the link to the full sized airplane designer, who doesn't mention airfoils. My friend is your basic model builder and has a very elementary knowledge of geometry. What he wants is to have someone to tell him "exactly" where the CG must be for his Ultimate.
However, we know that flying model airplanes is not an exact science. Still, it is very interesting to read all the posts here and I could use some of the pointers myself.
His plane is nearly finished. I don't like to see him discouraged from a bad crash if his plane is not properly balanced. As one person said, the drawing may be in error.
So, what should I tell him about the CG on the plans he purchased? Or should he go ahead and use the indicated CG on the plan and add the 36 oz. of lead?
Wan
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snipped-for-privacy@toast.net wrote:

So he makes it easy to remove when he's tuning it. There are numerous articles out there on telling where the center of gravity needs to go based on flight performance.
The test that the sailplane guys do is to get the thing trimed out at one speed, then dive. If it tends to pull out of the dive then the C/G is too far forward; if it tends to tuck under the C/G is too far back. This gives you neutral stability, which is just right if you're a good flyer with quick reflexes (trainers do _not_ put the C/G back so far). For an intermediate pilot -- put it in an intermediate spot.
--

Tim Wescott
Wescott Design Services
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Lyman Slack wrote:

The long story:
I recently put together an AT-6 Texan model. With about a 9.5" chord and positive taper the directions had me put the C/G at 1.5" back from the leading edge. That didn't seem right, so I ran the numbers and came up with a C/G about 3.5 or 4" back (that taper does make a difference). With the C/G toward the front of that range it was _way_ too noseheavy -- it ended up with a 1 ounce block of tungsten screwed onto the tailwheel mount, which brings the C/G back far enough that it handles nicely.
The short story: You can't always trust C/Gs in plans.
--

Tim Wescott
Wescott Design Services
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refer to Alan's Hobby, Model & RC Web Links http://homepages.ihug.co.nz/~atong / sub section : "Calculators, Conversion, Center of Gravity, Electric Flight , MAC & Servo Torque calculation charts."
regards Alan T. Alan's Hobby, Model & RC Web Links http://homepages.ihug.co.nz/~atong / .................................................................

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On 2 Apr 2006 14:06:36 -0700, snipped-for-privacy@toast.net wrote:

What brand and 'size' Ultimate is this ? Is it a kit or an ARF ? The reason I ask if it's a kit is that some people tend to build real heavy and can make for a tail heavy airplane. Many biplanes need a lot of nose weight...one that comes to mind for being notorious for this is the Sig Skybolt. 32 oz is a lot of weight for a .40 size aircraft but not so bad for a .120 size.
Also , is he putting the recommended engine on the aircraft or does he possibly have a a lighter one ?
I'm looking at plans for a .40 size Great Planes Ultimate and it shows the CG 2 3/8" from the LE at the fuse on the Bottom wing which looks to be about 25-28%. Don't have the aircraft to measure. If it would help , I have a book on RC Airplane design that shows a simple way to arrive at the CG. I could send you a sketch of it. Finding the CG is really not a complicated thing , but with this method you don't even have to know the wing area.
But , like most everyone else here , I would say follow the instructions , especially if it's a well known brand of aircraft. I can't recall ever having an aircraft that didn't balance where they said it should.
Ken Day
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Ken, keep that thought in mind. I won't see my friend until the Toledo Weak Signals Show (We live in Toledo). I do know he bought the plans and built his Ultimate from "scratch" and may not have selected the lightest balsa. The motor is a 150 cc gasolene engine and it could be on the light side, which may result in a tail heavy plane.
The reason I question the CG is there is a discrepancy between the amount of stagger of the wings on the plans and the actual built plane of at least 2 inches. That is, the plans show more stagger and the plane has less.
I would like to double check the measurements and come back for your further suggestions. I could be wrong.
Wan
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On 4 Apr 2006 23:52:57 -0700, snipped-for-privacy@toast.net wrote:

150 cc ?

Ken
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<snip>150 cc ?
Ken, you're right. I checked about the engine and it is a 3W 75cc gas burner. I had misunderstood my friend.
I still have to check the measurements of the stagger when I next see him.
Wan
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On 5 Apr 2006 18:05:14 -0700, snipped-for-privacy@toast.net wrote:

Wan , an airplane that requires a 75cc 3W would be in the neighborhood of 20 lbs or more. The weight your're speaking of to balance this thing is not uncommon at all. Didn't have any idea you were talking about a 1/3 scale or thereabouts.
Ken day
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Yes, it's a very large aircraft. But I think I will let my friend work it out by calling the designer of the plane about the CG. He had a rash of bad luck in the last couple of weeks, as he had crashed his Twist twice. The second time it was damaged beyond repair. Then the troubles he had with his Edge 540. Nearly lost it due to a glitch in the control system. When he thought he had replaced his warped tail feathers of the Edge, on his first test flight with the modifications, nearly lost control but saved it.
Thanks for all your help, but I don't want to suggest anyting to my friend that may lead to problems that may contribute to his already deep discouragement. And if he have problems after my suggestions with his Ultimate.....?
I had been looking into your site about your busy involvement in musical instruments and modeling history et all. You have been and still are a very active person.
With sincere thanks, Wan
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