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Old 03-04-2012, 08:54 AM   #1
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Default Balancing a low wing plane

Hi all

Just wanted to check out some opinions with you.

I have just completed my latest project, a low wing warbird, 56 inch span, tapered wing, and I am at the balancing stage.

I normally balance low wing planes upside down on my balancer, positioned on the recommended balance points about 1 inch from the fusalage on either side.

So, I get the plane balanced, and I did another check with the plane the right way up using the same balance point positions but on the underside of the wing.

Now its way out of balance. I am guessing there is a good reason for this, but I am interested in which gives the more accurate result.

Balance upside down or right way up?

Would appreciate any input.
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Old 03-04-2012, 09:03 AM   #2
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if you are balancing so that the fuselage sits perfectly level (as you should do) then either top or bottom will give identical results. The centre of gravity of a plane does not move when you turn the plane upside down.

For practical purposes it's easier to balance a low wing plane inverted because it will sit stable when balanced, not tend to tip one way or the other as you would find if balancing upright... So inverted is the usual way to do it, but the actual CoG position is the same either way.

Steve
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Old 03-04-2012, 09:32 AM   #3
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Hi Steve

Many thanks for the quick response.

I must admit I was expecting the balance to be the same both ways up, but as I mentioned, if I balance the plane perfectly level upside down, when I test again the right way up is is definitely nose heavy.

If I then rebalance the plane the right way up, and then check it upside down it has become tail heavy.

Just not sure which is the best reading to use.
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Old 03-04-2012, 10:19 AM   #4
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Lipo.. it really is impossible that the centre of gravity of the plane has moved due to turning the plane upside down. If you support the plane on the longitudinal CoG location it will balance, regardless of if it's inverted or right way up. I suspect what's happening is that when you balanced inverted the model was hanging ever so slightly nose down, it only takes a few degrees to make a difference. hanging nose down means that the true longitudinal CoG location is actually further forward than the point where you are supporting the model. Then when you try it 'right way up' the model tips to the nose.

Anyway.. if in doubt err on the side of slight nose heaviness.

Steve
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Old 03-04-2012, 01:31 PM   #5
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Just realise that the CG is a point in space, not even necessarily within the the physical object being checked. If that point is above the two points you are suspending the object from, any slight movement about the suspension points will put the CG either fore or aft of the points of suspension and the object will then continue to rotate until the CG is directly below the points of suspension. Now if you hang the object from one point and project of vertical line from that point then suspend it from a different single point and also project a vertical line from that point, where the two vertical lines intercept is the true CG no matter what the shape of the object is.
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Old 03-04-2012, 02:28 PM   #6
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If you have it right with the plane inverted (or right side up), then you're done. Close enough. Go fly it and see how it behaves. You probably measured differently on the two sides. Personally, I don't even measure warbirds. I just balance them on the spar. Hasn't missed yet.
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Old 03-04-2012, 04:23 PM   #7
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Lip - I have had the exact same findings. But only on some low wings. Usually they are done inverted just so the landing gear is not in your way. And I agree with every single one that has posted .. CG is CG upright or not. But, as I said, I've had the same issue as you. And I still don't know why. It isn't a little bit different, it seems to be a lot. I carefully checked the last one, right side up, and it flew fine, just a bit of normal trimming needed.
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Old 03-04-2012, 06:39 PM   #8
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Think about it ... when inverted - the fuselage is below and balance is easily sorted. It actually takes a reasonable amount to offset.

Turn it right way up - now the fuselage is above and to get an accurate CoG is very difficult. The slightest touch and away the model goes. Ultra sensitive.

I know wome will argue with this - but I submit that finding CoG with a low winger right way up is very hard and easy to get wrong. The inverted method is accepted by majoroty as the easier way to go.

For those interested .......... has anyone applied same two-way checking of a HIGH-winger ? Same result, but no-one mentions it or even checks it ? Because no need.

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Old 03-06-2012, 05:30 PM   #9
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Even more fun then that, a bipe or tri plane.
On a tri plane you would have 3 CG points normally, as the wings tend to be staggered.
Actually the CG point stays the same, it's where it's located on each wing that varies.

Balance on the center wing and it's perfect. Take the same measurement on the top wing and it will fall on it's tail, bottom wing would be the exact opposite.

The CG point stays the same it's wing location moves dependent on the wing location to the fuse.

Some planes you can "cheat", instead of adding or subtracting weight, you move the wing fore and aft to balance. Like on a Slow Stick for example.

When I die, I want to go like my Grandfather did, in his sleep...... Not screaming like the passengers in his plane.
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Old 03-07-2012, 01:39 AM   #10
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This method will work for any configuration.

http://home.mindspring.com/~the-plum...%20Machine.htm
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Old 03-07-2012, 02:33 AM   #11
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If you are wise you balance low wing planes inverted. High wing upright. Mid wing your choice - I usually do inverted.

It is wise because it allows you to take advantage of the pendulum effect. Even the experts agree....

See the pic from the GP balance stand.


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Old 03-07-2012, 11:28 PM   #12
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Balancing a low wing from the bottom of the wing creates an unstable balance point because the weight is on top of the cg wanting to make the plane fall in either direction. Put the weight under the cg and it creates a stable balance point.
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Old 03-07-2012, 11:57 PM   #13
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Alternatively you don't have to actually balance it at all. Just stand the plane on it's landing gear, if it a tail dragger put something under the tailwheel so that the fuselage is sitting perfectly level.

Insert a scale under the mainwheels (or for a large plane under each mainwheel wheel in turn, keeping the plane level at all times). Record the weight. Now put the nosewheel/tailwheel on the scales, again keeping the plane level.

The total of all readings should exactly equal the weight of the plane (obviously).

Take a datum point near the front of the plane such as the tip of the spinner or whetever. Measure back from this datum horizontally to the mainwheel axle and multiply that distance by the weight measured at the mainwheels. Now measure from the datum to the tailweel/nosewheel and multiply that distance by the weight measured on the tailwheel/nosewheel. You have now calculated the moments around your datum point due to the reaction loads on the wheels.

Add the two resultant moments together and divide by the overall weight of the plane. The answer you get is the precise distance from your datum point to the plane's actual C of G.

This is how C of G position is established for all real airplanes, from microlights to Super-jumbos.. so it's highly accurate. It's especially good for doing large models that would be difficult to support and balance on your thumbs, it also saves investing in fancy C of G machines or such like.

Steve
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Old 03-08-2012, 02:02 PM   #14
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Steve - with all due respect I say NO WAY am I going to go through all of that. Same with the Vanessa device. I need simple things, I am a simple man.
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Old 03-08-2012, 04:48 PM   #15
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Originally Posted by JetPlaneFlyer View Post
Alternatively you don't have to actually balance it at all. Just stand the plane on it's landing gear, if it a tail dragger put something under the tailwheel so that the fuselage is sitting perfectly level.

Insert a scale under the mainwheels (or for a large plane under each mainwheel wheel in turn, keeping the plane level at all times). Record the weight. Now put the nosewheel/tailwheel on the scales, again keeping the plane level.

The total of all readings should exactly equal the weight of the plane (obviously).

Take a datum point near the front of the plane such as the tip of the spinner or whetever. Measure back from this datum horizontally to the mainwheel axle and multiply that distance by the weight measured at the mainwheels. Now measure from the datum to the tailweel/nosewheel and multiply that distance by the weight measured on the tailwheel/nosewheel. You have now calculated the moments around your datum point due to the reaction loads on the wheels.

Add the two resultant moments together and divide by the overall weight of the plane. The answer you get is the precise distance from your datum point to the plane's actual C of G.

This is how C of G position is established for all real airplanes, from microlights to Super-jumbos.. so it's highly accurate. It's especially good for doing large models that would be difficult to support and balance on your thumbs, it also saves investing in fancy C of G machines or such like.

Steve
Unfortunately models rarely are like their full size counterparts ... where the U/C is maintained on its correct designed point. How often do you bend main legs on your models to get tracking / sit of model correct ....

The wing on top, two pencils or fingers method is the simplest and has worked for years ...

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Old 03-08-2012, 04:49 PM   #16
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Originally Posted by dkrhardy View Post
Steve - with all due respect I say NO WAY am I going to go through all of that. Same with the Vanessa device. I need simple things, I am a simple man.
We ought to start the "Simple Mans Club" ... as I totally agree with you !

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Old 03-08-2012, 06:07 PM   #17
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Originally Posted by solentlife View Post
Unfortunately models rarely are like their full size counterparts ... where the U/C is maintained on its correct designed point. How often do you bend main legs on your models to get tracking / sit of model correct ....

The wing on top, two pencils or fingers method is the simplest and has worked for years ...
Solent it really doesn't matter at all where the wheels are. After you check the CG (by any method) you can bend the wheels wherever you like, it wont change the CG. I totally agree that for smaller to medium size model two fingers is fine, that's all I use, but if you have a 20+lb low winger you try balancing on two fingers. As for it being 'complex'.. I guess that's a matter of opinion. All it takes is the ability to use a tape measure, scales, and to add multiply and divide simple mumbers. Anyone with basic numeracy should have no problem, my 11 year old child could do it easy enough. But as I said right from the get go, I'm not really suggesting this method for anything other than models that are too big and/or awkward to do the usual way. I'm really just throwing the idea out there, if you use it is entirely up to you.

Plus it's the only method endorsed by the FAA: http://www.faa.gov/library/manuals/a...-h-8083-1a.pdf
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Old 03-09-2012, 08:43 PM   #18
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"Simple Mans Club" Ilike it. From this point on it shall be SMC so no one else has to know what we really are. Jeeez, you think anyone might read this other than us?
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Old 03-19-2012, 11:18 AM   #19
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[QUOTE=JetPlaneFlyer;860480]Solent it really doesn't matter at all where the wheels are. After you check the CG (by any method) you can bend the wheels wherever you like, it wont change the CG................./QUOTE]

On my foamies where they are lightweight models - the sit and position of wheels does make a difference. I agree that it will not be enough to upset the plane in flight - but they do affect the COG.
On my EDF - the wheels make up 54gr of the AUW ... 28gr of that is in the nose wheel. That's on a 450gr 50mm EDF model.

Ideally the wheels should be close to CoG to allow pitch to work when on a ground run ... but often we bend mains fwd to stop the dreaded nose-over on taildaggers, bend mains back on 3-pointers to stop the tail rub ...

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Old 03-19-2012, 06:26 PM   #20
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Originally Posted by solentlife View Post
On my foamies where they are lightweight models - the sit and position of wheels does make a difference. I agree that it will not be enough to upset the plane in flight - but they do affect the COG.
On my EDF - the wheels make up 54gr of the AUW ... 28gr of that is in the nose wheel. That's on a 450gr 50mm EDF model.

Ideally the wheels should be close to CoG to allow pitch to work when on a ground run ... but often we bend mains fwd to stop the dreaded nose-over on taildaggers, bend mains back on 3-pointers to stop the tail rub ...
If the wheels get bent back it will ever so slightly shift the CG but I fail to see how that has any relevance to how you measured the CG in the firstplace (which was after all what my post was about)

If after locating the CG you go moving stuff around on the plane (the wheels or anything else) then the CG will move but that's true regardless of what method you used to measure CG position in the first place. My suggestion of using full size airplane practice to work out where your plane's CG will work fine regardless of where the wheels are, that's the point I was making.

Steve
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Old 03-19-2012, 07:04 PM   #21
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Originally Posted by JetPlaneFlyer View Post
If the wheels get bent back it will ever so slightly shift the CG but I fail to see how that has any relevance to how you measured the CG in the firstplace (which was after all what my post was about)

If after locating the CG you go moving stuff around on the plane (the wheels or anything else) then the CG will move but that's true regardless of what method you used to measure CG position in the first place. My suggestion of using full size airplane practice to work out where your plane's CG will work fine regardless of where the wheels are, that's the point I was making.

Steve
Fine - but go back and read your post rebuffing my wheels claim. You wrote :
Solent it really doesn't matter at all where the wheels are. After you check the CG (by any method) you can bend the wheels wherever you like, it wont change the CG
Which you have now agreed is NOT correct, agreeing as I said. I'm not bothered about the method, but my contention is that there are factors that are peculiar to models that are not to full-size.

To be honest as long as we get to a workable CoG - I don't give a monkeys how you do it... but two pencils or sticks do fine for me and I reckon about 99% of modellers .... I've even done such on large models .. over 10lbs, but not pencils !! rounded posts.

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Old 03-19-2012, 08:13 PM   #22
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Originally Posted by solentlife View Post
Fine - but go back and read your post rebuffing my wheels claim.
You got me Solent, bending the wheels could in theory make some marginal difference to CG position. But as you said, in practice, not enough to matter. If splitting hairs I should have said "wont change the CG significantly"

As i said, my post was not about how the change the CG, it was about how you measure it in the first place. The CG 'is where it is' so the 'taking moments' method is as valid as any other be it for a model or a full size A380 Airbus... But I'd be the first to agree it's far too much fuss for all but very large models.

I think on that we may actually agree?
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Old 03-19-2012, 09:05 PM   #23
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Originally Posted by JetPlaneFlyer View Post
You got me Solent, bending the wheels could in theory make some marginal difference to CG position. But as you said, in practice, not enough to matter. If splitting hairs I should have said "wont change the CG significantly"

As i said, my post was not about how the change the CG, it was about how you measure it in the first place. The CG 'is where it is' so the 'taking moments' method is as valid as any other be it for a model or a full size A380 Airbus... But I'd be the first to agree it's far too much fuss for all but very large models.

I think on that we may actually agree?
Och Aye that we do ...

On the subject of 380 Airbus ... with or without wings !!

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Old 03-20-2012, 03:47 AM   #24
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Steve, I like your method. The only question I have is; do the scales have to be real precise? The ones I have tend to be a few grams off. I suspect they would be in the ballpark.
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Old 03-20-2012, 07:20 AM   #25
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Originally Posted by Iron bottom View Post
Steve, I like your method. The only question I have is; do the scales have to be real precise? The ones I have tend to be a few grams off. I suspect they would be in the ballpark.
A few grams shouldn't make much difference unless the plane itself was a super micro, in which case your probably using the wrong method anyway. Just make sure that the fuselage is perfectly level (chock up the tailwheel on a tail-dragger) before you measure the loads on the wheels.

I cant take any credit for the method, it's how they have being doing real planes for 100 years!
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