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Yet Another CG Post ...

Old 03-29-2010, 05:23 PM
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SeamusG
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Default Yet Another CG Post ...

Some days my head hurts and some days it doesn't. Today my head hurts.

The picture below is very accurate profile of an LT-40.
    [*]The datum line (red line) is at 0 degrees or horizontal.[*]The wing incidence (green line) is at 1.5 degrees positive per the plans.[*]The CG range is from 3 1/2" to 4 1/4" behind the LE.[/list]What is considered a nose heavy plane? What is considered tail heavy?

    If the plane balances (datum line horizontal) anywhere behind the center of the CG range (3 7/8") is it considered tail heavy?
    And likewise, if the plane balances anywhere in front of the center is it considered nose heavy?

    Just askin' ....
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Old 03-29-2010, 05:37 PM
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dredhea
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Default RE: Yet Another CG Post ...

Actually, you have it backwards. If you need to move the balancer back to make the plane balance, then the plane is tail heavy, meaning just like it sounds, the tail is heavier than the nose when attempting to balance at a given point. As for the "Where should it balance?" question, that depends on your experience. If you are new to flying, I would recommend that the plane be balanced slightly ahead (1/4" to 1/2") of the center of the range. This will help to make the nose "Stick" on landing ( less likely to bounce back into the air). The farther back the plane balances, the more sensitive it will be to elevator input, to the point that the plane will become unflyable. Above all, NEVER go outside of the recommended range. I follow the old adage, " A nose heavy plane flies poorly, a tail heavy plane flies once."
Old 03-29-2010, 05:44 PM
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Default RE: Yet Another CG Post ...


ORIGINAL: dredhea

Actually, you have it backwards. If you need to move the balancer back to make the plane balance, then the plane is tail heavy, meaning just like it sounds, the tail is heavier than the nose when attempting to balance at a given point. As for the "Where should it balance?" question, that depends on your experience. If you are new to flying, I would recommend that the plane be balanced slightly ahead (1/4" to 1/2") of the center of the range. This will help to make the nose "Stick" on landing ( less likely to bounce back into the air). The farther back the plane balances, the more sensitive it will be to elevator input, to the point that the plane will become unflyable. Above all, NEVER go outside of the recommended range. I follow the old adage, " A nose heavy plane flies poorly, a tail heavy plane flies once."
dredhea - I cheated - I editted the post after I read the darn thing. Yea, if the CGis behind the "range center" point it's tail heavy and conversely if the CGis ahead of the "range center" it's nose heavy.

As this will be a used as a club trainer I'm looking at "nose heavy".

I'll target somewhere near the 3 1/2" mark.

Thanks for your input.

BTW - what technique(s) would you use to balance this plane? Place pieces of tape to mark the CGrange? A dab of epoxy at the front and rear of the range? Enquiring minds are soooo dangerous.

Old 03-29-2010, 07:35 PM
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Default RE: Yet Another CG Post ...

The heck with the balancing...how did you make that diagram?
Old 03-29-2010, 07:44 PM
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Default RE: Yet Another CG Post ...


ORIGINAL: tailskid

The heck with the balancing...how did you make that diagram?
I scan the plans, convert the images to a 2D structure using Microsoft Visio. They are all to scale within 1/16". I can recreate the plans using a printer. They can be converted to dxf files for CAD parts cutting. Mostly Iuse the big views to play with covering schemes.

pic 1 is a 2D of a 4Star60 wing
pic 2/4 are the prototypes of the current covering job

Yea - I need to get a life ...

Oh yea, I lay out rolls of covering colors and drag-n-drop the covering pieces onto the rolls to figure out how much covering I need and get an idea how to keep the scrap to a minimum.

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Old 03-29-2010, 08:02 PM
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Default RE: Yet Another CG Post ...

Wow! Now that's progress.....ya did good!
Old 03-29-2010, 08:07 PM
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Default RE: Yet Another CG Post ...

But keep in mind that "Nose-Heavy" and "Tail-Heavy" are relative terms.

What is perfect to my dad is nose-heavy to me, and what's perfect to me is tail-heavy to my dad.

Of course, there are extreme limits to both, but generally anywhere withing the "range" is a safe place to fly.

My rule of thumb is: Slightly nose down at the center of the range. Then fly it and adjust from there
Old 03-29-2010, 09:21 PM
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Default RE: Yet Another CG Post ...


ORIGINAL: SeamusG

BTW - what technique(s) would you use to balance this plane? Place pieces of tape to mark the CG range? A dab of epoxy at the front and rear of the range? Enquiring minds are soooo dangerous.

I use a marker on the plastic cover, and then, cover the mark with 3-M transparent tape.

The location of the CG or center of weight is about the stability margin, or horizontal distance between the CG and the center of lift and drag.
The CG must be located ahead of that center of drag, the more forward the location is, the more stable in pitch the model is (but the harder the elevator has to work).
Old 03-29-2010, 09:46 PM
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Default RE: Yet Another CG Post ...

MinnFlyer - so that I understand what you just said - if the plane is supported at the center of the CG range (3 7/8" from the LE in the case of the LT-40), then the angle of incidence of the wing will no longer be 1.5 degrees positive (up) but may be 0 degrees or 1.5 degrees negative (down)?

Most of my local experts just look at the plane when supported and say nose-heavy or tail-heavy. Not very accurate. We have no sheltered area at the field to field balance planes - too much wind & gusts so I do it "in the lab".

My method of madness for checking CG is to put a Robart incidence meter on the wing and support the fuse such that the meter reads 1.5 degrees (in the case of the LT-40). The meter is removed. Then I raise the plane from the rest and lower it back while supporting it with my finger tips at a specific location (target) that is within the CG range. It is very easy to sense whether the nose or tail hits the rest first. I just keep adding / removing heavy pellets until both ends hit the rest simultaneously.

lnewqban - I just found out that my 3M tape is 3/4" wide - exactly the width of the LT-40 CG range. I did just what you said - easy to feel with "sanded" finger tips. Thx.
Old 03-29-2010, 11:04 PM
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Default RE: Yet Another CG Post ...

If I remember....the LT40 should balance on the main spar. Don't worry about incidence - that should have been built 100% accurately just by following the instructions. If you pick up the plane by the spar and the tail drops, it is tail heavy. If the nose drops it is nose heavy - as long as the nose is level or slightly down - GO FLY!
Old 03-30-2010, 05:37 AM
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Default RE: Yet Another CG Post ...

Seamus, Yes. But don't use an incidence meter, it adds weight that won't be there when the plane is flying.

When I say, "The plane shoud sit at a slightly nose-down angle" what I mean is that it should sit in such a way that you can tell it is nose down and not level. So we are only talking a degree or two at the most.

At this point, the plane is "safe" to fly. Once you fly it, you can decide if it flies the way YOU like it.

For example, on a tail dragger, one of my tests is: Will it make a 3-point landing? As it's about to land, can I drop the tail enough that it lands on all 3 wheels at the same time, or does it refuse to drop the tail without gaining altitude? This is a sign that it needs more tail weight (or have weight shifted back).
Old 03-30-2010, 06:14 AM
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Default RE: Yet Another CG Post ...

I agree with MinnFlyer, while balance is important, it's not thatcritical. We have a youth program with several LT-40's. We check the balance by picking the plane up with our fingertips under the main spar, if the nose tips slightly down (slightly is key) it's good to go. And even though we haven't specifically used MinnFlyer's three point test (we convert most of the LT's to tail draggers) the "feel" of the plane on landing tells us if it's right.
Old 03-30-2010, 06:27 AM
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Default RE: Yet Another CG Post ...

WOW! and I thought that I had too much time on my hands. LOL . I wish I had the time to be this thorough. I probably waste about a third of all the covering I use, but I have carpal tunnel damage and I need a big piece of covering to grab onto to pull around corners and whatnot. Tried pliers, but I kept tearing the covering.
Old 03-30-2010, 09:44 AM
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Default RE: Yet Another CG Post ...

Minn - ya missed the quote "The meter is removed".

This exercise with the LT-40 is more practice than anything. I believe practice makes perfect. While it's not real important to accurately set the CG on a trainer I want to develop an approach that will work with more performance oriented planes. Guess that I've been reading too many Dean Pappas articles about trimming.
Old 03-30-2010, 12:10 PM
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Default RE: Yet Another CG Post ...

Yes, there is no "Scientific Method". This is why they give you a "Range" to shoot for instead of an exact spot.

I have even seen two identical planes that balanced differently to get the same flight characteristics
Old 03-30-2010, 12:32 PM
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Default RE: Yet Another CG Post ...

What I'm hearing is that ya get it close on the bench then fly-n-trim as an iterative approach. If there are any "static" adjustments that can then be made, do it on the bench, then repeat the fly-n-trim approach.

Thanks all for you input.

Old 03-30-2010, 12:52 PM
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Default RE: Yet Another CG Post ...

Yup
Old 03-30-2010, 02:36 PM
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Default RE: Yet Another CG Post ...

Hi!
If you want to learn how to set the C of G on more performance oriented planes ...Well you do it with your fingers under the wing! Simple as that! All straight winged planes with constant cord wings have the C of G at around 25-30% of the wing cord! Set the fingers there (under the wing) on both sides of the fuselage and see what happens...If the plane nose-dips slightly ...it's nose heavy! If the tail dips slightly...it's tail heavy! This method has been used for years by many of us experienced fliers...been around this hobby/sport for 35 years
When you gain experience you can even see directly where the C of G should be on most airplanes regardless of the wing shape by just looking at the plane!
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Old 03-30-2010, 04:16 PM
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Default RE: Yet Another CG Post ...

All of thease answers have been correct. As long as you're in the recommended range, you'll probably be alright since the recommended C of G range is usually a little on the conservative (aka nose-heavy) side. I was a bit surprised when I said never go outside the recommended range that some of the 3D guys didn't harpoon me. They routinely set the C of G back farther than is recommended by the designer. It helps to keep the tail under them in hovers and is the reason the planes snap around as quick as they do. I should have specified that you shouldn't go outside the range on a trainer. Trainers are rarely used for 3D flying. My 2cents on setting the C of G on a high performance airplane: Start someplace inside the recommended range, fly the plane and see how it flies, move the C of G back 1/8", repeat. Keep moving the C of G back until the plane starts to get a little too squirrely for your taste, then move the C of G forward 1/8". The plane should have the maximum elevator response that you can comfortably control. Mark this spot for future reference. I've never seen a CG Machine at the field. Fingers serve the purpose.
Old 03-30-2010, 04:39 PM
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Default RE: Yet Another CG Post ...

I gotta a big smile on my face when I type this "My fingers are FATTER than 1/8". Maybe I can take my knife and whittle the points of my index fingers down a bit. When it comes to the small incremental changes that you're talking about do you need some "hardware" help? If so - what kind?
Old 03-30-2010, 07:38 PM
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Default RE: Yet Another CG Post ...

Once I balance a plane, I rarely check the CG again (unless I change and engine or something)

Once I have flown it, I might think it need tail weight, so I'll shift the battery back if possible, or if everything is tucked away nicely, I'll just add a 1/4oz of lead to the tail (it's not a sin - really)

I'm not going to check the CG again because I don't CARE where it is. the plane needed tail weight, so I added it.

Now I'll fly it again, and maybe I'll add another 1/4oz to the tail. I just keep doing that until the plane flies the way I like.

Once it flies properly, I don't know where it balances. I don't care where it balances. The point is - it balances! That's all that's important.

Now... That having been said, the one time I WILL check the balance is if I have a plane that flies very well and I want to make a major change, like put a bigger engine, or something that I know will change the CG. Now I will balance it first to find the CG and rebalance after the change to make sure it's in the same place.
Old 03-30-2010, 07:38 PM
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Default RE: Yet Another CG Post ...

Remember a nose heavy plane may fly poorly, but a tail heavy plane only flies once [sm=cry_smile.gif]
Old 03-30-2010, 09:14 PM
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Default RE: Yet Another CG Post ...

1/8" at a time? you must be kidding.
Old 03-31-2010, 07:21 AM
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Default RE: Yet Another CG Post ...

On a trainer or a throw-around, I follow MinnFlyer's method above, never checking a second time, but adding weight or shifting the battery as needed until I get the right "feel". If I have a more precise flyer, I check the CG at home using the plumb-bob method and after getting the CG set to the proper starting point, I add enough weight to the tail to move the CG back 1/8", usually 1/4oz or so, depending on the plane. I now know that 1 unit of weight moves the CG back 1/8", so if I add weight at the field I know that the CG is moving 1/8" for every unit I add (yes, it linear). If I add enough weight to move the CG back, say 1/2", when I get home I remove the weight and try moving stuff back to move the CG without the weight. I then double check to make sure that the same amount of weight moves the CG the same distance as it did on the first go-round (it rarely changes) and flight test again. I keep adding weight until I don't like how touchy the plane is getting. I then remove the last weight I added I guess I should have described my method since I know that I can't measure the CG that accurately with my fingers. With my method I don't have to, I know where the CG is moving to beforehand.
I'll explain the "Plumb-bob" method since I know that somebody is going to ask. Attach a long string to the rear of the plane and to the front of the crankshaft or spinner. Make sure the string is strong enough to support the weight of the plane. Run the string over a cuphook (or other hardware) in the ceiling. Loop it once around the cuphook if you need to reduce slippage. Suspend a plumbob from the same point and adjust the length so that it nearly, but not quite touches the plane. Adjust the plane so that it is level (use this same method to balance laterally first). When the bob stops moving, it will be pointed at the CG, hopefully it's where you want it. If not, adjust the CG until the plumbob points to your desired CG when the plane sits level. Now add 1 unit of weight (1/4 oz) to the tail. Relevel the plane, wait for the bob to stop swinging and mark where it points to. Measure the distance between the marks. Remove the weight to move the CG back to it's original position. At the field, you can now add weights one at a time and the CG will move whatever distance you measured for each unit of weight you add. The only real advantage of using this method over MinnFlyer's "add weight until it flys right" method is that you will know where the CG is. As MinnFlyer pointed out however, if the plane is flying the way you want, do you really care where the CG ended up?
Old 03-31-2010, 11:00 AM
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Default RE: Yet Another CG Post ...

Em - 20 to 40 mph wings today AND I have a sling set up in the basement for lateral balancing. Guess I'll take my carpenter's plumb bob and add it to the mix, stir well (not shaken) and see what I end up with.

Thanks for the ideas.

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