CG question?
 

When setting up my CG per the instructions, I have the assembled plane with no fuel, balanced on two dowels.
I can move the plane forward and back quite a bit and still be "balancing" on the dowels.
When I say "balance" Imean it doesnt fall off the dowels! The nose can be high or low but still "balance" on the dowels.

Would I be correct to use a small bubble level on the tail surface to balance the plane with the tail surface level?

Icant leave the bubble there to balance because of the weight but Iused it to get the tail close to level while the plane is on the dowels...

Ibelieve the CG is best determined while flying. I think My Tiger 60 is nose heavy but I dont realy have the experience to be sure?

Its flying ok but I notice when I do a roll it wants to dive quite a bit during the roll.

Iknow I dont want to be tail heavy!


Steve

RE: CG question?
 

You're right that the final changes are made based on the way the plane flies. When I first balance a model, I do it so it balances with the nose slightly down.

Things to do to check your CG in the air are:
Fly inverted and see how much elevator you need to keep the plane from losing altitude. Should be very little.
Fly the plane and pull up 45 degrees. Then roll inverted; the nose of the plane should gently start to fall, over a period of a few seconds. It should not immediately start to drop and level out.

RE: CG question?
 

Just place it on the dowels and stand to the side and look at it. It should sit slightly nose-down. How much? Just enough that you can see that it's not level.

Now fly it.

A roll really won't tell you if a plane is nose heavy. The two most obvious things (to me) are:

1) it won't slow down for landing

2) it won't do a spin - it will just sort of spiral down.

If it flies the way you like, leave it. If not, shift the battery, or add small amounts of weight until it does fly the way you like, but you really don't need to put it back on  the balancer - that's just to get the initial setup.

RE: CG question?
 

I dont have much flying experience but the Tigerwas trimmed to fly hands off by a buddy at the field.
Trim settings are: (looking at the readout on my Spectrum DX7)
rudder = centered.
Aileron = halfway to the right.
Elevator = more than halfway down (or back)

When Icheck lateral balance it is heavy on the right side / wants to roll to the right.

So even with this lateral balance I have to add right aileron trim to fly level.

Is this an unusual amount of trim?

I'm having a Blast with it so I'll just keep flying but these trim adjustments intrest me... somethings crooked / not aligned right.


Steve

RE: CG question?
 

Go up to Lowe's, Home Despot, wherever, and purchase any of the following that you don't have on hand:

1) About 20'-30' of rope
2) A 3-4' long 1/2" wooden dowel
3) A plumb bob
4) Some kite string
5) A 1/4-20 bolt about 3" long, and a 1/4-20 blind nut
6) A 1x2 board.
7) A small string level
8) A pulley
9) A hook


Then, [link=http://home.mindspring.com/~the-plumber/Vanessa%20CG%20Machine.htm]use all this stuff to build a Vanessa CG Machine[/link].

It's simple, elegant, inexpensive, and will determine the CG AND the impact of any amount of weight added anywhere on the plane for ANY airplane. About a month ago, just to prove it was possible, I balanced a < 4oz foamy on it, and JUST YESTERDAY balanced a GP Ultimate 1.60...and you better believe I'll be balancing the 102" Edge on it soon.

RE: CG question?
 

The Venessa CG machine is the way to go.  It's really inexpensive to build and I couldn't believe how easy it is to use when we did it yesterday.  If you want to get your CG set to a certain point, this is the way to do it.

RE: CG question?
 

Something does sound crooked, but it's no reason NOT to laterally balance it. And you say that your aileron trim is halfway to the right, but is the right aileron higher than the left? If so, then something IS crooked.

First, balance it laterally.

Then, remove the wing, set the ailerons straight, and hold the wing in the center with the LE facing you. Now, this is a little tricky, but you need to close one eye, and slowly roll the wing so that the Trailing Edge slowly rises above the "horizon" of the spar.

You need to glance back and forth between the left and right side of the wing and BOTH trailing edges should appear at about the same time. I am guessing that the left TE (The one on your right as it's facing you) will appear first - meaning that the left wing's TE is higher than the TE on the right wing.

Now, look at both sides of the wing from the tip and see if you can tell if the right wing is twisted with the TE down, or if the left wing is twisted with the TE up (Or a little of both)

Now (You may need an extra set of hands for this) hold the wing down flat at the center and twist it the way you want it to go (The covering will wrinkle). While you're holding the twist, heat the covering to get out the wrinkles (Top and bottom).

Do the "horizon" check again and repeat if necessary

RE: CG question?
 

There is nothing wrong with using this machine, and it's true that it IS very accurate, but I look at it like using a measuring cup to test that your coffee cup will hold exactly a cup of liquid.

What I mean by this is that balancing your plane is not something that you need to go to any great lengths to achieve.

The initial balance is the ONLY time you need to actually check anything - and even then, it only needs to be close (as long as it's on the nose-heavy side)

The reason I say this is: The manufacturer gives you a "range" in which to balance your plane. Depending on the size of the plane, this range can be .5" to 1.5". So where in this range should you balance it?

Let's say that two people have identical planes and they both are balanced in the middle of the range. They both fly them, but one guy wants his plane to be more on the nose-heavy side, while the other wants his to be more tail-heavy. There's no need to re-balance it, just add small amounts of weight and fly it again until it flies the way you like.

RE: CG question?
 

I'll have to disagree with you here, Minn...but I think it's only a matter of how people think, really.

You're absolutely right...there's no reason you can't simply be "in the range", and then fly, adjust, fly, etc until it's where you want it. To be sure, where the CG "should" be is very much a subjective thing.

However, for several reasons, I still prefer having and recommending the Vanessa:

1) Some of us are weirdos :) We prefer to make known changes to the CG, and then "measure" results, rather than "hunt around" for a CG we like. It's really six of one and half a dozen of the other, admittedly...I just happen to think in terms of "Eh...I think the CG needs to come back 1/2" " as opposed to "I think i need another ounce in the tail".

2) While I agree with you that "close enough" usually is, if we combine 'close enough" with "not quite in line" or whatever...we can get pretty far out of whack. For beginners especially, but really all of us, it's pretty easy to miss by a significant margin when balancing some of the 'tougher" planes. A big bipe, or a long nose moment that makes it tough to get fingers under the wings, or trouble telling when the plane's level as the OP noted...any of these things can have you miss by enough to make the first few flights "adventurous". The Vanessa takes that out of the equation, and gives us some real precision.

3) Probably the biggest thing I see (and it's often argued with) is the ability to "experiment" with the Vanessa. Take a new scratch build for example...we need to move the CG quite a bit forward, maybe. Can't just "go fly and then adjust". And putting "some weight" here or there, and then back on the balancer, then 'some more weight" or "move that weight", and then back up, etc etc...real PITA there. :) The Vanessa let's me examine ALL the possible ways to achieve that change of CG, without ever coming out of the harness. Just drop a single ounce at a known distance from the Cg, quick twist of the dowel to re-level the plane, and it's nothing but math from there on out. Or, if you don't want to do math, you can add/remove/move weight all day long, and never pull the airplane out of the harness.

One last little tidbit too...especially appropriate in the beginner forum, imo.

I've found with students...and heck, even my own kids...that NOTHING teaches airplane balancing better than this thing. It's a WONDERFUL tool for understanding that WxA=M, and how that relationship impacts the airplane...they can SEE the airplane, quite literally IN THE AIR, and even press/lift on one end or the other to see how changes in CG effect the sensitivity of tail surfaces, pitch and yaw, etc.

============

Like I say...I agree with you for the most part...CG really is something VERY subjective (Within, of course, the limits of the aircraft), and there's no need for perfection. If the Vanessa were a few hundred bucks, I'd absolutely agree that it shouldn't be used.

But with it being cheap (< $20 in material), easy to assemble and use, versatile as heck, AND a pretty darn fine educational tool, I can't help but recommend it for anyone in the hobby.

RE: CG question?
 

I don't know how many times I've read in all these forums about people having CG problems.  Many times they don't understand CG and what it actually does.  The Vennessa contraption will show them. 

Sure, it isn't something you'll use every time you fly a model, but understanding why something works is half the fun of making it work.  For me anyway.

So for me, I thought it pretty interesting when I saw it used for the first time this past weekend.  To each his/her own.

RE: CG question?
 

Everything you say is true, and I agree that it's a great tool. I'd never tell someone NOT to use it, I just like to point out that it's not critical, because a lot of people - especially newbies - can get something stuck in their heads that can get them upset over nothing.

A good example of this is prop balancing. I can't tell you how many times I have heard of a newbie who has been waiting to fly his plane, but his prop balancer hasn't arrived and he thinks that he can't fly his plane because he hasn't balanced his Master Airscrew 10x7 prop. :D

RE: CG question?
 

If you pull the plane up to about 45 degrees, roll over inverted, and continue on that line, the plane should stay at the same angle or just faintly fall off the 45 degree angle. If it climbs more, the plane is tail heavy. If the plane starts to sink, it's nose heavy.

RE: CG question?
 

That's not doing a roll, is it?

And your method won't work on anything but a fully-symmetrical wing.

RE: CG question?
 

Looking at my trailing edge it looks like both ailerons are crooked. as if all the CA hinges are not centered in the aileron.
The diagram is how it looks from the rear.
If the inboard edges of the ailerons line up with the wings TE. The outer edge of the aileron DONT line up with the "outter fixed wing TE".
The diagram is exagerated, the amount off is 1/16" - 3/32"....

On the venessa CG machine, looks like something good to play with but Istill have my original question. how do you determine the plane is level?


thanks Steve

RE: CG question?
 

trying that image again.

RE: CG question?
 

Oddly enough, that will have little affect on the situation. Don't look at the ailerons, look at the TE and compare it with the spar.

If it is warped, it will look something like this

RE: CG question?
 

Thanks Mike but I have to have someone else look at it at the field. Other than what I described everything looks pretty good.
Not to worried about it, Seems to fly good...for me anyway.

About using the vanessa to check CG.
If the stabalizer is supposed to be 0 degrees incidence,does that mean it should be level when the planes balanced? or is incidence only relative to the wing?
Ican eyeball nose up or nose down but if I'm using this device to be more accurate I have to know what is level?

Steve

RE: CG question?
 

Thought I'd bring this back to life instead of starting a new one, because it's in line with something we discussed earlier in the thread.

Call it an "alternative use" of the Vanessa, and the sort of thing that makes the machine, for me at least, an invaluable tool. Rather than using it to FIND the CURRENT CG, we're going to use it to help GET the airplane TO the CG we want.

============

I'm currently assembling WildHare's 102.5" Edge. At that wingspan, weighing in the 25 lb range, as you can imagine, this isn't an "easy" model to set/locate/experiment with the CG on. It's getting to be time to put the gear in. Batteries, Rx, Regulators, Smoke system gear, etc. The question this raises, of course, is WHERE to put this stuff? Sure...there's plenty of room...but do i need to be thinking "toward the rear", or "as much up front as possible" or what?

Enter the Vanessa, which, when used creatively, not only gives me "an idea", but can downright pinpoint where I need to be looking to put all this stuff, to within just a couple inches.

Here's how:

First, the airplane is hoisted off the table in the machine. A level is set on the foredeck astride the CG we're after (center of the wing tube in this case). Obviously, the level can go anywhere you wish, but astride the CG means it, itself, won't impact the balance of the model. Certainly, the level should be placed somewhere with a 0 degree incidence angle. This is true (or at least 'close enough") of the foredeck on most airplanes.
http://images49.fotki.com/v1520/phot...MG_0829-vi.jpg

Next, we rotate the shaft of the Vanessa to bring the airplane to such an angle that the plumb bob is pointing at the INTENDED CG. Note, we don't care at this point if the airplane is level while indicating that CG. We just want the machine pointing at that location. Indeed, in the 2nd picture below, you'll see that the airplane is quite tail high, or "nose heavy".
http://images50.fotki.com/v1524/phot...jpg?1245775102
http://images50.fotki.com/v1523/phot...MG_0832-vi.jpg

At this point, it gets pretty simple. We just start stacking our gear on various locations on the airplane. What we're after is to bring the airplane to level. Note that the indicated CG won't change...just the balance of the airplane will. In short, when we bring the airplane to level, the ACTUAL CG will be right where we INDICATED we wanted it before.
http://images50.fotki.com/v1530/phot...MG_0834-vi.jpg

All you need to do is move batteries, or smoke pump, or whatever you're working with, around on the airplane. Nose needs to come up? Move a battery back a bit, check the level. Nose needs to come down? Move the smoke pump forward a tad. Whatever works.

At this point, once the airplane is level, and indicating the desired CG, just take a picture or make a few notes.

VIOLA! Now you can begin installing gear with a high degree of confidence that, once everything's in there, the airplane will be at, or very near (well within a bit of wiggle room) your desired CG.

===================

Certainly, I agree with MinnFlyer's basic assertion...that CG is often more "worried about" than it need be. Anything within the usually conservative range provided by manufacturers will almost certainly fly acceptably well initially...and from there it's most definitely a subjective "feel" thing. You certainly need to decide what traits YOU like in an aircraft, and then experiment with weight and balance to achieve those...and your final CG may well be a good bit outside of the suggested range, or considerably different from what another modeler may like.

And, indeed, each of us will think about changing the CG in different ways. You may fly an airplane and think "Huh...that 'feels' like it might need an oz of weight in the tail" while another may think "that 'feels' like the CG should go back half an inch". Again, it's a subjective issue, and largely modeler dependent.

I did, however, want to revisit this topic, and give a "real world" example of how the Vanessa can be used in 'creative" ways to do more than simply determine where a particular airplane balances at a given point in time.

Hope someone somewhere gets some benefit from the idea. :)

RE: CG question?
 

As a side note, slightly off topic, but wanting to give credit where due.

That pile of "stuff" on top of the canopy is a homemade smoke system based on the principles MinnFlyer discusses in his excellent [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=455]Got Smoke?[/link] article. It has served me quite well in two previous gassers (a US 41 and G62) and will find a new home in the Edge.

It's made from a simple windshield washer pump, micro servo, and common RC switch, cost < $30 to put together, and smokes like a freight train. :)

RE: CG question?
 

All great replies to the means of getting proper balance, which of course is definitly important. One thing I'm not sure of is the manner you are doing your roll. In order to do consecitve rolls it requires a tap of down elev each time inverted as well as tap of up when upright. Timing is the thing here. You can't just hold aileron and expect it to continue a roll without loseing speed and therefore altitude. Proper CG is not a cure all for everything each manuever requires proper input also. Maybe I'm barking up the wrong tree but since all replies centered on the CG just thought I'd throw that in.

RE: CG question?
 

A suggestion for you. You mention the trim settings needed . I suggest you make these adjustmants mechanically with your linkage so you can bring the trims on Tx. back to center

RE: CG question?
 

gboulton, Nice picture showing the Venessa in use. I'll be assembling one this weekend.
I started the original post questioning what part of the plane should be level when balancing.
I see where you have put your level, I dont really have anything flat like that on my tiger fuse to go by.
I balanced the plane on two dowels which worked ok but then I was noticing I could have the nose down or up and still balance on the dowels.
So what part of the plane should be level when doing this. I guessed that with 0 incidence on my wing and tail, they should be level?
So I was putting a small level on the tail (supporting it with my hand because of the weight of the level)

Also, My canopy will be over my CG so I guess I'll put masking tape on it to make any marks... But I'll lose them as future reference points when I remove the tape. Why do they say to balance low wing models with the canopy down.

Like I had posted, I still think I'm nose heavy but dont have enough flying experience to tell for sure.

I was talking to some of the guys about doing flat spins ( watching them do it ) And got as high as I dared and tried a couple. My nose always pointed down, It was more of a spireling dive down than a flat spin.

I'm going to add 1/4 oz weights to the tail ( 1 at a time ) and see how it flys.

I'll use the venessa to see how I balance before and after adding weight. If I like the way it flys I'll then go in and move my equipment around.

Steve

RE: CG question?
 

Really, any part of the plane that's at 0 incidence will do. That's why i mentioned the string level earlier...if you know the wings to be at 0 incidence, then simply put the string level on...well...a piece of string *heh*, and then tape the string to the wingtip, with the string running through the centerline of the wing (presuming a symmetrical wing). or through the chord line if not. It's ok to be a tad off.

Other places I've used include the tail, the bottom of the fuse (hang the airplane upside down), the top of the vertical stab, or even pinstriping on the side of the fuse.

While it may not be exactly "perfect" you can, ime, use pretty much any line that "looks straight" through the long axis of the airplane.

This, imo, is probably not the best way to go about it. Others may disagree, but I feel that I can't be precise enough "supporting something with my hand" to accurately offset the weight of a level. Instead, when i've used the level at a point well away from the intended CG, I do a bit of very simple math:

W*A=M

Weight * Arm = Moment

Basically, all that formula says is that for a 50 lb kid to balance a 100 lb kid 10 ft away from the middle of the see-saw, the 50 lb kid must sit 20 feet from the middle.

Weight is exactly that. The weight of the item in question, in this case your level.
Arm is the distance of the weight (measure to the CENTER of the item in the case of something long like a level) from the balance point, or CG in this case.
Moment is the product of multiplying the two. So, if you have an 8 oz level, and it's 25" from the CG, you have a moment of 200 oz-in

Let's say you have the level on the tail. Fine. To balance it, you need an equal moment somewhere FORWARD of the CG.

Perhaps there's a spot by the motor, for example, that you can set something. But the motor is only 10" away from the CG, perhaps. Ok, do the math. You need some as yet unknown weight * 10" arm to equal 200 oz-in.

Viola. A 20 oz weight will do the trick. :)

================

While we're on the subject, one of the handiest things I have in my shop is a big tray of old wheel balancing weights. Just go up to any tire shop near you, and tell them you'd like to take a few handfulls of old, used wheel weights off their hands. They'll be happy to give them to you. The weights come in sizes from 1/10th of an oz all the way up to 5 oz or more, and are usually clearly marked. They're small, and many are easily bent. They're GREAT for offsetting unwanted weight like the level in the example above, or for simulating weight you haven't yet installed, like future servos or batteries.

Taping the canopy for marks is fine...or you can tape or mark out on the wings if you wish. With something like the vanessa, you can easily get out at the wingtip, and sight down the line to see where the plumb bob is pointing. Take a look at the second picture in my post for an example.

As for balancing canopy down, there's a couple of things there. The major thing is that if you're balancing on fingers or dowels, you want the mass of the airplane BELOW the wings, so it's acting like a pendulum. It's not that the CG is "wrong" done the other way...it's just considerably easier to be more precise.

And that's much of the fun of this sort of thing, imo. :) As you fly more, you become better adept at identifying the subtle (or sometimes not so subtle *heh*) changes a small shift in CG can make to an airplane's characteristics in the air.

Perhaps you ARE "nose heavy"...but so what? Just remember...as several have pointed out, this is a VERY subjective thing. Given an airplane that does X with the CG at some point Y, then there are, indeed, some fairly predictable changes that will occur if you move the CG forward ("nose heavy") or rearward ("tail heavy"). But these aren't necessarily "bad" or "good" things (within reasonable limits, of course)...they just are what they are. See below.

Here's exactly that example. You're probably right...moving the CG back a bit would quite possibly flatten those spins out. It'll do a few other things too. :) The tail surfaces will likely become a bit more responsive to inputs, the plane may "float and thud" (floating to a near standstill, and then suddenly falling out of the sky) a bit more on landings, turns may require a bit more rudder...all kinds of neat things. Some changes may be so subtle you don't see them, others very pronounced.

Are any of these things "good" or "bad"? Not really. They're simply what they are. What makes them good or bad is whether YOU like them. YOU want the airplane to flatten out a bit in spins...so if you can move the CG a bit, and get it to do that...then that's as "good" as it needs to be. :)

All you can really do, as Minn suggested a while back...is play around. :) When the airplane does what you want, and you enjoy flying it, then it's "right"...regardless of what some manual or "expert" may say to the contrary. :)

Nothing wrong with that. There's all kinds of ways to play with the CG. heck, there's multiple ways to THINK about it. As i said above...perhaps you're a "X amount of weight in the tail" guy...or maybe you're a "Move the CG X distance" guy. Whatever works, man. :)

You might even consider just moving some things...perhaps slide a battery back a touch, or relocate a servo. Those impact CG as well, and have the advantage of not adding "dead weight" to the airframe.

All just stuff to play with. :)

And this, exactly, is why I consider the tool so valuable. It provides means to do all sorts of things, including just what you said. You can use it to see what kind of impact ANY change makes to the CG, and then also use it to 'record' that change, and recreate it later using another method.

===============

In any event, Steve, I probably should echo again what I believe Minn's original point was...

All this stuff is well and good...and for some of us odd-balls, it's actually an enjoyable part of the hobby to play around with weight and balance to some insane level of precision. There are, however, a million ways to skin this cat, and ultimately, "close enough" will, indeed, get the airplane in the air and flyable...and from there it's all a big pile of subjective preference anyway.

In other words...don't forget to have some fun in all of this. :)

RE: CG question?
 

After balancing all my prior planes on sticks, or fingers, my last build, a Top Flite P-47, I had to build the Vanessa to balance it, as I didn't have enough hands to try to balance it in my shop by myself. Now that I have it, I will always use it, as it is easier to see where the balance is, and I have made rope loops to fit all sizes of planes, (except my neighbors full scale Cessna). I made mine with nothing but scrap lumber and dowel, the only thing I bought new was the rope / cord. So, I have a total of about $5 into it. You may wish to try a small bubble level, they weigh very little.

RE: CG question?
 

I also suspect my tail surface has positive incidence? ( front is up )

I havent tried measuring it yet but when I epoxied it down I was trying to make it parallel with the wing and didnt notice the that the clamp in the back was tight (pulling the back of the stab down?) and the clamp in the front was loose (allowing the front to rise?). this would explain why I need a little up elevator trim to fly level???

I cant spend any money on an incidence meter but I read here its ok to block the plane up and take measurements of the LE and TE to see if one is higher than the other.

I dont know what I could do about it now but I'm curious...

If the stab has positive incidence could I adjust the wing saddle to match thus getting the wing and the stab to match?

And I do agree this may be splitting hairs, I'm flying the plane and having a ball with it but I find these details very interesting and can apply this knowledge when building the next plane...

It still scares me when the plane is heading towards the ground wether in a spin or just diving. But its getting better with more stick time...


Steve