Top_Gunn

My Feedback: (6)

Whit,

You're right; to find the balance point with a Vanessa rig you need the plane level, just as you need it level when balancing it on your thumbs. (We talk a lot about finding the CG, but we don't need to do that and we never do in practice.) It is true that the plumb bob on a Vanessa rig always points to the center of gravity, but knowing that doesn't tell you the balance point unless the plane is level because you don't know where on the imaginary line down the string through your fuselage the CG is. Your pictures showing the "wrong CG" aren't really showing the wrong CG, they're not showing the balance point at all. Unless the CG is a lot higher than it normally is (i.e., unless it is on the very top of your fuselage), the point on the top of a tilted fuselage on a Vanessa rig is not the balance point. All you know on a tilted airplane is that the balance point is somewhere on a line running diagonally (in a fore-and-aft sense) through the fuselage, and you need to know more than that. The balance point will be somewhere ahead of or behind the point on top of the fuse, depending on which way it's tilted.

What you are doing makes perfect sense to me.

ZAGNUT

My Feedback: (1)

Whit, when i said i take a "good guess" i was referring to a new design that's never been flown before. if you're building from plans you don't have to guess at anything, the designer has already worked everything out. the only thing you might have to guess at is what constitutes "level" when hanging the plane in the sling and that's most likely going to be the same attitude as shown on the plans....and as someone else pointed out, eyeballing it is going to get you closer than you need....even if you have a few grand in the model.

there is no such thing as a "correct" flight attitude either...unless you also know the exact speed, weight, elevation, etc., etc.

wphilb

My Feedback: (10)

Thanks, I think a little due diligence never hurt.

whit

Lnewqban

Whit,

Going by a horizontal tail is wrong, simply because what the designer cares about is the attitude of the model or real airplane for cruise speeds.

He wants the minimum drag for the more common speeds (imagine so many airliners burning extra fuel flying most of the time nose up or down).

Whatever attitude seems more natural to you should be the position for proper and accurate balance.

The incidence of the tail has nothing to do with the above considerations or the balance of the model.

The same designer selects that incidence solely based on the tail down force that he wants, without which the model would nose down (equivalent to elevator-up input) or wouldn't be stable in pitch (if no need for that force due to too aft CG (tail heavy model): model too sensitive to any elevator input).

How to obtain that negative lift from the tail?
We know that zero AOA = no lift, then some AOA is needed.
That AOA comes from the angle between the tail and the down-wash coming from the wing.

So, very frequently, the incidence of the tail respect to the datum line of the fuselage is zero, because the downwash is around 2 or 3 degrees pointing down and that creates enough tail down force, but it doesn't have to be horizontal for cases where the downwash angle is more or less than that.

radfordc

My Feedback: (14)

Just to help beat this horse to death. What you really want if for the plane to hang at the same attitude that it flys in level flight. For many airfoils this amounts to about 3-5 degrees of angle of attack...so this is what you should try to get in the Vanessa rig. If you eyeball the plane and say "yeah, that's what I want it to look like in level flight" you should be close enough. If you aim for slightly nose heavy you will be safer than a little tail heavy.

wphilb

My Feedback: (10)

Thank you both for your input, it is much appreciated! I also want to thank everyone else who weighed in, a healthy discussion requires a willingness to listen to varying viewpoints and a willingness to give same. 

Lnewqban

You are very welcome, Whit

DoubleDD

Don't understand this setup. With strings suspending the plane fore and aft of the main wing you are only finding the balance point between the ropes. One pic shows rope touching front of wing and touching back of wing. The other example shows the rope at front of wing and the other rope taped farther back from the back of the wing. Doing these two techniques on the same plane will give you two different C of G's will it not?

wphilb

My Feedback: (10)

Try it!

DoubleDD

Ok I re read the whole forum again and I got it. It's so simple but seems complicated. Gonna build one.

DoubleDD

Built one and it works awesome! Another question though, do you balance with a full tank or half a tank or an empty tank of fuel? I'm thinking half of a tank would give a more stable reading.

wphilb

My Feedback: (10)

I was always taught to balance with an empty fuel tank, assuming that the tank is forward of the CG. That way you will never burn off enough fuel to move the CG to an unstable position. The plane is at least correctly balanced or slightly nose heavy.

waters10

I built one of these 2 days ago and was doing some testing. I got one of my planes that I've been flying for a while and I was getting weird readings. Then I realized something that I don't recall reading in any of the Vanessa CG info out there. If the dowel is not parallel to the wings, and your plumb bob is hanging from one of the dowel legs, then you get a wrong reading. By how much, it depends on the angle that you have between the dowel and the wings.

It was late last night and I was trying to figure it out why sometimes my dowel is not parallel with the wings. I think it's due to the fact that I'm using a single loop on the fuselage. Every time I hoisted the plane and had to rotated the dowel to get the horizontal stab at 0, the plane would tilt to one side. I had to grab the plane and adjust side to side, when I did that, that's when the dowel started showing an angle to the wings. I tried to manually twist the whole thing and try to eliminate the angle, but it doesn't seem to work and it's not reliable, so I ended up putting the plumb bob hanging from the center of the T. The bad part is that now I have the plumb bob right on top of the fuselage and not on top of the wing, but at least should be a lot more reliable. The other thing I might try, is the 2 loops and hang from the wing, instead of fuselage.

Any thoughts?

wphilb

My Feedback: (10)

Pics please!  I had a similar issue but was able to twist plane and fix. I did tape rope to fuse, mostly to protect flaps. You can stretch tape line from CG marks on both wings over the top of the fuse and then the plumb bob hanging from the center of the T can be more accurately compared.

waters10

I could twist the plane and get it close. But my point is that this is not mentioned anywhere and can be easily overlooked. Going through some of the pics on this thread, you can see some show that condition! I got some cases of over 10 degree angle between dowel and wings and my CG was probably off by as much as 10mm. For a 60mm that I was measuring from the leading edge, 10mm is a lot. Plumb bob at the center works, so I'm not trying to say this method is no good.

I'm at work, so no pics to post right now. But you get the idea, right? It's like the whole thing is twisted and looking from the top or bottom, you see the dowel has an angle relative to the wings, or the dowel is not perpendicular to the fuselage, whichever makes more sense to you. Plane still balancing correctly though, so plum bob at center works. I'm pretty sure the 2 loops method is better in that regards, but I didn't think about that last night ...

ddfwaj

Hey guys hoping someone can help me with my new Vanessa setup. I have a FMS P51B and wondered where you guys would put the bubble level as it makes sense to me to put on the nose however I have read where people are putting it on the horizonal stab.

Appreciate any help anyone can offer!

Thanks,

Dennis

Tisoy909

My Feedback: (21)

Dennis,
I wouldnt worry too much about using a level, as many planes that would be balanced on a Vanessa rig doesnt even have a level or flat enough surface to put one on in the first place. I generally see to it that the horizontal stabilizer is generally parallel with the floor.

thebluemax

My Feedback: (151)

Dennis,
Try balancing it upside down. You will find a spot somewhere that is level and as Tisoy909 says you can double check it by seeing if it is parallel to the floor. I have found that even though you can balance mid-wings and low wings right side up I find it easier to do them upside down.I think Iget better results that way too. That is just my preference.

Battle Short

My Feedback: (5)

After reading this thread I will be making one of these.   I have not made it yet so am unable to test an idea i have. 

LesUyeda

"The CG would be at the intersection of the two lines indicated by the plumb bob."

In reality, when this thread says CG, they really mean center of balance, and are only concerned with the horizontal plane.

"I have found that even though you can balance mid-wings and low wings right side up I find it easier to do them upside down."

With the Vanessa, it really makes no difference, since the plane is supported above the mass.

"I wouldnt worry too much about using a level, as many planes that would be balanced on a Vanessa rig doesnt even have a level or flat enough surface to put one on in the first place. "

I try to find a horizontal place on the wall, like a joint of wood, a window sill, or the street surface down the way, etc, and align the horizontal stab with that.

Les

ddfwaj

Thanks for the awesome advice guys! I will def try the upside down method as it makes sense to balance this way and would probably help as it seems most of my planes have their CG marks on the bottom of the wings.

Thanks again!

Dennis

thebluemax

My Feedback: (151)

I know a lot of guyssayyou can balance them right side up and they actually will do it that way.Which is ok. Go with what works for you.

I just found from my personal experience Ihad better results doing them upside down.I still do the finger test as a last resort, double check.Just to see if there was any kind ofa difference. On a couple of planes I did right side upwhen doing the double check test somethingdid not seem right. So I did them again upside down and the double check test was good. Again that is something that is not really necessary to do but it is something Ido to just double check me. Guess it lets me sleep better at nght.
Good luck.....

Tisoy909

My Feedback: (21)

The upside down two finger method is the original way to do it. I was even making rigs so i can balance my 30cc gassers this way, it never fails. After 50 cc and up just go with the vanessa rig, in lieu of punching holes in wings

raptureboy

I found the first few posts more than adequate to build one, try it, and like it. It works just as explained and placing a small bubble level on the horizontal stab somewhere close to the high side of the airfoil will get you close enough not to crash your plane on the maiden because the cg was all wrong. Follow the plans and the instructions as written and you will have no issues. Keep it simple, keep it fun.

LesUyeda

" placing a small bubble level on the horizontal stab"

Watch out for the extra weight on the horizontal stab; that is a long ways from cg; i.e. a long lever arm.

Les