RE: Vanessa c.g. Rig: How to.
 

OK, my bad, did not read far enough along. Set the STAB to zero, leave the wing alone and the bob shows the current CG.

RE: Vanessa c.g. Rig: How to.
 

Correct. If you roll the rod supporting the airframe, relevel the tail. If you add or take away weight fore or aft, relevel the tail. Anytime anything is changed, relevel.

RE: Vanessa c.g. Rig: How to.
 

What about a design where the horizontal stabilizer is not 0° in the design?

Jim

RE: Vanessa c.g. Rig: How to.
 

Your in your own on that one.

RE: Vanessa c.g. Rig: How to.
 

"What about a design where the horizontal stabilizer is not 0° in the design? "

Think "arrow". The horizontal stab will fly level, unless you force it to do otherwise.

Les

RE: Vanessa c.g. Rig: How to.
 

That makes sense to me.

RE: Vanessa c.g. Rig: How to.
 

For example, what if the wing is at 0 and the tail at +2 degrees?

RE: Vanessa c.g. Rig: How to.
 

Just FYI, my father and I balanced the Meister using the fingter tip method (inverted and using more than finagers!) and got it to balance, then I ran the Vanessa rig trial and with the stab set level I got the same result, within a margin of error. The fuse was at a really nose down attitude but the two methods produced the same result.

I then "leveled" the fuse by eye and while the result differed to indicate a nose heavy condition, it did not shift that much maybe a 1/4"?

Whit

RE: Vanessa c.g. Rig: How to.
 

You should level the fuse thrust line, just like the model would appear on a drawing.

All you want is to achieve the recommended or specified distance between the vertical force lifting the model and the weight (also vertical).
By doing that, your model will fly with pitch stability.

RE: Vanessa c.g. Rig: How to.
 

"For example, what if the wing is at 0 and the tail at +2 degrees? "

The tail WILL fly at zero degrees, and the wing will be at -2 degrees.

Les

RE: Vanessa c.g. Rig: How to.
 

Yes, but in my case the wing is at +3.5 degrees and the stab at +2.5 degrees. When I set the plane up in the Vanessa and zero the stab, the plane is pointing down far more than it does in level flight so it seems off to me. That said the method generated a CG within measurement error of what I had calculated by other methods. I then arbitrarily leveled it as I am used to seeing all my other corsairs fly and the indicated CG moved forward only about 1/4" and remained within what I believe is an acceptable flyable CG for this plane.<div>
</div><div>I theorize that the stab may never be achieving zero incidence in flight, that it is counterbalancing the large nose up force of a flat bottomed wing set at 3.5 degrees positive, thus the plane flies in a slightly nose down attitude but never in cruise reaching zero stab incidence.</div><div>
</div><div>Just my non educated $.02
<div>
</div><div>Whit</div></div>

RE: Vanessa c.g. Rig: How to.
 

I think this is mostly right. Not every plane flies with the stab horizontal in level flight. If they did, you'd never need to use elevator trim. Most of them are pretty close, though. (On most planes, though, the stab doesn't offset a nose-up force: it prevents the nose from dropping.)

Nothing about this question is peculiar to the Vanessa rig. You'd have the same question if you were balancing a plane on your finger tips: should the stabilizer be level or what? It just comes up more often in the Vanessa discussions because the Vanessa rig is so accurate and so stable you get to notice small details like this.

RE: Vanessa c.g. Rig: How to.
 

At the risk of starting an argument, the error in locating the CG by missing the level by ±5° is probably going to be less than 1/8".  You will probably move the CG forward or back more than that after a few test flights anyway.  Just get it close and go fly it.</p>

RE: Vanessa c.g. Rig: How to.
 

I concurr, though I found it it be (guessing) about 1/4" perhaps less. Net net is I think it's flyable as is like you say.<div>
</div><div>Whit</div>

RE: Vanessa c.g. Rig: How to.
 

"Not every plane flies with the stab horizontal in level flight. If they did, you'd never need to use elevator trim. "

Only the ones that are not properly balanced and trimmed; if they are you never need to use elevator trim. Go back to my reference of an arrow.

Les

RE: Vanessa c.g. Rig: How to.
 

I'm sorry I don't your "arrow" reference, are you referring to the Dornier Arrow? <div>
</div><div>Whit</div>

RE: Vanessa c.g. Rig: How to.
 

I don't understand your arrow analogy. If you are saying that the movement of the plane through the air will always keep the stabilizer level because of the forces exerted on it by the air, that's just wrong. If it were right, every plane would fly with its stabilizer level, and elevator trim wouldn't ever be needed. Some planes (the Telemaster is an example) are designed to fly with their horizontal stabilizers at a noticeable angle to the horizontal. It's very likely true, though, that setting a plane's cg with the stabilizer level will be close enough to guarantee a safe first flight.

An arrow doesn't fly with its feathers parallel to the ground through the entire flight; if it did, it wouldn't rise and then fall.

RE: Vanessa c.g. Rig: How to.
 

wow, never seen so much confusion over something so simple....and as someone who designs pretty much everything i fly AND has been using this balancing method for over 20 years maybe i can clear a few things up.....maybe;)

where do you put the level? you can disregard any incidence or decalage or even the flight attitude of the plane UNLESS told so by the designer. more often than not the designer will go with the horizontal datum line on the plan...this is an arbitrary horizontal line and engine thrust, wing tail and flight attitude don't have to be parallel with it.....it's just an easy reference line to use while drawing AND locating the CG of the model after flight testing. a simple example is a lot of planes i build have a flat and level fuselage bottom when looking at the plan. when i first fly them i take a good guess at the CG (usually 25%) and then start moving the battery around until it flies like i want. i will then hang the plane above my level work bench and adjust the sling until the fuselage bottom is parallel to it and where the plumb bob points to is where i will mark the CG on the plan. horizontal stabs rarely fly level, an extreme example being the DR1 where the tail needs a lot of positive incidence to counteract all the drag way up above the thrust line of the plane.

and IMO the "machine" as presented here is nice but overly complicated for most planes. i use the same single loop sling that goes under the fuselage fore and aft of the wing but at the hanging point it just goes through a small loop at the bottom of the vertical line. plumb bob is also suspended from this loop. to adjust the attitude of the plane i just pull the tail or nose down and that pulls the lines of the sling through the small loop. to find the lateral CG i just level the model in the sling.

RE: Vanessa c.g. Rig: How to.
 

Zagnut,<div>
</div><div>I'm sorry but not helpful, no disrespect meant. I can't "take a good guess" on a multi thousand dollar plane, plus I have a CG that I know many many planes of this exact design have flown well at. So I need to determine, within say 1/4 to 1/8" where my CG actually is. How level the plane is, what attitude CAN affect this though how much is debatable. So I need to know what reference to use in determining what "level" is. </div><div>
</div><div>For my $ I've decided to test both stab zero and crutch zero and make sure it's flyable at both positions and adjust after flying. Right now stab zero is correct CG and crutch level is?? Stay tuned to this bat channel....</div><div>
</div><div>Whit</div>

RE: Vanessa c.g. Rig: How to.
 

No disrespect from me either; but, what Zagnut says will get you within your ownrequirements of 1/4 to 1/8" - easily. "Eyeball Level" is close enough to get the accuracy you need. Don't make somebody pull out the physics and the math here.

RE: Vanessa c.g. Rig: How to.
 

My reference to arrow, was to indicate that as in a bow and arrow, arrow; the tail feathers determine the path of flight. If you distort these tail feathers, the flight of the arrow will NOT be true to what you expect. The same is true of our aircraft. for example, If you mount the tail feathers of a arrow in an off axis orientation, the arrow will spin.

Les

RE: Vanessa c.g. Rig: How to.
 

Nobody is claiming that the stabilizer (with the elevator) doesn't affect the flight path. That's true, but it doesn't mean that the stabilizer is always horizontal in flight. Perhaps the clearest example is a plane with a T tail, in which the stabilizer is too high to be affected by downwash from the wing. If a plane like this has a symmetrical stabilizer (together with elevator) the stabilizer couldn't do its job of holding the back of the airplane down if it were level: it needs to have a negative angle of attack to keep the plane flying level.

Most planes (at least high-wing planes) with a stab in the usual place fly level with the stabilizer also level. They can do this because the stream of air striking the stabilizer is not level: it is angled downward. So the stabilizer's angle of attack is negative, even if the stabilizer is level.

RE: Vanessa c.g. Rig: How to.
 

The center of gravity (CG) on any airplane is independent of the angle of attack of the horizontal empennage.  The plumb bob on a Vanessa Rig will always point to the center of gravity of the aircraft no matter what the angle of the aircraft.  Don't try to make this more difficult than it is.

RE: Vanessa c.g. Rig: How to.
 

This is true, but not of any practical importance. The imaginary line extending from the end of the plumb bob will indeed pass through the CG. But because that line goes through the fuselage at an angle, you can't tell where the balance point is unless the plane is level because you don't know where on that line the CG is. If the CG is high, it's near where the line would enter the fuse; if it's low, it would be near where the line exits the fuse. If the fuse is tilted a lot, those points could be a couple of inches apart. I can't picture someone getting an accurate balance point reading on a large plane at a 45-degree angle on a Vanessa rig, can you?

To be sure, if you do it with the stab level, you will be close enough for a safe flight.

I'm not trying to make anything difficult. The practical and very simple answer is "do it with the stab more or less level." One person keeps insisting that all planes fly with their stabs level. That's not true, though it may not be very important.

RE: Vanessa c.g. Rig: How to.
 

I think if you read my earlier post I indicated that I tried both Stab zero (as recommended here) and an arbitrary "level" done by eye and got two different results but results that indicated that I was within a range that was acceptable for flight, and that I was going back to test the crutch level position and make sure that was also at an acceptable CG position. A little care before the flight of a large expensive plane is not a bad thing, yes?

As to the assertion that a Vanessa rig will always point to the CG regardless of the angle of the plane, then why level it? You are correct that it will always point down through the plane to the actual center of mass, but I can't measure inside the plane. I level the plane so that I can measure it. Designers usually tell you "CG7" back from wing LE at fuse." so that's where I need to be able to measure and that requires a "level" plane with a Vanessa. See the following set of photos below. I've been testing my Vanessa rig to better understand what issues/things can effect my results. This is on a plane with a known CG, marked in Red. The first photo shows the CG determined correctly, the next photos shown the plane at different attitudes and the resulting incorrect CG indications.

I would still like to know what the best method is for determining the "level" flight attitude of a plane so I can have the best shot at getting the correct CG indication on the surface I'm able to measure. I accept that we are not looking for results within more than 1/4" on a GS plane and that flight testing will be needed to correctly trim out the planes CG. I think that for most planes with a 0-0 wing stab incidence relationship either surface will suffice. for flat bottomed WW1 planes I think either wing surface or the stab will do. It's for planes with different wing and stab incidences of a significant amount that things get "interesting." Then I think an educated guess is probably the best you will get and you need to bear on the side of caution, (very) tail heavy planes flying once, yada yada. That is what I am trying to do.

Again, no disrespect meant, I come here to learn and I've carried out my own experiements to learn and I have shared them with the caveat that I am no expert and do not claim to be.

Whit