Ed Smith

Try This.

Ed S

majortom-RCU

My Feedback: (40)

Major benefits with the plumb bob approach are: accuracy; convenience (once you've got your rig); one guy can handle a big plane by himself; you can hang your model with everything mounted except battery and rudder servo(s), then position them very quickly forward or aft of CG to get the balance just where you want it, with little or no ballast required; you can check the lateral balance at the same time, and add a little wingtip ballast if needed. I use nylon straps for my sling instead of cord--gentler on the model's finish.

TLH101

My Feedback: (90)

Great for trainers and very light aircraft.

jaka

Hi!
Well! Should'nt be that difficult.

Ed Smith

Some questions about the suspension system.

When the aircraft is suspended, the wing incidence has to be that which is designed for flying, Correct? How is that ensured.

If the airplane is suspended in a nose/tail down attitude does the CG pointer still point to the correct CG position?

The CG of the airframe is a function of the mass of the airframe. The flying balance point is an aerodynamic function. Are the two in the same location?

I am seeking information. I am not trying to decry the method.

Ed S

majortom-RCU

My Feedback: (40)

For the plumb bob method in general, the pointer will point down through the fuselage to the three-dimensional CG, which is located somewhere in the interior of the model. But all you can go by is the visible point on the fuselage exterior. Therefore, for reasonable precision, you want the model to be straight & level. In the sling variation I use (without the dowel and twisting cord), I just slip the sling a little forward or back as needed to have the model hanging level in pitch and roll. With the Vanessa rig, I understand this function is performed by rolling the cord one way or t'other on the dowel.

Aerodynamic neutral point and CG are typically different, although they may coincide. If they're different, you make it up with adjusting wing incidence or decalage or control surface trim. What we call 'the CG' can be understood two ways. Any mass has its natural, physical CG. In setting up your model, you decide where you want that natural CG to be, and distribute your movable masses (battery, servos, ballast, etc.) to make your 'design CG' come out where you want it--let's say at 33% of mean aerodynamic chord, laterally centered on the fuselage. The aerodynamic neutral may be forward or aft of where you arbitrarily set your design CG, which you'll discover when you do your trim flight. So you test fly, trim surfaces for straight & level, and after observing and feeling how the model is flying, you nudge the CG to take out the trim, until you like how it flies. No more measuring, no more calculating, no more plumb bob, just fly it and change little things (wood prop or composite, heavy or light spinner, shift the battery, etc.) until you like the result. So the design CG is a first approximation, chosen for the purpose of positioning the heavy components for which you have some choice as to where to put them.

The hanging and plumb bobbing is only done once, and I do it with every component installed, with the usual exception of rudder servo(s) and battery. These I simply slide around (maybe with a little patch of double tape to keep them from rolling off a rounded surface), and the new configuration will naturally swing its new CG under the pointer. When the pointer comes to rest at the spot I have marked, I then figure out how to do the final installation of the variable components.

The old way I used to do it was to just put everything where the plans said, and then add lead to bring the balance to the manual's specifications (which are reliably nose-heavy, usually at 25% of MAC). The way I do it now lets me use necessary components instead of ballast, saving who knows how many ounces and even pounds of lead on a large model. So the possibilities for conserving lightness can be very considerable. If you're lucky, and the trial CG comes out very close without the battery, you can choose a lighter LiPo or whatever and put it right at or very close to the CG. If you're way off, you can make up a lot of balance with a heavier nicad and put it out on the engine mount beams or way back in the tail or anywhere in between. Sometimes this means cutting in a hatch to mount and remove the battery, which I've done, and seen it done on other people's pattern models.

Ed Smith

Thanks for the detailed explanation.

When you say the model should be hung straight and level do you mean the wing at 0-0 incidence?

Ed S

LesUyeda

The horizontal stab will fly at 0-0 unless you force it to do otherwise. The wing will fly at whatever incidence the design has dictated.

Les

Ed Smith

Maybe, but will it hang that way in the system described? I am pretty sure I could get the airplane to hang at any angle I want. Will the CG position always be the same? As yet nobody has answered that question.

Ed S

majortom-RCU

My Feedback: (40)

Ed, the answer from Les above is correct and pertinent. I don't typically get into the fine points of wing incidence unless the model is giving me problems. When I say straight & level, I just look at it as though I were flying it as in IMAC or pattern level flight, and then set my CG with the plane in that attitude. There is some illusion of precision here that doesn't really apply. Because you can measure down to 1/64" or 1 mm doesn't mean you've solved your ultimate problem. The whole point of the sling is to get me to my first approximation of CG with the least lead possible. But the final CG can only be settled on after flying the plane--easily ten or twenty flights before I'm satisfied. When all is said and done, I might be an inch away from where I started, but at least I start with a balance point somewhere in the ballpark.

My typical plane starts off at 0-0-0 for vertical thrust, wing incidence and tail incidence. If I can get it to fly the way I want while sticking to these parameters, I'm happy. A pattern or IMAC model I will usually put my initial CG somewhere between 30-33% of mean aerodynamic chord. For a long time I liked 33%, tried as far back as 35%, but my latest model seems to do better around 30%, so I'm still learning. I've never had a plane set up with CG so far back I couldn't fly it, but I have had planes with initial CG at the manufacturer's recommendation that were so nose-heavy it was all I could do to make it around once and land it.

You ask 'will it hang that way in the system described?' I have no experience with the Vanessa system specifically, but my approach uses the same principle. I just take two nylon straps with adjustable lengths, loop them, tie a piece of nylon cord connecting the two loops, then hang the cord over a screw-hook in the ceiling. The straps can go either around the fuselage fore and aft of the wing, or under each wing at the armpits. I adjust the loops as needed for everything to hang straight. If the plane is hanging nose down, I just lift the nose and the nylon cord slides over the skyhook and settles in the adjusted attitude. So you're right in saying you can get the plane to hang however you want it. But how you want it is straight and level, because you want the plumb bob to point to a mark you've put on the surface of the fuselage (top or bottom, depending on the type of model). You want the fuselage mark to be directly above the center of mass of the model, which is somewhere in the middle of the fuselage. So if you adjust the plane to a level attitude, the plumb bob will point straight down to the center of mass, because the whole thing is basically a pendulum which will come to rest with the CG straight down from the plumb bob.

This is one of those situations where it takes too many words to describe what is so simple if you just set it up and give it a try.

LesUyeda

Ed. Yes you can get the airplane to hang at any angle that you want, but what you want is the airplane hanging in it's flying attitude (horizontal stab at 0) and at the same time have the plumb bob pointing at the CG point specified on the plans. If the plumb bob is not pointed there, you add (or move) weight to accomplish this.

Les

Ed Smith

Thank you Les.

You said the magic words, "Horizontal stab at zero". All is now clear.

Ed S

BillS

If the rope doesn’t break size doesn’t matter. I have been using the balance cradle method for several years and the string broke on a 35-pound model. The fall to the floor was only a couple inches and softer than most landings. With a chain hoist and a big tree you could balance a full-scale airplane.

Bill

LordSirOb

Here is my Venessa rig. Works like a charm, and the parts were cheap. The Funtana came out tail heavy with all the servos in their stock location. I got my Funtana balanced with a pull-pull setup and NO extra weight.

mikenlapaz

Got two of them in shop and they work fine.
I use 1" wide strap for support for wings and a section of Spyder wire fishing line for lateral balancing (prop hub to rudder hinge.
Rigged with pulleys they serve as overhead storage of planes when not in use.

grubbyjeans

I'm in the process of converting a HobbyZone SC to a low-wing trainer. I have a question about locating the CG. I've used my version of the Vanessa rig and am not sure if I'm making a correct application. Take a look at the build thread and help me with the cg, if you will.

[link=http://rnew.lonestarfieros.org/SuperCub/]The build is located here.[/link]

I've only been able to move it about 1/2in. by placing a 7cell battery. I first placed the battery at the rear edge of the original wing location and leveled. The cg moved about 1/2in. to the rear.

I then moved the battery to the front of the original wing, just centered f>r over the lead edge and leveled. The cg then moved to about 1/2in. in front of the first location.

Is this plane properly balanced, or did I miss something.

Unless I misunderstood something, 3in. would be about 43%.

buck1856

My Feedback: (49)

I just read this and im going to make me one this weekend.Im putting together a 31% staudacher wich this would be perfect since im in the refurbishing stage.(picked it up built for $150.00 built but needed some improvments.)Im going to get some use out of this thread for sure,thanks for sharing!

majortom-RCU

My Feedback: (40)

Grubbyjeans, I'd have to say the photo showing the plumb bob pointing at 3" back from the wing leading edge is considerably too far back. You need to either get some weight out of the tail or add it to the nose. Lead ballast on the firewall if you must, but first try moving your battery as far forward as you can get it. Since you have a straight (not tapered) wing, you can just take somewhere between 25-30% of the chord back from the leading edge and use that to set up. If you can get 30% just by moving the battery or other components forward, I'd settle for that for test flying. If you're more forward than 25% back from leading edge, that will be pretty noseheavy, and will take a lot of elevator to keep it from diving. There are plenty of CG calculator web pages, just google for them. I'd give you a few good links, but I'm on a new computer, don't have the bookmarks handy.

As far as proper use of the Vanessa method, what your photo shows is just fine. Your rig works, now you just have to shift weight in the model to get the pointer in the 25-30% range back from wing leading edge.

DaddySam

My Feedback: (9)

I've used my Vanessa to balance my planes, but it's caused some damage to the thin trailing edge of the wing. I see in one of the photos in this thread that one user put one sling loop over the fuse in front of the landing gear and one behind the wing, maybe fastened there with tape? I'm curious if where the attachment points are on the plane affect the machine.
My Vanessa was made from thread reposted here. The dowel sits spanwise above the model. So if I did put my slings on the fuse, how are they rigged to the machine?
THanks
sam
AH, never mind. I looked at the first page and figured it out -- The fuselage sling is one continous loop!

feihu

If your model is too big and heavy for ypur fingertips, suggest using a ruler and a weighing scale, then calculate using the 2 equations and 2 variables method, thus:

Lets assume that that Connie weighs 25 pounds and the distance between the main landing gears and nose wheel is 24 inches. Also assume that the weight on the nose wheel is 5 pounds, and the weight on the two main wheels is 20 pounds. Then:

let x = distance from nose wheel to CG
let y = distance from main wheels to CG

equation 1 is x + y = 24 inches
equation 2 is 5x = 20y so we will write this another way x = 20y/5 and x = 4y

substuting eq. 2 into eq.1 we have 4y + y = 24 inches or 5y = 24inches
so y = 24/5 inches which is 4.8 inches forward of the main wheels to the CG
and from eq 1 x = 24 - 4.8 or 19.2 inches aft of the nose wheel to the CG

feihu

TLH101

My Feedback: (90)

OK, let me see Vannessa rig $5 parts and stores in a small space or, 3 "accurate" scale sets and a calculator, cost how much?
I guess if you are doing a model that weighs in excess of around 75lbs, that is a better option

jcterrettaz

My Feedback: (1)

Hello,
i build the Vanessa " machine"
I level my Mig 15 level and the BOB is pointing way tail heavy...I know if I fly with this Cg will be a very hard plane to Handle!?
What I'm doing wrong?
Thanks.
J-C

jcterrettaz

My Feedback: (1)

How I forget a picture[img][/img]
how you post a pic?

jcterrettaz

My Feedback: (1)

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majortom-RCU

My Feedback: (40)

I'm not experienced with swept back wings, nor with jets, but the photos look like the plumb bob is pointing pretty close to 1/3 of the aerodynamic chord, so it doesn't look all that tailheavy to me. Do you have a recommended CG from somewhere? And how does the plumb bob indication relate to that recommendation?