Cg-Both aft & Up/Down
 

I hope I ask this question correctly. I am working on a plane where the science guys are asking for the CG both aft and down. Not just the CG on the pitch axis; but also vertical, i.e., up and down. The plane is a 50% Cub. Does anyone know how to determine the CG in this manner. thanks.

RE: Cg-Both aft & Up/Down
 

No matter how you hang it, the CG is always below a single point of attachment.

RE: Cg-Both aft & Up/Down
 

For a 50% CUB, I can't think of a practical way to suspend the plane in a knife edge position and make it so you can move your point of suspension until you get the model to hang plumb.
I can think of ways that would require poking holes in the model, but doubt that you want to go that route.

RE: Cg-Both aft & Up/Down
 

Yes, I'm trying to clarify what they want. We have communicate by email. but I invisioned having to do the same hanging CG check for large planes, but doing it on it's side and finding the CG in that axis. But obviously I can't do that with a plane that has a 19 foot wing span.

RE: Cg-Both aft & Up/Down
 

it is clear they want the cg in a 3 dimensional plane. This is easy and can be found with minimal hanging and some math.You need a digital scale and a calculator and some means to hang the plane from its prop.level the plane and take the weight readings at each wheel.Using the front of the prop as the reference plane the following formula will help you.total weight of plane x X = (weight at left wheel x distance to reference plane) + (weight at right wheel x dist. To ref plane) + (weight at tail wheel x dist to ref plane) X is your cg from the ref plane.

RE: Cg-Both aft & Up/Down
 

now lets do the position of the cg in the roll axis. We use a similar concept as for the pitch axis above.here we know the distance between the wheels and the total weight carried by both wheels. we need to find the position bewtween the wheels where the cg sits. We assume symmetry of the wheels from the longitudinal centre line.Weight x distance between wheels= weight at right wheel x (x) + weight at left wheel x (X - dist between wheels)Solve for x and you have the cg location from the longitudinal axis.

RE: Cg-Both aft & Up/Down
 

lastly we want to find the height of the cg from the thrust line.This is the only hanging required. Use some masking tape to mark the thrust line of the model down the fuselage.Now suspend the plane from its prop blades.It will hang either with its thrust line perfectly verticaal indicating that the cg is on the thrust line or it will hang with the thrust line an angle. Where the vertical line intersects the position of the cg in pitch Will be the deviation of the cg from the thrust line. Measure it and you now have a position for cg in 3 dimensions.Done.

RE: Cg-Both aft & Up/Down
 

Well if you want to find the location of the true CG and not just the pitch related balance point then you really don't have much choice in this. There are two ways to do this. First would be that you kept a detailed and accurate log of every bit and piece used, it's weight, the weight of any glue or fastners used to mount it and it's exact location in three planes relative to the center line. You then do a moment and mass analysis for each of the thousands of parts and add up all the moments to find the CG location.

Or you can find somewhere high enough to suspend the model in various attitudes and use a plumb bob to find where th line passes through the fuselage. 4 or 5 of these and you'll be able to pin it down to within likely a 1/2 inch or better.

RE: Cg-Both aft & Up/Down
 

see previous posts, its not that complicated...

RE: Cg-Both aft & Up/Down
 

Yes, it is a little complicated, from the practical point of view, since Splais is dealing with a 50% scale plane.

The J-4 has 36'-2" of wingspan and 22'-6" of length and 1300 lb.

A 50% scale Should have 18'-1" of wingspan and 11'-3" of length and more than 100 lb probably.

In order to hang that baby, he will need to engineer a rig able to support that weight without damaging the structure and surface of the plane, and he will need a stable structure of more than 16' height.

RE: Cg-Both aft & Up/Down
 

The plane has a wing span of 19' and a length of approx 12'6". Without payload weighs about 70 pounds; with, around 100. I know about the moment and mass thing, but that's just out of the question (at least for anything even close to accurate). I understand what TimBle is saying and if they really need this I think I could hang it from someplace in the hanger.

It will hang either with its thrust line perfectly vertical indicating that the cg is on the thrust line or it will hang with the thrust line at an angle. Where the vertical line intersects the position of the cg in pitch Will be the deviation of the cg from the thrust line.

So let me see if I got this - if the Pitch CG is say 10" back from leading edge I draw an imaginary line vertically through this point. When I hand the plane by the prop, the point point at which the thrust line crosses the Pitch CG line is the distance they need above or below the thrust line. Correct? Wow, you guys are smart, :) thanks much.

RE: Cg-Both aft & Up/Down
 

For an informative discussion of the effect of shifting the CG upon a plane's performance see Dave Lockhart's column in the November 2010 issue of Flying Models. He also deals with how the vertical CG effects flight.

RE: Cg-Both aft & Up/Down
 

Yeah, I think Tim missed the bit about the half size issue.

Replace the prop with a shorter bar that is just a little wider than the fuselage max width point and lock it in place so it is dead on horizontal. Use a Y support off the ends of the bar. Then hang a plumb line from the end of the bar extending along the side of the fuselage such that it doesn't touch the sides or tail. The true CG would be located on that line. Mark the line with some tabs of masking tape and felt marker to establish that line. Following this the easy way to carry on would be to use the same supports to hang the model from the landing gear axles with another plumb line off the axle. Mark the line the same way and where they intersect you have your true CG position. At least in the two planes that matter.

Splais, the bit about weights and moments was intended as a totally impractical funny bit. I forgot to add the smiley after it as I'd intended.

RE: Cg-Both aft & Up/Down
 

As you can apparently suspend the entire plane....

RE: Cg-Both aft & Up/Down
 

I didn't mid the bit about 50% scale. I simply assumed that anyone with space to assemble such a monster wouldhave a building with th required tools such as a block & tackle for liting, A-frames or strong rafter for suspending parts.

After all, you seldom see such large scale plane being assemble in apartments (although I'm sure there someone who's tried) :)

RE: Cg-Both aft & Up/Down
 

Splais,

Before you hang the beast up, study the alternative shown in the attached scale drawing.
I have used the dimensions and weight that you have provided.

You will need wheels’ chock, a level, a plumb, a big square (optional), a ruler, a pencil and a helper.
First, level the airplane in a horizontal attitude, using the fuselage’s datum line or the stabilizer as reference. The fuel tank should be empty, but other than that, the airplane should be as ready to flight (weight-wise).

Secondly, extend and mark a vertical line from the CG down.

Then, ask a helper lift the tail to the point where the airplane is ready to flip over, i.e. when all the weight is on the main wheels. Of course, that is an unstable equilibrium balance that your helper will need to keep while you draw another vertical line. Use a big square (if the ground surface is flat and level) or a plumb from the line that connects the points where the wheels make contact with the ground.

The intersection point of both lines drawn on the side of the fuselage will tell you where the CG is located.

If the science guys require it, we can calculate the exact location of the CG using angles and equations instead of drawing lines on the side of the fuselage.;)

(For some reason, the attachments are showing up huge today, sorry[:o]. Same drawing is attached in PDF format)

RE: Cg-Both aft & Up/Down
 

Bmathews - don't laugh, one of the engineer types wanted me to start weighing all the little parts and taking distance measurements and god knows what kind of info would have come of that. [>:]

RE: Cg-Both aft & Up/Down
 

You could put a scale underneath the tail wheel, and raise the tail. When the scale reads zero, that's all the weight on the mains, and the c.g. will be directly above, as shown.
The plane will be tippy, as the c.g. will be some distance up from the axles.
Have something underneath the cowl at the front to take any over-rotation.

RE: Cg-Both aft & Up/Down
 

Likely he's an accountant at heart... or a sadist... :D

Some good ideas for options that don't require a barn are coming from Lnewqban and Sparky. The short'ish baselines will make it a trifle less accurate than longer ones but it should be able to ID the true CG to within a 1/4 inch with any decent degree of care. Just don't tell them that there is that much error of precision or they'll have you doing the weights and analysis spreadsheet on each part again.... :D

RE: Cg-Both aft & Up/Down
 

The wheels of the airplane have some effect on the vertical CG.
The method that involves tipping the plane until you reach equilibrium looks good at first glance, but those wheels could be made out of solid lead, or they could be made out of angel food cake and tipping the plane while resting on the wheels doesn't account for that.

The only way I know of that is dead on fool proof is to hang the model at the fore / aft CG from one of the wingtips. A "trapeze' stick attached to the wingtip at one end and attached to the belly of the plane at the other end is what you need. Now the plane can be suspended from the rafters with a line that ties off to the "trapeze" stick. Slide the rope along the length of the trapeze stick until the model hangs perfectly plumb.

RE: Cg-Both aft & Up/Down
 

Don't understand the point you are trying to make about the wheels. They are not soft and spongy; but rather rigid. There is no flattening. Are you saying that lead wheels, i.e., solid wheels are good but angel food wheels are bad. From looking at the drawing it doesn't seem the wheels matter at all. You could take the wheels off and set it on the axles it appears and get the same reading. Not arguing by the way; just discussing.

RE: Cg-Both aft & Up/Down
 

Reconsidering the idea shown in the drawing, I think that we are mixing two concepts: where the CG should be (for step 1), and where the CG actually is (for step 2).
We don't wnat the science guys to point that discrepancy out.:)

Combining better accuracy and actual balancing of the airplane with no hanging, we should use a scale, as suggested by Paul above, in order to know the exact weight that each wheel "feels".

We could substitute step 1 by weighting the three wheels while the airplane is in normal stand position (tail on the ground).
In that way, we would know the actual total weight of the airplane and we could calculate the actual position of the vertical line along which the actual CG is located.

For that calculation, we will need to measure the distance between the line on the ground that connects both contact points of the main wheels and the contact point of the tail wheel.
Then, we could place our square on that point and draw the first vertical line on the side of the fuselage.

Distance between main wheels' line and projection on ground of the CG (vertical line) = weigh tail wheel / (weight main wheel 1 + weight main wheel 2) x Distance between main wheels' line and tail wheel

Then, lifting the tail to any position, measuring the weight on the tail wheel, as Paul explained, allows us to use the same formula and to draw as many vertical lines as we want for as many attitudes as we want.
The actual GC will be located by the intersection of those lines.

RE: Cg-Both aft & Up/Down
 

The weight of the wheels effect the vertical CG. Remove the wheels and your CG will shift towards the top of the plane. Increase the weight of the wheels and the CG will shift towards the belly.

RE: Cg-Both aft & Up/Down
 

Very true,..............and also, the plane will weight less:)

Refering to your previous post, the weight that we abstract as concentrated in that single point that we call CG, is the addition of any weight above the ground contact surface, wheels' weight included.

Gotta go!![:o]

RE: Cg-Both aft & Up/Down
 

Ok, look, I know you guy are trying to help, and you are doing great, but I think you are maiking this too complicated - at least you are for me.

Let's assume I know were the Pitch axis CG is (I will, and I have the method I'm going to use). The only thing I need now is the easiest possible way to get a farely accurate, but not necessarily perfect, vertical CG.

Let me digress. I'm going to use two different mass & moment methods to get Pitch axis CG. One method takes weights and distance measurements from each mainwheel and tail wheel, calculates moments and gets CG by dividing total moments by total weight. The datum line for this method can be anywhere in front of plane at a fixed location; I'm using front of prop hub. The other method basically is the same but only calculates a moment from a datum line running through mainwheel axles to tailwheel and gets CG by taking tailwheel moment and dividing it by the total weight of the plane. For now please don't comment on these methods; just assume the Pitch axis CG is say 10" back from leading edge.

Now, back on point - the vertical CG along that Pitch axis CG line through fuselage. I totally understand the effect of wheel weight on location of Vertical CG. The method described in the middle of post #20 by Lnewqban is basically the same as I describe above. Now I need a method other than hanging the plane by the nose to get Vertical CG.
I do not understand this: Then, lifting the tail to any position, measuring the weight on the tail wheel, as Paul explained, allows us to use the same formula and to draw as many vertical lines as we want for as many attitudes as we want. The actual GC will be located by the intersection of those lines. Are you saying the Vertical CG shifts with plane attitude? I thought the CG was what it is and fixed as long as you don't change anything in the plane.

So does that take us back to the fact that method two in the big diagram of post #16 is a farely accurate, but not perfect, vertical CG.