rtstestpilot

My Feedback: (2)

You are absolutely correct, the aircraft must be shimmed up to flight attitude and not in a parked stance. Once you have the total weight of the plane the scale can be left under the tail/nose wheel with the blocking on top for stability, just tare the scale to zero it out with the blocking before putting the aircraft weight on it for that measurement. Also, the distances back from the datum (spinner or foremost part of the plane) MUST be taken in the same "flying" attitude.


Todd

Joe Fisher

My Feedback: (4)

Here is the actual weighing. The scales only read to 300# so the levers are used to read only 2/3rds.

Rob2160

Yes good point that I didn't mention in the instructions, measurement of the wheel from the datum must be taken in the flying attitude.

If you plan to do it with only one scale make sure the attitude of the plane is exactly the same as you measure each wheel. Having three scales would make it easier, especially if you want to test how the CG moves as fuel is added or used. (it can be done with one scale but will take longer)

Three of these would do the job. Around $20 each.

http://www.overstock.com/Home-Garden...AI&searchidx=1

rtstestpilot

My Feedback: (2)

Yes Joe, this is exactly what the spread sheet does, it just simplifies it greatly and does the math for you plus you only need the one scale. You could use just one scale on full scale aircraft but you would have to weight each wheel separately while propped up in the same flight attitude then add them together. It just means a little more work and I would think moving the scale with the weight of the full scale wouldn't be any fun so 3 makes positioning the aircraft a one and done thing.


Todd

abufletcher

So, if I understand the two different speadsheets available through this thread, they are both similar but approach the problem from a slightly different perspective. In the first, we add in the weights and moment arms and it calculates the CG position. In the second (the one from Todd), we also put in the weights and moment arms but it tells us what the weight at the nose or tail wheel SHOULD be for a CG that we select.

Is that about right, guys?

TFF

Models are weighed in flyable condition. Most real airplanes are weighed without people and other things, and those are adjusted when someone gets it by math. The airplane example needs passengers to be safe to fly but on the ground it will sit on its tail; just an example of something out of the ordinary. Most will be on the landing gear as designed.

rtstestpilot

My Feedback: (2)

Correct. My version is "plug and play" the spread sheet does the calculation for you and in a form that can be saved as is for future modifications or printed off. Basically, you sit down with a pencil and paper or calculator to get all the calculations or let the program do it for you. It helps that most of our large models can easily be weighed in one maneuver (I stand it on the nose and carefully and safely balance it - just make sure you aren't adding or subtracting any weight as you steady it). Or if this isn't practical, just weight each wheel with a couple of spacers of the same thickness as the scale under the uninvolved wheels in turn (if this is done, it doesn't matter if in flight attitude or not - it's just for the total aircraft weight and.

PC-6

Here is a link to a method I use.
http://www.rcscalebuilder.com/forum/....asp?TID=23164

Hope this helps.

Leif

rtstestpilot

My Feedback: (2)

Correct TFF. But they both do the same thing. The addition of all weight in full scale aviation is computed after the fact (fuel, passengers, cargo, etc.) because the other measurements are all known in advance and makes the calculations for the correct CG in flight. In our models, we aren't constantly changing cargo or passengers so or job is a little easier and more of a one time thing unless changes to the plane are made. Stability in the air is the #1 goal for both models and full scale.

Rob2160

Yes you are right. Here is one I did for a full size Hawker 900XP.

It also flags if you exceed the weight limit at any station and warns about fuel management to ensure CG is always in balance.

Note the interesting shape of the CG envelope - this is due to having an Aft Ventral fuel tank that will make it tail heavy if you don't have enough fuel remaining in the wings.

Play with the numbers and see what warnings you find..


https://www.dropbox.com/s/u7d24rgtf4...900XP.xls?dl=0

rtstestpilot

My Feedback: (2)

I forgot to tell you guys who I sent the sheet to that at the bottom left of the spread sheet are 3 tabs. One is the actual calculation sheet and the other 2 are directions - one for tail draggers and the other for nose wheels. You can also copy the calculation tab, rename it as a specific model, and have an archive of every one that you do at your finger tips.

Todd

rtstestpilot

My Feedback: (2)

Hey guys,

Rob2160 was able to make a drop box and put the spreadsheet in it. Here is the link -

https://www.dropbox.com/s/3c84g9e2d58rmue/RCF%20CG%20Spreadsheets.xls.zip?dl=0

Thanks Robert!

Todd

OldScaleGuy

My Feedback: (2)

This is very good information.
I do have a question though. There are several mentions of the plane being in flying attitude when weighing. How critical is that? Should we being checking the wing incidence with a meter to confirm is it exactly where it should be?

rtstestpilot

My Feedback: (2)

Flying attitude is critical but not to the degree you are talking. Let's think about it for a minute. Let's take any common tail dragger for example and say that the correct CG is hypothetically 2 inches behind the main landing gear and 2 inches down from the top of the wing and the CG is 6 inches above the bottom of the mains. We want the plane to be stable when it is flying. Make an imaginary triangle by drawing the longest leg straight up from the center of the main landing gear to the top of the wing (as it would be when flying) a fixed distance from the edge of the paper (this will be the datum or in the spread sheet - the nose) now plot the correct CG down and to the rear from the top point by 2 inches both ways. Now if you could leave the point down at the mains stationary and pivot the whole triangle back a few degrees as if it was in a parked position, the measurement from the datum to where the correct CG is now would be greatly different but the distance to the mains has stayed the same - royally screwing up the relationships and equations giving you a greatly different tail wheel weight in order to keep the CG as the neutral point. That's why it's important to have the aircraft in the flying attitude or as close to it as possible and your eyes should be close enough to tell you if it looks correct or not. Some aircraft will fly with a very slight nose down attitude and I mean very slight. Basically you are setting the neutral point (the most stable) in the aircraft to be ideal when flying since that is when it is important. I hope that answered your question and I didn't muddy up the water any more.


Todd

flycatch

My Feedback: (26)

http://www.ezbalancer.com/

I have both the large and small unit.

Joe Fisher

My Feedback: (4)

When you balance an airplane on your fingers by eye ball you level. If you move your fingers forward it will be nose up. So how you see level is what you are using and it usually works. The Rans is the only home built that I have finished that had a leveling means spelled out, so I just eye balled those airplanes. They fly just fine.

Rob2160

This looks interesting.

http://adamone.rchomepage.com/cg_calc.htm

abufletcher

Rob, I've seen that before, in other discussions of CG. What it does is suggest a theoretical CG point for any aircraft/wing design. That's useful, but different from calculating balance.

Len Todd

+ 2 on the EZBalancers. I also have the extensions.

However, I also made a balancer similar to the EZBalancer II for my first giant plane out of 3/8" plywood and balanced the 35% Extra. It flew spot on. Then, when I later got the EZBalancer II, I double checked the cg on the Extra and the CG was actually also spot on for the EXBalancer. It is really simple to make one up out of plywood and it works! Only thing is; The support of my homemade one did not allow it to slip under the Checkmate's top wing. So, ... that is when the EZBalancer II was ordered. It will do them all. No calculations! No support systems! No fussing around.

Rob2160

Agreed, yes it is for a different purpose than the subject we are discussing.

abufletcher

Personally, I would greatly prefer visual inspection. I want to SEE the model balancing and be able to move stuff around and see it actually changing the attitude of the model. Maybe that's old-fashioned of me. I also thought that it should be quite easy to make something like the EZ balance; it's ultimately nothing more than a base with free-swiveling support arms. Some ply wood and a pair of ball bearings should do the trick. On the other hand, this sort of balance seems better suited to aerodynamically "clean" models like Extras. It's harder to imagine using such a contraption with a fully-rigged 1/4 scale WWI 2-seater (with a scale airfoil and/or a scale wire TE).

As I said earlier, the weighing method requires that we TRUST THE MATH...or more specifically that we trust our own methods and measurements. But it does seem to have a lot of advantages. Still, I imagine I won't be able to resist a quick fingertip check afterwards.

Propworn

My Feedback: (3)

Why so complicated with the scales and such I make a simple fixture that goes around the whole wing and has a pivot point where the CofG is supposed to be. After the CofG is set a steel jig is inserted in the two wing jigs and adjusted to center and lateral balance is set. Works so far for 14 and 16 ft wing spans carrying up to 40 lbs of cargo.

We built these large span models with 12 to 14 inch cords very light weight and fragile. The airfoil was totally encapsulated in the balancing jig and suspended while weight was added observing for changes in balance or CofG.

I have done my personal models this way without any problems. If the wing is flat bottom or a more traditional airfoil chances are its more robust and a less complicated balancing jig would be necessary.

Models up to 65 lbs have been suspended successfully from the ceiling in my garage.

Dennis

abufletcher

Also a very nice idea. That seems like it would solve the problem with a fragile TE. Still, I suppose "complicated" is relative and depends or the individual modeler. Weighing the model and taking the measurements vs. making a set of custom fixtures for each wing airfoil.

And, not everyone has a garage with sturdy beams.

Propworn

My Feedback: (3)

We built a frame for field trials and competition to make sure the CofG was correct for each flight. It held the model but a few inches from the floor and allowed minor movements to get the CofG and lateral balance right on the money. This airfoil is fussy with balance and angle of attack to work to its maximum efficiency.

Smaller models and airfoils are suspended on a plate that supports from the bottom and wraps around the leading edge to balance the same way.

Larger more expensive models its well worth it to make the templates. Perfect for rechecking after repairs or changes in design or equipment.

Prime example was a fellow had a 50 cc Edge he had flown and tweaked for a few years. He had reached perfection for his style of flying but now was considering going electric. He did not want to start over with a new plane and spend all that time getting it to the stage of his present model. He was leery of converting it to electric as he felt it would be hard to keep everything the same. We made up a set of fixtures that locked the wings in place then found his balance point. We were a bit surprised to find it behind the recommended point in the instructions but he was happy with it so we left it right were it was. I had him remove the gas motor and the motor box and make a new motor box and mount the Axi motor in place. Wire tied the esc to the side of the motor box and assembled everything. A box for the batteries was constructed and with the batteries inside double sided tape was used to position the batteries on the airframe until the same CofG was attained. The airframe was opened up and modified to accept the battery box. There was enough room lengthwise in the battery box to move the batteries forward or back a small amount. Not only did the plane fly with the same feel but by sliding the batteries forward it flew a bit more precise in the aerobatic sequence and moving it towards the rear helped with freestyle sequences. The gas motor has been sold and the pilot has not looked back. Saved him the cost of a new plane and the disappointment of selling his old favorite.

Dennis

Dennis

Lone Star Charles

I have always found the three point weighing system to be the most accurate and foolproof. I have a little postal scale that I think cost about $20 with an accuracy of 0.1 oz. and a maximum capacity of 80 lbs. I use spacers under the other two wheels to maintain the correct flight attitude. Maybe one of these days I will get two more of these scales, but since I don't do weight and balance often, I probably won't.

One other minor point is this: If you have a big ceiling fan or other air handling device that is blowing on your model, turn it off when you do weight and balance. If you don't, it may affect your results.