abufletcher

Just out of curiosity I'm wondering if the two main wheels could be weighed together. Since they both (presumably) have the same geometry relative to the datum location, does it matter to weight them separately? Many WWI aircraft have a quite narrow undercarriage and so both wheels could be placed on the scale at one time. Or if the stance is too wide, a board could be placed on the scale (and the weight tared out).

rtstestpilot

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It doesn't matter. There are only 2 measurements in weight that have to be taken - complete aircraft and the tail/nose wheel. If not weighting the whole aircraft at once (as with our models) then the sum of the 3 points of contact with the ground can be measured all at once or as the sum of any combination (as long as all 3 are taken) of the landing gear.

Todd

rtstestpilot

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I'd like to add some information that I had just sent acerc for any of you guys that may be playing with or using the CG calculator I posted earlier.


When you open it, note that there are 3 tabs down on the bottom left of the page. One sheet is directions for nose wheeled aircraft, another is directions for tail draggers, and the 3rd it the actual spreadsheet (it can be copied, renamed as one of your models and kept as a permanent record for that aircraft with the original used as a "master".) The upper left of the spreadsheet has a conversion table from fractions to decimals for the linear measurements. All you will need to accomplish your goal in a decent scale, the spreadsheet, some blocking to prop the aircraft up into its flight attitude, and a large enough flat and level surface (your build table) to set it all up on.
I have a magnetic build table similar to yours and I have marked a center line down the long length (center of aircraft) and a perpendicular line across on the short dimension several feet from one end that I center the aircraft and main wheels on respectively. Then use a carpenters square and a quick square to transfer the nose tip (datum for our purposes), desired CG, and nose/tail wheel positions down to the table for spot on accurate length measurements (all with the aircraft propped up in the flight attitude). Just make sure that if you are adding/subtracting weight to balance as opposed to simply moving components in the aircraft, the total aircraft weight will have to be adjusted appropriately. It may take several adjustments when altering total weight since it affects the wheel weight also but I can get it perfect within 3 to 5 adjustments to total weight if that makes sense. If adding weight, you can literally tape or place the added weight to the aircraft to find out where it will have to be placed or keep messing with the amount of added weight (altering total weight appropriately) to get it in the place that works best for you. I have, on a couple models, placed a specific amount of weight back at an easy to get to former position in the aircraft (on the outside) to get the proper wheel weight and exchange that amount for an equal amount in lead shot and epoxy (or small amount of gorilla glue) mixture and jigged up the aircraft in a vertical climb position so the mixture would settle and adhere to the former until set.

I used to double check the accuracy (with help from a friend or two) by mechanically balancing (some form of the "fingertip method") but no longer do this because it has always been spot on. I would suggest (as the directions suggest) that you also do this a few times for your own piece of mind till you are confident with this method. I haven't looked back and use this method on everything 1/3rd scale or larger for ease of use and accuracy. Unless you are careless with where you do this and with the propping up of tail wheel models in the flight attitude, there is absolutely no chance of damage from any jig used to suspend or hold the model up since it is only being supported by it's natural and designed means of support.

Lone Star Charles

The answer to your question is generally, yes. That does assume that both of the main gears are located at the same datum.

The only thing that might matter is that when using that method (or total weight and nose/tail wheel weight) you will not get any information about whether the plane is balanced laterally. That is not usually a problem anyway and is not solved by using other balancing systems like Vanessa or EZBalancer either.

Boomerang1

Thinking about it the Vanessa WOULD probably work for lateral balance.

Loop around each wing, wings level, pointer should point to the middle of the model.

One wing heavy if you adjusted loops to level the wings the pointer would be offset.

Something to try next time I balance a model!

John.

4ptroll

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PM sent

abufletcher

Well, there are no more parts left, so I guess I'm done. Using the very crude (and difficult) fingertip method, it is roughly in balance now. Fine tuning will have to wait until April when I return from a trip to the US with my Harbor Freight balance.

Propworn

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I was under the impression that hanging a full size from the CofG and lateral center of balance was not practical and that is why the scales. Even for large models suspending it is still practical. No calculations, no spread sheets just pick the measured CofG and observe how the aircraft hangs. How much simpler and fool proof can it get?

Dennis

rtstestpilot

My Feedback: (2)

The question of being practical is not the issue. It's mainly an issue of not damaging the aircraft during the balancing procedure (there are a number of aircraft that would damage easily especially with frail trailing edges). People that are thinking that a spreadsheet is difficult or too complex to use should not be afraid of them because it is simply not true. I personally have and happily use a Vanessa rig, and/or a couple of different EZ balancer style balancers; all very good and accurate tools that I now use for smaller aircraft which also happen to be a little more robust due to their size/weight ratio. I have gone to using a spreadsheet on every aircraft that is 1/3 scale or larger because it can be easier for the lone builder (me) to safely manipulate the aircraft, and as said before, with no chance of damage to the aircraft.

WacoNut

I will have to try the scale weight measurements on my 1/3 WACO,
I suspended the model to check the CG and it was tricky to say the least. The model weighs in the neighborhood of 60 lbs. I had to add several pounds to the nose to get the balance within range. It was difficult to suspend the model and not damage anything due to the weight. I used a 2 ton crane with 2" straps to disperse the weight and it still scared the (&%^ out of me but I got it done.

Will be interesting to see what the scales say.
Anthony

abufletcher

No it's not. Not for everyone. For example, in my case, I live in a traditional Japanese house meaning no big American garage with big, strong rafter beams. There is only one room in the house that is large enough to assemble my 3m wingspan model and even then there is only one location in that room that works. And as far as I can tell there is no beam in the ceiling at that point from which I could suspend any serious weight. Yeah, I suppose I could construct some sort of support framework (like you did to use at the field) but then the idea of this method being "easy" is out the window. And, like others have said, there is a great danger of damaging the model, unless we use something like your airfoil fixtures, which again means a step away from "easy."

And heck if a "speadsheet" is too scary, just draw a little table on a napkin and get out your pocket calculator.

But, I'm with you that I'd sure like to SEE a balanced model before I fly it. I'm not sure quite how I'll manage that. When I hoisted the model at the CG last night just to get a rough idea I could already hear the wood underneath the wing creaking.

abufletcher

BTW, very roughly the model weighs 11kg (24lbs).

OldScaleGuy

My Feedback: (2)

But, I'm with you that I'd sure like to SEE a balanced model before I fly it. I'm not sure quite how I'll manage that. When I hoisted the model at the CG last night just to get a rough idea I could already hear the wood underneath the wing creaking.

That is why in you need a piece of 1/8 plywood 5 X 10 (or something of that nature) to spread the load out at each wing tip and lift slowly when using the finger tip and lift method.

abufletcher

Yeah, I get that. But, you know, I also worry about putting such a heavy load on just the outer foot or so of each wing. This is the part beyond the struts so it would be relying entirely on the internal structural strength of the wingtip. I'm wondering about putting the ply sheets in-board (where you'd put your fingers for one-man finger balancing) and the build a simple fulcrum of some sort.

otrcman

Edited. See below.

otrcman

To answer your original question, Abu, yes you can weight your model just like "big boys" do it. You can do it with just one scale, providing it's fairly accurate. But it's better to use at least two scales. One scale would be employed to weigh the entire airplane. The other scale would be to weigh the tailwheel or nosewheel, whichever you have on your model. I've weighed full scale airplanes in sizes ranging from a B-52 down to light planes, and the principle is pretty much the same. The only "fiddly" part is that you have to measure the locations of the main wheels, the tailwheel, and some key point of structure quite accurately while the model is in it's weighing position.

Step 1. Chose some repeatable configuration for your model. That means knowing (and recording) what's in the plane and what's not in the plane at the time of weighing. A good choice is to have the plane absolutely ready for flight, no missing parts whatsoever, but with the fuel tank empty. Doing this avoids fiddly arithmetic down the line.

Step 2. Choose some solid, repeatable location on the structure that is easy to access. A good choice on most planes is the leading edge of the wing if it's a taildragger or the tip of the spinner if it's a nosedragger. It's easiest if you choose a location that's ahead of all the wheels. That's your "jig point". Write it down for future reference.

Step 3. Chose some means of repeatably leveling the airplane. There isn't any hard and fast rule on what you chose for level, just get the plane so it is pretty much in a level flight attitude and select a spot that is solid and repeatable. If you block the plane up to what appears to be level flight and place your level on top of the horizontal stab, that will be fine. If you have a digital level, so much the better. Check the level while it's resting on top of the horizontal stab and write down the number. If you level the plane so the stab angle reads zero, that will be very convenient. I know, I know, the stab incidence may not show zero on the plans. That's not critical for this process. Just be sure you know how to repeat the position for any future weighings.

Step 4. Now, determine the total weight of the plane. At this point you can use one scale or two, or three. The plane doesn't have to be level. It can be in a three point attitude and won't make any difference just for total weight. The important thing is that the scales need to be accurate to within about 1% of whatever the weight shows. If the main wheels or total plane show 24 pounds, then that scale ought to be accurate to about 1/4 pound. If the tailwheel weight is around one pound, that scale should be accurate to about 1/8 ounce. If you used one scale for the total, write it down now. If you used two scales, add the weights and write that number down.

Step 5. At this point you know how much the plane weighs, but you don't know where the CG is. Finding the CG is next.

Step 6. Level the model (Step 3) with just the small scale beneath the tailwheel. You already know the total weight, so there's no need to weigh the mains again.

Step 7. Record the weight of the tailwheel with the model level.

Step 8. Subtract the tailwheel weight from the total weight. This is the mainwheel weight. Write it down.

Step 9. Now comes the fiddly part. With the model still leveled, hang a plum bob from the jig point down to your work surface. Here is where using the wing leading edge is convenient. A nut on a thread makes a good plumb bob.

Step 10. Measure the horizontal distance from the jig point to the main wheel axles. Write it down !

Step 11. Measure the horizontal distance from the jig point to the tailwheel axle (or ground contact point for a taliskid). Write it down.

Step 12. Now comes the arithmetic.

a. Multiply the mainwheel weight (Item 8.) times the distance from the jig point to the main wheels (Item 10.) Write it down.

b. Multiply the tailwheel weight (Item 7.) times the distance from the jig point to the tailwheel (Item 11.) Write it down.

c. Add items 12a and 12b. Write it down.

d. Divide Item 12c by the total weight (Item 4.) Write it down.

Step 13. Item 12d is the distance from the jig point to the CG. If you chose the wing leading edge for your jig point, then this is your final answer. If you chose some other jig point (say, the tip of the spinner) you will have to measure from the tip of the spinner to see where your CG is.


This all sounds very complicated. It's really pretty simple, but you have to take it step by step. I'll show an example in a succeeding post.

Dick

otrcman

OK, here is a simple example of the process shown in post #66.


1. Measure the total weight of the plane. Any attitude is OK. 160 ounces ? Write it down.

2. Level the plane with the sensitive scale beneath the tailwheel. 12 ounces ? Write it down.

3. Subtract the tailwheel weight from the total weight. That's the mainwheel weight. 160 - 12 = 148 Write it down.

3. With the plane still level, measure the horizontal distance from the jig point to the main wheels. 5 inches ? Write it down.

4. With the plane still level, measure the horizontal distance from the jig point to the tailwheel. 40 inches ? Write it down.

5. Multiply 148 ounces times 5 inches. 148 X 5 = 740 Write it down.

6. Multiply 12 ounces times 40 inches. 12 X 40 = 480 Write it down.

7. Add 740 to 480. 740 + 480 = 1220 Write it down.

8. Divide 1220 by 160. 1220 / 160 = 7.625.

9. Congratulations. You CG is located 7.625 inches behind the jig point.

Note: This example assumes that your chosen jig point is forward of all of the wheels and forward of the presumed CG. If you have chosen a jig point that is AFT of any wheel, then the measured distance needs to be written down as a negative number. Then when you multiply the minus distance times the plus weight, you have a negative answer. Remember your ninth grade algebra ? Mrs. Smith told you it would come in handy some day.


Dick

abufletcher

The neat thing about the spreadsheet that rstestpilot made available is that it works "backwards" in that you put in the total weight and distances and CG and then it automatically calculates what the weight SHOULD be at the tailwheel location for the model to be in balance. This means that once you have all the numbers you can put the scale under the tail and just move stuff around until the tail weight is what it's supposed to be for a given CG.

Again, there's a difference between calculating where the CG currently is and using a fixed CG (based either on designer recommendations or aerodynamic calculations) to calculate how the model should be balanced (at that CG). In other words, it is features of aeronautical design like the MAC and wing sweep and so on that determine an ideal CG range. Balance is the process of setting up the airplane so that it's neither nose-heavy nor tail-heavy at an aerodynamically-determined CG. Since what we want here is how to balance at a pre-determined CG, the spreadsheet is ideal.

otrcman

Yes, the spreadsheet is a real labor saver. But one must be careful to not substitute the spreadsheet for thinking. If a person were to jump into the spreadsheet method without actually understanding the process, he might overlook one of the basics. An example might be to weigh the tailwheel with the airplane not level to the same reference that was assumed in the spreadsheet.

My long form explanation in post #66 wasn't meant to imply that it is the only way to measure center of gravity location. It was to show the basic principles involved. Once you are aware of the underlying principles, then you can mechanize the process by any means that you desire.

Remember the old computer maxim: Garbage In = Garbage Out.

Dick

abufletcher

You're absolutely right and thanks for taking the time to write out the step by step instructions. I plan to check my own math against the spreadsheet math, not because I don't trust the spreadsheet but because I want to see whether my own thinking is right. There is a recommended CG given in the ARF assembly manual (4.9" back from the LE) and I'll consider that in light of the usual 20-30% MAC recommendation (on a simple equal cord wing). In fact, that 4.9" is more towards the rear of that range. I might split the difference and select 25% as the desired CG point. I will then use that position in determining the balance.

Again, (for others) note that to you FIRST have to establish a CG (based on aeronautical engineering) before determining if the model is BALANCED at that point.

abufletcher

Another way of saying this is there are many possible CG locations for any given aircraft or model, some of them more suitable for certain things than others (and some with likely disastrous consequences). But for each of those possible CG locations there is ONE AND ONLY ONE correct balance. There are never numerous "balances" for a given CG. Those people who say they like to "balance with the nose a bit down" are actually just saying that they like to place their fingertips (or whatever) behind the point at which the model would actually be balanced.*

*BTW, I'm one of those people who use this crude "rule of thumb" to give myself a little margin of safety on maiden flights since it's much better to be nose-heavy than tail-heavy on a maiden. On my smaller models I like to see a downward angle of about 5-10 degrees.

ColinW

There's a thread on the jet page talking about this, and it might be worth a look at this site: http://www.xicoy.com/catalog They do a meter that is similar to full size, and does the calcs for you

MajorTomski

Abufletcher, you may not have to wait till April. Just like TFF said back in post #8, put your .1KG scale under the mains and your gram scale under the wing and run the numbers just the way they come out. Then plug in the weight on the mains plus 0.1kg and minus 0.1 Kg and see how much that moves the final answer on the CG.
You stated that the total weight came in at 11 Kg. Works out nice let's say when we actually get it on two scales it comes in reading the mains at 10.1 Kg and the tail comes in at 1.0 Kg.
Run the spread sheet for this pair then plug 10.0 or 10.2 Kg in the spread sheet and see how far it moves the CG.

Did you scratch this Paulistinha or is the The World Models ARF?

abufletcher

To get the rough weight I just used an uncalibrated bathroom scale (stepped on the scale myself then with the model in my arms). It has a digital display that reads in 0.1kg increments but I doubt very much that it would match the readings on some other bathroom scale. I'll wait until I have a proper scale. As Dick reminded me in his post about: Garbage in, garbage out.

ARF. I still can't bring myself to spend a minute of building time on anything other than a WWI model. And even this ARF is primarily a way of getting some flying time on the Seidel 770 (and experience flying larger models) as training for a future 1/4 scale 2-seater (Strutter).

Propworn

My Feedback: (3)

I don't know but looking at your work space if you have room to assemble the model then there is enough room to build a balancing jig that you can take down an put away. It would work for any smaller model you would build as well.

Dennis