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CG calculation by weight

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CG calculation by weight

Old 04-10-2015, 12:13 PM
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I just like using my fingers......

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Old 04-12-2015, 11:42 AM
Lone Star Charles
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Originally Posted by speedracerntrixie View Post
Charles, would you care to explain to me how to determine precision CG placement on a model that you have yet to fly?

Certainly - Let's use your 40% Extra (nice plane by the way) for our example and that you want the CG set precisely at 30% of the MAC. A simple weight check of each wheel will give you a very precise weight of each wheel. You can also measure the precise distance from the aircraft datum to each axle. Multiplication of the weight of each wheel times its distance from the aircraft datum will give you the moment for each of the three points. The sum of the three moments divided by the total weight of the aircraft will give the precise location (arm) of the current CG.

Let's say that the answer that you got turns out to be that the CG calculates to 31%. We will call that CG(31%) and its corresponding moment as MOM(31%). It's easy enough to calculate the 30% CG moment by just multiplying the weight of the aircraft times the desired arm = MOM(30%). If the aircraft datum is selected to be at the front of the spinner, you will notice that MOM(30%) is less than MOM(31%). Subtract MOM(30%) from MOM(31%). Divide that difference by the weight of the battery. This will be the precise distance that the battery should move forward in order to achieve a precise CG at 30% of the MAC.

One move and you are done!

Now, let's go fly. If you are not satisfied with the CG at 30%, you know exactly how much to move the battery to get any change in CG location that you want - precisely.

To be perfectly honest, this method is a quite a bit more cumbersome and only slightly more accurate than the thumb balance. But it is precise.

Hope this helps. Cheers.
Old 04-12-2015, 07:27 PM
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Today I test flew my T.F. Cessna 182 ARF with the "stone age" C/G set up. Sweet...two clicks of up elevator, three clicks of right aileron and that was it. Nice bird., successful maiden flight. It was overcast and a little windy today and I did manage to bent the nose gear a little landing (I'll take the blame for that). It still has the known to be weak stock nose gear and it was not my best set down so I'll straighten it and try it again before going for a nose gear upgrade. This is a third hand, used, "bargain priced" 182 that I rescued; 3 months of on and off repair and redoing paid off with another nice airplane in the hanger.
Old 04-14-2015, 06:48 PM
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Charles thanks for the complement and taking the time for the explanation. I did know that you are able to figure out exactly where you want to place your CG and easily make the adjustment. This is of course dependent that you know where it should be. Lets continue to use my Extra as an example. By making some quick measurements I was able to figure out that the wing tube is at 33% MAC. Initially I set my CG at the forward edge of the wing tube. After a few initial test flights it was apparent that I needed to move the CG forward. What I observed was that with the application of rudder to either direction the airplane wanted to tuck towards the gear. This would happen in horizontal, vertical and knife edge flight. As this airplane is intended to be flown in IMAC contests this trim situation needed to be corrected. What was happening is that down trim was being used to compensate for the rearward CG. For whatever reason when rudder is applied it makes that down trim more effective. The solution is to move the CG forward. Question is how much? there are a few things that will influence this. Cowl shape, how much % area is the stab in relation to the wing. Stab height, fuselage shape all are contributors. At that point one must resort to trial and error. In my case I moved the dual 5000 mah RX batteries forward 8" and got very good results but moving them an additional 2" got me spot on. Now the tuck is all but gone and a TX mix removes it completely. I could have gone farther forward with the CG and removed it 100% that way but then other flight attitudes would have suffered. Trimming is always a balance, take or add here and it affects something else. Larry commented on his 182 having a rough landing. It would be a nice experiment to know where his CG placement is. I have found in almost all cases a nose heavy airplane is more difficult to land then one with the CG placed closer to a neutral point. In Larry's case if the airplane is landing a bit fast and requires a fair amount of elevator on final and flair then he sould experiment with moving the CG back a small amount at a time.
Old 04-14-2015, 08:19 PM
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I had the C/G on the Cessna 182 set exactly at 4 5/16" per the Top Flight instructions. I have another T.F. 182 (this one kit built) also set @ 4 5/16" which flys nice so I didn't do the 25-33% MAC calculation, I just went with 4 5/16". Now, I only made one landing and it was not by the book so I will fly it some more to evaluate the C/G before any changes are made. I will normally land this type of plane with the mains touching down before the nose wheel but because of circumstances and pilot decisions that didn't happen and the nose wheel hit first slightly bending the nose gear wire and knocking the steering out because of thread failure in the steering arm set screw. This is a used plane and I didn't catch the damaged threads in the steering arm. Not being able to steer it was the reason I stopped at one flight, not the bent gear wire as the bend was very minor. That and incoming foul weather let me call it a successful maiden flight and returned to the shop to address the nose gear issue. It's fixed now and will be reflown next time I fly. I was flying at a sod farm where we can land any direction we want, not just on a fixed direction preset runway. This is usually cool but this time it hurt me (pilot's decision) as I came in on final from my right side, pretty normal, than made a right turn into the fairly stout wind so my view of the plane and landing was from directly behind the plane and my speed and flare judgment was poor. I should have just landed it as a crosswind landing so I had a good view of the plane but I didn't. No harm of any consequence, I just had to call it a day with only one flight. This weekend is not looking good weather wise with 80% rain in the forecast. All in due time.
Old 11-05-2017, 09:05 PM
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Grosbeak, Thanks for posting this spreadsheet. We have a guy in our club that has a Xicoy that does the weight-moment calculations for you. Many of us have used it to calculate CG on our larger planes. I'm doing a scratch/ Cad design/ build of a 104" Eindecker and really didn't want to cart it over to his house for balancing. This spreadsheet will fill the bill perfectly. Just like his Xicoy minus the $400. Thanks, Dave
Old 11-06-2017, 06:25 AM
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I also use an "Xicoy like" system using 3 cheap scales. Here is a thread with pics of how I do it: http://www.rcuniverse.com/forum/tips...ge-planes.html

Yes, you can set the CG by lifting the model with your fingers at the 25% point...that gets you close, but gets hard to do with really big planes like my 75% Eindecker... https://www.rcgroups.com/forums/show...&postcount=633

Yes, you can set the CG using a "machine"....CG machine or Vanessa rig. I have both a Vanessa rig and this one: https://www.rcgroups.com/forums/show...asy-cg-machine
Old 11-06-2017, 08:02 AM
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Thanks Rad. Those are all great threads. My Xicoy guy is gonna be bummed he spent the $$.
Old 02-16-2018, 07:34 AM
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Default Agree with dpetsel!!

I fly full scale to and this is same method we use... It's fool proof and I don't have a couple thousand dollar gasser hanging in from the ceiling, yikes!. And why spend $$$ for the xicoy! If you don't like the math make an Excel spreadsheet 😁
Old 03-27-2020, 05:27 AM
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Wow! You are a real scientist that works for people.. Thx dude! Oh, by the way not so long I faced some problems with the calculation of my vehicle. So turns out that my weights were in Kilograms but not in lbs... So I've decided to make some search and turns out that there is a converter that works online and convert 175 lbs to kg. It can calculate any amount of units that you want. I am very glad to these guys that I can do the work distantly on different devices... So if anyone wants it, give it a try...
Old 05-03-2020, 05:35 AM
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Default Spreadsheet needs Macro correction

Originally Posted by grosbeak View Post
Take another look... CG(s) is the specified CG - that can't come from the calculation but must be provided as input, from the manual, 25-33% estimate, WAG, whatever. CG(a) is the ACTUAL CG as calculated by the weights on the wheels and the measurements you took. If CG(a) is smaller than CG(s), the actual CG is further forward than the specified CG and the plane is nose heavy; vice versa and it's tail heavy.

Note also W(tr) - the required weight at the tail wheel for CG(a) to equal CG(s) (as long as you are moving components around inside the airframe and not adding or removing weight).

Everyone has their favourite method. This one is dead simple for me to use and does not require me to suspend my model from the ceiling, which is why I will continue to use it.
To grosbeak,
I am familiar enough with Excel to appreciate what you did. In my line of work, now retired, we had to have someone independently verify Excel formulas. You reference Tom's PDF as the source. I plugged in Tom's numbers and verified that calculations are correct, but the IF Macro declaring the "Tail Heavy" "Nose Heavy" and "Balanced!" is backwards. Your Template should be reversed, namely it should read. =IF(INT(B16)<0,"Nose heavy",IF(INT(B16)>0,"Tail heavy","Balanced!")) NOT =IF(INT(B16)<0,"Tail heavy",IF(INT(B16)>0,"Nose heavy","Balanced!")). This was true for a Tail Dragger, Tom's plane. Actually reverse for Trike Gear.

Recommend adding another separate IF Macro for the Trike Gear scenario which is opposite as your original template macro was for the Trike Gear.
"=IF(INT(B16)<0,"Nose heavy",IF(INT(B161)>0,"Tail heavy","Balanced!"))" for Tail Dragger
"=IF(INT(C72)<0,"Tail heavy",IF(INT(C72)>0,"Nose heavy","Balanced!"))" for Trike Gear

In addition, I recognized the sensitivity of unit precision used and how it affects the macro statement. Using inches would declare Tom's plane balanced, and as he stated, it was Tail Heavy. Therefore adding a note that one should use perhaps cm for large planes and mm for small planes, the macro would work better because you are using the INT command which "rounds down" the calculation to a whole integer, a.k.a. number without decimals.

My conclusions of your spreadsheet are as follows:
1. Description "Tail Heavy" and "Nose Heavy" was reversed in macro. (For a Tail Dragger) Try it by putting in large weight in tail which makes macro read "Nose Heavy." Leave alone for Trike Gear.
2. Using Tom's plane as example, formulas are correct.
3. Units used, i.e. in. vs. mm greatly impact macro statement, NH, TH, or Balanced! Because INT command rounds down.
4. Use cm or mm based on plane size, mm if small plane. Inch units too large.
5. Moving weights different than adding weights; proportions change
6. If CG(d) in cell B16 is positive, then plane is Tail Heavy (for Tail Dragger, Tom's plane) Opposite statement for Trike Gear. because cell B13-B8 = CG(d) the calculated CG.
7. If CG(d) in B16 is negative, then plane is Nose Heavy (for Tail Dragger) Opposite macro statement needed for Trike Gear. (as originally listed)
8. CG location based on plans or manual recommendation is a place to start. Actual flying determines aerodynamic CG and pilot preference. Being more TH affects static margin and plane usually flies with quicker control response. but susceptible to stall at slow speeds Flying upside down and seeing how plane reacts decreasing throttle usually tells you if CG location is aerodynamically correct. It should glide and descend slowly.
9. You made a simple calculator automating the simple algebra / math used to calculate CG by weight. This is great for me, especially with large planes because they hurt my fingers when I lift them!

PS I would also change cell format to see at least 1 decimal if not 2 as a matter of precision and avoid rounding errors by Excel.

Last edited by szempruw; 05-03-2020 at 08:56 AM.
Old 01-13-2021, 09:39 PM
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Just found this Thread and joined RCUniverse to comment.

I found the spreadsheet useful. it's not perfect, but it is very close to what I've been scribbling on paper. On models that are getting greater than 10 lbs, I don't like to balance them, repeatedly, as I'm adding weight. The Venessa Rig certainly works, but you've got to have the real estate available for the hoisting rig. I prefer the weighing method. It's a very solid, accurate, and safe way to determine the current CG of the plane. The other respondent (I know that it's been years ago) who was wanting the spreadsheet to spit out what the CG is supposed be... that's nuts! Where the CG is supposed to be is a completely different spreadsheet that determines the MAC, based on the the wing geometry. That's a totally different algorithm than for balancing and determining the actual CG.

I have a couple of comments on the spreadsheet.
1. the cell, in which the user can select trike or tail-dragger, does nothing that I can see. It should be tied into the cell that declares NEUTRAL, TAIL HEAVY, or NOSE HEAVY. Tail heavy and nose heavy are the wrong polarity for tail draggers.
2. The cell that states how much the weight should be on each Main Wheel, in order to balance, is pretty worthless. It'd be much more useful, if there was a cell for the distance from the Main Gear to the Nose, and using that for the weight location predicts how much weight needs to be added to the nose to achieve the desired CG location. I will probably attempt to add this to my copy of the spreadsheet.

I find the spreadsheet useful and appreciate you putting it together.
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Old 06-22-2021, 04:06 AM
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I cannot understand why not using just your index fingers to balance your airplanes ?! It 's so easy to do!
This method works for nearly all airplanes. I have been using it since I started R/C flying in 1975.
But as "speedracerntrixie" said earlier in this tread. the first CofG placement is just a guess, you must then fly the plan several times. moving the battery, servos and receiver to obtain the "perfect" CofG setting.

Last edited by jaka54; 06-22-2021 at 04:08 AM.
Old 06-22-2021, 05:32 AM
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Originally Posted by jaka54 View Post
I cannot understand why not using just your index fingers to balance your airplanes ?! It 's so easy to do!
Once you get a plane that is too long and/or heavy to get your index fingers into the proper position, you'll understand. Then it is time to build fancy balance stands, Vanessa rigs, or to do the math.....
I've used the weighing method many times now, and it can be very precise to help set the initial CG before first flight. The bigger and heavier the model, the more precise it can be. And the math can be done to tell you what should be moved/added, where, to get it to the desired point. For traditional "smaller" models fingertips are just fine.

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