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CG calculation by weight

Old 04-06-2015, 07:31 AM
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grosbeak
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Default CG calculation by weight

I like to calculate the CG of my tail draggers by weight, and I have created a new and simplified version of the spreadsheet to do so. All credit for the method I use goes to Tom Hastie's article.


The spreadsheet is available to view by all at CG Calculation template; you have my permission to copy it and use it as you like.


Here's a look at the layout.





Here are the steps I use to calculate CG:


First time with this airplane


  1. Enter the model name and details
  2. Rename the tab to the current date
  3. Measure and record the lengths of D and CG(s)
  4. Measure and record the weights of W(p), W(s) and W(t)
  5. Note the calculated values for W(tr) and CG(d); the closer you can get W(t) to W(tr), the closer CG(d) will be to zero (perfectly balanced)



Subsequent times with this airplane


  1. Make a copy of the most recent tab
  2. Enter the model details (I record the changes since the last CG calculation)
  3. Rename the tab to the current date
  4. Measure and record the weights of W(p), W(s) and W(t)
  5. Note the calculated values for W(tr) and CG(d); the closer you can get W(t) to W(tr), the closer CG(d) will be to zero (perfectly balanced).



Notes:


  • It is best to move components (e.g., batteries) to achieve the desired weight if possible, but ballast may be required.
  • I prop the tail up to weigh the plane in an attitude of straight and level flight but it's not absolutely necessary to do so
  • Remember that this is just a starting point; you will likely need to fine tune the CG after flying



Feel free to respond here or PM me if you have any questions.

Last edited by grosbeak; 04-06-2015 at 08:05 AM.
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Old 04-07-2015, 07:56 PM
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52larry52
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grosbeak, I didn't reply last night to your C/G calculation formula cause I couldn't even began to follow it, it's way too complicated and I'am way too stupid! I thought I would wait until someone else replies and explains what your talking about. Well no one else replied so I guess no one else could figure it out either ! I'll have to stick with the 25-33% of the M.A.C. system for my C/G calculations. "K.I.S.S.", Neat chart tho.
Old 04-07-2015, 08:08 PM
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52larry52
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Oh my God, I just reread your post and saw that this is the "simplified version". I didn't catch that on the first read last night, but now that I know it's been "simplified", I really feel stupid! Yea, I'am being a little, no make that a lot sarcastic. Seriously my friend, that's seems like a long way to go for a C/G answer.
Old 04-07-2015, 10:10 PM
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52larry52
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OK, I figured it out.....The whole thing with the complicated C/G formula was an April Fools Day joke and in Canada April fools day is on the 7th not on April 1 st. like here in the states. Right??
Old 04-08-2015, 06:11 AM
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Jennifer Curtis
 
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The formula is actually a simple one.

Start by measuring the distance from a fixed point
on the plane. I use the end of the prop shaft or spinner
because it makes the math simple (no negative numbers)
and it works for tail draggers and tricycles.

Multiply distance from the fixed point back to the main axle
and the weight on the main gear.

Multiply distance from the fixed point back to the tailwheel
(or nosewheel) axle and the weight on that wheel.

Add these two numbers and divide by the total airplane weight.

The result is the distance from the FIXED POINT to the CG.

Jenny

ps.

It doesn't matter what units you use to measure and weigh
as long as you don't switch in the middle. (Don't weigh the
whole airplane in pounds and the tail in ounces or grams.)

Don't try to measure with mixed units like 2 pounds 4 ounces
or 2 feet 3 inches. Use 2.25 pounds and 2.25 feet
(or 36 ounces and 27 inches).

It does matter if you don't raise the tail on a tail dragger
to make the plane level. If you don't raise the tail when
you are weighing, the CG will be slightly farther forward
when the plane is in level flight.

Last edited by Jennifer Curtis; 04-08-2015 at 06:20 AM.
Old 04-08-2015, 10:16 AM
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speedracerntrixie
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I guess Im with Larry. I just dont see the value of going through this process for an initial setting that I will be changing a couple times before I am happy with its placement. Now if you knew exactly where you wanted to place the CG and expected no adjustments it may be useful.
Old 04-08-2015, 10:35 AM
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Jennifer Curtis
 
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I find it particularly helpful on planes I have trouble lifting.
It also tells EXACTLY where the CG is.

With a little more math it can be used to figure exactly
how much weight to add and where to add it to get the
CG where you want it.

Jenny
Old 04-08-2015, 03:57 PM
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speedracerntrixie
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Jenny, that is my point. It will tell you exactly where the current CG is but will not tell you where it needs to be. It will tell you how much weight to add to get it at the estimated location. The only way to get the CG where the airplane performs best is to go fly the airplane and then make adjustments. Even then there is no mathematical formula to tell you how much of a CG shift is required to gain what trim result. I set my CG to a safe location and fly the airplane. I make adjustments based on how the airplane flies. There is no formulas that will do the same.
Old 04-08-2015, 05:16 PM
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Top_Gunn
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Originally Posted by speedracerntrixie View Post
Jenny, that is my point. It will tell you exactly where the current CG is but will not tell you where it needs to be. It will tell you how much weight to add to get it at the estimated location. The only way to get the CG where the airplane performs best is to go fly the airplane and then make adjustments. Even then there is no mathematical formula to tell you how much of a CG shift is required to gain what trim result. I set my CG to a safe location and fly the airplane. I make adjustments based on how the airplane flies. There is no formulas that will do the same.
That's true. But you do need to know where your CG is, don't you? This is one way to find out. (I use a Vanessa rig, myself. That way I can hang the plane on it and take a coffee break. No math to hurt my head, either.
Old 04-08-2015, 05:39 PM
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52larry52
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Well, Glad to see we got some dialogue going on this. I thought I was going to be left hung out to dry messing with grosbeak. It looks like us non-engineer types will use the 25-33% then adjust method and the engineers with slide rules, errrr, I mean calculators will use long over the top math calculations to get to the same point. I'am still listening and learning, keep going.
Old 04-08-2015, 06:27 PM
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speedracerntrixie
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Originally Posted by Top_Gunn View Post
That's true. But you do need to know where your CG is, don't you? This is one way to find out. (I use a Vanessa rig, myself. That way I can hang the plane on it and take a coffee break. No math to hurt my head, either.
Yes good point. I suppose what I am trying to get across is that you can use these equations to get in the ballpark based on where you think your CG should be and then adjust after a few test flights.
Old 04-08-2015, 07:08 PM
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radfordc
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Full size planes use the computational method to establish the CG....cause it's hard to find two guys with really strong index fingers to lift under the wingtips. While it works for any size plane, it's a big "finicky" for models. The Vanessa rig is certainly easier and quicker.
Old 04-09-2015, 07:05 AM
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grosbeak
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Originally Posted by speedracerntrixie View Post
Jenny, that is my point. It will tell you exactly where the current CG is but will not tell you where it needs to be. It will tell you how much weight to add to get it at the estimated location. The only way to get the CG where the airplane performs best is to go fly the airplane and then make adjustments. Even then there is no mathematical formula to tell you how much of a CG shift is required to gain what trim result. I set my CG to a safe location and fly the airplane. I make adjustments based on how the airplane flies. There is no formulas that will do the same.
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.
Old 04-09-2015, 04:10 PM
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speedracerntrixie
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I don't think I'm missing anything here. Then again I have been flying for almost 40 years and the way I tune CG may be somewhat old school. I set CG by how the airplane is flying. Not what someone decided to write in a manual. Take your Sbach for example. What does it do when you put it into knife edge? If the CG is correct it should have a very slight tuck to the landing gear. If its nose heavy it won't tuck and may actually pitch towards the canopy , landings will be fast and pitch trim will change dramatically with speed changes. Do you set your CG once per your calculations and leave it there or do you tune it as part of your trimming process?
Old 04-09-2015, 05:44 PM
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Hey there speedracerntrixie, lighten up a little, will ya? You guys are saying the same thing.

If you look at Grosbeak's first post, in the last line of the Notes section he says " Remember that this is just a starting point; you will likely need to fine tune the CG after flying"

Seems to me that is exactly what you are saying, is it not?

Grosbeak is trying to give modelers an alternative way to get "in the CG ballpark" as you say, with an added benefit of giving you the total weight of your airplane at the same time. He's not saying that this method is the be-all and end all, he's presenting another option.

I too have been flying for over Forty years, but I'm not afraid to have somebody show me something new.

Regards,

Dennis

Last edited by DennisD; 04-09-2015 at 05:47 PM.
Old 04-09-2015, 09:16 PM
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52larry52
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grosbeak, I am starting to see what you're doing here but still don't see the value in doing all that weighing and calculating. Yes, I am very old school in my approach to building and set up methods. I really do try to operate by the "Keep It Simple Stupid" motto. Having said that, I have an observation, a question and a comment. Observation; It seams that in your chart all you gain between line #9 and line #14 is a theoretical amount of weight needed to balance the plane to the previously given C/G point. I call it a "theoretical" amount of weight because in actual practice you can't always get the ballast to be added directly over the tail wheel (if that's where it's needed, or it could be forward somewhere sometimes). The physical layout and construction of the plane will influence where the weight can be safely added, so that weight number may be a little off. Question; do you weigh all three landing gear points at the same time (needing three scales) or can you weigh each point one at a time and just prop up the other two points as you move the scale around? Comment; When I C/G balance a plane I first get it close to the point it won't fall or slide off the balancer (at the previously given or determined C/G point) then add 1/4 once segments of lead weight to the area of the plane needed to bring the plane to a level pitch position on the balancer. Real easy and fast to do by adding or removing lead segments right on the balancer. Then attach the lead you just measured out in the spot you had it, double check it on the balancer one more time and your done, at least until you fly it and tweek it to where it flys like you want it to. Again my way is a hands on mechanics way, not an engineer's method to get to the same point. And BTY, my feelings were hurt that you ignored my attempt early on in this thread to stoke it up when no one replied to it . I hope you took my April fools day comment in the fun way it was intended. Do you do that April fools day thing in Canada also? Regards
Old 04-10-2015, 06:00 AM
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You can weigh each wheel on a separate scale if you
wish, or do one at a time while propping up the other
wheels.

I usually only weigh the tailwheel (or nosewheel) with
the plane assembled. I weigh all the parts separately
(wing, fuse, any other parts) and add them to get the
total. Subtract the tailwheel weight to get the main
gear weight. If you do this you need to remember
to include EVERYTHING, like rubber bands, screws,
wing rods, struts, flying wires, etc.

It may sound like a lot of trouble, but on a really
big plane it is actually a whole lot easier than
muscling a big plane around the living room.

Jenny
Old 04-10-2015, 06:41 AM
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Double post

Last edited by DennisD; 04-10-2015 at 06:52 AM.
Old 04-10-2015, 06:46 AM
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Originally Posted by 52larry52 View Post
It seams that in your chart all you gain between line #9 and line #14 is a theoretical amount of weight needed to balance the plane to the previously given C/G point. I call it a "theoretical" amount of weight because in actual practice you can't always get the ballast to be added directly over the tail wheel (if that's where it's needed, or it could be forward somewhere sometimes)
Larry,

I will try to explain your above statement.

Lines 9 to 11 are the actual weights taken at the scales under the gear.

Lines 12 to 14 are values calculated by formulas resulting from these weights.

So what line 14 tells you is that if line 11 is less than line 14, your plane is nose heavy, and if line 11 is more than line 14, your plane is tail heavy.

The difference between the two is the amount of weight you have to move either forward or backward in order to balance perfectly.

The weight does not have to be directly over the tailwheel, it just has to add or subtract weight on the tailwheel scale until Line 11 equals Line 14.

Hopefully this makes it clearer to understand?

Regards,

Dennis

Last edited by DennisD; 04-10-2015 at 06:51 AM.
Old 04-10-2015, 07:05 AM
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Originally Posted by DennisD View Post
Larry,

So what line 14 tells you is that if line 11 is less than line 14, your plane is nose heavy, and if line 11 is more than line 14, your plane is tail heavy.

The difference between the two is the amount of weight you have to move either forward or backward in order to balance perfectly.



Regards,

Dennis
Here is what I see wrong with those two statements. Both of those statements assume that you can achieve the perfect balance statically. Not possible. Most kits and ARFs will give you a starting point and for 99% of the airplanes out there it is very easy for you to verify that location by simply picking up the airplane at the desired balance point. You will have to make adjustments from there. If you are not then your CG is not set to optimal. Jennifer makes a point that some airplanes are not easily picked up due to size. Well you have to reach under the airplane and pick up part of it to place a scale right. My 40% Extra was initially balanced by making a sling that went around the wing tube and up to a pulley on my garage door frame. I pulled the airplane a few inches from the ground and moved the RX batteries around until it hung level. I knew that the wing tube was 30% of the MAC. After two battery placement adjustments the CG is perfect for that particular airplane.

I have nothing against people using the scales and weights method, just don't sell it as easier and more accurate. Dennis, as you obviously fly IMAC you should know that it takes a few flights to get CG sorted out.
Old 04-10-2015, 07:39 AM
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Lone Star Charles
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Nice analysis, Grosbeak. I use a similar routine except that I use the front of the spinner as "station 0" and then define all distances aft as positive. I only have one little postal scale that I move from wheel to wheel while blocking the unmeasured wheels so that the plane stays level. Like you, I find it nice to know the total weight of the airplane as well as exactly how much weight to shift (or add) so that I can get the CG where I want it. Although the "trial and error" method is very valid, and close enough for most models, I like the precision that your method offers.
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Old 04-10-2015, 08:12 AM
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speedracerntrixie
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Charles, would you care to explain to me how to determine precision CG placement on a model that you have yet to fly?
Old 04-10-2015, 08:32 AM
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52larry52
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Thanks to all for helping me understand this approach to C/G setting. When I first read grosbeak's post and chart it was truly over my head and I couldn't see any value in going through all that. I do now understand what you're doing at each step and why it is good in some cases. I thankfully don't have the too big a plane for the work space problem that Jenny points out so I have not faced that. A rascal 110 and a 1/4 scale Cub are my biggest and both are less than 15 lbs or so and I have a large open area in my shop. I do have an electronic scale (one only with a 5 lb max) that I will play around with under a T.F. 1/5 th scale Cessna 182 that I just balanced old school just to get a feel for this approach. It's the "old dog, new tricks" thing and I am the old dog!
Old 04-10-2015, 09:25 AM
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grosbeak
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Speed, I do indeed feel that the specified CG is just a starting point. I did a LOT of tuning with the SBach to get it right. But you do need a starting point, agreed? It's got to be in the ballpark if you want it to survive the first flight. Why not try to nail it down to the exact specification? Bear in mind that I've been in the hobby a tenth of the time you have and I haven't built up the intuition and confidence to, ahem, wing it. :P

I agree that there isn't such a thing as perfect static balance - you can only strive to match the specs on the ground, and it will only be perfect in flight if you happen to be very lucky... it COULD happen!

As for picking up the plane... the bigger they are, the harder it gets!

- - -

Dennis, thank you for the kind words. I think we're all looking at this from our own points of view.

- - -

Larry, sorry I hurt your feelings, Yes we have April Fool's day here in the Great White North. Dad's birthday, as it happens! :P

Regarding your point, specifically on line 14, I have failed to be clear here - that number works as long as the total weight stays the same. In other words, changing the balance by moving stuff around inside the fuse. If you add ballast, no matter where, you need to measure and enter all the weights again.

I do weigh all three wheels at a time (I think the scales were about $15 each) but one at a time is fine as long as you prop the other wheels to the height of the scale. Note the CD jewel cases mentioned in the original article (link in Post #1).

As for the balancer, I have one... but all of my planes since the first trainer have had a WS of 6'+... and it's very tricky to maneuver large low-wing planes onto a balancer upside-down (as they should be).

- - -

Charles, I appreciate that precision and accuracy are not the same thing!

- - -

To all... I wouldn't try to sell my approach as the easiest or best way to help you get your plane to its correct CG... I'm just refining a method that I happen to like.
Old 04-10-2015, 11:15 AM
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52larry52
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OK, I'am gonna "tap out" on this one (I quit, I'am done)! I went down to my shop, in my large basement, got out my 5 lb electric scale that I use for weighing parts, put my 12.4 lb T.F. Cessna 182 on with equal height blocks under the other wheels to keep it level and the scale went into "error" mode as the main gear weight on each wheel exceeded the 5 lb max of the scale that I have. Nose gear weight was 15 1/2 oz so each main had 5 1/2 + lbs and I couldn't do the exercise. May try it again another day with a smaller model and then again I may not bother. An unnecessary exercise for me! I going to fly the T.F. Cessna on Sunday and have no doubts about it being very flyable with My "stone age" method of initial C/G setup. I have a similar kit built T.F.Cessna 182 and am quite comfortable with it's flight. Cheers

Last edited by 52larry52; 04-12-2015 at 07:10 PM. Reason: Error in weights quoted

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