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Old 12-23-2012, 03:39 AM   #1
mclarkson
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Default Guestimating CG during a build - any tips?

I've come to enjoy scratch building over the past couple of years and have had some success.

One thing that vexes me, though, is CG placement. I'll expect the plane to come out, say, nose-heavy so I put the servos, etc., way way back only to have the plane come out slightly tail-heavy. So now I'm in a scramble to shift the battery farther forward than I'd designed for, etc.

I never seem to know where the CG is going to come out until I've built the thing and, by then, it's too late. ("Man, I wish I'd put those wing servos way at the front instead of way at the back!")

I don't know if I'm being clear at all. Maybe the only way is to build it, realize how you ought to have built it, and then build it again, but I thought I'd ask if there were any tips or tricks.

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Old 12-23-2012, 03:52 AM   #2
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Here's a good example from a scratch build I've just started.

How far forward the motor goes will make a big difference to the CG. But, because of the way I'm building it, the motor mount will have to go on fairly early, which means I'll have to cut the CF arrow shaft (fuselage) at some point. I could put the motor anywhere in a fairly wide range but, until I get farther along in the build, I won't know if I should have mounted it farther forward or farther back. By then, of course, it's too late.


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Old 12-23-2012, 04:15 AM   #3
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Originally Posted by mclarkson View Post
I've come to enjoy scratch building over the past couple of years and have had some success.

One thing that vexes me, though, is CG placement. I'll expect the plane to come out, say, nose-heavy so I put the servos, etc., way way back only to have the plane come out slightly tail-heavy. So now I'm in a scramble to shift the battery farther forward than I'd designed for, etc.

I never seem to know where the CG is going to come out until I've built the thing and, by then, it's too late. ("Man, I wish I'd put those wing servos way at the front instead of way at the back!")

I don't know if I'm being clear at all. Maybe the only way is to build it, realize how you ought to have built it, and then build it again, but I thought I'd ask if there were any tips or tricks.
Normally on most front wing planes, you are always going to come out tail heavy, so install the servos as far fowards as possible, on wing planes like your building, estimate the CG, drill 2 small 1/8" holes at the cg area on the wing, get 2 sharp pencils and mount them in a 2x4 or 2x6 pointing straight up and the same width as the 2 holes you drilled in the wing, lay the wing on the 2 pencils in the holes in the wing, now lay and tape the electronics on the top of the plane, you will have to move them around to find the balance for the cg, a little nose heavy is best for a maiden flight, then adjust from there when flying, Hope that helps, Chellie

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 12-23-2012, 07:49 AM   #4
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I know where you're coming from ... have same problem ...

Look at the QB10 I'm knocking up ... I've decided to put the servos at back end of fuselage ... I reckon I will be tail heavy as the model is conversion from Glow job. BUT I also hope that changing from Lite Ply original to foam will help me.

I like to build larger battery areas just forward of CoG point - that way I can adjust weight by larger or smaller battery pack ...

I have to say that I just build and then see what it comes out as ... luckily so far I haven't had to resort to anything drastic .. they all come out OK. Maybe it's luck, maybe it's me not realising that my eye is better than I thought ?

Nigel

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Old 12-23-2012, 09:28 AM   #5
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If you know or can reasonably estimate the weight of all the main parts (wing, tail-feathers, fuselage, battery, motor, servos etc) then you can do it by mathematics.
  • Use some datum point, like the tip of the spinner (use a point right at the front to avoid getting negative numbers in the calc).
  • Take moments for each components (moment = weight of component x distance of the centre of the component from datum), do this for all components.
  • Add all the individual component moments together.
  • Divide result by total weight of the plane.

The figure you get is the distance from the datum point to the CG of the plane. If it doesn't come out where you want it to be move components around and re-do the calc. The math really isn't as bad as I've made it sound, it shouldn't take more than a few minutes. This method is basically the same way all engineers calculate CG for stuff like real planes during design, or even giant structures like offshore oil platforms etc where CG is critical (been there done that).
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Old 12-23-2012, 12:19 PM   #6
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One thing I try to do is note any ballast used in a build thread, in setting of the CG. If more folks did that, then there would be more info to research, when building a similar airframe with similar construction. After building about 60 models of widely varying airframes and construction, you get a really good feel just off the top of your head. The one thing that I had been thrown on in the past, but don't get thrown on now, is covering/painting. Generally the tail area including the feathers on an airplane requires far more covering/paint than the area in front of the CG. If you have the CG set perfectly before covering, then it's usually thrown a good bit rearward after covering. The same thing goes for the wing, as the majority of the wing area is behind the CG.

One thing I don't understand is why folks won't run pushrods, to avoid mounting servos rearward. With a few supports, you can put them in the cowl if necessary to avoid ballast, and not have any issues with slop. My Guillows 16" FW190 actually came out nose heavy, with the tail servos and battery box in the cowl, and an outrunner mounted on the front of the battery box. Moving the batt rearward a bit solved the issue, but it showed that even the most difficult subjects do not require ballast. The short nose moment Avro Type F that I recently built has the servos mounted in directly behind the cowl, with no ballast needed.
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Old 12-24-2012, 02:51 AM   #7
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But do we not first have to find the center of pressure and the desired CG location?
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Old 12-24-2012, 03:33 AM   #8
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I was presuming that, from the start, I know more or less where the final CG ought to be ...

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Old 12-24-2012, 05:44 AM   #9
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My Mentor used to check and recheck the c/g on his designs as he built them, moving the motor/battery/servos around as the build progressed, then installed the components with a bias towards nose-heavy. One thing to remember: a nose-heavy model only requires 1/4 the ballast for correction!
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Old 12-24-2012, 06:11 AM   #10
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Originally Posted by TLyttle View Post
My Mentor used to check and recheck the c/g on his designs as he built them,
I'm guessing he built i.c. power? i.c. give a lot less scope for adjusting CG as you dont have a big heavy battery to slide back and forth. I'd be disappointed if I had to add ballast to an electric model.
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Old 12-24-2012, 10:44 AM   #11
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Originally Posted by JetPlaneFlyer View Post
I'm guessing he built i.c. power? i.c. give a lot less scope for adjusting CG as you dont have a big heavy battery to slide back and forth. I'd be disappointed if I had to add ballast to an electric model.
You bet. Shouldn't happen often. I think the last time I ballasted was the sheeted Ansaldo SVA, where an occasional model requires ballasting when built with scale features. Most of the time it can be avoided if built properly.
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Old 12-24-2012, 12:33 PM   #12
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Originally Posted by JetPlaneFlyer View Post
If you know or can reasonably estimate the weight of all the main parts (wing, tail-feathers, fuselage, battery, motor, servos etc) then you can do it by mathematics.
  • Use some datum point, like the tip of the spinner (use a point right at the front to avoid getting negative numbers in the calc).
  • Take moments for each components (moment = weight of component x distance of the centre of the component from datum), do this for all components.
  • Add all the individual component moments together.
  • Divide result by total weight of the plane.
The figure you get is the distance from the datum point to the CG of the plane. If it doesn't come out where you want it to be move components around and re-do the calc. The math really isn't as bad as I've made it sound, it shouldn't take more than a few minutes. This method is basically the same way all engineers calculate CG for stuff like real planes during design, or even giant structures like offshore oil platforms etc where CG is critical (been there done that).
This is Beam Theory and use of Moments - works fine ...

Aircraft load using this ... Ships load using this .... It works.

For me - even though I was trained as a Ships Navigator / Cargo Officer - just build and then install gear. So far it's worked out fine. My SE5 .. QBee .. F15 ... F16 .... etc. have all come out fine. I think it comes down to the "what looks right ... usually is right" factor.

I remember years ago - at flying site a guy arrived with a radical model ...

Large nose / cockpit sort of BD5 style ... a flat wing .... and a large V tail with pusher prop. Now this model was quite large and had a 40 IC motor in it.
I looked at it .. others looked at it ... and then guy asked if anyone would test fly it. Myself and a few others checked out CoG etc. and I for one walked away not wanting to upset the guy. By eye - I considered it too short in length ...

Heres a video that has it in the first minute or so ... (no need to watch all unless you'd like to see model flying UK in 1980's ?)


The model was a disaster .. the 'eye said it was wrong' and the 'eye' proved to be right.

Nigel

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Old 12-24-2012, 05:29 PM   #13
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Originally Posted by mclarkson View Post
I've come to enjoy scratch building over the past couple of years and have had some success.

One thing that vexes me, though, is CG placement. I'll expect the plane to come out, say, nose-heavy so I put the servos, etc., way way back only to have the plane come out slightly tail-heavy. So now I'm in a scramble to shift the battery farther forward than I'd designed for, etc.

I never seem to know where the CG is going to come out until I've built the thing and, by then, it's too late. ("Man, I wish I'd put those wing servos way at the front instead of way at the back!")

I don't know if I'm being clear at all. Maybe the only way is to build it, realize how you ought to have built it, and then build it again, but I thought I'd ask if there were any tips or tricks.
Being as I'm too idle to do complicated math, what I do is to aim to put the big wooden bits together as soon as possible, plus placing the expensive bits. Motor will have to be bolted in, but ESC,battery and servos can be dropped into near enough positions. There are fudge factors to be applied - covering, for one, so cover the tailfeathers first, if possible. But it can give you an initial idea of how things are going.

Like most stuff in our hobby, it gets easier after a few shots at it! Good point is, you're thinking about this - which is way better than looking at a fully finished model with a CG aft of the trailing wondering what to do about it.

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Old 12-24-2012, 05:38 PM   #14
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Derek,
Funnily enough that's what I'd do most of the time too. The time I might start doing the math is if I was drawing up a proper plan and wanted to have the best chance of getting everything in the right place before I started building. Often even when doing a plan you don't need the math because you can compare it to a similar build and get it pretty close. The mathematics option is just another tool in the box.

Steve
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Old 12-24-2012, 08:12 PM   #15
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Originally Posted by JetPlaneFlyer View Post
Derek,
Funnily enough that's what I'd do most of the time too. The time I might start doing the math is if I was drawing up a proper plan and wanted to have the best chance of getting everything in the right place before I started building. Often even when doing a plan you don't need the math because you can compare it to a similar build and get it pretty close. The mathematics option is just another tool in the box.

Steve
Taking this further ... an old trick was to take a similar model plan and adjust to the design / shape you desire ... keeping moments, placements similar. Models and full size have been flying for many years now and unless we go full Fly by Wire as in Stealth etc. - we will be adhering to standard concepts and ratios ....

Nigel

222kph PKJ,EDF Concorde, Mini4,Mig3,T45,PKJ twin,ME109,Edge540,Cessna182,Skymaster Biplane,F15,F16,Badius,Ultimate,SE5,Qbee10,450 Heli,V911,J3 Cub Founder 9x forum: http://groups.yahoo.com/group/Flysky_RC_radio/
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Old 12-31-2012, 02:51 AM   #16
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If it's really weird in shape, I cut a small cardboard scale of the shape and toss it.

Provides amazing results
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Old 12-31-2012, 03:24 AM   #17
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Originally Posted by mclarkson View Post
I've come to enjoy scratch building over the past couple of years and have had some success.

One thing that vexes me, though, is CG placement. I'll expect the plane to come out, say, nose-heavy so I put the servos, etc., way way back only to have the plane come out slightly tail-heavy. So now I'm in a scramble to shift the battery farther forward than I'd designed for, etc.

I never seem to know where the CG is going to come out until I've built the thing and, by then, it's too late. ("Man, I wish I'd put those wing servos way at the front instead of way at the back!")

I don't know if I'm being clear at all. Maybe the only way is to build it, realize how you ought to have built it, and then build it again, but I thought I'd ask if there were any tips or tricks.
On my last couple of scratch built models, I've built up the left and right panels of the models nose 3 or 4 inches to long. Then the tail, wing, landing gear, servos are temporarily installed.

And last, the motor, ESC and battery pack are placed on top of the fuse, and slid back and forth until the thing balances at the CG. Then the firewall in glued into place at the proper location. Make certain the battery pack can be shifted back and forth an inch or two to tweak the balance point.

Rather crude, but it does work.

DennyV
Retired and the days are just too short, busier than ever!
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Old 12-31-2012, 08:42 PM   #18
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I for one am confused and amazed that someone who says they are a builder and pilot is unable to plug in physical traits
of their model and click a button to see if it will fly. That's one I can't rationalize.

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Old 12-31-2012, 10:17 PM   #19
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Was that directed at me? My question wasn't related to figuring out where the CG ought to be on a given model (eg 32mm behind the LE) but rather on figuring out where the CG was going to actually end up.

Especially if I'm building some funky scratch build like this one:


Should I shove the electronics all the way forward? Or all the way backward? Should the motor be 3" from the LE of the wing? Or 4.5"?

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Old 12-31-2012, 10:22 PM   #20
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Originally Posted by Flite-Metal View Post
I for one am confused and amazed that someone who says they are a builder and pilot is unable to plug in physical traits
of their model and click a button to see if it will fly. That's one I can't rationalize.
I think the OP wanted to know how to predict where the actual CG will be during the design phase so as to not have to add dead weight to get the model to balance properly.

It's never too late to have a happy childhood.
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Old 12-31-2012, 10:25 PM   #21
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Mclarkson,
Cut a cardboard copy of the aircraft, toss it gently, add washers unitil it glides smoothly

When it performs well, you will know the sweet spot and you not have trashed your model in the process

I did this with the Heinkel I am working on now, its so goofy looking I had no idea were to start
Happy New year
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Old 01-01-2013, 05:23 PM   #22
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http://adamone.rchomepage.com/cg_calc.htm For reg planes and this one is for canards http://adamone.rchomepage.com/cg_canard.htm joe
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Old 01-01-2013, 06:08 PM   #23
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Originally Posted by mclarkson View Post
Should I shove the electronics all the way forward? Or all the way backward? Should the motor be 3" from the LE of the wing? Or 4.5"?
On a 'plank' flying wing you usually need to move everything as far forward as possibly to get a flyable CG. The CG needs to be well forward 15-20% chord is a good starting point.
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Old 01-02-2013, 01:17 PM   #24
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This is extremely interesting in that everyone assumes you cram everything forward of a declared CG point.

After one learns to fly and pays particular attention to weight and balance either with a kit or some simple
planeform like a quickie or a pattern layout...you notice a direct relationship between fore and aft of CG
distribution of weight.

Moving an electric or recip motor should be the last consideration unless you placed an excessively small or
large one on the airframe...unless in the case of a multi you will have the accumulative mass accomplishing
with small movement what requires a greater distance forward with a single. Multi's are problematic...which
brings us to how do you "observe" cause and effect of distribution or redistribution of mass.

OK, acquire a "scrap" piece of particle board at Home Depot from their rip station. It should be about 1 ft. sq.
Measure the center point then draw a line through the centerpoint dot at right angles to one side. Divide the
line into thirds and drill a pair of holes at the location of the two points designating the center third.

Diameter of these holes is dependant on the diameter of the steel rod or all thread you purchase to go into
them. White glue rod or all thead into holes and let cure.

Two points can have pencil erasers placed over the sharp ends. The two sharp ends are the support point for
placing the model onto the two position points derived from using a CG calculation program or manual formula.

Typically you want to first balance the model right side up. This permits you to easily slide/relocate batteries to
accommodate what is assumed to be minor distances. The cowl on the front of the "scale" model will constrain
movement of the motor.

Always balance the model with the retracts in the "up and locked" position.

Always balance with a prop(s) on your model....the props you plan to fly with.

Always "first" do a weight and balance of the model "dry" if it is a recip or turbine.

..............THEN.............

After recording where the internal components are placed....

Turn the model upside down and rebalance it.

If a CG is too close to the declared location on plans or determined by calculation it will be "more" sensitive to
elevator control movement than if the CG were a wee bit ahead of that location.

Hope this elementary CG device will permit you to manage placement of internals in your models and eliminate
"guessing" between crashes.

Here's a PVC balance frame you can purchase at Home Depot



....Just scoot'n along...

Ed Clayman
"Flite-Metal For The Look Of The Real Thing"
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Old 01-02-2013, 03:49 PM   #25
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Originally Posted by Flite-Metal View Post
This is extremely interesting in that everyone assumes you cram everything forward of a declared CG point.
So have you ever built and balanced a 'plank' type flying wing that has no fuselage as such?, like the one in the photo

You do usually need to cram everything as far forward as possible in these designs because there is very little available 'lever arm' ahead of the CG. If you want to calculate it all mathematically then I posted the method on the first page of this thread, it's all to do with moments.

Regarding the balancing rig. The thread is not about how to check where the CG is on a model. It's about how to decide where to place components during design of a scratch built model in order to achieve the desired CG. The assumption is that you already know where you want the CG to be.
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