Find your Centre of Gravity!

by alibopo | November 11, 2013 | (26) Posted in Tips

How to find your plane’s true Centre of Gravity.

 

Hey! Hasn’t this been done-to-death before!

Listen - most times when folks talk about CG they mean that point on the wing where the plane’s manufacturer told them to stick their fingers so they can tell if the plane is tail or nose heavy.

Assuming the information the manufacturer or designer gave you is correct - you get this right and the plane will fly reasonably well. But there’s a little bit more to it than that.

That ‘wing’ CG is not the plane’s true Centre of Gravity - usually*.

* [It can be, or very close to it, on flying wings, and mid-wing aircraft like 3D planes. Plus - the term "'wing' CG" is just something I made up to help explain this. Read this bit really fast like an advertising disclaimer.]

The ‘wing’ CG is just a little in front of a point where an imaginary vertical line representing ALL the LIFTING forces created by the plane cuts through the wing.

LIFT is produced when the plane is moving, but to make things easier for us the ‘wing’ CG is calculated so that it can be used when the plane is sitting still.

Acting in the opposite direction (due to gravity) is the combined weight of all the plane’s parts. Again, to make this easier, this is gathered together into a single force acting through a line which (from bitter experience) always points straight down.

The trick of balancing a plane is to get the two opposing forces ‘LIFT’ & ‘WEIGHT’ to act near-enough through the same vertical line. Though for reasons of stability it is normal to create a slight 'nose heavy' condition that is constantly balanced by a small 'down' force created by the tail surface. If LIFT and WEIGHT are in too perfect a balance, the plane will be inherently unstable.

Looking at the extremes; if the forces don't act through the same line, it’s a bit like pedalling a pushbike – the forces try to rotate the plane about the centre of the ‘pedal crank’ (a point between the lines the two forces are acting along.)

Of course, once the plane has rotated to a position where the two forces line-up with each other - the rotation stops.This ‘stable position’ could be when the plane is pointing  upwards….

 … or downwards.

Flying downward tends to keep the plane moving – which is GOOD (within limits) as it keeps air moving across the wings (providing lift) and across the elevator (allowing control.) As mentioned earlier a little nose-down attitude is desireable to create stability - but too much can 'overwhelm' the elevator making the plane unresponsive and difficult to fly.

A nose-up attitude tends to slow the plane down - which is BAD, as the plane loses both lift and control. There's loads of other BAD stuff happens, but I'm keeping it simple here.

Within limits, this is why experts advise flying a little 'extra' nose-heavy until you get the feel for a plane (it also covers the possibility that the ‘wing’ CG the manufacturer gave you isn’t quite right).

It’s usually easier to achieve balance by changing the WEIGHT distribution, as the position of the LIFT component is hard to shift once a plane is built. In other words you’re more likely to move the battery (or any other ballast) than shifting the wing.

Hang on! Hang on! If the CG marked on my wing allows me to balance the plane for flight, surely that’s all I need. What’s this ‘other’ CG and why should I bother with it?

This ‘other’ CG is the true Centre of Gravity of the whole plane; the point where EVERYTHING balances. If you could tie a string to that point (usually impossible), and hang the plane on the string, it would sit at any angle you left it - regardless of orientation - it would just sit there, motionless, hanging on the string.

OK, that’s… interesting, but when would I need to know this true CG?

Well, generally speaking, you want the thrust of the motor to act through the true CG, so that when you apply more power it doesn’t cause the plane to rotate and pitch up or down. Changing the line of thrust can be achieved by angling the motor.

You’ve probably seen an example of this – the distinctive angling of a motor on a pusher plane that has the motor above the wing. Yes, they did it for a reason – it wasn’t just to stop the prop hitting the fuselage.

OK there are other factors like drag to consider, but knowing where the true CG is gives a good starting point for tuning a plane for efficient flight.

So if it’s worth knowing - how do I go about finding my plane’s true CG?

Well, it’s actually quite easy - you let gravity do all the work. First load up your plane for flight and get it balanced on that ‘wing’ CG.

Now pick a convenient point to hold your plane from and lift it by that point. Gravity will cunningly take charge and place the true CG directly below your fingers.

You can use a bit of string with a weight on it to help you see this better. (A laser level showing the vertical will also work.) Line the string up with where you’re holding the plane and mark where the string (or laser line) crosses the fuselage. The true CG lies somewhere on this line.

Hold the plane at another point and repeat the process. Where the two lines cross is where the true CG of the plane is (if you peeled away half the fuselage).

You can add more lines, and it could get a little more accurate – but if you do it right each time, you’ll find that the lines just keep crossing at the same point.

That point gives you a good indication of where the Centre of Gravity of the plane is...

                  ...which, among other things, can help when deciding on the best thrust angle for your motor.

Here's a real-life example. I used this exact technique to find my true CG for my Smash Drone, and then adjusted the angle of the swappable pod to direct the thrust through the CG. The plane flew a lot better after the adjustment. Josh Bixler suggested the same modification to the Fowl Flyer (7.00 min in to the video) after the first test flight.

This ‘hanging’ technique can be used to find the centre of gravity of ANY regular or irregular object.

COMMENTS

apnewton on November 18, 2013
Great article. Simply explained, I learned stuff.
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alibopo on November 18, 2013
Couldn't ask for more! Here in Scotland we're on the gloomy decline into winter - you're on the cheery way to summer. I'll be expecting some sunny-weather vids to keep us warm. Cheers. :)
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Bedrock on November 18, 2013
Good piece, thanks. Anyone would think you were some sort of teacher...oh wait a minute...:-) I've got a Bixler to build for the spring (I'm in Wiltshire, however from Lanarkshire!) and have been wondering why the prop was angled like that.
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alibopo on November 18, 2013
Hi Bedrock, yes, a teacher - now retired. Best of luck with your flying!
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squishy on November 19, 2013
This is a really good article but the title is entirely incorrect, it should read "How to find the proper CG of an aircraft in relation to the center of pressure in order to achieve inherent balance of the horizontal axis" I hate the phrase "Find the CG" and if you want to answer that question, it's easy, you put your fingers under the airplane. But your article answers much more than that, it's title should reflect that. I hate your title..I hate that phrase..
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alibopo on November 19, 2013
Hi Squishy, fair comment - but how many Flite Testers would have clicked on an article with THAT description? :)
I felt it was easier to get a handle on the empirical nature of the exercise IF I fudged some of the facts. I also fudged the whole resolution of component forces issue when the wing was inclined. Ach weel - as we say in Scotland!
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squishy on November 19, 2013
Maybe put the title in quotes and then explain it in the opening. I want to help spread knowledge and put a stop to traditions that were born of ignorance. I know what you mean though, you have to speak the language of your audience in order to reach them. I just see this as an opportunity to point our the error of that phrase.
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alibopo on November 19, 2013
Hi Squishy, I know where you're coming from, but that's a pretty steep hill to climb. Just about everyone in this 'hobby community' - including the manufacturers in their product descriptions - accepts common use of CG to mean the finger balance point. I think this is a no-win scenario. I'm happy enough with the article as it stands. Thanks for your input though. Cheers.
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jd7792 on December 12, 2013
Here Here I agree....I would not have read it in the first place and probably have thought quite differently about you than I do right now.
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Captain JiB on November 21, 2013
Thanks brother ! Its help me to find correct CG for my new scratch build cessna 182.
Cheers.
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alibopo on November 21, 2013
Great! Just to be sure we're on the same page - the method I've shown finds the true CG - not the 'wing balance point' commonly referred to as the 'CG'. If it's a scratch build without a known wing balance 'CG' you need to make a first estimate - the suggestion is between 25-30% of chord from the front of the wing. Using the Flite Test/Armin wing all my scratch builds seem to fall on or a tiny bit less than the 25% mark, so I tend to go for that, plus make it a little nose heavy just to be sure. My most recent maidens have worked well at that initial setting, though on some I have shifted them back a little once I get the plane trimmed and it's obvious it is nose heavy. Get back to me if you're now totally confused :)
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jd7792 on December 12, 2013
Really good article. Kept to simple english and with respectful regard for those of us that try to learn something evertime we read something. Well Done! Hope as many people as possible read this article.
JD
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alibopo on December 12, 2013
Thanks for that! Cheers.
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Pat on December 12, 2013
Thanks for this well laid out bit of info, much appreciated. I love learning stuff. I don't care what the title is, what you call it, or what I'm supposed to call it, I am just glad to know it!
Cheers!
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L3Grasshopper on June 18, 2014
Great article, practical too
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Mukhdi on September 16, 2014
Deat alibopo ... Can we find the CG without the wing installed? .. Im trying to balance scratch FT Racer
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alibopo on September 17, 2014
Hi Mukhdi, if it is built to the Flite Test plans the CG (balance point for flying) is already known for this plane. There is no point in finding a CG without the wing installed because the weight and position of the wing changes the final result. Just get your plane finished and fit the wing. Once everything is fitted you can balance the plane using the weight of your battery by shifting the battery backwards or forwards. For this 'low wing' design you turn the plane upside-down and put your fingers at the points specified on the wing (it appears as a small circle on the top of each wing in the plans - quite close to the fuselage). See this done in the build video at the 53 minute point. David demonstrates checking the balance, though he doesn't mention the exact position he's placing his fingers, (and also you can't see his fingers) but he is putting them on those two little circles. Balance the plane so that the nose is very slightly lower than the tail. This is a great flying plane. Hope you get it in the air soon. Cheers. :)
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obonfa on September 11, 2018
Hi folks, I found another article in this same forum talking about the thrust line through the CG at the CORD line, I'm confusing now, which one is the correct?
Link for the other thread:
https://www.flitetest.com/articles/Motor_angles_for_pusher_planes
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Find your Centre of Gravity!